Environmental Geology of the 

Front Range Urban Corridor and 

Vicinity, Colorado

By WALLACE R. HANSEN and ELEANOR J. CROSBY

With a section on PHYSICAL PROPERTIES and PERFORMANCE 

CHARACTERISTICS OF SURFICIAL DEPOSITS and ROCK UNITS in the 

GREATER DENVER AREA By RALPH R. SHROBA

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1230

A descriptive summary of geologic conditions 

in a region of varied physiography 

and rapid urbanization

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON : 1982
UNITED STATES DEPARTMENT OF THE INTERIOR 

JAMES G. WATT, Secretary

GEOLOGICAL SURVEY 

Dallas L. Peck, Director

Library of Congress Cataloging in Publication Data

Hansen, Wallace R., 1920-

Environmental geology of the Front Range Urban Corridor and vicinity, Colorado.

(Geological Survey Professional Paper 1230)

Bibliography: 99 p.

1. Geology Front Range, Colo. and Wyo. 2. Geology Colorado Denver region. 3. Environmental protection Front Range, Colo. and

Wyo. 4. Environmental protection Colorado Denver region. 5. Sediments (Geology) Colorado-Denver region-Testing. 

I. Crosby, Eleanor J. II. Shroba, R. R. III. United States. Geological Survey. IV. Title. V. Series. 

QE92.F7H36 624.1'51'097883 81-4280 AACR2

For sale by the Superintendent of Documents, U.S. Government Printing Office 

Washington, D.C. 20402
CONTENTS

Abstract ....................

Introduction .................

Geographic factors and area . 

The topographic map .......

Landforms ..................

Front Range ..............

Hogback belt ............

Colorado Piedmont .........

Piedmont lowlands .......

Flood plains and terraces 

Mantled lowlands ......

Piedmont uplands ........

Page

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Bench-and-valley uplands ............................... 10

Low rolling uplands .................................... 10

Complex uplands ...................................... 10

Composite uplands ..................................... 11

Climate .................................................... 11

Geology .................................................... 12

Descriptive outline of rock units .............................. 12

Geologic history ............................................ 15

Introduction ............................................ 15

Precambrian time ........................................ 15

Early and middle Paleozoic time ............................ 15

Late Paleozoic and early Mesozoic time ...................... 18

Present distribution of upper Paleozoic to middle Mesozoic

rocks ............................................... 21

Pre-Laramide, late Mesozoic time .......................... 21

Present distribution of pre-Laramide, upper Mesozoic rocks . . 23

Latest Cretaceous to early Eocene time ...................... 23

Middle and late Eocene time ............................... 25

Oligocene time .......................................... 25

Present distribution of the Oligocene rocks ................. 26

Miocene, Pliocene, and Quaternary time .................... 26

Geologic structure summary ................................... 26

General setting ............................................ 26

The Front Range .......................................... 27

Denver Basin .............................................. 29

Subordinate folding in the Denver Basin ..................... 31

Faults in the Denver Basin ................................ 31

Urban mineral resources ...................................... 31

Source areas .............................................. 32

Gravel and sand ........................................... 32

Sources of gravel and sand ................................. 34

Deposits underlying flood plains and terraces ............... 34

South Platte River ................................... 34

Cache La Poudre River ............................... 34

Big Thompson River ................................. 34

St. Vrain Creek ..................................... 34

Left Hand Creek .................................... 35

BoulderCreek ....................................... 35

South Boulder Creek ................................. 35

	Page

Urban mineral resources Continued

Coal Creek ......................................... 35

Beebe Draw ........................................ 35

Clear Creek ......................................... 35

Bear Creek ......................................... 36

Plum Creek ......................................... 36

Cherry Creek ....................................... 36

Monument Creek and Fountain Creek .................. 36

Upland gravel deposits on pediments and high-level Tertiary surfaces ...................................... 36

Conglomerate ....................................... 37

Clayrock .................................................. 37

Sources and production ................................... 38

Development problems and prospects ....................... 38

Productive rock units ..................................... 39

Dakota Group ......................................... 39

Benton Shale .......................................... 39

Pierre Shale ........................................... 39

Laramie Formation .................................... 39

Arapahoe Formation ................................... 40

Dawson Arkose ........................................ 40

Expandable shale .......................................... 40

Gypsum .................................................. 41

Igneous and metamorphic rocks .............................. 41

Igneous rock ............................................ 42

Metamorphic rock ....................................... 42

Vein rocks .............................................. 42

Limestone ................................................ 42

Manitou Limestone and associated rocks ..................... 43

Lykins Formation ........................................ 43

Niobrara Formation ...................................... 44

Sandstone and conglomerate ................................. 44

Lyons Sandstone ........................................ 45

Dakota Group ........................................... 45

Site rehabilitation .......................................... 45

Gravel and sand pits ...................................... 47

Clay pits and stone quarries ................................ 47

Coal ....................................................... 48

Subbituminous coal ........................................ 49

Lignite ................................................... 49

Surface effects of past underground mining ..................... 49

Oil and gas .................................................. 51

Soil problems ................................................ 54

Land development on expansive soils .......................... 54

Land development on eolian soils ............................. 55

Soils containing organic matter ............................... 55

Mountain soils and bedrock .................................. 56

Mass wasting ................................................ 57

III
IV CONTENTS

Hydrology ..............

Flooding ..............

Urbanization and runoff 

Ground water .........

Valley-fill alluvium .....

Pediment gravels .......

Terraces ..............

Eolian sand ...........

Bedrock aquifers .......

	Page 

............................... 58

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............................... 62

............................... 63

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............................... 64

............................... 64

............................... 64

Mountain land development and ground-water supplies .......... 65

Lakes .................................................... 66

Seismicity ................................................... 66

Physical properties and performance characteristics of surficial

deposits and rock units in the Greater Denver Area .......... 67

Introduction ............................................... 67

Eolian deposits ............................................ 68

General description ...................................... 68

Physical properties and performance characteristics ............ 68

Flood-plain and terrace alluviums ............................. 70

General description ...................................... 70

Physical properties and performance characteristics ............ 71

Pediment alluviums ........................................ 73

General description ...................................... 73

Physical properties and performance characteristics ............ 74

Slope deposits ............................................. 75

General description ...................................... 75

Physical properties and performance characteristics ............ 76

Manmade deposits ......................................... 76

General description ...................................... 76

Physical properties and performance characteristics ............ 77

Sedimentary rock units ........................................ 77

Slightly to highly expansive and weakly indurated units ........... 77

General description ...................................... 77

Physical properties and performance characteristics ............ 78

Sedimentary rock units Continued

Moderately to highly expansive and weakly indurated units ....... 79

General description ...................................... 79

Physical properties and performance characteristics ............ 80

Nonexpansive and weakly to moderately indurated units ......... 81

General description ...................................... 81

Physical properites and performance characteristics ............ 81

Nonexpansive and nonindurated units ......................... 82

General description ...................................... 82

Physical properties and performance characteristics ............ 82

Nonexpansive to slightly expansive and moderately to strongly indurated units .......................................... 82

General description ...................................... 82

Physical properties and performance characteristics ............ 83

Igneous and metamorphic rock units ............................ 83

General description ......................................... 83

Weathering ............................................... 83

Units commonly overlain by thick, slightly to highly weathered

regolith ............................................... 84

General description ...................................... 84

Physical properties and performance characteristics ............ 84

Units commonly overlain by thin, slightly to moderately

weathered regolith ..................................... 85

General description ...................................... 85

Physical properties and performance characteristics ............ 85

Units commonly overlain by thin and discontinuous, unweathered

to slightly weathered regolith ............................. 85

General description ...................................... 85

Physical properties and performance characteristics ............ 86

Units affected by faulting and igneous intrusion ................. 86

General description ...................................... 86

Physical properties and performance characteristics ............ 86

References .................................................. 86

ILLUSTRATIONS

FIGURE 1. Map showing place names ............................................................

2. Map showing landforms ..............................................................

3. Photograph showing sequence of strata in roadcut through the hogback at Mount Vernon Canyon

4. Photograph showing Devils Backbone, near Loveland .....................................

5. Maps showing late Paleozoic and Laramide uplifts ........................................

6-11. Photographs showing:

6. Precambrian gneiss and pegmatite in rock cut ......................................

7. Fountain Formation at Red Rocks Park ...........................................

8. Ripplemarks in Kassler Sandstone Member .......................................

9. Dinosaur footprint in South Platte Formation ......................................

10. Steeply dipping Fox Hills Sandstone on Green Mountain ............................

11. Denver Formation near Alameda Parkway, Lakewood ..............................

12. Sketch map showing Denver Basin and adjacent mountain ranges and uplifts .................

13. Photograph showing Big Thompson anticline near Loveland ................................

14. Map showing selected structural features ................................................

15. Sketch section along the 40th parallel ...................................................

16. Photograph showing extent of gravel extraction along Clear Creek ...........................

17. Photograph showing urban refuse backfilling an abandoned gravel pit near Clear Creek .........

18. Photograph showing Overland Park and Ruby Hill Park, Denver ...........................

Page 

4

7

8

9

18

19

20

22

23

24

25

27

29

30

31

33

46

48
CONTENTS V

Page

FIGURE 19. Map showing distribution of coal in the Denver Basin ................................................................... 50

20. Photograph showing extreme ground subsidence over an old underground coal mine at Marshall ............................... 51

21. Map showing distribution of oil and gas fields and coal ................................................................... 53

22-29. Photographs showing:

22. Wall damaged by stairstep corner fracturing ..................................................................... 54

23. Inactive dunefield southeast ofGreeley .......................................................................... 56

24. Deeply weathered grus west of Colorado Springs .................................................................. 57

25. Soil creep of clay-bearing rock, Jefferson County .................................................................. 58

26. Rockfall on eastbound Interstate 70 ............................................................................. 58

27. Damage caused by 1965 flood in Denver ......................................................................... 60

28. Flooding in Englewood, 1973 .................................................................................. 60

29. Debris-avalanche scar on Hunt Mountain near Castlerock ......................................................... 62

30. Map showing traces of known Quaternary faults ........................................................................ 67

31. Surficial deposits and their approximate ages for the classification of fine-grained materials .................................... 69

32. Plasticity chart .................................................................................................... 70

33. Lithologic subdivisions of the Pierre Shale ............................................................................. 78

TABLES

Page

TABLE 1. Rock units in the Front Range Urban Corridor, Colorado ................................................................ 16

2. Summary of Cretaceous coal occurrences in the Laramie Formation, Denver region .......................................... 51

3. Record discharges of selected streams, Colorado Piedmont area ........................................................... 61

4. Grain-size scale ................................................................................................... 70

5. Grain-size distributions of surficial deposits ............................................................................ 71

6. Unified Soil Classification chart ...................................................................................... 72

7. Atterberg limits and Unified Soil Classification group symbols for surficial deposits ........................................... 73

8. Volume change, swelling pressure, and unconfined compressive strength of surficial deposits .................................. 74

9. Grain-size distributions of rock units .................................................................................. 79

10. Atterberg limits and Unified Soil Classification group symbols for rock units ................................................. 80

11. Swelling pressure and unconfined compressive strength of rock units ....................................................... 80

METRIC CONVERSION FACTORS

Multiply To obtain

	Kv metric units ' U.S. customary units

millimeter (mm) 0.0394 inch (in)

centimeter (cm) 0.3937 inch (in)

meter (m) 3.281 foot (ft)

kilometer (km) 0.621 mile (mi)

square kilometer (km2) 0.386 square mile (mi2)

cubic meter (m3) 35.311 cubic foot (ft3)

ton (t) 1.1023 ton (U.S. short)

milligram per liter (mg/1) 1 part per million (ppm)

kilogram per cubic meter (kg/m3) 0.06243 pound per cubic foot (lb/ft3)

millimeter per hour (mm/h) 0.0394 inch per hour (in/h)

liter per minute (1/min) 0.26418 gallon per minute (gal/min)

kilonewton per square meter (kN/m2) 20.8333 pound per square foot (lb/ft2)

joule 0.00095 British thermal unit (Btu)

radian (rad) 57.31 degree

ENVIRONMENTAL GEOLOGY OF THE FRONT RANGE URBAN 

CORRIDOR AND VICINITY, COLORADO

By WALLACE R. HANSEN and ELEANOR J. CROSBY

ABSTRACT

Some geologic processes in the Front Range Urban Corridor have 

resulted from human activities; others are quite natural but have 

been enhanced artificially. Environmental stress intensifies when 

land development involves easily disturbed sediments such as loess, 

eolian sand, organic loam, and expansive clay. Adverse environmental impacts can sometimes be mitigated if geologic knowledge is applied to the solution of land-use problems.

The Front Range Urban Corridor, centered on Denver, straddles 

the boundary between the Front Range of the Southern Rocky Mountains and the Colorado Piedmont section of the Great Plains. This 

boundary is defined by a zone of sharp-crested foothills or hogbacks. 

The mountains are underlain by a crystalline complex of Precambrian 

igneous and metamorphic rocks, some as old as 1.75 b.y. (billion 

years). The hogbacks and piedmont are underlain by sedimentary 

rocks ranging in age from Cambrian to Holocene; about 25 different 

major rocks formations and many surficial units are recognized. 

Physical properties vary widely from unit to unit. Although the piedmont is more subdued topographically than the mountains, it has 

varied relief and form.

For this report, the long geologic history of the area is divisible into 

eight time intervals of unequal length. The longest interval, Precambrian time, is further broken by plutonic events 1.75-1.70 b.y. ago, 

1.45-1.39 b.y. ago, and 1.04-0.98 b.y. ago. Orogeny accompanied the 

first and second events. Three ensuing intervals of relative crustal 

stability were each followed by orogeny, in Pennsylvanian, Late 

Cretaceous, and Miocene time. In and after Miocene time, faulting, 

uplift, erosion, and deposition led to the elevation of the present Front 

Range and the shaping of the mountains and piedmont.

Two major features dominate the geologic structure of the Front 

Range Urban Corridor the Front Range uplift on the west and the 

Denver Basin on the east. These features are regional in size and extend far beyond the limits of the corridor. The Front Range is a broad, 

flat-topped anticline, but erosion has removed the sedimentary cover 

from the crustal part of the fold, and the internal structure of the 

Precambrain core is very complex. The boundary zone between the 

uplift and the Denver Basin is a steep monocline along which the 

sedimentary rocks are tilted, faulted, and, locally, folded into subordinate anticlines.

The Colorado part of the Denver Basin is broadly asymmetrical, 

having its axis much closer to the west border of the basin than to the 

east. At the deepest part of the basin, Precambrian rocks are more 

than 2,300 m below sea level. The general simplicity of the basin is 

modified by subordinate folds and faults, especially in the northern 

part.

Urban mineral resources are extracted from within the Front Range

Urban Corridor for local use. These primarily industrial products include gravel, sand, crushed stone, riprap, clay, dimension stone, 

limestone, and earth fill. Other products extracted, but not necessarily for local use, include coal, natural gas, and petroleum. To be competitive, urban mineral resources must be obtained close at hand, 

often within metropolitan areas. Urban growth, which creates the demand and consumes the products, sometimes curtails production by 

preempting the unextracted resources for other land uses.

Extensive deposits of gravel, sand, sandstone, fine-grained igneous 

rock, clay, and expandable shale underlie the Colorado Piedmont. The 

Hogback belt yields clay, limestone, dimension stone, crushed rock, 

landscaping rock, and gypsum. The mountain area is a source of 

crushed rock, riprap, gravel, sand, earth-fill material, and weathered 

granite (grus). Most high-quality gravel is extracted from beneath the 

flood plains or adjacent low terraces of the major streams. Less 

sought-after gravels cap higher terraces and pediments.

Most urban mineral extraction leads to the excavation of open 

pits, and what happens to the pits after extraction is completed has 

long-term significance to the community. In 1973 the Colorado 

Legislature enacted into law a bonded reclamation plan for such 

mined areas. Some pits have been backfilled with urban refuse and 

reclaimed as valuable real estate; to the extent that such a pit can be 

rehabilitated for other use, it is a community asset rather than a 

liability. Problems that result from solid-waste disposal in landfills, 

such as ground subsidence, noxious gas generation, and groundwater pollution, can be minimized by engineered design. Alternatively, exhausted pits can be converted to urban lakes and open 

space.

Coal of Late Cretaceous and Paleocene age underlies parts of all 

counties in the Front Range Urban Corridor. Little if any coal is 

now being mined, but a resumption of mining in the future seems 

likely. Most remaining Cretaceous subbituminous coal is too deep to 

be removed by surface mining, but a huge resource of strippable 

Paleocene lignite is in the Denver Formation just east of Denver. Individual beds of strippable lignite as much as 8 m thick lie generally 

less than 60 m below the surface. Past underground mining of subbituminous coal at shallow depths in the Boulder-Weld and Colorado Springs coal fields has caused ground subsidence which could 

present problems to subsequent land use.

The Denver Basin is one of the leading oil-and gas-producing 

areas of the Rocky Mountain States. By far the largest gas field in 

the basin is Wattenberg, which lies mostly in the Front Range Urban Corridor between Denver and Greeley and has the highest 

estimated ultimate gas productivity in Colorado. Oil has been produced in the Denver Basin chiefly from areas east of the corridor, 

but many wells inside the corridor are very productive. The most 

productive reservoir rock, for both oil and gas, is the "J" sandstone 

member of the South Platte Formation of the Dakota Group.
ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

Various kinds of soil problems, particularly with respect to 

highways and lightly loaded buildings, increase costs of preventive 

or precautionary construction and damage repair. Foremost are 

problems caused by expansive soils, which may create swell 

pressures exceeding 15 metric tons per square meter. Other problems are related to eolian soils, soils rich in organic matter, and 

residual or transported mountain soils. Mass wasting is closely 

related to soil problems, although it commonly involves bedrock 

also. Urbanization is spreading into areas where mass wasting is 

likely but can be minimized by care in site selection and treatment.

Urbanization has affected and been affected by the hydrology of 

the Front Range Urban Corridor. Urbanization has altered stream 

flows, the frequency and magnitude of floods, channel characteristics, scour and sedimentation, ground-water conditions, and water 

quality. Episodic flooding is perhaps the foremost natural hazard in 

the area, on the plains as well as in the mountains. Flooding is characterized by uncommonly high velocities and short durations. The 

seasonal distribution is distinctly bimodal May through 

September with peaks in the first week of June and the first week 

of August. Astonishing precipitation rates have accompanied some 

storms, the maximum recorded being 610 mm (millimeters) in one 

day, mostly in about 3 hours on May 30, 1935, at Elbert. Much 

ground water is available for use in the Front Range Urban Corridor 

in valley-fill alluvium, pediment deposits, terrace deposits, eolian 

sand, and bedrock. Bedrock aquifers contain vast quantities of 

ground water suitable for many purposes; the principal aquifers are 

in formations such as the Laramie, Fox Hills, Arapahoe, and 

Dawson Formations, but many mountain households obtain water 

from fractures in the Precambrian bedrock.

For a semiarid region, the Front Range Urban Corridor contains a 

surprising number of small lakes. Many are natural water bodies 

that have been enlarged by damming. Most are irrigation reservoirs, but with increasing urbanization, uses are changing. As centers of real-estate development and water-related recreation, lakes 

enhance the value of adjacent property.

The swarm of earthquakes that followed the injection of waste 

water into the deep disposal well at the Rocky Mountain Arsenal 

caused concern about possible seismic hazards in the area. Except 

for those tremors, however, few earthquakes larger than MM 

(modified Mercalli intensity) III have been experienced in the Front 

Range Urban Corridor in 100 years. Because no fault offset has 

been found in deposits younger than Sangamon(?) age (125,000 

years ago), the frequency of earthquakes strong enough to cause 

ground breakage has been very low.

INTRODUCTION

Since the late 1960's there has been a growing nationwide awareness of the importance of geologic and 

hydrologic information to the design and implementation of plans for urban land use. More than ever before 

people now realize the part that the earth sciences play 

in their daily lives. Nowhere, perhaps, has this awareness grown faster than in the urbanizing areas along 

the east slope of the Front Range in Colorado, centered 

on Denver and spread north to south from Fort Collins 

and Greeley to Colorado Springs. An alert technical 

community in the universities, engineering firms, businesses, and governmental agencies and a knowledgeable citizenry have been supported by legislative actions intended to assure that geologic and hydrologic

factors are considered in land-use planning and development. Laws enacted by the State Legislature have 

addressed ground conditions as related to land development, open-pit extraction of gravel, sand, coal, limestone, and other commercial mineral deposits in populous counties, reclamation of mined lands, and land 

development in areas of geologic hazard.

Progress has been made, but more work is needed. 

The flood plain concept is still widely misunderstood. 

Buildings are still being damaged by expansive soils. 

Urban growth is still encroaching on mineral resources 

and wildlife habitats. Nonrenewable resources that 

could be recycled are still being discarded. High concentrations of pollutants still enter the biosphere.

Environmental stress focuses on the urban setting. 

It increases when human activity alters the biosphere, 

as in the urban setting where most human activity is 

concentrated. For countless centuries the Front Range 

landscape slowly evolved under the influence of climate and weather, organic change, and geodynamic 

forces. Hardly perceptible day-to-day changes were accented by occasional exceptional events a flood, a 

rockfall, a landslide, or a debris avalanche. But as a 

result of human activities, the Front Range Urban Corridor has seen a greater rate of change since the time of 

pioneer settlement in the last century, than at any time 

in the previous 30 million years.

Along the Front Range Urban Corridor, environmental stress intensifies when land development involves 

certain easily disturbed sediments such as loess, some 

eolian sand, organic loam, and expansive clay. It intensifies when excavations involve marginally stable hillslopes or penetrate the water table, when mineral extraction (gravel, clay, stone, coal) disturbs natural processes, or when urban development alters the hydrologic regimen. Some geologic/geomorphic processes in 

the Front Range Urban Corridor have resulted partly 

or wholly from human activity. Subsidence over coal 

mines, for example, has caused a collapse topography 

that has adversely altered the local landscape and 

could damage superincumbent structures. Some other 

processes are quite natural but in the urban environment have been heightened artificially stream discharge and runoff are increased, for example, as are 

erosion and slope forming processes.

Human impacts on the environment tend to interact 

and multiply. For example, urbanization fosters highway construction and vice versa. Construction aggregate is excavated from a gravel pit that later is backfilled with urban refuse that reacts with seepage and 

generates leachate that seeps into the ground water. 

Noxious gases generated in the landfill escape to the 

atmosphere. Urbanization and, agricultural practices 

alter the soil chemistry and the natural runoff, which
INTRODUCTION

in turn affect water quality, water-table levels, erosion, 

and sedimentation. These in turn modify the habitat of 

aquatic organisms and the hydrologic amenity (Leopold, 1968). Adverse environmental impacts can sometimes be mitigated if geologic knowledge is applied to 

the solution of land-use problems. Environments 

change, but environmental quality can be maintained 

and enhanced if the cities, towns, highways, canals, 

reservoirs, airports, plowed fields, irrigated farms, 

mines, quarries, landfills, and myriad other enterprises 

of the Front Range are compatible with the natural dynamics of the biosphere. Four hundred years ago Sir 

Francis Bacon observed that "Nature to be commanded must be obeyed." But nature to be obeyed 

must also be understood (Robinson and Spieker, 1978). 

This report summarizes the results of U.S. Geological Survey activities in the Front Range Urban Corridor area during the last decade. Though the field of 

activities is broad, the summary is generalized and 

nonspecific. It is intended more to call attention to 

basic data than to answer questions. The fairly numerous literature citations in the text serve as departure 

points for readers interested in more thorough pursuit 

of specific topics. Many more papers could have been 

cited. A truly vast literature (Chronic and Chronic, 

1974) has accumulated through the years on the geology, hydrology, and related fields of the Front Range 

Urban Corridor.

GEOGRAPHIC FACTORS AND AREA

The Front Range Urban Corridor is neither a 

political nor a geographic entity, and its boundaries 

are indefinite. It means different things to different 

people. As limited by the l:100,000-scale topographic 

maps, its boundaries are lines of cartographic convenience drawn to conform to standard lines of latitude 

and longitude while at the same time encompassing 

most of the area of probable urban expansion within 

the foreseeable future.

As thus drawn, the area is about 62 km (kilometers) 

wide west to east through most of its length and about 

218 km long north to south. Near Colorado Springs it 

is about 33 km wide. It contains 79 7|-minute 

topographic quadrangles and has an area of nearly 

12,000 km2 (square kilometers). However, some planners and legislators in Colorado have come to regard 

the 'Front Range area" in a broader sense than that of 

this report. For administrative and statutory purposes, they have included cities such as Pueblo that are 

separated geographically from the more or less continuous urban corridor of cities and towns to the north. 

Pueblo has not yet shared the rapid growth of its 

neighbors to the north at the foot of the Front Range.

The Front Range Urban Corridor straddles the 

boundary of the Front Range, which is a major subdivision of the Southern Rocky Mountains, and the 

Colorado Piedmont section of the Great Plains (Fenneman, 1931, pi. 3). The Colorado Piedmont, insofar as 

the Front Range Urban Corridor is concerned, separates the Rocky Mountains from the High Plains and 

contains most of the State's population (fig. 1). 

Urbanization, however, is spreading into the mountains, particularly west of Denver but also west of 

other corridor communities. Although the entire area 

is drained by the South Platte and Arkansas Rivers 

and their tributaries, the mountains and plains communities have widely differing environments. Geology, 

hydrology, topography, soils, vegetation, climate and 

weather differ greatly from the plains to the 

mountains.

Because of its complex physiographic and geologic 

setting and its rapid growth rate, therefore, the Front 

Range urban Corridor faces a host of land-use problems. Many of the problems have a geologic/geomorphic basis and the potential for large environmental 

impacts. They are related to foundation conditions and 

soils, slope stability, mineral extraction, urban hydrology, waste disposal, and perhaps, seismicity.

THE TOPOGRAPHIC MAP

The topographic map of the Front Range Urban Corridor consists of three sheets identified from north to 

south as the Boulder-Fort Collins-Greeley Area (sheet 

1), the Greater Denver Area (sheet 2), and the Colorado 

Springs-Castle Rock Area (sheet 3). At the time the 

Front Range Urban Corridor project was started as a 

pilot study, much consideration was given to the kind 

and scale of topographic map on which to plot data 

drawn from all the diverse elements of the program. 

The basic topographic map would be an important 

product on its own merit it would be widely used by 

people with varied needs and requirements. This 

judgement later proved to be correct, and the Greater 

Denver Area map soon had to be reprinted to meet the 

ongoing demand.

Several innovations were incorporated into the topographic map to serve the special needs of the project. 

One of the most important was scale. Other considerations contour interval, map detail, line weight, preparation time, and cost estimates all depended on 

scale. Subdivision planners might prefer the largest 

scale available, but regional planners require a smaller 

synoptic scale for showing regional relationships. The 

scale is determined by the need.

In view of the size of the Front Range Urban Corridor and the degree of detail desired, a scale of
ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO
INTRODUCTION

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ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

1:100,000 for the base maps was chosen after consideration of such scales as 1:95,040 (1.5 mi/in, (miles per 

inch)), 1:96,000 (8,000 ft/in, (feet per inch) and a fraction of the standard 1:24,000 scale), 1:125,000 (a multiple of the widely used quarter-million scale), and 

1:126,720 (2 mi/in.). A scale of 1:100,000 had not previously been used by the U.S. Geological Survey, although it is widely used abroad and is now coming into 

general U.S. Geological Survey use. It preserves a synoptic map quality lacking at larger scales, without 

sacrificing the accuracy and detail missing from 

smaller scales. With the map spread out on a conference table, the user can view at a glance the geographic 

features of the entire Front Range Urban Corridor.

A scale of 1:100,000 was preferred to other nearly 

equal scales (that is, 1:96,000) because it is a step 

toward the metric system of measurements: 1 cm (centimeter) on the map equals 1 km (kilometer) on the 

ground. The metric scale, moreover, is particularly 

suited to computer treatment of map information. At 

the smaller scale of 1:125,000, which is not metric, the 

contour density in the mountains probably would have 

required an interval of no less than 200 ft (feet), which 

is manifestly too large for the plains. The finally 

adopted 100 ft interval with local supplemental 50-ft 

contours well portrays the regionwide topography.

For the best possible fit of map data to the base, a 

transverse Mercator projection was designed by cartographer Wesley S. Hupp, U.S. Geological Survey 

(retired). Because of the curvature of the earth, no flat 

map of a large area completely lacks distortion. The 

transverse Mercator projection, however, provides approximate equal-area coverage with minimal distortion 

for an area that is elongated north to south like the 

Front Range Urban Corridor. A meridian rather than 

the Equator is the circumference of tangency here the 

105th meridian, which bisects the area. In effect, the 

105th meridian of the projection is in contact with the 

globe, and all other points on the ground are projected 

up to the imaginary plane of the projection in proportion to their distance from the circumference of tangency. Map distortion increases away from the 105th 

meridian but is negligible for an area the size and 

shape of the Front Range Urban Corridor.

Distinctive nonstandard treatments were used to 

show map features of special concern to the land-use 

planning community. Roads and streets are shown as 

single lines so that their map widths are approximately true to scale. Heavy-duty roads and paved 

streets are shown in red; light-duty roads, unimproved 

dirt roads, and trails are shown in black. This scheme 

eliminates the need for the rather arbitrary pink tint 

used to denote urban areas on conventional maps the 

welter of red street lines creates its own pink tint.

Other map features that have specific land-use significance are shown by distinctive tints also for example, 

cemeteries, parks, recreation areas, airports, Federal 

installations, and woodlands. Standard colors are used 

for other map features: blue for water, brown for contours, and black for lettering, political boundaries, 

selected buildings, railroads, and marginal data.

LANDFORMS

The eastern slope of the Front Range section of the 

Southern Rocky Mountains forms the westernmost 

part of the urban corridor. It is paralleled along much 

of its length by a narrow zone of still lower sharpcrested foothill ridges, sometimes called the hogback 

belt (fig. 2). The part of the corridor east of this belt is 

in the western part of the Colorado Piedmont.

FRONT RANGE

Within the corridor, the eastern part of the Front 

Range rises as much as 1,450-1,525 m (meters) above 

the adjacent piedmont, reaching elevations of 

2,700-3,000 m above sea level. Even higher mountains 

stand farther west. The sharp contrast between the 

mountains and the piedmont is a result of uplift and 

erosive resistance of the mountain belt. Narrow ridges 

and deep canyons characterize the mountain front, but 

many of the canyons widen upstream, the slopes flatten, and the ridge tops expand into rolling uplands. 

These subdued areas contain most of the small mountain towns and outlying mountain residential developments. Evergreen is such a development southwest of 

Denver on Bear Creek. Bear Creek flows through a narrow canyon along much of its length, but Evergreen 

and other small communities fill wide places.

HOGBACK BELT

The hogback belt along the mountain front rises 

from a few meters to as much as 300 m above the western border of the Colorado Piedmont. The hogbacks are 

the edges of steeply tilted, resistant sedimentary 

strata (fig. 3) that once covered the crystalline core of 

the Front Range. As the mountains rose, the sedimentary cover was eroded away, leaving the tilted strata at 

the mountain flanks. These strata extend far eastward 

at much gentler inclinations beneath the Colorado 

Piedmont and, still farther east, beneath the High 

Plains.

The basal beds of the hogback sequence can be seen 

lying against the crystalline rocks in many places 

along the mountain front, such as Red Rocks Park 

west of Denver. The hogbacks are separated from one 

another by narrow valleys cut into less resistant rocks
105°22'30" 

40°37'30" 105W

LANDFORMS

104°37'30"

40°00'

39°22'30"

EXPLANATION

Front Range 

Hogback belt

Colorado Piedmont 

Piedmont lowlands 

Floodplains and terraces

Mantled lowlands

Piedmont uplands 

Bench-and-valley uplands

Low rolling uplands 

Complex uplands 

Composite uplands

38°37'30"

FIGURE 2. Map showing landforms of the Front Range Urban Corridor.
ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

Dakota Group

Morrison Formation

Ralston 

Creek \ 

Formation

South Platte Formation ,

J*

FIGURE 3. Sequence of tilted strata at Interstate 70 roadcut through the hogback at Mount Vernon Canyon, Jefferson Co., July 1972, looking southward.

(fig. 4). Most of these valleys are floored with soil and 

rock debris from the adjacent ridges; landslides and 

rockfalls are common. Hill slopes commonly are 

steeper toward the mountains than toward the plains; 

they range from a few degrees to nearly vertical.

COLORADO PIEDMONT

The Colorado Piedmont section of the Great Plains 

extends along the Rocky Mountain front from a boundary near the Wyoming State line to an indefinite 

margin beyond Pueblo in southern Colorado. The piedmont is bounded on the east by the western border of 

the High Plains section of the Great Plains. The westernmost point of the High Plains is about 113 km 

southeast of Denver, and, thus, the High Plains are entirely east of the area, although many people mistakenly think that Denver and other corridor cities are 

on the High Plains. Only the western part of the piedmont, moreover, is within the Front Range Urban Corridor, but that part of the piedmont contains most of 

the corridor population.

In the geologic past, the High Plains extended west 

across what is now the piedmont to the mountain 

front, but the old surface has since been dissected by 

erosion so that none of it remains in the Front Range 

Urban Corridor. Some parts of the divide between the 

Arkansas and South Platte Rivers (Palmer divide and 

a few other areas), however, are close to the original 

High Plains surface. In the dissection process, erosion 

and down cutting have alternated with deposition. 

Loose sediment moved by wind also helped shape the 

piedmont. These processes have constructed a rolling 

surface that is subdued compared to the mountains 

but is more varied than the western High Plains.

The South Platte River drains northeastward across 

the northern three-quarters of the corridor. Fountain 

Creek drains southward in the southern quarter to join
LANDFORMS

FIGURE 4. Devils Backbone, near Loveland. Steeply dipping beds are on southwest flank of Big 

Thompson anticline. Resistant strata form hogbacks; easily eroded beds form strike valleys. 

Low-altitude aerial photograph, looking northwest, by Brent N. Petrie, planning consultant, 

April 1973.

the Arkansas River nearly 50 km south of the corridor 

boundary. Between the two drainage systems, the 

Palmer divide (the corridor segment of the South 

Platte-Arkansas divide) reaches above 2,300 m. 

Minimum elevations on the corridor are about 1,400 m 

on the South Platte east of Greeley and about 1,645 m 

on Fountain Creek south of Colorado Springs. The 

regional gradient from Palmer divide to exit points of 

the two principal streams is less than 2 percent (less 

than 10 m/km) to the south and less than 1 percent to 

the north. Many local landforms, however, are much 

steeper.

In this report, the landforms of the Colorado Piedmont are divided into lowlands and uplands. These 

broad groups are further divided into several categories. With few exceptions the boundaries between land 

forms are gradational.

PIEDMONT LOWLANDS

FLOOD PLAINS AND TERRACES

The piedmont lowlands include flood plains, terraces, and mantled lowlands. Flood plains are 1.5-3 km 

wide along the lower reaches of the South Platte River 

and its major tributaries in the northern half of the corridor. They decrease in width upstream and disappear 

from some of the smaller drainages. In the Fountain 

Creek drainage system in the southern part of the corridor flood plains are less than 1 km wide. The widths 

cited here are based on the approximate extent of 

100-year flooding, as calculated by McCain and 

Hotchkiss (1975a, b, c). The broader flood plains are 

nearly flat bottom lands of very low relief. Slight surface irregularities are due to incised or abandoned
10 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

channels and very low terraces, or to gravel pits and 

other human works.

One or more terraces border many of the flood plains 

at heights of about 3 m to 21 m (Colton, 1978; Trimble 

and Machette, 1979a, b). Terraces are more than 3 km 

wide along the lower reaches of the South Platte River, 

but elsewhere they are mostly less than 1 km. Well-preserved terraces are nearly flat; others are variously 

dissected.

MANTLED LOWLANDS

The mantled lowlands are in the northeast part of 

the Front Range Urban Corridor. The mantled lowland 

surface has very low irregularities related largely to 

wind deposition and erosion. Beebe Draw, between 

Greeley and Brighton, and the Eaton-Ault area north 

of Greeley are segments of former stream valleys 

covered by eolian (wind-blown) sand and silt. Beebe 

Draw contains a former channel of the South Platte 

River. Broad tracts of dune sand have been stabilized 

in most places by natural vegetation or cultivation, 

but in some places blowouts are occasionally activated 

by strong winds. Water entering the mantled lowlands 

moves in the subsurface or in manmade canals.

PIEDMONT UPLANDS

The term "uplands" is used here for all interstream 

areas on the Colorado Piedmont above the levels of 

flood plains and bordering terraces or above the mantled lowlands. The several upland categories are named 

according to their predominant landforms, but they 

contain other smaller areas, and the various landforms 

commonly grade into one another. With these qualifying factors in mind, we may divide the Colorado Piedmont into bench-and-valley uplands, low rolling uplands, complex uplands, and composite uplands.

BENCH-AND-VALLEY UPLANDS

The name "bench-and-valley uplands" is here applied to a zone about 3-16 km wide just east of the 

hogback belt. A bench, in this rather free usage, is a 

nearly flat tongue of land that slopes generally eastward at a low angle from the hogbacks or the mountain 

front. Some benches widen away from the mountains, 

as at Rocky Flats south of Boulder, and many are 

notched marginally by gullies. Bordering slopes are 

gentle or steep and smooth or gullied. Heights may be 

60-120 m, but mostly are less. Where erosion has 

severed a bench from an adjacent hogback or mountain 

front, the free-standing, flat-topped landform is a mesa 

or, if the summit area is very small, a butte.

Nearly all benches, mesas, and buttes in the benchand-valley uplands are capped with gravel that was

deposited by streams flowing out of the mountains in 

the geologic past, when the benches were the valley 

floors.

Valleys between benches have been partly or completely stripped of a once more extensive gravel capping. Present-day streams are small, and they only intermittently have enough water to cause erosion. 

Where tilted strata crop out, a complex of low ridges, 

flats, and hollows contrasts with the smoother surfaces of the benches.

Bench-and-valley topography is well formed in the 

U.S. Air Force Academy area north of Colorado 

Springs, in the Rocky Flats area south of Boulder, and 

in and north of Boulder. Table Mountain north of 

Boulder is a good example of a gravel-capped mesa in 

the bench-and-valley zone. (North and South Table 

Mountains at Golden are mesas capped by lavas rather 

than gravel.)

LOW ROLLING UPLANDS

Low rolling uplands extend from the northern 

boundary of the Front Range Urban Corridor southward 24-29 km beyond the center of Denver and from 

the bench-and-valley uplands on the west to the east 

edge of the corridor. With few exceptions, the relief is 

lower than on other parts of the piedmont. In interstream areas, the proportion of bordering slope to upper surface is generally small except at the transition 

to the bench-and-valley uplands. This transition is 

especially noticeable north of Denver along the west 

side of the South Platte River to about the latitude of 

Boulder.

The subdued topography of much of the low rolling 

uplands is caused by a cover of eolian silt and sand, 

which is extensively farmed. In a few places, however, 

the soil cover is too thin to mask the irregularities of 

underlying bedrock which, locally, is exposed. Isolated 

remnants of gravel top a few of the higher hills.

The western and southern margins of the low rolling 

uplands generally correspond with the limits of 

widespread eolian silt and sand. These margins are 

about 1,555-1,585 m above sea level near Fort Collins 

and 1,770-1,860 m south of Denver. The lowest areas 

are slightly above 1,400 m, in the South Platte valley 

near Greeley. Local relief rarely exceeds 75 m and in 

most places is less than 30 m.

COMPLEX UPLANDS

The term "complex" refers to a diversity of landforms controlled by variations in bedrock. Generally 

the surface is rocky or is covered by residuum 

weathered from the bedrock itself. The bedrock surface 

is mantled in only a few places by eolian sediments and
CLIMATE 11

ancient river gravels. The complex uplands extend 

from the southern boundary of the low rolling uplands 

south across the Palmer divide, where elevations are 

above 2,300 m, and down the shorter slope south of the 

divide to elevations between 2,135 and 2,170 m. The 

southern boundary is the approximate upper limit of 

eolian mantle in the adjoining composite uplands division of the piedmont. Locally on the north slope of the 

Palmer divide the complex-uplands surface reaches 

west to the mountain front, breaking the continuity of 

the hogback ridges.

The complex uplands are divisible into two major 

groups of landforms having many variants. The more 

conspicuous forms are mesas, buttes, and transitional 

features. Some larger, free-standing buttes and mesas 

rise 150 m or more above their surroundings, but many 

are much lower. Typically they are capped by cliffforming rimrocks of sandstone, conglomerate, or 

volcanic rock overlying weaker slope-forming sedimentary rock. Upper surfaces are slightly undulatory, but 

where erosion is far advanced, the summits are reduced 

to narrow ridges or points. Included in this moreconspicuous first group are mesalike uplands that 

merge with the uncapped less-conspicuous interdrainage ridges of the second group of landforms. The 

largest of the incompletely developed mesas extends 

from Castle Rock east and southeast beyond the 

border of the corridor. Cherry Creek and some of its 

tributaries have notched or cut completely through the 

northern bluffs of this mesa to depths of as much as 90 

m; East Plum Creek and its tributaries have shaped 

the western margin.

The second group of landforms in the complex upland consists of interdrainage ridges without resistant 

cappings underlain by easily eroded sandstone, mudstone, and claystone. The resultant topography contrasts with the broad-topped mesas, and it also differs 

from the interstream areas of the low rolling uplands 

to the north. The landscape is much more closely 

dissected than most of the low rolling uplands, where 

minor drainage courses are so shallow that they are not 

shown by 10-ft (3-m) contours on a topographic map. 

Also, in the ridge-dominated areas of the complex 

uplands, slope width generally exceeds summit width, 

except high on the north side of the Palmer divide, 

where broad ridge summits dominate an open rolling 

landscape. Even there, however, many of the ridges 

have definable crest lines.

The upper south slope of the Palmer divide is slightly 

steeper in most places than the upper north slope, and 

it supports a dense coniferous forest; much of the north 

slope is open forest or grassland. The forest on the 

south slope tends to obscure details of dissection and 

ground roughness, although the uppermost south

slope is about as dissected as the uppermost north 

slope. In and below the eastern part of the forest fringe 

on the south slope, however, shallow dissection has 

produced an intricate pattern of gullies separated by 

many knobby ridges.

COMPOSITE UPLANDS

The area east of Monument and Fountain Creeks, 

from the southern border of the complex uplands to the 

southern border of the Front Range Urban Corridor, 

contains landforms characteristic of all of the other 

divisions of the piedmont. Because no one landform 

predominates, this division is designated as composite 

uplands. Remnants of ancient river gravels may once 

have been continuous with gravels on the bench-andvalley uplands to the west. Parts of the area resemble 

the low rolling uplands in the northern part of the corridor; eolian sand and silt cover much of both places. 

Elsewhere, as in the complex uplands, differences in 

bedrock dictate the landform; examples are the sandstone bluffs north and northeast of Colorado Springs 

(Palmer Park, Austin Bluffs, and other unnamed 

bluffs) and the canyonlike valley of Jimmy Camp 

Creek. Flood plains and terraces also are present.

CLIMATE

Astride the boundary between the Great Plains and 

the Southern Rocky Mountains, the Front Range Urban Corridor has a varied climate. Many local differences result from the infinitely varied topography 

and aspect (exposure) and from variations in altitude 

in the different parts of the area. Natural variations, 

moreover, are modified by the climatic effects of urbanization, which not only alter local temperature, circulation, and cloudiness, but also degrade the air quality around the larger communities.

The climate of the Colorado Piedmont is semiarid 

temperate continental, sometimes referred to as a 

"middle latitude steppe" (Trewartha, 1961, map facing 

p. 8). In parts of the adjacent foothills the climate is 

subhumid. Mean annual precipitation within the corridor ranges from about 280 mm near Greeley 

on the piedmont to more than 650 mm in the mountains west of Boulder. Monthly mean temperatures on 

the piedmont range generally from about 21 ° C in July 

to about -2° C in January. The maximum for any one 

month may vary from about 38° C in summer to about 

16° C in winter. Extreme low temperatures range from 

around -30° to -40° C. Temperatures are nearly 

always cooler in the mountainous parts of the corridor 

than on the plains, but winter temperatures are sometimes lower on the plains when shallow, blanketlike inversions spread across the plains but fail to reach into 

the mountains.
12 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

A capsule summary of the climate of the Front 

Range Urban Corridor (Hansen and others, 1978, p. 58) 

includes the following:

 Highly variable weather elements (temperature, 

humidity, cloudiness, precipitation, and wind) along 

the Front Range and the Colorado Piedmont; areas 

of higher altitude in general have lower mean temperatures, greater cloud cover, greater precipitation, 

and greater variability in other weather elements.

 Strong seasonal variations of most weather 

elements.

 General semiaridity at the lower altitudes, with 

average rainfall of 300-350 mm per year on the Colorado Piedmont and 400-500 mm or more in the 

mountains.

 About 70 percent of the precipitation in the warm 

season between April and September.

 Completely clear weather over most of the area on 

about half the days.

 Cloudy weather less than a third of the time over 

two-thirds of the area.

 A persistent, well-defined heat island over the 

Greater Denver area caused by broad expanses of 

heat-absorbent and heat-radiant material, such as 

streets and buildings, and by the release of artificially generated heat.

 A persistent climatic ridge extending eastward 

across the Colorado Piedmont in the vicinity of the 

Palmer divide, caused by the orographic effect of the 

divide on local air masses.

 A north-south belt of occasionally very strong foehn 

or chinook winds, especially in winter, concentrated 

in the foothills and the immediately adjacent 

piedmont.

 Frequent temperature inversions along the valleys 

of the South Platte River and Fountain Creek, 

especially in winter.

GEOLOGY

The geology of the Front Range Urban Corridor is 

presented here in three forms. The first is a tabulation 

of formations, designed to show quickly the relations 

of the rock units in the southern, central, and northern 

parts of the corridor (table 1). The second is a listing of 

the rock units, from younger to older, with a brief description and notes on thickness and distribution for 

each unit. The third section is a narrative account of 

the geologic history of the corridor and, in part, the 

surrounding areas. These presentations are based on 

the geologic maps of the Front Range Urban Corridor 

(Colton, 1978; Trimble and Machette, 1979a, 1979b) 

and on the extensive geologic literature that preceded 

these maps. Only a few of the many papers available

have been cited mostly publications that summarize 

or supplement the main body of knowledge regarding 

the Front Range Urban Corridor. These papers provide 

additional citations from the literature.

DESCRIPTIVE OUTLINE OF ROCK UNITS1

Quaternary

Lower level alluvial deposits (Holocene and Pleistocene) 

Variably sorted gravel, sand, silt, and clay; cobbly to bouldery 

along major streams, becoming sandier eastward; poorly to 

well sorted, well stratified; soil profiles poorly to well 

developed. Form valley-fill terraces. Major units, from 

younger to older: post-Piney Creek alluvium and Piney Creek 

Alluvium, dark gray to brown, humic (about 2-11 m thick 

and less than 12 m above present stream levels); Broadway 

Alluvium, yellowish brown (less than 2-38 m thick (locally including Louviers) and 0-13 m above present stream level); 

Louviers Alluvium, yellowish to reddish brown (9-12 m thick 

and 2-21 m above present stream level).

Upper-level alluvial deposits (Pleistocene)

Cobbly, bouldery gravel near mountain front and along major 

streams, finer grained (sand, silt, and clay) on tributaries and 

eastward; generally poorly sorted, moderately to well compacted, stratified; commonly contains calcium carbonate that 

coats and cements gravels; soil profiles well developed, some 

truncated by erosion. Deposited as thin pediment gravels and 

thicker alluvial-fan or terrace gravels on stream-cut surfaces 

of bedrock (pediments). Major units, from younger to older: 

Slocum Alluvium (6-7 m thick and 24-40 m above present 

stream levels); Verdos Alluvium (3-6 m and 60-76 m); Rocky 

Flats Alluvium (3-15 m and 73-107 m); Nussbaum alluvium 

(2-24 m and 167-213 m); pre-Rocky Flats alluvium (1-2 m 

and 103-115 m).

Eolian sand, silt, and clay (Holocene to upper Pleistocene) 

Loess (silt and subordinate fine sandy silt and clay) and sheets 

or dunes of fine to coarse sand, less than 1 m to 15 m thick, 

blanket much of the corridor east of the foothills. Source of 

commercial sand near Colorado Springs.

Other surficial deposits (Holocene and Pleistocene) 

Landslide deposits, active and inactive: rock and soil debris, 

bouldery to sandy with a variable content of silt and clay; 

usually formed by moderately slow to very rapid downslope 

mass movement induced by gravity and, commonly, water 

saturation. Colluvium: unconsolidated rock and soil debris 

slowly moved downward and deposited on slopes by gravity 

and slopewash processes. Includes talus at base of steep 

cliffs. 

Tertiary

High-level gravel deposits (Pliocene to Oligocene) 

Rounded to subangular pebbles, cobbles, and boulders generally 1-2 m in maximum dimension but as much as 6 m in 

diameter, in a sandy matrix. Stones are chiefly granite and 

other Precambrian rocks, but include Tertiary volcanic rocks 

in the southern part of the corridor; 150-300 m above present 

main streams.

' Compiled from Braddock and others (1970), Colton (1978), Crosby (1976), Grose (1960), 

Larson and Hoblitt (1973), Maberry and Lindvall (1972), Maughan and Wilson (1960), Pipiringos and O'Sullivan (1976), Scott (1972d), Scott and Wobus (1973), Trimble and Machette 

(1979a, b), Tweto (1975, 1976), Tweto and Sims (1963), Van Horn (1976), Walker and Harms 

(1972), and Weimer and Land (1972a, b).
GEOLOGY 13

Tertiary Continued

Castle Rock Conglomerate (lower Oligocene) 

Bouldery cobbly gravel in a sandy matrix; includes mudstone, 

claystone, siltstone, and sandstone in eastern part of corridor. 

Stones are Precambrian rocks and subordinate chert and Tertiary volcanics; gravel matrix well cemented by silica, locally 

less well cemented near base. Generally less than 15 m thick.

Wall Mountain Tuff (lower Oligocene)

Light-gray, fine-grained rhyolitic volcanic rock (74 percent 

silica) composed mostly of devitrified glass shards. Formed 

as an ash-flow sheet; probably less than 15 m thick.

Quartz monzonite (Eocene)

Small irregular intrusive bodies and stocks of fine- to mediumgrained quartz monzonite in the northwest part of the corridor, associated with the Colorado mineral belt (Tweto and 

Sims, 1963).

Green Mountain Conglomerate (Paleocene) 

Conglomerate, in part andesitic, and subordinate sandstone, 

siltstone, and claystone; contains Paleocene pollen and fossil 

plants. Thickness about 198 m. Preserved only on Green 

Mountain, south of Golden.

Lava flows and small intrusive masses (Paleocene) 

Dark-gray porphyritic lava flows, three on North Table Mountain and two on South Table Mountain at Golden (Van Horn, 

1976, p. 47), are tentatively correlated with an intrusive dike 

of similar composition near Ralston Reservoir that may have 

been the feeder for the flows. Other intrusive rocks, mostly 

sills, between Golden and Lyons are of about the same age. 

Maximum thicknesses of individual flows range from about 

18.3 m to about 52.4 m. The Ralston dike and the Valmont 

dike east of Boulder were called alkalic basalt by Larson and 

Hoblitt (1973); other geologists have used the names monzonite or shoshonite for the intrusives and mafic latite or 

shoshonite for the flows (Van Horn, 1976; Scott, 1972d) Larson and Hoblitt designated other intrusives, northward 

toward Lyons, as rhyodacite. 

Tertiary and Mesozoic

Dawson Arkose (Paleocene to Upper Cretaceous) 

Dominantly lenticular, locally crossbedded, white to yellowishgray or brown arkosic conglomerate or coarse sandstone, 

siltstone, and olive-brown to variegated claystone. Contains 

lignite in Adams and Arapahoe Counties and the Kassler area 

and two zones characterized by andesitic pebbles and finer 

detritus near Colorado Springs. Maximum thickness about 

750 m. In part may be laterally equivalent to Green Mountain 

Conglomerate, Denver Formation, and Arapahoe Formation 

in the Denver metropolitan area; data published by Soister 

(1978, p. 227, fig. 2) indicate that the lower member of the 

Dawson Arkose is no older than late Paleocene, the main 

body of the Dawson is of Eocene age, and the formation 

overlies the Denver Formation where both units are present.

Denver Formation (Paleocene to Upper Cretaceous) 

Brown to olive fluvial claystone, siltstone, sandstone, and conglomerate; interbedded mudflows contain pebble- to bouldersize blocks of rock in a clayey to sandy matrix. The Denver is 

derived predominantly from andesitic volcanic rock, locally 

as much as 95 percent. The tuffaceous sandstone and finer 

grained rocks contain montmorillonite that swells when wet 

and may damage houses, roads, and other construction. 

Thickness as much as 280 m. Formation largely restricted to 

the Denver metropolitan area. Probably equivalent to and intertongues southward with the middle and upper parts of the 

Dawson Arkose; at Golden is interbedded with Paleocene 

lava flows in North and South Table Mountains. Upper part 

contains mammal bones, lower part contains dinosaur remains and fossil leaves and wood.

Mesozoic

Arapahoe Formation (Upper Cretaceous)

Lenticular deposits of gray to brown fine- to coarse-grained 

sandstone, siltstone, claystone, and thin pebble beds in the 

upper part; coarser sandstone and pebble-to-cobble conglomerate in the lower part. Sedimentary rock fragments exceed 

igneous and metamorphic rock fragments in the lower conglomeratic beds; proportions are reversed in the upper beds. 

Formation grades into lower part of Dawson Arkose. Contains ironstone concretions and dinosaur bones. Thickness 

about 120-150 m.

Laramie Formation (Upper Cretaceous)

Upper part mostly claystone and siltstone or shale interbedded 

with subordinate sandstone and thin local lignite beds; beds 

commonly pinch out within a few kilometers. Lower part contains more sandstone than siltstone or claystone and includes 

several subbituminous coal beds as much as 4.3 m thick, but 

usually thinner, above the basal sandstone. Contains fossil 

plant remains. Thickness about 170-300 m. Some geologists 

believe the basal sandstone is part of the Fox Hills 

Sandstone.

Fox Hills Sandstone (Upper Cretaceous)

Interbedded gray to brown sandstone and shale; massive or 

crossbedded sandstone predominates in upper and lower 

parts of formation in various localities. Limestone concretions locally in shale; calcareous iron-stained sandstone concretions in the generally poorly to moderately cemented sandstones. A transition zone of marine to nonmarine beds contains marine invertebrate fossils and finely broken plant 

fragments. Thickness about 18-152 m.

Pierre Shale (Upper Cretaceous)

Olive-gray to brown marine shale, siltstone, and silty sandstone; bentonitic shale in the lower part of the formation, 

limestone masses that weather into conical mounds (teepee 

buttes) in the middle part; calcareous concretions and calcareous shale in some zones. In the northern part of the corridor, 

upper, middle, and lower shale units are separated by an upper sandstone unit (Richard, Larimer, and Rocky Ridge Sandstone Members) and a lower ridge-forming Hygiene Sandstone Member; the Terry Sandstone Member is within the 

middle shale unit. A transition zone at the top of the formation grades upward from shale to increasingly more sandstone. Thickness of the formation is more than 2,600 m in the 

north but thins to about 1,143-1,585 m in the south.

Niobrara Formation (Upper Cretaceous)

Smoky Hill Shale Member: yellowish-gray to yellowish-brown, 

thin-bedded, soft, calcareous shale and interbedded thin, impure, commonly chalky limestone. Locally contains thin bentonite and selenite layers. Thickness 70-162 m. Contains 

marine fossils. Fort Hayes Limestone Member: yellowishgray, thin- to thick-bedded, dense, hard limestone. Contains a 

few thin, calcareous shale partings and bentonite. Thickness 

6-12 m. Contains marine fossils.

Benton Shale (Upper and Lower Cretaceous): equivalents of Benton are Carlile Shale and Greenhorn Limestone (Upper 

Cretaceous) and Graneros Shale (Upper and Lower 

Cretaceous). Carlile Shale: olive-gray to dark-gray, noncalcareous, silty, claystone and partly sandy siltstone in the 

northern part of the corridor; gray-brown, hard calcarenite, 

gray, silty sandstone, and yellow-gray, soft, calcareous shale 

in the southern part. Greenhorn Limestone: dark-gray limestone and olive-gray, calcareous, silty claystone and siltstone 

in the north; black to light-gray, noncalcareous clay shale, a 

few thin limestone interbeds, and several bentonite beds in 

the Golden area; and dense gray limestone, calcareous shale, 

and shaly calcarenite containing a basal marker bed of bento-
14 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

Mesozoic Continued

Benton Shale Continued

nite in the south. Graneros Shale: dark-gray to black, noncalcareous siltstone, claystone, or clay shale. Total thickness is 

about 90 m (south) to 200 m. Sparse marine fossils.

Dakota Group (Lower Cretaceous)

South Platte Formation: Two or three sandstone members 

(names vary with area), light-gray to brown, thin- to thickbedded, commonly crossbedded and locally ripple-marked, 

fine- to coarse-grained, soft to very hard; dark-gray to black 

clay shale or claystone and gray to white refractory clay 

lenses or beds between the sandstone members. Thickness 

commonly about 8-11 m. Nonmarine and marine fossils indicate that this formation was deposited in shifting deltaic 

and shallow-marine environments. Resistant sandstone 

forms the most prominent hogback ridge along the mountain 

front and in places also forms a secondary ridge. Lytle Formation: Light-colored, lenticular, crossbedded, partly conglomeratic sandstone interbedded with sandy or silty, 

greenish to varicolored claystone. Thickness commonly 

15-30 m.

Morrison Formation (Upper Jurassic)

Varicolored gray, maroon, and green siltstone and claystone 

and thin beds of sandstone, limestone, marl, and conglomerate. In the central part of the corridor, the formation 

contains a light-gray thin bedded, crossbedded, or massive 

commonly lenticular basal sandstone 3-12 m thick. Total 

thickness about 68-122 m. Contains fossil algae, freshwater 

mollusks, dinosaur bones, and plant remains.

Ralston Creek Formation (Upper Jurassic) 

Greenish-gray to varicolored siltstone, claystone, and sandstone; associated with impure freshwater, partly algal 

limestone in some areas and with disseminated or bedded 

gypsum in others. Commonly contains grains, nodules, 

streaks, or thin beds of red to gray jasper. Reported maximum thickness is about 6 m near Colorado Springs and 

Boulder, and about 31-84 m in areas between. Some geologists correlate the upper part of the Ralston Creek with the 

lower part of the Morrison Formation, and the lower part of 

the Ralston Creek with the Canyon Springs Member of the 

Sundance Formation north of Boulder.

Sundance Formation: Windy Hill Member (Upper Jurassic); Pine 

Butte and Canyon Springs Members (Middle Jurassic) 

Windy Hill Member: Gray to brownish gray thin-bedded ripplemarked sandstone with shale partings. About 3-5 m thick. 

Tongues out north of Loveland (Pipiringos and O'Sullivan, 

1976). Pine Butte Member: massive, light-yellowish-gray 

sandstone, 1-3 m thick. Recognized in the corridor only north 

of Fourmile Canyon Creek north of Boulder (Pipiringos and 

O'Sullivan, 1976). Canyon Springs Member: calcareous, 

crossbedded, fine-grained sandstone, pink to grayish white, 

about 3-10 m thick. Grades southward into lower part of 

Ralston Creek Formation in vicinity of Boulder.

Jehu Formation (Upper Triassic)

Pinkish-gray crossbedded, fine- to medium-grained, calcareous 

sandstone, 6-43 m thick. Present in corridor only north of 

Fourmile Canyon Creek north of Boulder. 

Mesozoic and Paleozoic

Lykins Formation (Triassic to Permian)

Maroon, grayish-red, and reddish-brown thin- to thick-bedded 

silty shale and calcareous siltstone and claystone; some green 

siltstone layers; silty sandstone; and gray to pale-red and 

reddish-brown wavy laminated limestone. Members, from 

youngest to oldest, include Strain Shale Member of LeRoy 

(1946), Forelle or Glennon Limestone Member, and Bergen

Mesozoic and Paleozoic Continued

Lykins Formation Continued

Shale, Falcon Limestone, and Harriman Shale Members of 

LeRoy (1946). Total thickness about 55 m (south) to 205 m 

(north). 

Paleozoic

Lyons Sandstone (Permian)

Yellowish to pinkish-gray to red, crossbedded, quartzose sandstone; includes arkosic conglomerate and conglomeratic 

sandstone in parts of the central and southern corridor, and 

subordinate tabular sandstone and thin beds of mudstone 

farther north. Thickness is 213-244 m near Colorado Springs, 

108 m near Boulder, and 6 m near Fort Collins where all but 

the uppermost part of the formation has graded northward 

into the Satanka Formation.

Satanka and Ingleside Formations (Permian) 

Red siltstone and fine-grained, thin-bedded, ripple-laminated 

sandstone (Satanka) and red, fine- to medium-grained, 

crossbedded, calcareous sandstone (Ingleside). Thickness 

about 103 m. Present only in northern quarter of corridor.

Fountain Formation (Lower Permian and Pennsylvanian) 

Maroon, reddish-brown, and gray crossbedded, arkosic sandstone, conglomeratic sandstone, and conglomerate interstratified with horizontally laminated to thick-bedded, 

dark-red or green micaceous siltstone or mudstone and minor 

gray to pinkish-gray quartzose sandstone. In the Colorado 

Springs area, a basal unit, the Glen Eyrie Shale Member, consists of about 30 m of gray sandstone, sandy shale, and black 

shale. Thickness of the formation is about 1,340 m in the 

southern part of the corridor, 210 m in the northern part. The 

upper part of the nonmarine Fountain Formation grades into 

the partly marine Ingleside Formation in the northernmost 

part of the corridor.

Leadville Limestone (Mississippian) and Williams Canyon

Limestone (Devonian)

Dark-gray, hard, dense limestones, about 40 m thick. Present in 

the southern part of the corridor only.

Harding Sandstone (Middle Ordovician)

Gray to white, fine-grained, thin-bedded sandstone and reddishbrown, sandy shale; about 20 m thick. Present in the 

southern part of the corridor only.

Manitou Limestone (Lower Ordovician)

Reddish-gray to buff, thin- to thick-bedded or massive, dense or 

granular limestone; in part nearly pure limestone, in part 

dolomitic, locally sandy. Thickness about 57 m. Present in 

southern part of corridor only.

Peerless Dolomite (Upper Cambrian)

Dark-red, finely to coarsely crystalline, thin-bedded dolomite. 

Thickness about 12 m. Present in southern part of corridor 

only.

Sawatch Sandstone (Upper Cambrian)

White, pink, reddish-brown, and locally green-mottled, fine- to 

coarse-grained, hard sandstone, thick to thin bedded, partly 

crossbedded; contains some conglomerate. Thickness about 

7.6 m. Present in southern part of corridor only. 

Precambrian

Pikes Peak Granite (Proterozoic Y)

White to orange-pink, medium- to coarsely crystalline biotite or 

biotite-hornblende granite, granodiorite, and quartz monzonite. Weathers to disaggregated mass of consitituent 

minerals. Age about 1,040 m.y. Several variants identified in 

southern third of corridor.

Silver Plume Quartz Monzonite (Proterozoic Y) 

Yellowish-orange to reddish-gray, equigranular, fine- to 

medium-grained biotite-muscovite quartz monzonite; locally 

medium to coarse grained, porphyritic. Age about 1,440 m.y.
GEOLOGY 15

Precrambrian-Continued 

Boulder Creek Granodiorite (Proterozoic X) 

Light- to dark-gray, medium- to coarse-grained gneissic 

granodiorite; locally porphyritic and may include some 

quartz monzonite. Some quartz monzonite in the southern 

part of the corridor is slightly younger than the Boulder 

Creek. Tonalite (quartz diorite) in the northern part may be 

slightly older than the Boulder Creek. Age about 1,710 m.y. 

Pegmatite (Proterozoic Y or X)

Very coarse grained lenses, dikes, and irregular intrusive bodies 

composed mainly of quartz, feldspar, and mica. May be 

genetically related to one, two, or all three of the major 

Precambrian intrusive rock units of the corridor. 

Metamorphic rocks (Proterozoic X)

Several varieties of gneiss, schist, quartzite, amphibolite, hornblendite, and other rocks metamorphosed from preexisting 

sedimentary or igneous rocks.

GEOLOGIC HISTORY

INTRODUCTION

The geologic history of the Front Range Urban Corridor, which spans at least 1 3/4 b.y. (billion years), has 

been divided for this report into eight time intervals of 

unequal length. By far the longest interval is the Precambrian. It also is the oldest, the most complex in 

terms of geologic processes, and the most obscured by 

subsequent events. A brief discussion of the Precambrian is followed by a longer description of later times 

during which relatively stable episodes alternated with 

intense geologic disturbances in and near the Front 

Range Urban Corridor.

In preview, a broad, low plain lay submerged at the 

end of Precambrian time beneath a shallow seaway in 

the region of the Southern Rocky Mountains. Three ensuing intervals of relative crustal stability the second 

and the third successively shorter than the first were 

followed by orogeny (mountain building) and other 

crustal disturbances. The first orogeny began in the 

Pennsylvanian Period, the second late in the Cretaceous Period, and the third in the Miocene Epoch of the 

Tertiary Period. Even under the comparatively stable 

conditions of early and middle Paleozoic time, gentle 

sagging and crustal upwarping reshaped the shallow 

seaway during its several advances and withdrawals 

across the region. Similar changes in areas of erosion, 

flooding, and deposition followed in Mesozoic time 

after the late Paleozoic uplift of the ancestral Rocky 

Mountains and before the late Mesozoic rise of the 

Laramide mountains (fig. 5). With the onset of the 

Laramide orogeny, the sea withdrew for the last time. 

The third and shortest interval of crustal stability was 

in the Eocene and Oligocene Epochs, when late Eocene 

erosion and deposition were followed by extensive 

Oligocene outpourings of lava and volcanic ash. In and 

after Miocene time, faulting, uplift, erosion, and 

deposition led to the elevation of the present Front

Range and the shaping of the mountains and the 

piedmont.

PRECAMBRIAN TIME

At the end of Precambrian time (about 570 m.y. ago), 

the Southern Rocky Mountain region was a low plain 

eroded to the crystalline roots of earlier formed mountains. These roots were a complex of gneiss, schist, and 

quartzite derived from pre-existing sedimentary, volcanic, and plutonic igneous rocks. They had been metamorphosed under conditions of deep burial, intense 

pressure, and high temperature (fig. 6), and they had 

been invaded by granitic magmas during three major 

periods of intrusion: Boulder Creek Granodiorite, 

Silver Plume Quartz Monzonite, and Pikes Peak Granite intrusions. Numerous compositional variants and 

countless pegmatite dikes accompanied the major 

intrusions.

EARLY AND MIDDLE PALEOZOIC TIME

In early and middle Paleozoic time, the Southern 

Rocky Mountain region was a moderately stable area 

of low relief within the interior of the continent. Gentle 

warps and sags allowed shallow epicontinental seas to 

enter and leave the region, most often from the west or 

northwest, but sometimes also from the east and 

southeast. Erosion of high land farther west, of low 

eastern border lands, and of ephemeral islands and 

peninsulas within the inundated region supplied sand 

and mud to the seas and tidal flats; the marine waters 

themselves yielded calcareous and dolomitic sediments. The resulting sandstone, shale, and carbonate 

rocks were generally less than 300-450 m thick (Berg, 

1960; Rothrock, 1960). Numerous low-angle discordances or irregular surfaces between sequences of beds 

record times and places of slight unwarping, nondeposition, and erosion. Silurian through Middle Devonian 

rocks were long thought to be entirely absent from this 

region, but blocks of fossil-bearing Silurian limestone 

found in the rocky fillings of ancient volcanic vents 

near the Wyoming-Colorado border indicate that Silurian marine rocks once covered at least part of the 

Southern Rocky Mountain region. All other traces of 

their presence seem to have been removed (Chronic and 

others, 1969).

Lower to middle Paleozoic rocks within the Front 

Range Urban Corridor have a maximum probable 

thickness of less than 150 m. They are now exposed 

only along the mountain front of the Rampart Range 

and in the embayment of the mountain front at Manitou Springs west of Colorado Springs. To the east they 

are deeply buried in the subsurface. If they were once 

more extensive in the corridor area, as seems likely, 

they were removed by erosion before deposition of the 

later Paleozoic sediments.
16 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO 

TABLE 1. Rock units in the Front Range Urban Corridor, Colorado

ERATHEM

CENOZOIC

MESOZOIC

SYSTEM

QUATERNARY

TERTIARY

CRETACEOUS

SERIES

Holocene 

and Pleistocene

Pliocene and Oligocene

[

Lower Oligocene

Eocene

Paleocene

Upper

Lower

COLORADO SPRINGSCASTLE ROCK AREA

GREATER DENVER AREA BOULDER-FORT COLLINSGREELEY AREA

Lower level alluviums (terrace deposits); upper level alluviums (pediment 

gravels and associated deposits); windblown sand, silt, clay; other surficial deposits (landslide, colluvium, talus)

High-level gravel deposits

Castle Rock Conglomerate

Wall Mountain Tuff

Dawson Arkose1

Laramie Formation

Fox Hills Sandstone

Pierre Shale

Niobrara Formation

Carlile Shale

Greenhorn Limestone

Graneros Shale

Dakota Group

Castle Rock Conglomerate

Wall Mountain Tuff

Green Mountain 

Conglomerate

c Table Mtn.Shoshoo5 .2 nite - Lava flows, 

> TO intrusives gfc - - - ----- 

0 o

LL

Arapahoe Formation

Laramie Formation

Fox Hills Sandstone

Pierre Shale

Niobrara Formation

Carlile Shale

Greenhorn Limestone c jS

Graneros Shale

Dakota Group

i0 i

Quartz monzonite

*V1

Small intrusive bodies

r . 4

Laramie Formation

Fox Hills Sandstone

Pierre Shale

Niobrara Formation

Carlile Shale

Greenhorn Limestone cjS

Graneros Shale

Dakota Group

'Data published after preparation of this part of the report indicate that the lower member of the Dawson Arkose is no older than late Paleocene, the main body of 

the Dawson is of Eocene age, and the formation overlies the Denver Formation where both units are present (Soister, 1978, p. 227, fig. 2).
GEOLOGY 

TABLE I. Rock units in the Front Range Urban Corridor, Colorado Continued

17

ERATHEM

MESOZOIC

PALEOZOIC

SYSTEM

JURASSIC

TRIASSIC

PERMAIN

PENNSYLVANIAN

MISSISSIPPIAN

DEVONIAN

SILURIAN

ORDOVICIAN

CAMBRIAN

SERIES

Upper

Middle

Lower

Upper

Upper

Middle

Lower

Upper

PRECAMBRIAN 

(Proterozoic 

Z, Y, andX)

COLORADO SPRINGSCASTLE ROCK AREA

Morrison Formation

Ralston Creek Formation

;

Lykins Formation

Lyons Sandstone

Fountain Formation

Leadville Limestone

Williams Canyon Limestone

Harding Sandstone

Manitou Limestone

Peerless Dolomite

Sawatch Sandstone

GREATER DENVER AREA

Morrison Formation

Ralston Creek Formation

Lykins Formation

Lyons Sandstone

Fountain Formation

BOULDER-FORT COLLINSGREELEY AREA

Morrison Formation

Sundance Formation: 

Windy Hill Sandstone, 

Pine Butte and 

Canyon Springs Members

Jelm Formation

Lykins Formation

Lyons Sandstone

Satanka Formation

Ingleside Formation

Fountain Formation

Pikes Peak Granite, Silver Plume Quartz Monzonite, Boulder Creek 

Granodiorite, and associated intrusive masses; pegmatite lenses, dikes, 

and irregular bodies related to major intrusives

Metamorphic rocks, including gneiss, schist, quartzite, amphibolite, 

hornblendite, and others
18 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

B

Upper Cambrian Sawatch Sandstone overlies Precambrian rocks discontinuously along the Rampart 

Range and grades upward into the dolomite, dolomitic

FIGURE 5. Maps showing late Paleozoic and Laramide uplifts in 

the Southern Rocky Mountains. From Tweto, 1975, p. 5, 22, 28; 

courtesy, The Geological Society of America, 1975. A. Late 

Paleozoic uplifts (shaded) of ancestral Rocky Mountains (modified 

from Mallory, 1960, 1967). Outlines of main bodies of Precambrian rocks as now exposed shown for reference. B. Major 

Laramide uplifts classified according to time of first rise. Horizontal rules, Cretaceous, older than 75 m.y.; coarse dots, Cretaceous, 

70 to 65 m.y.; diagonal rules, Paleocene; fine dots, Eocene. C. 

Laramide igneous and mineral belts. X, intrusive bodies of known 

or inferred Laramide age; solid black areas, andesitic sedimentary 

rocks and flows; V, minor occurrences of andesitic detritus; 

shaded strip, mineral belt as defined by Laramide intrusive bodies 

and productive mining districts. Outlines of main bodies of Precambrian rocks as now exposed shown for reference.

sandstone, and shale of the Upper Cambrian Peerless 

Dolomite. In places outside the corridor, the Peerless 

grades up into Lower Ordovician Manitou Limestone, 

but near Colorado Springs the Peerless and Sawatch 

were beveled by erosion before the Manitou was deposited. The Manitou, therefore, cuts across the Peerless 

and Sawatch, and it lies on Precambrian rocks to the 

south (Berg, 1960, p. 11).

Near the southern boundary of the corridor, Harding 

Sandstone of Middle Ordovician age overlies the Manitou, but northward the Williams Canyon Limestone 

overlies the Manitou. The Williams Canyon is an unfossiliferous limestone that generally has been regarded as Mississippian, but it has been correlated by 

Baars (1972, p. 92) with rocks deposited in a Late 

Devonian sedimentary basin in western Colorado.

In the Colorado Springs-Manitou Springs area, the 

Williams Canyon Limestone is overlain by the Leadville Limestone of Mississippian age. The Leadville 

Limestone was the last formation deposited in the 

Front Range area before the uplift of the ancestral 

Rocky Mountains.

LATE PALEOZOIC AND EARLY MESOZOIC TIME

An increase in crustal unrest in the Southern Rocky 

Mountain region probably began in Late Mississippian 

time with regional uplift and withdrawal of the seas 

westward and southeastward. By Early Pennsylvanian time, the major structural elements of the 

ancestral Rocky Mountains (fig. 5) were defined but 

were not yet developed to their ultimate height or 

lateral extent. These elements were the ancestral Front 

Range highland (trending slightly more northwest 

than the present Front Range), the Apishapa highland 

to the south and southeast of the Front Range, the Uncompahgre highland southwest of the Apishapa, and 

the subsiding basins that bordered or separated these 

highlands (Mallory, 1975, p. 268-269; Wilson, 1975, p. 

247, 251). The highlands provided most of the sediments that filled the basins to or above sea level in 

Pennsylvanian and Permian time. The older Paleozoic
GEOLOGY 19

" ^7* / SJr.f, 'S f j - " %J£ ."SB Ttr'." !*>-' « ij». » i 1 ' k'W^Sfc.fH- ¥**- 1 ''

»*"K WP^r.* -^ ^ " ^-W7 ^ ?-< S , '^ j fc -' , * %*V B3T C^'-1^ im.--.^ 4V^ / «- V> 'yT / y>* *

^fPV * *? *«^«4-'^ ^ J ^ ' » ^»'""^M^ * ^

^ -- i-> $ * *^^H^ JgK**** Ji^ ^ " -^KI^^ ^ " i * **

FIGURE 6. Precambrian gneiss (darker layers) and pegmatite (lighter layers) exposed in a roadcut 

along Interstate 70 in foothills of the Front Range at Mount Vernon Canyon west of Denver. 

Long vertical drill-hole scars indicate that the cut was excavated by the presplit method, 

which is used in competent rock. Roadcuts provide the best exposures of the old Precambrian 

complex. November 1971.

rocks were removed from the highlands and the Precambrian rocks were deeply eroded. As mountain 

height and the supply of detritus were reduced, marine 

carbonate and saline deposits formed over wide areas, 

though not notably so in the Front Range Urban 

Corridor.

The geology of the Front Range Urban Corridor has 

been influenced mainly by the development and 

destruction of the northern and eastern ancestral 

Rocky Mountains. Early Pennsylvanian uplift in the 

northwestern Apishapa highland and to a lesser extent 

in the northern Front Range highland supplied mud 

and some arkosic sand to alluvial plains that stretched 

to a shallow sea in eastern Colorado (Wilson, 1975, p. 

250-251). Uplift of the Front Range highland yielded 

coarse arkosic sediment, and downwarping of the adjoining eastern basin reached a maximum in Middle 

Pennsylvanian time. Thereafter, diminished crustal activity in Late Pennsylvanian through Permian, Triassic, and Jurassic time was interrupted only briefly by 

resurgent local uplift or regional warping. Erosion 

reduced both height and area! extent of the mountains 

until only isolated low hills remained above the levels 

of plain and sea.

Shorelines fluctuated widely throughout the late 

Paleozoic and early Mesozoic. At times, lagoons or 

bays of the eastern sea became saturated with calcium 

sulfate, owing to circulation barriers, and anhydride 

mud and some purer lenses of anhydrite (later altered 

to gypsum) were deposited. The many changes in the 

landscape are recorded by disconformities; more than a 

dozen erosion surfaces are recognized in the Southern 

Rocky Mountain region within the Middle Triassic to 

Upper Jurassic rock sequences alone (G. N. Pipiringos, 

oral commun., 1978).

Within the Front Range Urban Corridor, more than 

1,500 m of arkosic red beds, the Fountain Formation 

(Pennsylvanian and Permian; Mudge, 1967; Wilson,
20 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 7. Fountain Formation at Red Rocks Park, Jefferson County, 1965. Irregularly bedded arkosic sandstone and conglomerate are 

interbedded with finer grained ferruginous sandstone (darker layers). These rocks formed from sediments that were derived from the 

ancestral Rocky Mountains and accumulated on an alluvial plain in Pennsylvanian and Permian time. Photograph by J. R. Stacy, U.S. 

Geological Survey.

1975), accumulated near Colorado Springs on alluvial 

fans at the mountain front where deposition kept pace 

with or exceeded subsidence. Streams carrying sand, 

pebbles and cobbles meandered across muddy flood 

plains (fig. 7). The Fountain Formation thinned northward within the corridor and intertongued with a fluvial or marginal-marine sand of the Ingleside Formation (Lower Permian; Maughan and Wilson, 1960, 

p. 37).

The Lyons Sandstone (Permian), about 100 m thick 

near Lyons, was deposited on the Fountain Formation 

between the highland to the west and the marine 

evaporite basins to the east. The formation covered 

much of this corridor area. In the north, all but the up-
GEOLOGY 21

permost beds of the Lyons Sandstone graded into the 

estuarine or tidal-flat sand and silty sand that became 

the lower part of the Satanka Formation (Maughan 

and Wilson, 1960, p. 37). Near Lyons, the light-colored, 

partly cross-bedded sandstones of the Lyons Sandstone have been interpreted as wind-drifted dunes interbedded with stream-laid sands (Walker and Harms, 

1972, p. 279). Crossbedded sandstone and arkose in the 

Golden-Morrison area may have been deposited by 

streams partly within reach of tidewater (Weimer and 

Land, 1972a). Near Colorado Springs the Lyons 210 

and 245 m thick in this area grades up from massive 

red sandstone through arkose into white to red cross 

bedded sandstone (Grose, 1960, p. 190). The arkose 

resembles the Fountain Formation, but with local exceptions is finer grained than the Fountain. Gradual 

lowering of the ancestral Rocky Mountains by erosion 

reduced stream gradients and, consequently, diminished the ability of streams to carry coarse sediments 

to the sea.

The height of the source areas continued to lower 

and the average grain size of deposited material continued to decrease through Late Permian and into 

Early Triassic time. The Lykins Formation, which was 

deposited next, is more uniform laterally than are 

deposits in the Fountain and Lyons. Brick-red mudstone, fine-grained sandstone, and thin limestone and 

gypsum beds are products of coastal plain, shoreline, 

and partly evaporitic shallow marine environments 

(Weimer and Land, 1972a, p. 295-296; Van Horn, 

1976, p. 13).

The corridor contains no rocks of Middle Triassic 

through Lower Jurassic age other than crossbedded 

sandstone of the Upper Triassic non-marine Jelm Formation, which is exposed from northeastern Boulder 

County into Wyoming (Pipiringos and O'Sullivan, 

1976). The Jelm in the corridor area is believed to have 

been deposited when the northern part of the waning 

Front Range highland underwent minor uplift during 

Middle or Late Triassic time (M. R. Mudge, in McKee 

and others, 1959, p. 17). This event is suggested partly 

because the Jelm is coarser grained than the underlying Lykins and partly because it rests on a surface 

eroded into the Lykins. In late Paleozoic time the 

highland continued to erode elsewhere in the corridor, 

and ultimately the Jelm itself was partly removed.

The sea then returned from the north in a series of 

Middle Jurassic advances, partial retreats, and readvances. The crossbedded basal Canyon Springs Member of the Sundance Formation and a more massive 

younger Pine Butte Member were spread across the 

northern part of the Front Range Urban Corridor area. 

Near Boulder, the basal sandstone passes southward

into the lower part of the Ralston Creek Formation, 

which probably accumulated near the shoreline (Pipiringos and O'Sullivan, 1976; G. N. Pipiringos, oral 

commun., 1978). No comprehensive study of the 

Ralston Creek Formation has been published. Many 

geologic reports, however, note that red and varicolored mudstones, thin sandstones and limestones, 

and gypsiferous beds of this formation crop out intermittently from Ralston Reservoir northwest of Denver 

to the southern border of the corridor and that the formation also occurs in the subsurface to the east.

As the sea again withdrew in Late Jurassic time, the 

Morrison Formation spread across the low interior of 

the continent, including the area of the Front Range 

Urban Corridor. Varicolored clayey mud, sand, fresh 

water lime mud, and volcanic ash from sources far to 

the west were deposited in lakes and streams. (See 

Oriel and Craig, 1960.)

PRESENT DISTRIBUTION OF UPPER PALEOZOIC 

TO MIDDLE MESOZOIC ROCKS

Today, upper Paleozoic to middle Mesozoic rocks 

are exposed mainly in the narrow hogback belt along 

the mountain front, but they underlie much of the Colorado Piedmont at depth. Most conspicuous is the 

Fountain Formation, seen in the maroon monument 

rocks of the Garden of the Gods west of Colorado 

Springs, at Perry Park and Roxborough Park south of 

Denver, in Red Rocks Park west of Denver, in The 

Flatirons at Boulder, and in less well known exposures 

throughout the length of the corridor. The lighter colored Lyons Sandstone also forms a locally conspicuous 

outcrop, but most of the other formations are poorly 

exposed in gently sloping hillsides or valley floors 

below the high hogback of the Dakota Group, described in the following paragraphs.

PRE-LARAMIDE, LATE MESOZOIC TIME

The last marine invasion of the continental interior 

spread south from the Arctic in late Mesozoic time. 

Early in the Cretaceous Period, it joined briefly with a 

sea extending northward from the Gulf of Mexico. A 

continuous seaway in Late Cretaceous time was finally 

drained by the Laramide uplift. The floor of this interior seaway subsided most deeply near its western 

margin in western Utah and eastern Nevada. Mountains rising to the west shed more than 3,000 m of sediment into the trough and kept the depositional surface 

above or near sea level, but a central zone of deeper 

water in western Colorado received relatively little 

sediment. Farther east, shallower water extended into 

the midcontinent on a broad shelf, and here limited
22 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 8. Oscillatory ripple marks in upturned beds of the Kassler Sandstone Member of the 

South Platte Formation, Dakota Group, exposed in the wall of a claypit near Interstate 70, 

Jefferson County. Photographed in November 1974. The ripple marks were formed in shallow, 

standing water. Desiccation cracks and dinosaur tracks (fig. 9) elsewhere in the Kassler indicate ephemeral emergence and drying. The Kassler is the highly productive "J" sand in oil 

and gas fields of the Denver Basin.

deposition was often interrupted by temporary withdrawals of the water (fig. 8). Local upwarping and sagging of the sea floor caused many changes in the position of the shoreline and in the directions of sediment 

transport. Marine and nonmarine sediments intertongued repeatedly (Kauffman, 1977) as the shoreline 

shifted back and forth.

The Front Range Urban Corridor lay generally in the 

transitional hinge zone between the shallow-water 

eastern shelf and the deeper water zone to the west; 

but in the several stages of Cretaceous deposition, environments in the corridor ranged from alluvial to offshore marine. The Lower Cretaceous Dakota Group

began to accumulate as lenticular channel sands and 

gravels and as alluvium on a flood plain (Lytle Formation). Sediment was supplied from the west or southwest. The overlying South Platte Formation (fig. 9) 

was deposited in coastal-plain channels, delta channels 

and bays, shallow marine waters, and on flood plains 

and tidal flats; the sea advanced toward the south or 

southwest (Weimer and Land, 1972b).

Marine detrital mud, lime mud, and sand accumulated in the corridor through much of the Late Cretaceous, and at times volcanic ash was introduced from 

far western sources. These deposits constitute the 

Graneros Shale (partly Lower Cretaceous), Greenhorn
GEOLOGY 23

Limestone, Carlile Shale, Niobrara Formation, and 

Pierre Shale. They have a combined maximum thickness of more than 3,000 m in the northern part of the 

corridor (Colton, 1978) and about half of that thickness 

in the south (Trimble and Machette, 1979a and b). In 

some sections of this report the name Benton Shale is 

used with reference to undifferentiated rocks of the 

Graneros Shale, Greenhorn Limestone, and Carlile 

Shale.

The many changes in the vertical sequence of Upper 

Cretaceous rocks in the corridor resulted from at least 

five major expansions and contractions of the seaway 

(Kauffman, 1977). Its final withdrawal was accompanied by a progressive eastward shift of a zone of 

shoreline sands, now the Fox Hills Sandstone, and, 

close behind the retreating strand, by the coal swamps 

and flood plains of the Laramie Formation (fig. 10). 

The Fox Hills and Laramie added another 150-400 m 

to the geologic section. The deposition of nonmarine 

sediments was associated with the draining of the sea 

and the beginning of the Laramide orogeny. In the corridor, the Laramie Formation represents a transition 

from pre-Laramide nonorogenic sedimentation and 

Laramide orogenic sedimentation.

PRESENT DISTRIBUTION OF PRE-LARAMIDE, 

UPPER MESOZOIC ROCKS

The upper sandstones of the Dakota Group form the 

most prominent of the major hogback ridges along the 

mountain front. Some ridges are 150-300 m high. 

Locally the Fort Hays Limestone Member of the 

Niobrara Formation sustains a lower ridge east of the 

Dakota, and sandstones in the Pierre Shale form ridges 

30 m high or more in places between Boulder and Fort 

Collins. Pierre Shale is the bedrock in much of the corridor south and southeast of Colorado Springs and, 

together with the Fox Hills Sandstone and Laramie 

Formation, it underlies the northern third of the corridor east of the hogback belt. In most of the central 

part of the corridor these formations are concealed 

beneath younger rocks.

LATEST CRETACEOUS TO EARLY EOCENE TIME

Laramide mountain building began about 70 m.y. 

ago in central and western Colorado. About 67 m.y. the 

Front Range (fig. 5) began to rise along the trend of the 

late Paleozoic Front Range highland. The uplift nearly 

coincided with the present Front Range in the south, 

but to the north it diverged northwest from the trend 

of the present mountain front. The amount and type of

uppermost Cretaceous and Paleocene sediments shed 

from the range suggest that rapid erosion kept the 

mountains from reaching the height of the present 

Front Range. Uplift in early to middle Eocene time 

steeply tilted the Paleocene and older sedimentary 

rocks along the mountain front south of Boulder, and 

steep reverse faults displaced the mountain flank. 

North of Boulder, the eastern part of the present Front 

Range east of the rejuvenated late Paleozoic highland (fig. 5) probably did not develop until Paleocene 

time. At that time, northwest-trending folds and reactivated faults of Precambrian origin began to raise a 

series of en echelon blocks along the present mountain 

front. In northward sequence, each block was offset 

northeast, creating the indentations that contrast with 

the smoother curve of the front farther south. (See 

Tweto, 1975, p. 25-26, 31-33; Epis and others, 1976, p. 

323-324.)

Early Laramide crustal disturbance was accompanied by intrusive and extrusive igneous activity. 

This activity diminished in later Laramide time, but 

resumed in Oligocene time and persisted into the 

Miocene. During the Late Cretaceous and Tertiary, the 

first intrusive bodies invaded a zone, now called the

'"** *:^~ -^ *' -&*'*'^&t< '^f ~^«~?.^ f - '?*-.

'^S^0^K *%&*:]£&.?

J It *'*L

'!&

,y

.,

FIGURE 9. Dinosaur footprint in the South Platte Formation of the 

Dakota Group at Alameda Parkway near Morrison. Print is about 

44 cm long front to back. Sharp claw marks indicate a carnivorous 

dinosaur; it was walking on its toes, leaving no heel print. May 

1968.
24 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

*.. , -

FIGURE 10. Steeply dipping Fox Hills Sandstone exposed near Alameda Parkway on the south side of Green Mountain, Jefferson County. 

Person is at the Pierre Shale-Fox Hills Sandstone contact. Fox Hills Sandstone-Laramie Formation contact is at right. Photograph by 

J. R. Stacy, U.S. Geological Survey, 1965.

Colorado mineral belt, that extended from southwesternmost Colorado northeast across the Front Range 

to a point near Boulder. Many of Colorado's ore 

deposits are related to these intrusions. Volcanoes 

erupted within the Colorado mineral belt (Laramide 

mineral belt of Tweto, 1975, fig. 4) and in adjacent 

areas. The eruptions and rapid erosion of the rising 

mountains contributed sediments to lakes and streams 

over a broad area. (See Tweto, 1975, p. 21-25.)

As the area of Laramide uplift enlarged eastward in 

latest Cretaceous time, erosion of about 3,000 m of Upper Cretaceous Pierre Shale and older sedimentary 

rocks provided fine sand, silt, and clay to form the 

Laramie Formation east of the mountains (Tweto, 

1975, p. 19-20). Coarser sediments, possibly brought 

from a source west of the mountains, were incorporated into the upper part of the Laramie in the southern 

part of the corridor. The coarse texture of the Dawson 

Arkose (Upper Cretaceous and Paleocene), which was 

laid down on the Laramie in much of the southern half

of the corridor, records greater uplift and more intensive erosion of the mountains. Coarse arkosic sediment 

derived from the Pikes Peak Granite was deposited as 

conglomerate in the Dawson close to the present edge 

of the mountains. In the Denver area, the Arapahoe 

Formation graded southward into the lower part of the 

Dawson. The Arapahoe also contains conglomerate at 

its base and locally at higher levels (Van Horn, 1976). 

Both formations pass eastward into finer grained lenticular beds of sand, silt, and clay.

The Denver Formation (Upper Cretaceous and Paleocene) overlies the Arapahoe and intertongues with part 

of the Dawson. It is distinguished by its content of 

Precambrian detritus and Upper Cretaceous and 

Paleocene andesitic sediment, which was carried by 

streams from volcanic sources to the west (fig. 11). All 

traces of the volcanoes have since been eroded. The 

Dawson Arkose, which is partly a time equivalent but 

is generally nonvolcanic, reportedly contains one 

Cretaceous and one Paleocene zone of andesitic
GEOLOGY 25

material near Colorado Springs. This material is 

thought to have come from a volcanic source west of 

South Park and about 120 km west of Colorado 

Springs (Scott and others, 1976, p. 324). Recent data 

indicate that the upper part of the Dawson is of Eocene 

age (Soister, 1978). Dark-colored lava flows (Table 

Mountain Shoshonite) in the Denver Formation of 

North and South Table Mountains at Golden are 

believed to have come from vents associated with intrusive masses of similar rock between Golden and 

Lyons. The most probable sources were the Ralston 

Reservoir intrusive (Larson and Hoblitt, 1973, p. 4) or 

one or more of the smaller monzonite intrusives just 

northwest of North Table Mountain (Van Horn, 1976, 

p. 53).

South of Golden at Green Mountain, the Denver Formation is overlain by the Green Mountain Conglomerate of Paleocene age, which probably is equivalent to 

beds in the Dawson Arkose. The Arapahoe and Denver 

Formations and Green Mountain Conglomerate may 

never have been deposited in the corridor north of 

Boulder, or they may have been eroded from there during late Paleocene-early Eocene uplift (Tweto, 1975, p. 

33).

MIDDLE AND LATE EOCENE TIME

Laramide uplift of the Front Range declined sharply after early or middle Eocene time, when igneous activity also diminished. But erosion continuing through 

late Eocene time, without compensating uplift, leveled 

off a broad, low, east-sloping plain known as the late 

Eocene erosion surface. Remnant hills a few hundred 

meters high and river channels added character to the 

scene. Locally, the plain merged with a much older preMesozoic or pre-Paleozoic surface exhumed by Eocene 

erosion. Paleobotanical evidence indicates that the 

regional altitude was no more than 900 m above sea 

level. Alluvium accumulated on the plain locally and in 

stream valleys or downfaulted blocks below the 

general level of the surface (Scott, 1975, p. 230-231; 

Epis and others, 1976, p. 324-328).

In the Front Range Urban Corridor, the late Eocene 

erosion surface extended across the Front Range 

eastward onto the Great Plains without much of a 

break in slope at the mountain front. The broad summit of the Rampart Range is the most extensive approximation of the late Eocene surface remaining 

within the corridor. There the surface may have been 

protected from erosion in Oligocene time by a cover of 

volcaniclastic rocks. In Miocene time, east-flowing 

streams were deflected by the rise of the Rampart

* -£>?-

r* - *%*<+*

FIGURE 11. Denver Formation exposed near Alameda Parkway in 

Lakewood, showing friable, unevenly bedded, clayey, volcaniclastic sandstone. Clayey sandstone such as this has a high shrinkswell potential. Gravel at top of outcrop is of Quaternary age. 

Photograph by J. R. Stacy, U.S. Geological Survey, 1965.

Range fault blocks. Later erosion stripped off the 

Oligocene deposits except from a few depressions in 

the old surface. Erosion cut to a depth of a few tens of 

meters below the Eocene surface in many places, but 

the general form of the surface is well represented by 

the Rampart summit. Parts of the old surface remain 

to the north also, beyond the South Platte River, but 

much of it there has been destroyed by continued erosion (Scott, 1975, p. 232-233.)

OLIGOCENE tlME

The volcanic quiescence that followed the 

Laramide uplift in Eocene time ended in early 

Oligocene time with eruptions in the southern part of 

the present Sawatch Range (about 125 km west of the 

southern part of the corridor). Silicic ash flows of the 

Wall Mountain Tuff spread eastward and northeastward from South Park toward and beyond Castle 

Rock. Volcanic eruptions continued into Miocene time, 

diverting and damming streams in some areas and 

locally impounding large lakes (see Epis and others, 

1976, p. 328).

Within the Front Range Urban Corridor, the Wall 

Mountain Tuff is overlain by the Castle Rock Conglomerate. The Wall Mountain Tuff flowed down the 

late Eocene slope, following stream channels and, in 

places, overtopping interstream areas. West of Castle
26 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

Rock, its northeastward path was at least 8-9 km wide 

and its maximum thickness was about 15 m. Before 

the end of early Oligocene time, the Wall Mountain 

Tuff was channeled by streams and buried by the Castle Rock Conglomerate, a mixture of volcaniclastic 

debris and boulders, cobbles, and finer grained 

material from Precambrian sources (Izett and others, 

1969; Epis and others, 1976; Trimble and Machette, 

1979a, b).

PRESENT DISTRIBUTION OF THE OLIGOCENE ROCKS

Remnants of the Castle Rock Conglomerate crop 

out irregularly in a large area in southeastern Douglas 

County and in isolated small mesas and buttes elsewhere in the county. In a small area south of Castle 

Rock and elsewhere on some mesas and buttes, the 

conglomerate rests on the Wall Mountain Tuff. On 

some buttes, however, the Wall Mountain is the cap 

rock and the Castle Rock Conglomerate is absent.

MIOCENE, PLIOCENE, AND QUATERNARY 

TIME

The Southern Rocky Mountains began to rise to 

their present heights early in Miocene time. Uplift increased in the Pliocene but decreased in the Pleistocene 

and probably the Holocene. Block faulting was the major mechanism, and differential uplift of adjacent 

mountain and valley blocks in parts of the mountain 

region amounted to several hundreds to several 

thousands of meters. Some of the movement was along 

reactivated faults of Laramide, Paleozoic, or Precambrian origin. In this process, the Eocene erosion surface was fragmented, many stream courses were interrupted and deflected by fault scarps, and valleys were 

choked with detritus from blocks that eroded as they 

rose. Volcanism and intrusive activity associated with 

the Colorado mineral belt persisted into Miocene time. 

In Pleistocene time, a climate cooler and wetter than 

that of the Tertiary brought valley glaciers to the high 

mountains and extensive destruction of the Eocene 

erosion surface. (See Epis and others, 1976, p. 334; 

Scott, 1975; Soister, 1967; Tweto, 1975, p. 22.)

In Miocene time streams that had flowed across the 

rising Rampart Range were deflected by the scarps of 

major faults, such as the Ute Pass fault. North of the 

South Platte River, however, streams such as Clear 

Creek were able to maintain courses that probably had 

been established in Laramide time (Scott, 1975, p. 

233). Patches of high gravels along the canyon walls of 

Clear Creek and other large Front Range streams are 

thought to be Miocene valley fills that escaped erosion 

during Pliocene and Quaternary uplift. By late Quaternary time, Clear Creek Canyon and other canyons were 

eroded to depths of 400-500 m below the Eocene erosion surface. (See Scott, 1975, p. 240-241.)

As the rate of uplift declined in latest Pliocene or 

early Pleistocene time, so did the rate of erosion along 

the streams flowing eastward from the Front Range. 

Streams that emerged from canyons at the mountain 

front alternately eroded and aggraded. Pediments 

were cut on bedrock and, in times of reduced stream activity, gravels accumulated on the pediments. Each 

pediment and its gravel cap were lower and less extensive than the preceding ones. The principal pediment 

gravels, from highest to lowest, have been named 

Nussbaum, Rocky Flats, Verdos, and Slocum Alluviums. These gravels are best preserved on nearly flattopped, dissected benches or mesas along or near the 

mountain front or just east of the hogbacks in the 

bench-and-valley uplands section of the Colorado Piedmont. Alternating deposition and partial erosion, without pediment cutting, formed three or more terraces in 

the piedmont valleys of the corridor at levels between 

the lowest pediment gravels and the present streams. 

The main terrace deposits, in descending order, are 

Louviers Alluvium, Broadway Alluvium, and Holocene 

alluvium (in part called Piney Creek Alluvium). The 

alternate (and perhaps partly simultaneous) erosion 

and deposition that produced pediments, terraces, and 

their gravel caps may have been responses to cyclic 

climatic change, intermittent uplift, or both. (See 

Hunt, 1954, p. 113; Scott, 1975, p. 242-244.) The pediment and terrace alluviums probably correlate in part 

with cycles of Pleistocene glaciation in the higher 

mountains west of the corridor.

The landscape of the Front Range Urban Corridor is 

still changing. Streams are eroding, transporting, and 

redepositing sediment. Silt and clay are being carried 

east beyond the corridor, even to the Gulf of Mexico. 

Frost action, interstitial water, and the force of gravity 

move weathered rock slowly down slopes, locally causing landslides. Winds are agents of erosion and deposition. Strong northwest winds have carried sand and 

silt across much of the corridor north and east of 

Denver and northeast of Colorado Springs. Most of the 

sand has come from flood plains on the Colorado Piedmont. Large stabilized dune fields extend south from 

Greeley to Brighton. Some dunes have been reactivated locally in blowouts northeast of Denver. In the 

northern part of the corridor, shallow basins scooped 

out by the wind now contain lakes or reservoirs. Northeast of Colorado Springs, dune sand is being extracted 

commercially.

GEOLOGIC STRUCTURE SUMMARY 

GENERAL SETTING

Two major features dominate the geologic structure 

of the Front Range Urban Corridor the Front Range 

uplift on the west and the Denver Basin on the east
GEOLOGIC STRUCTURE SUMMARY

105°

27

NORTH

AND

MIDDLE

PARKS

100

I

200

I

300 KILOMETERS

FIGURE 12. Sketch map showing the Denver Basin (contoured in hundreds of meters on the top of the Precambrian 

rocks) and adjacent mountain ranges and uplifts. Shading indicates outcropping Precambrian rocks. Western Colorado mountains are not shown. Generalized from Cohee and others, 1961.

(fig. 12). These features are regional in size and extend 

far beyond the limits of the urban corridor. The Front 

Range, both as a mountain range and as a structural 

feature, is about 300 km long north to south and is 

about 40-65 km wide. It extends from the Canon City 

area on the south to about the Wyoming State line on 

the north. Including the Laramie Mountains, a northern extension in Wyoming, the Front Range is about 

480 km long.

Structurally and topographically, the limits of the 

Front Range are clearly marked. The Denver Basin, on 

the other hand, is less sharply outlined except on the 

west where it meets the mountains. The basin is at

least 560 km long north to south and is at least 420 km 

across. From its deepest part near Denver, it extends 

north into Wyoming and Nebraska, east into Nebraska 

and Kansas, and south to the Apishapa uplift south of 

Pueblo, Colo.

THE FRONT RANGE

A general outline of Front Range structure is provided here for a better understanding of the geologic 

framework of the Front Range Urban Corridor. The 

complex geologic structure of the range could be fully 

described only in a monographic treatise beyond the
28 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

scope of this report. The Front Range uplift is a broad, 

flat-topped anticline. Tilted sedimentary rocks form a 

belt of hogbacks on the east flank of the range. Erosion 

has removed these rocks from the crestal part of the 

fold and, in the process, has deeply exposed the complex Precambrian core of the range.

The boundary zone between the Front Range uplift 

and the Denver Basin is a steep monocline the 

hogback belt along which the sedimentary rocks commonly are tilted 25°-45°; in places the rocks are much 

steeper or are even overturned, especially along faults. 

Dips are much flatter within the basin. The boundary 

zone is fractured in many places by high-angle reverse 

faults that dip westward beneath the mountain flank. 

They include the Golden, Jarre Creek, Rampart Range, 

and Ute Pass faults. Some workers have concluded 

that the faults diminish in dip surfaceward and in 

places flatten to rather low angles, although the 

evidence is not generally compelling.

Most investigators believe that the Front Range was 

elevated in Laramide time by essentially vertical uplift 

and that the mountain flanks and border faults coincide with zones of deep-seated, high-angle fractures in 

the basement. Van Horn (1976, p. 97) summarized 

evidence for steep fracturing along the Golden fault. 

Berg (1962, p. 2025) concluded that the overturned 

limb of the monocline near Golden is the principal 

mountain-flank structure and that the Golden fault 

cutting the monocline is but a secondary feature. Total 

displacements by faulting and monoclinal folding are 

measured in thousands of meters. Stratigraphic offsets 

may be much less. Some of the faults were reactivated 

by mountain uplift in early Miocene to early Pliocene 

time (Taylor, 1975, p. 224). This uplift gave the range 

much of its present configuration.

Several folds are present along the mountain-basin 

boundary, particularly in the northern part of the 

Front Range Urban Corridor (fig. 13). Some of these 

folds pass downward or laterally into faults. (For example, see Hunter, 1955; Boos and Boos, 1957, figs. 

4-7,13; Scott and Cobban, 1965; Braddock and others, 

1970; Colton, 1978.)

The interior of the Front Range is broken by countless faults that vary greatly in size, orientation, and attitude (fig. 14). On the east flank of the uplift, but extending 30 km or more back into the mountains, is a 

belt of en echelon fractures that trend northwest to 

north-northwest (Tweto, 1976; Bryant and others, 

1978). This belt is best defined in the middle part of the 

range where, because of brecciation and gouge zones, it 

exerts influence on the drainage and topography. The 

fractures originated in Precambrian time and probably 

predate the Pikes Peak batholith, but they have had 

recurrent movement at least through Laramide time 

(Levering and Goddard, 1950, p. 79; Boos and Boos,

1957, p. 2638; Tweto and Sims, 1963, p. 997, 1001; 

Tweto, 1977, p. 12). Many of these faults extend from 

the uplift through the hogback belt, but most die out 

rapidly in the Denver Basin within a kilometer or less 

of the mountain front.

Front Range structure began to evolve in Precambrian time and grew progressively more complicated 

with repeated tectonic and magmatic events. Severe 

deformation of a thick sequence of already metamorphosed sedimentary/volcanic rocks about 1.75 to 1.70 

b.y. ago was accompanied by high-grade metamorphism and plutonic activity. This episode brought the 

gneissic and schistose rock essentially to its present 

form (Hedge and others, 1967, p. 551; Peterman and 

others, 1968, p. 2277). These metamorphic rocks are intricately folded and strongly foliated, and the fold 

directions and foliation trends vary widely. The dominant trends, however, are west to northwest, and steep 

dips predominate.

Batholiths and smaller plutons of the Boulder Creek 

Granodiorite were intruded widely in the Southern 

Rocky Mountains 1.75-1.70 b.y. ago, modifying earlier 

structures. Subsequent structural events and features 

were then overprinted on these rocks. Widespread plutonic activity followed 1.45-1.39 b.y. ago (the Silver 

Plume and Sherman family of plutons), also accompanied by deformation, and again 1.04-0.98 b.y. ago 

(the Pikes Peak batholith) (Peterman and Hedge, 1968; 

Hutchinson, 1976, p. 74-75).

Structural modifications continued through Phanerozoic time, essentially to the present. The rise of the 

ancestral Front Range in Early Pennsylvanian time 

drastically altered sedimentation patterns across the 

region and set the stage for the Laramide events to 

come (p. 12). Laramide uplift was accompanied by 

igneous intrusion and volcanic activity. Stocks and 

smaller plutons rose through the core of the Front 

Range, and lavas flowed out on the surface. Dikes and 

sills intruded the east flank of the range in a discontinuous belt from Lyons on the north to Green Mountain on the south.

Present structural patterns were broadly defined by 

the time of the Laramide orogeny, when the buried 

worn-down roots of the Pennsylvanian mountains were 

re-elevated (Tweto, 1975, p. 6). The mountains as we 

know them today, however, really took form during 

Neogene time (Taylor, 1975), that is, the latter part of 

the Cenozoic Era. Erosion had truncated the Laramide 

structure of the Front Range by late Eocene time (Epis 

and Chapin, 1975, p. 46), and remnants of that erosion 

surface are still well preserved in many places along 

the west border of the Front Range Urban Corridor. 

Starting in early Miocene time, the present landscape 

began to take form as the Eocene erosion surface was 

displaced both up and down along faults. In places, 

movement on faults continued into the Quaternary
GEOLOGIC STRUCTURE SUMMARY 29

N

FIGURE 13. Aerial view of the Big Thompson anticline near Loveland; a subordinate fold on the east flank of the Front Range, showing the 

eroded core of Precambrian crystalline rocks (Milner Mountain) and the sedimentary sequence of the fold. West is at the top of the 

photo. Faulting has partly deleted beds from the west limb of the fold. The area shown is about 12 1/2 km across. pC, Precambrian 

rocks; pPf, Fountain Formation; Pi, Ingleside Formation; Ps, Satanka Formation; Ply, Lyons Sandstone; "fcpl, Lykins Formation; 

J~fcj, Jehu Formation; Jm, Morrison Formation; Kd, Dakota Group; Kb, Benton Shale (equivalent to Graneros Shale, Greenhorn 

Limestone, and Carlile Shale); Kn, Niobrara Formation; Kp, Pierre Shale. High-altitude aerial photograph by U.S. National 

Aeronautics and Space Administration, February 9, 1970.

(Scott, 1970; Kirkham, 1977). Earthquake epicenters 

along certain faults, such as the Golden fault and the 

Rampart Range fault, suggest that intermittent movement is still in progress (Simon, 1969).

DENVER BASIN

Because of its long history of oil and gas production 

and its high potential for further development, the 

Denver Basin has received much geological and geophysical attention. The published literature is 

voluminous, but only selected papers are cited here.

The Denver Basin is compound in that it has two 

widely separated structural lows one centered near 

Denver and one near Cheyenne, Wyo. (fig. 12) with at 

least 450 m of structural relief between them. The two 

parts, sometimes referred to as the Colorado part and 

the Wyoming or Julesburg part of the Denver Basin, 

are separated by a broad structural ridge trending east 

from about Loveland or Fort Collins toward Greeley, 

Colo. The following remarks apply to the Colorado part

of the basin only.

A steep, partly overturned west limb and a very 

gently sloping east limb make the basin strongly 

asymmetrical (fig. 15), and its axis or trough line, 

therefore, (fig. 12) is much closer to the west border of 

the basin than to the east. Contours drawn on the surface of the crystalline basement (MacLachlan and 

Kleinkopf, 1969, pi. 1) indicate that the basin axis at 

that horizon is about 13 km from the mountains and 

the low point of the basin is near Castle Rock. At that 

point, the Precambrian surface is more than 2,300 m 

below sea level. Inasmuch as the basin rim on the 

Precambrian, southeast from Castle Rock, is less than 

600 m below sea level, the enclosed negative relief on 

the Precambrian is more than 1,700 m. On the west, 

about 6,600 m of structural relief exists between the 

crest of the Front Range and the deepest part of the 

basin (Anderman and Ackman, 1963, p. 170).

At successively higher stratigraphic horizons, the 

axis of the basin shifts eastward from its position on 

the Precambrian surface. On the Laramie-Fox Hills
30

105°30'

ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

T05°OD' 30' 104°00' 103°30'

40°30'

40°00

30'

39°00'

38°30'

EXPLANATION

Mountain-front contact between Precambrian crystalline rocks (west) and 

younger sedimentary rocks"

 Fault within and near the 

mountains. Named faults 

are mentioned in text

 Structure contours on top of 

Laramie-Fox Hills aquifer 

(Cretaceous). Elevation in 

feet above sea level

I, Approximate top of 

. Laramie Fox Hills 

\ aquifer

0 10 \ 20 30 KILOMEFERS

FREMONT

FIGURE 14. Map showing selected structural features in and near the Front Range Urban Corridor. Modified from Tweto (1976), and

Romero and Hampton (1972).
URBAN MINERAL RESOURCES 31

W

METERS 

5000 -i

CONTINENTAL

x' ^x

/DIVIDEX

Top of

Crystalline 

Basement

SEA LEVEL

-4000

FIGURE 15. Schematic section along the 40th parallel showing the asymmetry of the Denver Basin and its relation to the Front Range. 

Sedimentary rocks are indicated by stippling. Vertical scale is greatly exaggerated. Dashed lines are faults.

aquifer (Cretaceous), the axis is about 25 km east of 

the mountain front, and the low point of the basin is 

beneath Cherry Creek Reservoir, about 35-40 km 

north of Castle Rock.

SUBORDINATE FOLDING IN THE DENVER BASIN

The general simplicity of the Denver Basin is 

modified by subordinate folds and faults. Small anticlines are numerous in the northern part of the Colorado part of the basin, and many of them yield petroleum. Anticlinal noses and structural terraces are common also. Several of these noses plunge diagonally 

southeastward from the Front Range uplift. Some of 

them have local closures in Paleozoic or Mesozoic 

rocks, and some are breached by erosion to their 

Precambrian cores (for example, see Hunter, 1955; 

Braddock and others, 1970, and fig. 13).

Structure contours on a datum in the Fox Hills 

Sandstone-Pierre Shale transition zone suggest little 

likelihood of subsidiary folding in the basin southeast 

of a line from Boulder Creek to the South Platte River 

near Greeley (Romero, 1976, p. 5). Romero's map (pi. 5) 

shows one local closure at that horizon 18 km southeast of Greeley, however.

FAULTS IN THE DENVER BASIN

Tweto (1975, p. 19) has shown that the east limb of 

the Denver Basin began to form as part of the regional 

depositional basin of the Pierre Shale in Late Cretaceous (Montana) time. Initially, the axis of this depositional basin was in western Colorado, far to the west of 

the Front Range. The steep west limb of the present 

Denver Basin formed later, during the Laramide uplift 

of the Front Range. In this process, the west limb was 

deformed and faulted along the new basin margin at 

the mountain front; deformation of the east limb was 

minimal. Eastward from the west margin of the basin,

faults are uncommon except in a zone about 18 km 

wide that trends northeast about 40 km from Rocky 

Flats past Firestone, Frederick, and Dacono.

Natural exposures of this fault zone are generally 

poor, except in a few localities. Individual faults are 

documented mostly by displacements in coal mines or 

by seismic profiles and well logs (Spencer, 1961; Davis, 

1974, p. 10). Most of the faults trend northeast to 

north-northeast, but some trend north to northwesterly (Spencer, 1961; Colton and Lowry, 1973; Trimble, 

1975; Colton, 1978). In general, they define a series of 

horsts and grabens, with displacements of as much of 

150 m, but mostly less (Spencer, 1961). Spencer (1961) 

suggested that the faults might be related to reactivation of the northeast-trending Ralston shear zone, 

which is a structure of Precambrian age in the adjacent 

Front Range (Lovering and Goddard, 1950, pi. 1, 2). 

Weimer (1973, p. 53: 1976, p. 214) and Davis (1974, p. 

52; Davis and Weimer, 1976, p. 280) ascribed the displacements to growth faulting within delta-plain unconsolidated sediments in Late Cretaceous time, controlled by recurrent movements on deep-seated basement faults. The faults appear to have been active during deposition of the Pierre Shale, Fox Hills Sandstone, and Laramie Formation, inasmuch as these stratigraphic intervals are reportedly thicker in the 

grabens than on the horsts.

URBAN MINERAL RESOURCES

The term "urban mineral resources," as used here, 

means those industrial mineral products extracted in 

or near the urban area primarily for use within the 

area. As commonly defined by the extractive industry, 

these products include gravel, sand, crushed stone, 

riprap, clay (for brick, tile, and sewer pipe), and to a 

lesser extent dimension stone, limestone for varied 

purposes, and earth fill. Products that might be in-
32 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

cidentally extracted from within an urban area but not 

necessarily for local use, such as coal, natural gas, and 

petroleum, are not included here but are discussed in 

later sections of this report.

Like all other metropolitan areas, Denver and its 

satellite towns require large quantities of industrial 

mineral products to sustain growth and fill recurring 

needs. Most of these products are consumed in new 

construction and in maintaining and repairing existing 

facilities. In the rapidly expanding residential communities of the Front Range Urban Corridor enormous 

quantities of material are consumed by new growth. 

These products are used in the buildings, houses, 

streets, parking lots, drainage works, and so on that 

make up the physical framework of the community.

All are high-consumption, low-unit-value products 

whose price to the consumer depends greatly on haulage distance to the market. To be competitive, they 

must be obtained close at hand, often within the 

metropolitan area itself. Ironically, urban growth, 

which creates the demand and consumes the products, 

sometimes curtails production by preempting the 

unextracted resources for other land uses.

Groups along the Front Range have successfully 

prevented the extraction of industrial mineral products from some parts of the corridor, citing increased 

industrial traffic, air pollution, and reduced visual 

amenity as neighborhood concerns. Proper balance is 

being sought between demand for raw materials and 

other community wants and desires. At that point, the 

problem of supply and demand for urban mineral resources leaves the field of earth science and enters the 

socioeconomic political arena.

Local or nearby sources along the Front Range Urban Corridor fill all or most present needs for gravel, 

sand, crushed stone, riprap, clay for brick and tile, 

dimension stone, and earth fill. Supplies of limestone 

suitable for sugar refining, gypsum for agriculture and 

construction, and clay for high-quality refractory products are inadequate to fill the local demand, and these 

products are supplied in large part from outside 

sources (Crosby, 1976).

Data have been obtained from many published and 

unpublished sources. Publications that provide data 

on all or several of the nonmetallic resources of the corridor include Larrabee and others, (1947), Vanderwilt 

and others (1947), Argall (1949), Del Rio (1960), U.S. 

Geological Survey (1964), Colorado Bureau of Mines 

(1896 through 1965, 1966 through 1971), and Colorado 

Division of Mines (1972 through 1974) for Colorado; 

and U.S. Bureau of Mines (1965) and Brobst and Pratt 

(1973) for the United States as a whole. Unpublished 

information was obtained from files of public record of 

the Colorado Division of Mines and through the cooperation of Coors Porcelain Co., Denver Brick and Pipe 

Co., Environmental Lime Co., A. P. Greene Refractories Co., Robinson Brick and Tile Co., and Van Howe 

Ceramic Supply Co.

SOURCE AREAS

Along the urban corridor, the mountain area is a 

source of crushed rock, riprap, and small amounts of 

other nonmetallic mineral products such as gravel, 

sand, and earth-fill material. Locally, weathered 

bedrock, particularly granite grus, is used as road 

metal or aggregate for bituminous road surfacing.

The hogback belt flanking the mountains yields a 

wider range of materials: clay for brick, tile, sewer 

pipe, and other structural products; limestone for cement, crushed rock, and small amounts of dimension 

stone; sandstone for dimension stone, crushed rock, 

brick and cement additives, and landscape rock; and 

gypsum for use as a cement retardant (Crosby, 1976).

The Colorado Piedmont section of the Great Plains, 

where most of the population of the Front Range Urban Corridor is concentrated, contains extensive deposits of gravel, sand, sandstone, fine-grained igneous 

rock for use as crushed-rock aggregate, and clay and 

expandable shale the latter usable for processing into 

lightweight (bloated) aggregate (Crosby, 1976).

GRAVEL AND SAND

The Front Range Urban Corridor contains a large 

resource of gravel and sand that could fill needs far into the future at present rates of consumption for use as 

aggregate, subgrade material, pervious fills, and other 

construction needs, provided that the resource is adequately conserved (Soule, 1974, p. 3, 18). Extensive 

areas underlain by high-quality gravel and sand, however, have been lost to production, especially in the 

Greater Denver Area, by urbanization across the resource or by zoning for nonextractive land use (fig. 16).

Estimates by Soule (1974, table 5) indicated that 

gravel and sand consumption in the Front Range Urban Corridor in 1974 equalled nearly 10 t (metric tons) 

per person per year. This rate was more than twice the 

national average and, in all probability, is higher now 

than it was then. Projected consumption between 1974 

and the year 2000 is about 227 million metric tons, according to Soule (1974, table 5). Supplies in the Front 

Range area should fill that need if the resource is not 

lost to other land uses.

In recognition of the continuing need for gravel and 

sand, the Colorado Legislature in 1973 enacted a law 

(House Bill 1529) to prevent land uses that would interfere with the eventual extraction of commercial 

deposits of gravel and sand in the heavily urbanized
URBAN MINERAL RESOURCES 33

FIGURE 16. Valley of Clear Creek (lower left to upper right) in Greater Denver Area, showing extent of gravel extraction as indicated by 

water-filled pits (black). Some pits have been backfilled and reclaimed. Although considerable gravel has been removed, much more remains and, in part, is still recoverable; however, some gravel deposits have been built upon. Area shown is 10.8 km across, left to right. 

North is at top. Interstate 25 trends north to south in right side of view. Interstate 70 is near bottom of view. High-altitude aerial 

photograph by U.S. National Aeronautics and Space Administration, February 9, 1970.

counties of the state. Most such counties are in the 

Front Range area. This law also contains provisions 

for the reclamation of exhausted open pits. The law 

does not apply to lands zoned for other uses prior to 

enactment.

Most high-quality gravel in the Front Range Urban 

Corridor is extracted from beneath the flood plains of 

the major streams or from adjacent low terraces 

(Ching, 1972, p. 20; Shelton, 1973; Colton and Fitch, 

1974; Trimble and Fitch, 1974a; Schwochow and 

others, 1974, map). These gravel deposits are correlated with late Pleistocene (Bull Lake and Pinedale) 

glaciations in the nearby mountains. On the plains 

they are referred to as the Louviers and Broadway 

Alluviums (Hunt, 1954, p. 104; Scott, 1960, p. 1541). 

Some gravel and sand are still being deposited in the

stream channels, largely through reworking of preexisting deposits.

Older gravel deposits of generally inferior quality 

cap higher terraces and pediments. These deposits are 

less sought after for commercial uses, except locally 

for road material, because the stones tend to be 

weathered or coated with calcium carbonate, or both, 

and because the deposits commonly contain unacceptably high proportions of interstitial calcium carbonate 

and silt. There are exceptions, and some deposits can 

be upgraded by crushing and washing.

Most excavations in or near valley bottoms extend 

down to or below the water table. Pits have been 

developed in the valleys of nearly all major streams of 

the area.

In general the lithologic character of a gravel deposit
34 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

reflects rather closely the composition of parent bedrock upstream in the mountains (Colton and Fitch, 

1974), which in turn affects the character and quality 

of the aggregate. Proportions of initial constituents 

vary, depending on durability of individual rock types 

and distances from source. Most of the gravel in the 

Colorado Springs area, for example, is derived from the 

adjacent Pikes Peak batholith, which is exposed in an 

area of 3,400 km2 (Bryant and others, 1976, p. 17) and 

which yields gravel of generally inferior quality, owing 

to a high content of coarse-grained feldspar.

SOURCES OF GRAVEL AND SAND

The following descriptive summary is abstracted 

from Colton and Fitch (1974) and Trimble and Fitch 

(1974a, b). A potential source of gravel as defined by 

these authors contains 20 percent or more of granuleand pebble-size stones (smaller than 6.4 cm, but retained on a number 10 U.S. standard sieve (larger than 

2 mm). Schwochow and others (1974) also provided 

brief summaries.

DEPOSITS UNDERLYING FLOOD PLAINS AND TERRACES 

SOUTH PLATTE RIVER

Much of the flood plain of the South Platte River in 

the Denver area has been covered by urban development, and large volumes of high- quality gravel have 

been eliminated from use. Gravel has been much used 

upstream from the junction of Bear Creek, however, 

and downstream from the junction of Clear Creek, and 

there has been extensive early extraction along the 

South Platte between these tributaries. This gravel 

contains abundant stones of granite, quartz, and 

pegmatite, and lesser amounts of gneiss and schist.

Pebble counts (100 pebbles per count) of samples 

from the Fort Lupton, Platteville, and La Salle areas 

show that about 70 percent of the stones are granitic, 

10 percent are gneiss, 10-20 percent are quartzite, and 

a few percent are sandstone, tuff, basalt, and gabbro. 

The composition of the stones changes little between 

Brighton and Greeley.

Deposits along the South Platte River flood plain 

and adjacent terraces are about 50 percent sand and 50 

percent granules and small pebbles. The size distribution changes little between Brighton and Greeley. Well 

records indicate that coarser material may be present 

locally in the deeper parts of the deposits. Overburden 

is silty sand about 0.5-1 m thick. Windblown sand on 

the terraces is as much as 4.5 m thick locally but 

generally is only a meter or so.

In the Denver area, the deposits range from about 

6-11 m in thickness, but in Brighton they are as much 

as 15 m thick. Farther north, thicknesses range from

15 m near Fort Lupton to 33 m near La Salle and 

Greeley.

CACHE LA POUDRE RIVER

The Cache La Poudre River valley has been the major source of gravel for Fort Collins, Windsor, and 

Greeley since the area was settled. The river and its 

tributaries drain mountain areas underlain mostly by 

gneiss, schist, and granite. Pebble counts from deposits west of Greeley indicate the gravel is 76-85 percent granitic rocks, 2-11 percent quartzite, 2-7 percent rhyodacite, and 3-4 percent gneiss.

The average diameter of the stones diminishes from 

about 2.5 cm just north of Fort Collins to less than 6 

mm (millimeters) just northwest of Greeley.

Thickness of the gravel under the flood plain and terraces ranges from 6 m northwest of Fort Collins to 15 

m northwest of Greeley. Drill records indicate greater 

thicknesses locally in buried channels (Ching, 1972; 

Shelton, 1973).

BIG THOMPSON RIVER

Much high-quality gravel and sand have been produced along the Big Thompson River south and east of 

Loveland. The Big Thompson and its tributaries Redstone Creek, Buckhorn Creek, and Little Thompson 

River drain mountainous areas of varied granitic 

rocks, pegmatite, quartzite, gneiss, and schist. In the 

hogback belt these streams particularly Redstone 

and Buckhorn Creeks drain extensive areas of sandstone, siltstone, and shale that tend to degrade the 

quality of the gravel. Little gravel has been produced, 

therefore, from these tributaries or, for the same 

reason, from the Little Thompson River.

Along the Big Thompson the gravel ranges from 

about 4 to 12 m in thickness, including generally less 

than a meter of sandy overburden.

ST. VRAIN CREEK

Deposits along St. Vrain Creek have been the source 

of gravel for the Longmont and Lyons areas. St. Vrain 

Creek and its tributaries drain mountain areas underlain largely by Silver Plume Granite. Pebble counts 

from deposits between Lyons and Longmont indicate 

that stones are 50-75 percent granitic rocks, 20-40 

percent gneiss, 5-15 percent porphyry, 5-20 percent 

sandstone, and as much as 15 percent quartzite. Downstream, the percentage of granitic rocks decreases and 

the percentage of quartzite increases.

The average size of stones decreases downstream 

from the Lyons area to the confluence of St. Vrain 

Creek with Left Hand and Boulder Creeks. Near 

Lyons, the diameter of more than 50 percent of the 

stones is more than 2.5 cm, but in pits 8 km east of 

Longmont, the average diameter is 9.5 mm.
URBAN MINERAL RESOURCES 35

The thickness of the gravel is known locally along St. 

Vrain Creek. For example, test pits a few miles east of 

Lyons indicate as much as 6 m of gravel. Pits in the 

Longmont area indicate that the gravel is generally 

about 3 m thick but may be as much as 4.5 m locally.

LEFT HAND CREEK

Extensive deposits just east of the Front Range near 

Altona have been used for many years by the Boulder 

County Road Department and others, mostly for highway material. Sixty percent of the area drained by Left 

Hand Creek and its tributaries is underlain by Boulder 

Creek Granodiorite and Silver Plume Quartz Monzonite. Most of the remainder is metamorphic rock. Pebble counts from Table Mountain indicate that the 

stones are 30-50 percent granitic (including 

pegmatite), 15-35 percent sandstone, 10-55 percent 

quartzite, 5-40 percent gneiss or schist, and 5-10 percent fine-grained igneous rock.

At Altona, the gravel consists largely of boulder and 

cobble gravel. It decreases in size eastward to the 

vicinity of the confluence of Left Hand and St. Vrain 

Creeks where the average diameter of stones is 2.5 cm. 

The thickness is as much as 4.5 m near Altona and 

about 3 m near the confluence.

BOULDER CREEK

The deposits of Boulder Creek have been the major 

source of gravel for the Boulder area for many years. 

Boulder Creek and its tributaries (excluding South 

Boulder Creek, discussed separately) drain mountain 

areas largely underlain by Boulder Creek Granodiorite, 

biotite gneiss, some schist and quartzite, and a small 

area of Silver Plume Quartz Monzonite. Pebble counts 

near Boulder indicate that 50 percent of the stones are 

granite, granodiorite, or quartz monzonite, 20 percent 

are gneiss, and 15 percent are pegmatite. The percentage of granite, granodiorite, quartz monzonite, and 

pegmatite decreases downstream to 25 percent at the 

confluence with St. Vrain Creek; gneiss appears to be 

more durable, and its percentage doubles from 20 percent near the mountain front to 40 percent (20 km) 

downstream. Quartzite stones increase from 20 percent near the mountain front to 30 percent 

downstream.

The average size of stones decreases downstream 

from the foothills. Between Boulder and Liggett the 

average diameter is 32 mm, whereas near the confluence with St. Vrain Creek the average is 16 mm.

Generally, the deposits are thickest near the mountains, and commonly, but not everywhere, they also 

taper from the center of the flood plain toward the 

edge. Measured thicknesses are 4.5-7.6 m near

Boulder, 3 m 0.8 km northwest of Liggett, and 2.4- 

37 m 3.2 km northwest or Erie.

SOUTH BOULDER CREEK

South Boulder Creek emerges from the mountains a 

few kilometers south of Boulder and flows northeastward. It drains a terrain of mostly Boulder Creek 

Granodiorite but partly Coal Creek Quartzite (Boos 

and Boos, 1934) these rock types predominate in the 

high-quality gravel of the valley fill. Granule and pebble gravel constitute about 50 percent of most 

deposits; sand and oversize gravel (cobbles and 

boulders) make up about 25 percent each.

COAL CREEK

Coal Creek flows from the mountains a few 

kilometers south of South Boulder Creek, and it, too, 

drains a terrain of Boulder Creek Granodiorite and 

Coal Creek Quartzite. Quartzite predominates. East of 

the mountains, Coal Creek has cut down through an 

older cobbly, bouldery (80 percent), quartzitic fan 

deposit into the underlying sedimentary bedrock. Cobbles and boulders, at least in large part recycled, 

therefore, constitute nearly half of the gravel along' 

Coal Creek. Granules and pebbles form about a third, 

and the remainder is sand.

BEEBE DRAW

This ancient abandoned buried channel of the South 

Platte River along the east edge of the area contains 

thick deposits of sand and gravel. The channel is 

slightly more than 1.6 km wide. Well records and some 

test drilling by the U.S. Geological Survey (Smith and 

others, 1964) indicate that the deposits may be as 

much as 15 m thick. They are mantled by windblown 

sand and silt as much as 9 m thick under some dunes. 

The water table under Beebe Draw is near the surface.

Stone types from Beebe Draw are known only from 

terrace remnants 2 or 3 km south of Lower Latham 

Reservoir. Pebble counts indicate that 59-61 percent 

of the stones are granitic, 19-30 percent are quartzite, 

4-7 percent are gneiss, and a few percent are sandstone, conglomerate, and porphyry. The character of 

the deposit at depth is unknown.

CLEAR CREEK

The valley of Clear Creek has been a major source of 

gravel for the Greater Denver Area for many years. 

Much of this deposit has been exhausted, but much 

high-quality gravel remains, although a large part of it 

has been preempted by urban growth. Clear Creek 

drains an area of mostly metamorphic and partly 

granitic rocks. Granitic rocks, however, constitute
36 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

about 60-70 percent of the gravel and metamorphic 

rocks only 15-30 percent. This reversal of proportions 

between source area and gravel constituents probably 

reflects the difference in resistance to abrasion and 

disintegration between the granitic rocks and the metamorphic rocks. Near the mountain front and for 

several kilometers downstream, cobbles and boulders 

form about a third of the gravel, granule and pebble 

gravel about half, and sand and fines the remainder. 

The percentage of oversize material cobbles and 

boulders decreases downstream; near the confluence 

with the South Platte River, the gravel contains little 

oversize material. The amount of sand in the deposit is 

about constant. The thickness of the deposit is about 

6-9 m, but it is only 2-3 m near the margins of the 

valley.

BEAR CREEK

Bear Creek and its tributary, Turkey Creek, drain an 

area of predominantly gneiss and schist intruded by 

large bodies of granitic rock. Less gravel has been extracted along Bear Creek than along either Clear Creek 

or the South Platte River. More than 25 percent of the 

gravel near the mountain front is cobbles and boulders, 

but the percentage decreases rapidly downstream and 

is negligible only a few miles from the mountain front. 

The granule and pebble fraction is consistently near 50 

percent. The deposit commonly is 4.5-6 m thick.

PLUM CREEK

Plum Creek flows north into the South Platte River 

south of Denver. Its east fork heads on the north side 

of the South Platte-Arkansas River divide (Palmer 

divide) near Palmer Lake; its west fork heads in the 

mountains west of Larkspur, with some tributaries 

heading in the mountains farther north. Much of the 

gravel along Plum Creek has been derived from the 

Dawson Arkose, which is almost entirely reworked 

granitic material. Gravel derived from the mountains 

also consists of granitic material, inasmuch as the 

mountainous part of the drainage is underlain by Pikes 

Peak Granite. Gravel in the east fork drainage contains pebbles of Wall Mountain Tuff, a dense silicic 

volcanic rock that is likely to react deleteriously with 

high-alkali cement if used for concrete aggregate. 

Granule and pebble sizes compose about half the 

gravel along Plum Creek and sand about half. The 

gravel commonly is about llm thick but locally is as 

much as 18 m thick.

CHERRY CREEK

Cherry Creek heads at the Palmer divide east of 

Monument and flows north into the South Platte River

in Denver. Its gravel is derived from the Dawson 

Arkose and the Castle Rock Conglomerate and consists mainly of granitic detritus and minor volcanic 

fractions. Mostly small granules and pebbles compose 

about 50 percent of the gravel. The gravel is 6-15 m 

thick.

MONUMENT CREEK AND FOUNTAIN CREEK

Fountain Creek flows southeast from the mountains 

west of Colorado Springs through the city. Monument 

Creek joins Fountain Creek from the north. The gravel 

along both streams consists mainly of granitic 

material derived from the Pikes Peak Granite and the 

Dawson Arkose. Granules and pebbles make up about 

50 percent and sand, 25-40 percent, along both 

streams. The remainder of the gravel is fines and oversize stones. The valley of Monument Creek contains little gravel north of Colorado Springs. South from Colorado Springs the gravel is about 3-15 m thick and 

locally is as much as 25 m thick. It is thickest along the 

east side of Fountain Creek southeast of Colorado 

Springs.

UPLAND GRAVEL DEPOSITS ON PEDIMENTS AND HIGH-LEVEL 

TERTIARY SURFACES

Most of the gently sloping benches just east of the 

mountain front and many of the drainage divides in 

the eastern part of the area are mantled by Pleistocene 

gravel deposits that lie 25-30 m or more above stream 

level. In the mountains, scattered patches of older Tertiary gravel lie hundreds of meters above present day 

main-stream levels. Most of the upland gravel was 

deposited by Quaternary streams emerging from the 

mountains, shifting their courses from side to side and 

probably merging laterally. The deposits, therefore, 

reflect the character of the rocks in adjacent drainage 

areas of the mountains to the west. At Rocky Flats, 

east of Coal Creek Canyon, for example, the deposits 

contain as much as 80 percent quartzite, which was 

derived from Precambrian terrain in the mountains.

The amount of oversize material (cobbles and 

boulders) in the upland gravel is highly variable and is 

as much as 25 percent. Thus deposits at Rocky Flats 

and on benches adjacent to Bear Creek contain large 

amounts of oversize material, but deposits adjacent to 

the South Platte River or to Plum Creek contain little 

or none and, generally, more than 50 percent granules 

and pebbles. Thick soil profiles generally contain much 

clay and caliche (calcium carbonate). Long weathering 

has partly or completely disintegrated many stones, 

but the quality of the gravel can be improved by 

washing, crushing, and screening.

Generally, only small amounts of these deposits 

have been used locally for highway or road construe-
CLAYROCK 37

tion. An exception is the deposit on Table Mountain, 

9.7 km west of Longmont, where the Boulder County 

Road Department has a large pit. This coarse deposit 

is as much as 5 m thick. About half the stones are 

granitic rocks, about 20 percent are red sandstone, 15 

percent are gneiss, 10 percent are porphyry, and 5 percent are quartzite. About half the stones exceed 5 cm 

in diameter.

Some upland gravels are remnants of long, narrow 

stream deposits. An example is a discontinuous line of 

small deposits, including Riley Mound, on the drainage 

divide between the South Platte River and Beebe 

Draw. These deposits are mostly about 1.5 m thick, 

but some of them are as much as 4.5 m thick. They are 

deeply weathered and partly cemented by caliche. Pebble counts suggest that prolonged weathering altered 

many stones to clay and sand. The gravel is 50-65 percent quartzite, 25-35 percent granitic rocks, about 5 

percent gneiss, 5-12 percent sandstone, and the remainder other rock types such as conglomerate and 

dark, fine-grained igneous or metamorphic rocks. 

Linear, discontinuous gravel deposits that trend 

southeastward from the foothills toward Longmont 

once filled ancient channels of the Little Thompson 

River.

A few patches of older gravel 3-6 m thick lie 100 m 

or so above stream level in the mountains along West 

Monument Creek. These deposits contain stones that 

are mainly Pikes Peak Granite. They generally contain 

more than 50 percent granules and pebbles and little 

oversize material, but chalky calcium carbonate is 

abundant, especially in the upper few feet. Considerable chemical decomposition degrades their quality, 

but despite their poor quality, they have been used extensively in the Colorado Springs area.

CONGLOMERATE

Conglomerate (cemented gravel) is a generally inferior source of gravel in the Front Range Urban Corridor. Three formations are potential sources, the 

Green Mountain Conglomerate west of Denver, the 

Castle Rock Conglomerate, and parts of the Dawson 

Arkose. The Green Mountain Conglomerate forms the 

upper part of Green Mountain. The lower 15 m of this 

deposit is a cobble-and-boulder conglomerate that contains pebbles of andesite, gneiss, pegmatite, quartzite, 

and sandstone (Scott, 1972a). Andesite decreases in 

abundance upward. The lower 15 m of conglomerate is 

overlain by 46 m of sandstone and conglomerate, overlain by 75 m of interbedded thin-bedded clay stone, 

siltstone, sandstone, and conglomerate, overlain by 

about 60 m of conglomerate at the top.

Well-cemented Castle Rock Conglomerate underlies 

the divide between Plum Creek and Cherry Creek. This

conglomerate is generally less than 15m thick. Most of 

the pebbles are granite, gneiss, quartzite, and white 

vein quartz (Richardson, 1915). Chert pebbles and 

rhyolitic ash-flow tuff are less common. Although the 

deposit contains some boulders 30 cm or more in diameter, most stones are only slightly larger than 5 cm. 

The sandy matrix is cemented by silica, hence, most of 

the formation would have to be quarried and crushed 

for reduction to aggregate. Locally, however, the basal 

part of the formation is less well cemented and is 

scraped up for direct use on secondary roads.

Conglomerate is extensive in two areas of the Dawson Arkose. One is in the northwesternmost exposures 

of the formation in Douglas County, and the other extends from a point southeast of Parker to the El Paso 

County line. Granitic stones predominate; little 

material is oversize.

CLAYROCK

Clays and clay products have been produced in and 

near the Front Range Urban Corridor for more than 

100 years. The major manufacturing centers are in 

Denver and Pueblo; brick kilns also operate near Longmont and Valmont in Boulder County. Pits and subsurface mines that supply these centers are within 80 

km of Denver and Pueblo. Within a marketing radius 

of about 480 km, the two centers supply much of the 

Rocky Mountain region and parts of the adjacent 

plains with brick, tile, sewer pipe, and other clay 

products.

Refractory products made in the Pueblo area are 

blended from fire clay mined locally and Missouri fire 

clay or Arkansas bauxite. Porcelains for technical use, 

now manufactured in Golden in Jefferson County, 

combine out-of-State clays (kaolinite from Florida and 

North Carolina, ball clay from Tennessee and England) 

or use aluminum oxide in place of clay. These porcelains are distributed throughout the country. The fire 

clays of the Denver area, formerly used for refractory 

purposes such as lining metallurgical furnaces, are now 

blended with other, lower quality clays to increase 

structural strength and hardness of sewer pipe and 

some brick and tile (Patterson, 1964). Pottery clays are 

similar to fire clays in their high firing temperatures, 

but they require very low iron content (to avoid unwanted coloration) and high plasticity; they have been 

produced in both the Denver and Colorado Springs 

areas. Formerly they were used for the industrial 

porcelains made in Golden as well as for pottery. Most 

deposits of pottery clay in the urban corridor, however, 

are very small; consequently, with a few exceptions, 

art pottery and flower pots for commercial use are 

made of local brick clay that is blended with pottery 

clay from other States.
38 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

The clayrocks of the corridor vary greatly in 

character from one rock stratum to another and within 

each stratum. Some characteristics permit the manufacturing of an exceptionally wide range of finished 

clay products of differing structural properties and colors; others preclude commercial use. Clays from a 

dozen or more localities may be blended for one product, or one or two clays may suffice.

SOURCES AND PRODUCTION

Clayrock in the urban corridor is produced mainly in 

two areas: (1) in or a few miles east of the hogback belt 

from Boulder south to northwestern Douglas County; 

and (2) in northeastern Douglas County, in northwestern Elbert County (Kiowa-Bijou Creeks area), and near 

Calhan in El Paso County. The Kiowa-Bijou basin and 

Calhan areas are outside the urban corridor, but are 

within the source area for Denver manufacturers of 

brick, tile, and pipe. The productive strata in these 

areas are discussed in the following text. Clay mines 

and pits in these units were comprehensively reviewed 

by Van Sant (1959). The geology of clayrock in the 

Dakota Group in the central and northern part of the 

corridor has been discussed in detail by Waage (1961).

Statistics on clayrock production by rock units or for 

specific areas of the urban corridor have not been published. An industry estimate of recent eastern Colorado production attributed a little less than one-half 

the production to the Laramie Formation, about onethird to clay lenses within the Dawson Arkose, and 

smaller amounts to the Dakota Group, the Benton 

Shale, the Arapahoe Formation, and the Pierre Shale. 

This assessment does not include Pierre Shale that has 

been mined for the production of lightweight aggregate in the Rocky Flats area in northern Jefferson 

County. The U.S. Bureau of Mines, however, includes 

production from the Pierre Shale for aggregate in its 

annual statistics on the output of clay and shale in Jefferson County. For 1971, the bureau reported 376,5711 

valued at $763,000 from Jefferson County (probably in 

major part from the aggregate operation) and 13,509 t 

valued at $19,000 from Boulder County. Tonnage and 

value from Douglas, Elbert, and El Paso Counties were 

included with figures for three other counties outside 

the urban corridor for totals of 98,851 t and $214,000 

(Kuklis, 1971).

DEVELOPMENT PROBLEMS AND PROSPECTS

As population and industry expand in eastern Colorado, the need for brick, tile, and pipe must be supplied from new as well as old workings. Clayrock exposed in outcrop or under shallow soil cover has been 

the chief source of past production. Future production

will come from a small part of the areas of outcrop, and 

will be determined by interrelated factors of geology, 

economics, and competitive land use.

The maximum extent of an operation may be limited 

by the lateral limits of commercial quality or thickness 

of clay, by overburden too thick for stripping at 

prevailing clay prices, or by availability of mining 

rights. The minimum objective for major operators 

may be a deposit that can provide 9,000 t per year for 5 

years and requires little or no selective mining. On this 

scale of operation, a deposit about 2-3 m thick in nearly flat lying rocks may require excavation of less than 

a hectare. Certain multiple-bed operations of 6-16 ha 

(hectares), as estimated in part from topographic 

maps, in the steeply tilted Laramie Formation north 

and south of Golden, have been active for periods 

longer than 5 years.

Overburden in strip-mining areas consists of gravel, 

sand, soil, or noncommercial bedrock. The thickness of 

unconsolidated surficial deposits may be as much as 

20-25 m or more in alluvial valleys such as that of the 

South Platte River and Fountain Creek, and in a few 

upland areas between drainages. In much of the urban 

corridor east of the mountains, however, surficial 

sediments are less than 10 m thick and in many places 

are only 1-5 m thick (Hamilton and Owens, 1972a; 

Smith and others, 1964).

The amount of overburden that the mine operator 

can afford to remove varies directly with the thickness 

of the minable clay and inversely with hauling distance 

to the manufacturing plant. Recent allowable costs of 

less than $4.00 per metric ton for most structural clays 

when delivered to the plant site dictate that the ratio 

of overburden to clay thickness should not exceed 5:1 

at a distance of 48 km or 3:1 at 100 km from the plant. 

These figures do not include the possible reduction of 

cost through marketing of sand or gravel from the 

overburden, or coal from the lower part of the Laramie 

Formation, and the cost of restoring the land surface 

after excavation of the clay is not included. (These 

statements are based on 1974 and earlier data.)

The most obvious restriction on areas of potential 

production of clay (and other nonmetallic mineral 

resources) is spreading urbanization. Front Range Urban Corridor base maps show urbanization plus lands 

set aside for military reservations, other government 

installations, recreation areas, flood-control reservoirs 

and reserves, and airports. Not shown are crop and 

grazing lands, which account for the largest nonurban 

use in the corridor east of the mountains, especially 

north of Denver (Driscoll, 1974a, b), urban fringe areas 

of predictable growth, and areas undeveloped but 

already zoned for residential and industrial use.

In addition to the above factors that affect clay pro-
CLAYROCK 39

duction in the urban corridor, the location of the outcrop, history of development, and distinctive geologic 

character of the deposit also influence potential yield. 

Rocks of the Dakota Group and the Benton Shale (or 

its equivalents; the Graneros, Greenhorn, and Carlile) 

are accessible only in the narrow zone of the hogback 

belt, where the rocks are steeply tilted and locally 

faulted or folded; locally they are omitted at the outcrop by faulting. To date, mining of the Pierre Shale 

and Laramie and Arapahoe Formations has been 

almost entirely within the hogback belt, but these formations also are accessible to the east in the piedmont. 

The western margin of the Dawson extends to the 

mountain front locally between Denver and Colorado 

Springs, but the Dawson is exposed and mined in a 

much wider area south and east of Denver. Widespread 

exposure provides the possibility, but not the promise, 

of greater future production.

PRODUCTIVE ROCK UNITS

DAKOTA GROUP

The Dakota Group is the only major source of refractory clay in eastern Colorado. Intensive mining of 

Dakota clays in Douglas, Jefferson, and southern 

Boulder Counties from the 1860's to the present has 

substantially reduced the easily accessible supply of 

high-quality refractory clayrock. Remaining known 

reserves are mostly in properties currently in production or held by major producers. Additional clayrock of 

commercial quality exists in previously worked areas 

at depths below present economic limits of mining and 

in areas where mining rights are not available. Before 

1909, Dakota clays mined in the Colorado Springs area 

supplied part of the material required for local brick 

factories; thereafter, the growing city obtained its clay 

products mainly from Denver or Pueblo. Limited production of pottery clay continued in the Colorado 

Springs area (Argall, 1949).

The strongly refractory quality of much of the 

Dakota Group clay in the Greater Denver area, as well 

as in the Pueblo area, is attributed to a very high content of the clay mineral kaolinite. North of Coal Creek, 

northwest of Denver, kaolinite decreases markedly and 

illite is the dominant clay mineral (Waage, 1961, p. 

20-23). Very little clay has been taken from the 

Dakota in this northern area. Nonrefractory clay from 

the illitic part of the Dakota would have to compete 

with similar clays from other deposits closer to 

Denver. Also, Dakota clays are less well exposed in the 

northern part of the corridor than in the central and 

southern parts and, therefore, both their quality and 

quantity are more difficult to evaluate.

BENTON SHALE

The basal part of the Benton Shale (equivalent to 

Graneros Shale), directly above the uppermost sandstone of the Dakota Group, is mined and blended with 

other brick clays. It is produced both north and south 

of Golden in conjunction with clay of the Dakota or is 

mined independently. Small amounts of pottery clay 

occur at the base of the Benton (Vanderwilt and others, 

1947, p. 239-240), but the higher parts of the formation are not used in the manufacturing of clay products 

because they contain large amounts of limy shale, limestone, and siltstone.

PIERRE SHALE

The Pierre Shale is the thickest clay-producing rock 

in the corridor, ranging from about 900 m in part of the 

Colorado Springs area to 2,000-2,500 m in the northern counties. Although much of the Pierre is sandstone 

and siltstone, and the clayrocks now make only a 

minor contribution to the brick industry, the formation is widely accessible in the northern and southern 

parts of the corridor and has attracted the interest of 

brick manufacturers in a time of anticipated shortage 

and high cost of fuel. Improvements in technology now 

under investigation may allow use of the Pierre in making brick at relatively low firing temperatures. If successful, brick from Pierre Shale can be substituted in 

part for present higher temperature products that require more fuel. Early brick plants in the Colorado 

Springs area used local Pierre Shale, but they have 

been inactive for many years. Use of the Pierre for expanded lightweight aggregate is discussed in the section on "Expandable shale."

LARAMIE FORMATION

The Laramie Formation is much thinner than the 

Pierre, averaging about 75 m in thickness near Colorado Springs and 180-215 m near Denver; it has been 

described in various localities as containing 26-40 

clayrock subunits, interbedded with sandstone, siltstone, and, in the lower part, subbituminous coal. Most 

or all of the clayrock units have been mined in one or 

several localities, and some of the most extensively 

worked units, in the lower part of the formation, contain clay of low-grade refractory quality. Sandstone in 

the lower part of the Laramie is a source of sand added 

to the clay to control shrinkage in firing and can be 

mined in some places with the clays in a single-pit 

operation. At present the Laramie provides more 

structural clay than any of the other clay-bearing units 

in the urban corridor and may be the largest source for 

the foreseeable future.
40 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

ARAPAHOE FORMATION

The Arapahoe Formation crops out in a narrow belt 

west of Denver and in a wider area northwest of the 

city. The formation is 75-120 m thick and contains 

silty claystone in some places, although much of it is 

sandy or locally conglomeratic and is not suitable for 

clay products. Within its limited area of outcrop, the 

formation has produced small amounts of structural 

clay. One clay pit east of Morrison is in the Arapahoe, 

and a recent development has been reported north of 

Golden.

DAWSON ARKOSE

The Dawson Arkose was deposited contemporaneously with and subsequent to the Arapahoe Formation and the overlying Denver Formation. These 

formations were deposited by laterally shifting 

streams issuing from the Front Range. The deposits 

differ in composition and texture depending on the 

mountain source rocks from which they were derived. 

The Dawson Arkose was deposited along ancient 

streams that drained Pikes Peak Granite terrane west 

of the Castle Rock-Colorado Springs area (Richardson, 

1915; Waage, 1952). The arkosic (feldspar-bearing) conglomerate, sandstone, and subordinate amounts of 

finer grained sediments (including clay) are thickest as much as 600 m on parts of the divide between 

the South Platte and Arkansas Rivers. In much of the 

area of greatest thickness, however, only the uppermost few hundred feet crop out. Most of the clay pits 

are in this upper part of the Dawson on the north flank 

of the divide in the Louviers-Parker-Castle Rock triangle within the corridor and in the Kiowa-Bijou basin 

area east of the corridor. Toward the north and south 

limits of the Dawson, erosion has cut away the higher 

parts of the formation, and the remaining rocks have 

yielded little or no commercial clay except in the 

Calhan area (east of map limit) northeast of Colorado 

Springs.

In the clay-producing units olaor than the Dawson 

the clayrocks are variable in thickness and lateral 

continuity, but tend to persist at certain levels within 

the formations. The Dawson, in contrast, contains few 

laterally persistent beds or zones of either clay or other 

rock types. The clayrock seems to be distributed randomly within the coarser sand and gravel. Within a 

single deposit the Dawson clays may vary in color 

from gray through green, yellow, and red, and streaks 

and layers of sand, silt, or mica flakes are common 

(Waage, 1952, p. 386). Little selective mining is attempted, and the unsorted products are sufficiently 

valuable in the making of brick and tile to justify hauling distances that exceed those for nearly all other 

clays mined in Colorado for Denver plants. An eastward increase in fine sediments in the Dawson suggests that additional clay resources may exist near or 

beyond the eastern edge of the urban corridor.

The clay deposits near Calhan are south of the South 

Platte-Arkansas divide, about 65 km northeast of Colorado Springs (Richardson, 1911; Van Sant, 1959, p. 

63). The clay-bearing zone is in lakebeds near the base 

of the Dawson. It is about 30 m thick and extends 

4.8-6.4 km along the outcrop below surficial gravels of 

Quaternary age. Thicknesses of clay that can be extracted without selective mining of high-grade material range from 1-3.5 m. Like the smaller stream-laid 

deposits to the north, the clays near Calhan show wide 

variation in color, texture, and quality. Selected 

samples have low-grade nonrefractory to high-grade 

refractory ratings. For a number of years after initial 

production in 1903, the clay was sent to Pueblo for furnace linings, firebrick for buildings, and sewer pipe. 

More recently it has been used in the Denver area for 

structural clay products; its usefulness in mixture with 

other clays apparently justifies the unusually long 

hauling distance. Unworked high-grade refractory clay 

bodies are said to be in the area, but presumably they 

will not be developed while Dakota clays of comparable 

quality are available closer to the Pueblo manufacturing center. No other lakebed clays have been reported 

from the Dawson Arkose.

EXPANDABLE SHALE

As a resource category, expandable shale for use as 

lightweight aggregate for structural and thermal- or 

acoustical-insulation products is closely allied to 

clayrock. Expandable shale is obtainable from some of 

the same rock units and is similarly dependent on a 

high content of clay minerals (hydrous aluminum silicates). Expandable shale differs from clayrock in its 

physical properties and minor constituents, in the 

nature of its processing, and in the final products toward which processing is directed.

Although several rock types are suitable for making 

lightweight aggregates including claystone; shale; 

slate; volcanic rocks such as scoria, cinder, pumice, and 

perlite; diatomite; and vermiculite only expandable 

shale and claystone are abundant in the Front Range 

Urban Corridor. The Pierre Shale has been used recently in the urban corridor, and the Benton Shale and 

Arapahoe Formation are potential sources.

Processing involves crushing the shale and rapid 

kiln heating to 980°-1,200°C. In this temperature 

range the rock becomes viscous and finely porous
IGNEOUS AND METAMORPHIC ROCKS 41

(frothy) liy the expansion of gases (CO2, SO2, H2O) 

formed within the rock. The cohesive cellular finished 

product has a volume two or three times that of the 

original material: a density of 480-960 kg/m3 (kilograms per cubic meter), in contrast to 2,400-2,700 

kg/m3 for sand and gravel aggregate. Furthermore, the 

expanded material has good structural strength and 

valuable thermal- and acoustical-insulating properties 

(Bush, 1964,1973). One shale-expansion plant supplied 

by an adjacent quarry in the upper part of the Pierre 

Shale, though not presently active, operated for about 

15 years in the Rocky Flats area between Golden and 

Boulder. It has a reported capacity of several hundred 

thousand metric tons per year and probably can supply requirements within a 320 km radius for the 

foreseeable future.

The suitablity of shale for commercial production of 

expanded lightweight aggregate cannot be evaluated 

in the field because many small differences in physical 

and chemical properties that may be critical in thermal 

processing must be kiln tested. In general terms, the 

most favorable rocks seem to be shales that are dark 

(gray, black, green), not extremely thinly laminated, 

and largely composed of illite and montmorillonite the minerals dominant in clay-bearing rocks used 

for structural rather than highly refractory clay products. High concentrations of iron, alkalies, organic carbon, carbonate minerals, and silt may adversely affect 

the processing of the shale, but some of these components, finely and uniformly disseminated, are essential for the formation of the expansive gasses (Bush, 

1973).

Because of its great thickness, the Pierre Shale could 

provide adequate resources of expandable shale for 

many years. It is 915-1,525 m thick in the Colorado 

Springs area and 2,135-2,440 m in the northern part of 

the urban corridor, and it is widely exposed.

GYPSUM

Gypsum (hydrous calcium sulfate, CaSO4 -2H2O) has 

been mined in the northern part of the Front Range Urban Corridor since the early years of this century for 

use as cement retardant, plaster of pans, plasterboard, 

other construction products, and as an agricultural soil 

conditioner Output has always been slight in comparison with that of several other nonmetallic minerals, 

and increased production seems improbable. Most of 

the deposits are small or of nonuniform quality, and 

adequate supplies of gypsum are readily available from 

other sources in and out of Colorado.

The Lykins Formation in the hogback belt near the 

mountain front contains most of the gypsum in the 

northern part of the corridor. The Ralston Creek Formation, adjacent to the Lykins on the east, contains 

subeconomic deposits south of Morrison in Jefferson 

County, in the Perry Park area of Douglas County, and 

in and south of the Colorado Springs area in El Paso 

County.

In Larmier County, gray to white, massive, finely 

crystalline gypsum in the lower part of the Lykins 

forms lenticular beds 0.3-15 m thick and mostly a few 

hundred meters long. Beds rarely are as long as 1.5 km. 

Gypsum of minable grade in many places is interbedded with and grades laterally into impure red gypsum 

and shale (Withington, 1964, p. 193). A small amount 

of anhydrite (nonhydrated calcium sulfate) is associated with the gypsum in this area. Anhydrite is considered a contaminant in gypsum used for plaster 

products, but it is not detrimental in small amounts 

with gypsum used as a cement retardant or as a soil 

conditioner (Williamson, 1963, p. 3, 9; U.S. Bureau of 

Mines, 1965, p. 416).

The only gypsum deposits now worked in the urban 

corridor are on the flanks of a partly eroded fold in the 

Lykins Formation about 8 km west of Loveland. Production has been nearly continuous since 1896. These 

deposits formerly supplied a local plaster mill; more recently they have provided gypsum used as a setting retardant in cement. A lenticular mass about 12m thick 

and sloping at a low angle has been quarried for about 

0.8 km along the east limb of the fold; a shorter westlimb pit is in two other beds of gypsum, each about 7.5 

m thick, and dipping about 45°. All three beds pinch 

out at shallow depths. Similar gypsum deposits flank 

folds in the lower part of the Lykins north of the 

mapped area, two of which have produced gypsum in 

recent years (Story and Howell, 1963, p. 258-260; Colorado Division of Mines, 1973). In areas south of 

Loveland, the Lykins is reported to contain little or no 

gypsum.

Near Perry Park, southwest of Castle Rock, gypsum 

was quarried in the Ralston Creek Formation from 

1898 to 1901 (Argall, 1949, p. 228). West of Colorado 

Springs, gypsum beds 6-18 m thick and partly of good 

quality (although interbedded with claystone), occur in 

the Ralston Creek Formation at Glen Eyrie, in the 

Garden of the Gods near Fountain Creek, and locally 

farther south. Until 1907, the Fountain Creek deposit 

intermittently supplied gypsum for a plaster mill near 

Colorado City (Withington, 1964, p. 193).

IGNEOUS AND METAMORPHIC ROCKS

Igneous and metamorphic rocks of the urban corridor have supplied small amounts of nonmetallic 

materials for various uses over the past century including: crushed or broken rock (concrete aggregate,
42 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

riprap, road metal, roofing granules, precast construction panels, and other uses); dimension stone (building 

stone, paving blocks, monuments); landscaping rock; 

and minerals for chemical and metallurgical uses. The 

role of these rocks in the economy of the area may grow 

larger as more crushed rock is needed to supplement 

declining supplies of gravel. Igneous and metamorphic 

rocks have been mapped and described in conjunction 

with reviews of gravel resources of the urban corridor 

by Trimble and Fitch (1974a, b), Colton and Fitch 

(1974), and Schwochow, Shroba, and Wicklein (1974); 

the following statement was summarized from these 

publications.

IGNEOUS ROCK

The igneous rocks east of the Front Range are 

dominantly fine grained; those within the Front Range 

are dominantly coarse. Each group, however, includes 

a range of textures and compositions that affect commercial use. East of the mountain front, near Golden, 

three dark, fine-grained lava flows (Table Mountain 

Shoshonite or mafic latite) in the upper parts of North 

and South Table Mountains were first used as highquality cobblestone and as building stone, also of good 

quality, but not popular because of its dark color. In recent years this rock has been quarried for tough and 

durable riprap and crushed for road metal and nonreactive concrete aggregate. Chemically and physically 

similar but somewhat coarser grained rock has been 

quarried from a dike near Valmont northeast of 

Boulder and from the Ralston dike northwest of 

Golden. Another crushed-rock quarry operates in a 

rhyodacite sill about 3 km southwest of Lyons. Other 

small bodies of igneous rock in the Golden-BoulderLyons area may be suitable for similar use. Lightcolored rhyolitic Wall Mountain Tuff caps small buttes 

and underlies broader areas on mesas near Castle Rock 

and south to the Douglas County-El Paso County line. 

The Wall Mountain Tuff has provided building stone 

for homes and small office buildings in Denver, Colorado Springs, and Pueblo and has been used for road 

metal. It contains material that may be reactive with 

common cement and would be suitable for concrete aggregate only in low-alkali cement.

Coarse-grained plutonic igneous rocks, whose outcrops occupy nearly two-thirds of the east slope of the 

Front Range within the corridor, are now of economic 

interest chiefly as sources for nonreactive aggregate 

and other crushed-rock uses. Until the early 1930's, 

these rocks were quarried in a few places for building 

and monument stone; limited quarrying of monument 

stone has continued in the northern part of the area. In 

many places the coarse-grained igneous rocks are disintegrated by weathering to depths of about 4-10 m 

and are used locally for surfacing mountain roads 

(Scott, 1963). Unweathered igneous rock is available 

for crushing from three widespread rock formations 

and from others more locally. The three major units are 

Boulder Creek Granodiorite extending north from the 

Coal Creek area; Silver Plume Quartz Monzonite, 

mainly west and southwest of Denver; and a nearly 

continuous mass of Pikes Peak Granite between the 

South Platte River and Little Fountain Creek. The 

Silver Plume is rated highest for unweathered crushedrock aggregate because tightly interlocking mineral 

grains resist abrasion and facilitate crushing to controlled size. The tendency of the Pikes Peak to separate 

along grain contacts may cause excessive breaking 

during crushing but results in effective natural disintegration. The Boulder Creek is intermediate in rating. 

Suitability for crushing may vary in different areas of 

each formation. Quarrying of unweathered rock requires blasting.

METAMORPHIC ROCK

Metamorphic rocks underlie a third or more of the 

Front Range area along the west margin of the urban 

corridor. These rocks are mostly coarsely layered to 

finely laminated gneiss and schist and, in the Coal 

Creek area, well-bedded, hard quartzite. For crushing, 

gneiss is generally better than schist. The Coal Creek 

Quartzite is a potential source of high-quality crushed 

rock. Quarrying any of these rocks requires blasting. 

Recent production of crushed metamorphic rock has 

depended in part on accessibility and closeness to the 

Greater Denver area market. In the foothills west of 

Denver gneiss is quarried for riprap and aggregate.

VEIN ROCKS

Tabular bodies of vein or dike rocks, cut the 

crystalline core of the Front Range within and west of 

the urban corridor. Usually coarse grained, these rocks 

have furnished small amounts of feldspar, mica, 

quartz, and fluorite for industrial use. In general, the 

Front Range deposits are not now economically competitive with sources outside the State.

LIMESTONE

Limestone is quarried in the Front Range Urban Corridor mostly for crushed rock and portland cement. In 

the past 100 years or more, the limestone has also been 

quarried for mortar, agricultural lime, poultry grits, 

sugar refining, metallurgical processes, and other industrial and municipal uses.

Many industrial applications require conversion of
LIMESTONE 43

high-purity limestone, containing as much as 95-97 

percent calcium carbonate, to quicklime (anhydrous 

calcium oxide, CaO) or slaked lime (the hydrous oxide, 

Ca(OH)2). Conversion may be an integral part of the 

manufacturing process, as in sugar refining, or the 

user may buy commercially prepared lime. Sources 

outside the urban corridor in Colorado, Missouri, 

South Dakota, and Utah supply much of the local need 

for very high calcium limestone and prepared lime. 

Economically accessible supplies in the corridor area 

are no longer adequate. Some byproducts from recent 

sugar refining and from old lime ponds at refineries in 

the corridor have found a market as soil stabilizers 

along newly constructed roads and runways and in 

treatment of water supplies, sewage, and mine and mill 

waste.

Limestone resources of the urban corridor are accessible in or near the hogback belt and in a westward 

extension of sedimentary rocks in the Front Range 

near Colorado Springs. Of the several rock units within 

the area that contain limestone, only the Paleozoic formations in the southern third of the corridor, and the 

more extensive Cretaceous Niobrara Formation have 

had sustained economic importance. The Glennon 

Limestone Member (LeRoy, 1946) of the Lykins Formation has been quarried to a limited extent. Like 

records of other resources of low unit value and intermittent extraction, the records of limestone production 

in the corridor are incomplete.

MANITOU LIMESTONE AND ASSOCIATED ROCKS

In the southern part of the urban corridor, the 

westernmost and oldest source of limestone is the 

Manitou Springs area west and northwest of Colorado 

Springs (Scott and Wobus, 1973). Smaller outcrops extend along the mountain front north to Perry Park and 

south beyond the area. The productive formations are 

the Manitou Limestone (the major source of limestone 

but probably absent at Perry Park) and Williams Canyon and Leadville Limestones in the Manitou Springs 

area and locally north to Perry Park. The Manitou is 

about 56 m thick near Manitou Springs, 85 m near the 

Air Force Academy, and less than 30 m in several 

other outcrop areas. In the Manitou Springs area, the 

Manitou Limestone is underlain by 4.5-12 m of Peerless Dolomite and overlain by about 40 m of Williams 

Canyon and Leadville Limestones. These other limestone and dolomite units are quarried with the 

Manitou near Colorado Springs.

The carbonate rocks of this area are not uniform in 

composition or, consequently, in utility and value. In 

the two large quarries at the mountain front northwest 

of Colorado Springs and in some of the smaller quarries in the Manitou Springs area, quarrymen in past 

years have separated beds of almost pure calcium carbonate, as much as 3-3.7 m thick, from softer dolomitic (high-magnesium) beds and hard, impure, siliceous limestone. Color differences afford a basis for 

separation. Until the early 1940's, the high-calcium 

layers provided lime for sugar refining, which requires 

97 percent or more calcium carbonate and strength to 

withstand the weight of 15-25 m of crushed rock in 

vertical kilns. Limestone from the Colorado SpringsManitou Springs area supplemented production from 

the Ingleside Formation, north of the urban corridor. 

The Ingleside has been the principal in-State source of 

limestone for eastern Colorado sugar refiners since the 

turn of the century (Berlin, 1964, p. 203). (The Ingleside extends into the northern part of the corridor but 

contains no commercial limestone there.) The highpurity limestones of the Manitou Limestone also were 

a source of prepared lime for the general market at 

least until 1968. Most recently, lime processed in the 

Colorado Springs area was marketed for use in soil 

stabilization and treatment of sewage and industrial 

wastes.

Since about 1950, two large mountain-front quarries 

northwest of Colorado Springs have produced crushed 

rock for concrete aggregate, road metal, hot-mix paving, gravel, riprap for dams, and miscellaneous construction-industry uses. These uses require breakage 

into clean, hard, roughly equi-dimensional pieces that 

can withstand heavy loads and a minimum of the costly selectivity needed in quarrying high-purity, colorfree limestone for many chemical uses. Some of the 

limestone, however, has a high content of chert, which 

is deleterious in concrete aggregate because of a possible reaction between the chert and cement. Such a 

reaction weakens the finished product. Also, some of 

the carbonate rock is not durable enough for use as 

high-quality crushed rock.

LYKINS FORMATION

The Lykins Formation is mostly easily eroded sandstone and sandy shale, less than 61 m to more than 183 

m thick, in a narrow zone in the western part of the 

hogback belt, between the more-resistant ridgeforming rock units. The Glennon Limestone Member 

(LeRoy, 1946) of the Lykins Formation is a hard, impure limestone layer 2.4-9 m thick in the lower part of 

the formation. The Glennon is characterized by wavy 

or "crinkled" bedding and an orange-pink to reddishbrown color that have made it attractive for use as 

decorative dimension stone. It has also been crushed 

for driveway surfacing. In earlier years the Glennon 

was burned for local use as agricultural lime and mor-
44 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

tar, but because of its high content of silt, sand, chert, 

and iron, it is no longer competitive. The Lykins is currently less important as a source of limestone than as 

the only presently productive source of gypsum in the 

urban corridor.

NIOBRARA FORMATION

Limestone and limy shale from the Niobrara Formation have been used since 1899 by a cement plant near 

Portland, 32 km west of Pueblo, and since 1927 by a 

plant near Laporte, 6.4 km northwest of Fort Collins. 

Cement production began in 1970 at a plant and 

quarry a few kilometers southeast of Lyons on the 

Niobrara outcrop belt. Another quarry about 8 km 

northeast of Lyons was opened in 1974 to supply the 

plant southeast of Lyons. Past production from the 

Niobrara has come from many pits and small quarries 

along the length of the urban corridor, where the rock 

was burned locally for agricultural lime or for mortar. 

The basal limestone of the Niobrara has been quarried 

in a Douglas County locality for use in several of the 

more than 40 small foundries that have been operated 

in the Denver area (Scott, 1963b, p. 115; Carter, 1964, 

p. 204).

As exposed in the eastern part of the hogback belt, 

the Niobrara Formation consists of a lower (western) 

member called the Fort Hays Limestone and an upper, 

thicker member called the Smoky Hill Shale. The Fort 

Hays contains 5.5-12 m of limestone in beds as much 

as 0.6 m thick and in places exceeding 90 percent 

calcium carbonate; very thin interbeds of limy shale 

separate the limestone beds. The Smoky Hill consists 

of limy shale and impure, commonly chalky limestone, 

more than 150 m thick south of Colorado Springs, and 

about 90 m thick near the north end of the urban corridor. The northward thinning is not uniform.

Where the Niobrara is quarried for making cement, 

the Fort Hays averages about 85 percent calcium carbonate and the lower part of the Smoky Hill averages 

50-65 percent. The two are blended to attain the 75-78 

percent calcium carbonate content required for portland cement (Wolfe, 1964, p. 182). Shale in the Smoky 

Hill supplies the necessary alumina and part of the 

silica for the cement; additional silica is added from 

crushed sandstone from the Dakota Group. The upper 

part of the Smoky Hill contains pyrite and other impurities that preclude its use for cement. In some 

places in the northern part of the mapped area, the 

Smoky Hill contains enough kerogen (solid hydrocarbons that yield oil when undergoing destructive distillation) to approximate a low-grade oil shale. Efforts to 

remove the kerogen to reduce air pollution from the cement plant add to the technical problems of cement 

production.

Economic factors have restricted quarrying for cement to broad outcrops of flat to moderately tilted 

beds (less than about 25°) and thin overburden. These 

conditions exist for the Niobrara within the corridor (1) 

on parts of an en echelon fold from the Lyons area 

northward and (2) in the extreme southern part of the 

mapped area, in and adjacent to the Fort Carson 

Military Reservation. In the northern area, the steeper 

flanks of the folds dip 30°-70° or more.

Large-scale use of limestone from the Niobrara of the 

urban corridor for industrial processes or products 

other than cement seems unlikely in the foreseeable 

future. Commercial quantities of rock containing the 

97 percent or more calcium carbonate needed for sugar 

refining and production of open-market lime, or the 95 

percent or more needed for most metallurgical processes, are not generally present. Some additional rock 

of metallurgical quality may be available in Douglas 

County. Presumably, if local demand existed, lime or 

powdered limestone of about 80 percent calcium carbonate could be obtained for agricultural use from the 

Fort Hays wherever that member is also chemically 

suitable for cement.

SANDSTONE AND CONGLOMERATE

Sandstone from 10 or more rock units in the Front 

Range Urban Corridor has met widely varying needs 

for dimension stone, crushed rock, riprap, landscaping 

rock, silica rock, and sand in the past century. Quarrying operations, however, have been small at many locations, commonly intermittent, and often short lived. 

Records are incomplete.

Dimension stone, for supporting walls and foundations of buildings, was probably the principal use of 

sandstone until the early 20th century, when steel and 

reinforced concrete largely replaced stone for weightbearing parts of large structures. Builders stopped 

using some sandstones such as the Fountain Formation and the Fox Hills Sandstone because these rocks 

lacked the strength and resistance to weathering 

available from more durable sandstones from parts of 

the Dakota Group and the Lyons Sandstone. Hard, 

conglomeratic sandstone in the Dawson Arkose in the 

Castle Rock-Colorado Springs area and Sawatch Sandstone in Ute Pass west of Colorado Springs also have 

been quarried for structural use. The Ute Pass locality 

provided a strong and durable, reddish, green-mottled 

sandstone in slabs as much as 30 cm thick and a few 

meters long. Later use of sandstone in buildings has 

been confined mostly to exterior facing and interior 

decorative applications; slab and crushed sandstone 

are also used for walks, patio paving, and landscaping 

(Lindvall, 1964; Sharps, 1963). Crushed or broken 

stone for road fill or riprap in dam construction is quarried locally.
SITE REHABILITATION 45

As a source of silica sand for special industrial purposes such as glass making, urban corridor sandstones 

are only locally pure enough (high in silica and free 

from contaminants such as iron) and soft enough 

(weakly cemented). Sand prepared from local sandstone is presently used mainly in making brick, tile, 

and cement.

The following sandstones have been used in recent 

years:

LYONS SANDSTONE

The Lyons Sandstone crops out near the western 

edge of the hogback belt. It ranges from 215-245 m in 

thickness near Colorado Springs to less than 30 m near 

the north end of the area. It is suitable for either 

dimension stone or crushed rock where it is red to pink, 

uniformly fine to medium grained and nonconglomeratic, well cemented, thinly and flatly bedded, and splits 

easily into large slabs of uniform thickness. It is less 

used, either crushed or in slabs where it is gray to 

white or yellow, variable in grain size, poorly 

cemented, or thickly crossbedded. Light-colored, weakly cemented Lyons has been a local source of silica 

sand. Mudstone and siltstone interbedded with the 

sandstone in the Greater Denver Area reduce its commercial potential.

In the past, the Lyons Sandstone was quarried for 

dimension stone in the Colorado Springs area as well 

as in the northern part of the corridor. In later years, 

nearly all production for facing of buildings, flagstone, crushed stone, riprap, and other minor 

uses has come from quarries near Lyons and farther 

north. Many quarries have been opened but few if any 

have operated on a full-time basis or for long periods. 

Only about four to eight quarries are active in any one 

year. Moderate but persistent output can be expected 

to continue for many years.

Although the Lyons Sandstone in the central part of 

the hogback belt is generally inferior to that in the 

north and south for most uses, the local softness and 

very light color were turned to advantage early in this 

century when sand for colored glass bottles and furnace bottoms was obtained from a bed in the upper 

part of the formation near Kassler in northwestern 

Douglas County. The amount of high-purity soft stone, 

however, is too limited to encourage further development of glass-sand production.

DAKOTA GROUP

Sandstone from the upper part of the Dakota Group 

(South Platte Formation) provided dimension stone for 

many of the older buildings in eastern Colorado because of its hardness, strength, bedding that facilitated cutting of large slabs, resistance to weathering,

and gray to light-brown color. Quarries operated 

southwest of Colorado Springs and in the Greater Denver area in the late 19th century. More recently, soft, 

white sandstone in the upper part of the Dakota both 

north and south of Kassler has been used as molding 

and core sand by foundries in the Denver area; ironstained layers have been removed before crushing 

(Scott, 1963, p. 115). Since the 1970 opening of a cement plant southeast of Lyons, nearby outcrops of 

Dakota have supplied sand to control silica content of 

the cement. The Dakota has also been used as riprap in 

dams and as landscaping rock. Less sandstone is now 

quarried from the Dakota than from the Lyons.

SITE REHABILITATION

Nearly all urban industrial mineral extraction in the 

Front Range Urban Corridor leads to the excavation of 

open pits. A minor exception has been the underground mining of clay from certain hillside galleries in 

the Dakota hogback north of Golden where the clay 

has been block-caved and removed through adits. In 

any event, what happens to an excavation after 

mineral extraction stops has more long-term significance to the urban community than has the actual 

mining process. Past failure to rehabilitate excavations has led to visual blight and environmental 

degradation. For reasons of public safety, sanitation 

and health, visual amenity, and property value in 

short, the quality of life in the vicinity of industrial 

mineral excavations rehabilitation of exhausted pits 

is becoming a part of planned sequential land use. For 

example, gravel production may be followed by conversion of pits to lakes or by filling of pits for construction 

sites or park development. In recognition of this need, 

the Colorado Legislature in 1973 enacted into law a 

bonded reclamation plan.

Some worked-out pits in the Greater Denver area 

have been backfilled with urban refuse and reclaimed 

as valuable real estate. Major shopping centers and 

municipal facilities occupy some of the high-value 

reclaimed lands that contribute to the economic base 

of the community. Among these areas are Cherry 

Creek Shopping Center, the Denver Coliseum, and 

McNichols Arena-Mile High Stadium area. Other reclaimed lands have been converted to parks, such as 

Ruby Hill. To the extent that an exhausted pit can be 

rehabilitated for other use, the pit is an asset to the 

community rather than a liability.

The practice of backfilling excavations with urban 

waste is widespread in the Front Range Urban Corridor and undoubtedly will continue in the future. The 

potential for pollution, however, should be evaluated 

(fig. 17). Backfilling is a sound sequential land use provided that (1) pollution does not reach ground water, or
46 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 17. Urban refuse backfilling an abandoned gravel pit near Clear Creek in Arvada Waste-disposal practices as illustrated 

here, besides causing visual blight, attract vermin, generate gases, and contaminate the ground water. March 1976.

(2) pollution of shallow ground water is prevented by 

appropriate engineering design (McCollough and 

Pacey, 1971, p. 45), or (3) the leachates aqueous 

chemical solutions from the fill are acceptably diluted 

in the ground-water reservoir within acceptable 

distances of the land fill, or (4) the community is willing to accept pollution of the shallow ground water, as 

landfilling has been widely practiced in the area 

(Hansen, 1977, p. 45), or (5) due consideration is given 

the problems of gas (that is, methane) generation and 

subsidence.

The rate of natural dilution of leachate in ground 

water has not been monitored in detail in Greater 

Denver or in many other places in the United States 

for that matter (Schneider, 1970, p. F5). Schneider (p. 

F5, F6), however, has cited documented ground-water 

pollution in various places in Europe many meters distant from land fills and many years subsequent to 

dumping. The leachates that undoubtedly are being 

generated at numerous land fills in the Front Range 

Urban Corridor probably travel through pervious 

gravels well beyond the disposal sites, in concentrations that limit potential use of the water (U.S. Public 

Health Service, 1962). And the leachates probably 

degrade the surface water as well as the ground water.

Microbial and chemical decomposition of waste 

material in landfills, besides producing leachate, 

generates a variety of gases that can escape the landfill. One of the more common reactions in an aerobic environment is the production of carbon dioxide from the 

breakdown of cellulose. The CO2 in the presence of 

water produces carbonic acid, which reacts with many 

components of the landfill, in turn raises the hardness 

of the water, and increases the biochemical oxygen demand (Schneider, 1970, p. F4). Anaerobic decomposition of cellulose and other organic substances leads to 

the production of methane, which is odorless, and ammonia or hydrogen sulfide, or both. All these reaction 

products can make their way into the hydrologic 

regimen or escape to the atmosphere. To help identify 

off-site environmental problems related to landfills in 

the Greater Denver area, the Geological Survey has 

prepared a map identifying known landfills in that area 

(McBroome and Hansen, 1978).

Gas generation and subsidence commonly occur 

together. As the fill material decomposes, it loses 

volume and simultaneously generates gases. Carbon 

dioxide evolves at a declining rate as the available free 

oxygen is consumed. Conversely, methane production 

increases until the nutrient level declines to the point
SITE REHABILITATION 47

where gas output decreases. If a landfill contains appreciable sulfate, such as gypsum board, hydrogen 

sulfide might result from reactions with carbonic acid. 

The gases move outward and upward from the landfill into the surrounding media in response to pressure 

gradients and available escapeways. They may vent to 

the atmosphere or may accumulate in basements, 

sewer trenches, or other openings. Methane is lighter 

than air; carbon dioxide is heavier. In concentrations 

of 5-15 percent, methane is explosive. Lateral migrations of as much as 340 m have been documented and, 

because of large-scale ground subsidence, buildings 

and other structures have been endangered or damaged (Russ Herman, Raymond Vail and Associates, 

oral and written communs., 1978).

GRAVEL AND SAND PITS

When gravel or sand extraction is widespread below 

the water table, the operation lowers the water table 

significantly and alters the hydraulic gradient between 

adjacent pits. Inasmuch as the hydraulic gradient is a 

sloping surface, and the pond surface is flat, the water 

table is lowered upgradient adjacent to the pond by an 

amount equal to the gradient times the length of the 

pond. For example, if the gradient is 10 m/km and the 

pond is 0.4 km long, the drawdown is 4 m. Gradients 

and ponds of this order are commonplace along creeks 

near the mountain front. At an operating pit the drawdown usually is greater, inasmuch as pumps commonly are needed to keep the water from rising into the 

working area.

When a pit is depleted and extraction stops, the 

water surface in the pit stabilizes at a level close to 

that of nearby surface drainage downgradient from the 

pit. Some pits are worked to within a few meters of 

drainage; commonly a narrow levee is left to preserve 

the drainage course. The water budget of a pit pond, 

therefore, can be rather complex, depending partly on 

the slope and direction of the hydraulic gradient, the 

permeability of the gravel, the regimen of the nearby 

stream, and the evaporation rate, which, in this area, 

ranges from about 97 to 117 cm per year (Meyers, 

1962, pi. 3).

All the major streams along which most gravel is being extracted in the corridor area have histories of 

repeated intense flooding (Follansbee and Sawyer, 

1948; Hansen and others, 1978, p. 43). The wisdom of 

sequential use by backfilling and reclamation on flood 

plains, therefore, is questionable unless such sites are 

reclaimed for conforming uses that will tolerate intermittent flooding. Such potential uses include parks, 

golf courses and athletic fields, open space, parking 

lots, and, where the topsoil has been saved, renewed 

agriculture (fig. 18). Although many comprehensive

community plans incorporate this concept, resistance 

in the form of citizen opposition to restrictions on land 

use has been voiced in public hearings and in letters to 

newspapers.

A nonpolluting alternative to landfilling that has 

high potential value to the community is to preserve 

and landscape the ponds themselves, modifying their 

shore contours as needed to make them attractive and 

safe for water-oriented leisure-time use. Such potential 

use includes boating, swimming, warm-water fishing, 

and onshore picnicking. This alternative has been advocated for the valley of Clear Creek by the City of 

Wheat Ridge, the Professional Engineers of Colorado, 

and the Colorado Chapter, American Society of Landscape Architects ("Action plan for Clear Creek," undated leaflet). This alternative also is supported by the 

National Sand and Gravel Producers Assocation 

(Soule, 1974, p. 18; Bauer, 1965; Schellie, 1963). 

Similar plans have been proposed for other gravelbearing areas such as the valley of Boulder Creek.

Normally, this alternative calls for public ownership, 

although private development has had some success. 

Water-filled gravel pits along St. Vrain Creek near Interstate Highway 25 have been stocked with warmwater fish and made into a state park administered by 

the Colorado Division of Wildlife. The subject of urban 

lakes and their value in real estate is outlined by 

Rickert and Spieker (1971), who pointed out that 

suitable development can greatly enhance the value of 

adjoining property (see also David, 1968). Because of 

the semiarid climate of the Front Range Urban Corridor, natural lakes are few in number, mostly small in 

size, and often inacessible to the public (Ficke and 

Danielson, 1973; Danielson, 1975). The scarcity of 

open water in this environment enhances its value. Artificial lakes along the Colorado Piedmont, therefore, 

are potential real community assets, just as the many 

attractive natural water bodies are in cities such as 

Seattle, Wash.; Greater Boston, Mass.; and Minneapolis-St. Paul, Minn.

CLAY PITS AND STONE QUARRIES

Reclamation of clay pits and stone quarries presents 

problems that are similar to those faced in the sequential use of gravel pits. Large hillside quarry scars, 

however, may not be amenable to restoration, certainly 

not in terms of anything resembling the landscape 

prior to quarrying, and treatment, if any, is a difficult 

problem in landscape architecture.

Pits and shallow quarries along the Front Range, on 

the other hand, offer a chance of nearly complete 

reclamation, and the concept of sequential use can be 

built into the utilization procedure. The pits and 

shallow quarries are prime locations for well-
48 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 18. Overland Park (background) and Ruby Hill Park (foreground), Denver. Viewed toward the southeast. Foreground area is a 

former waste-disposal site. Old clay pits on Ruby Hill were backfilled with refuse, regraded, and landscaped. Overland Park, on the 

floodplain of the South Platte River, also contains landfills. Overland Park has been inundated repeatedly without serious flood 

damage. October 1975.

engineered sanitary landfills. The chief constraint on 

reuse, the potential of offsite water pollution, is 

minimal and readily manageable in this setting 

because (1) surface runoff is minimal, (2) lowering or 

penetration of the water table in such areas is unlikely, 

and (3) the permeability of the enclosing wall rocks is 

very much lower than that of a terrace gravel along a 

major drainage, for instance. Migration of pollutants 

from the disposal site, therefore, could be expected to 

be slow and minimal. Moreover, backfilling with solid 

waste fulfills a community need for waste disposal.

Sequential use of pits and quarries along the mountain front presents some problems. Optimum compaction of solid waste in a steepsided pit or quarry is difficult, especially near the walls, partly because of the 

high and variable compressibility of most waste 

material and partly because mechanical compactors 

cannot maneuver easily near the walls, especially in 

long, narrow excavations. Moreover, slow decomposition of cellulose-based wastes gradually reduces the 

volume of the fill, and subsidence is likely for a long 

period of time. Past performance in the Greater 

Denver area shows that subsidence is a potential problem for buildings or other rigid structures sited on

backfilled pits or quarries. Some form of open space, 

thus, seems to be the preferred sequential use for 

reclaimed clay pits and stone quarries along the Front 

Range.

COAL

Coal underlies parts of all counties in the Front 

Range Urban Corridor and has been mined from most 

counties in the past (figs. 19 and 21). Little if any coal 

is being mined in the area now, but a resumption of 

mining seems likely in the future. A very large coal 

resource still remains even though more than 100 

million metric tons has been extracted (Lowrie, 1966, 

p. 9). Considerable socioeconomic and environmental 

impacts have resulted from past mining and would 

result from mining in the future.

A good summary of past coal production in the 

Denver region, embracing all of the Front Range Urban Corridor and including both Cretaceous and Tertiary coal, is given by Landis (1959), together with 

reserve estimates and brief descriptions of the coal, 

field by field.

A more detailed description of a Tertiary zone containing thick lignite beds in the Denver Basin north-
COAL 49

east, east, and southeast of Denver is given by Soister 

(1974). Soister (1965a, b, c; 1968a, b; 1972) also has 

published several reports on individual coal-bearing 

quadrangles.

SUBBITUMINOUS COAL

The greater part of past coal production in the 

Denver region has been from beds of subbituminous 

coal, ranks B and C, from the Laramie Formation of 

Late Cretaceous age (Landis, 1959, p. 159-166). 

Distribution of this coal is indicated in table 2. Subbituminous coals of ranks C to B range from 8,300 to 

13,000 Btu (British thermal units). Coal of less than 

8,300 Btu is classed as lignite (American Society for 

Testing Materials, 1939).

A general idea of the depth of subbituminous coal 

beneath much of the Denver Basin can be inferred from 

maps by Romero and Hampton (1972) that show the 

approximate configuration and depth of the LaramieFox Hills aquifer. The coal-bearing beds are in the 

stratigraphic interval that immediately overlies the 

aquifer. Most of this coal is too deep to be mined by 

surface methods, and much of it is deeper than 300 m. 

The shallowest coal is peripheral to the basin; the 

deepest is southeast of Castle Rock at a depth of about 

830m.

LIGNITE

A huge resource of lignite, much of it strippable, is in 

the Denver Formation (Upper Cretaceous and 

Paleocene) just east of Denver (Soister, 1974). The 

lignite is in the Paleocene part of the formation. Using 

drill data, Soister found lignite within 300 m of the 

ground surface under an area 121 km long and 56 km 

wide from a point just northeast of Denver in Adams 

County south-southeast to a point several kilometers 

south of Calhan in El Paso County. The lignite-bearing 

zone ranges in thickness from 100 m or so to as much 

as 150 m. Individual beds commonly range from less 

than a meter to as much as 8 m; the thickest known 

bed, east of Denver near Watkins, has a gross 

thickness of 16.6 m, including non-coal partings. In 

most places, this bed is less than 60 m below the surface. Soister has traced it for at least 29 km, and he 

estimated its volume weight at 1.1 billion metric tons. 

Soister further estimated that in beds at least 1.2 m 

thick within 300 m of the surface the lignite resource is 

about 18 billion metric tons. Soister (1974, p. 41) suggested that most of the lignite cannot be exploited 

economically by conventional mining methods (except 

perhaps the thick, strippable beds). He regarded in situ 

gasification as most promising because of the numerous noncoal partings. Liquifaction or reconstitution, or 

both, are possible alternative methods, especially if the

non-coal partings (clay) could be utilized as a 

byproduct. Under the old methods of mining, perhaps 

half the lignite extracted was discarded.

SURFACE EFFECTS OF PAST UNDERGROUND 

MINING

Most mining in the Denver region has been from coal 

beds in the Laramie Formation at depths ranging from 

zero at the outcrop to as much as 150 m. At the 

shallower depths, considerable ground subsidence has 

followed extraction, and in some places the timing and 

extent of subsidence has been nearly unpredictable. 

Particularly in the Colorado Springs and Boulder-Weld 

fields, urban growth is encroaching on areas where 

underground coal mining has been followed by ground 

subsidence (Colorado Springs Planning Department, 

1967; Colton and Lowrie, 1973; Hansen, 1976, p. 

97-100).

Collapse into mined-out workings has reached to the 

surface of the ground in some places, and the resultant 

pseudokarst topography has disrupted surface installations such as buildings, ditches, and railroad tracks. 

Where the depth of mining was shallow several 

meters or so the room-and-pillar patterns of working 

can be discerned in the collapse pattern at the surface 

(fig. 20). In some places the effect on the ground has 

been very disruptive. At shallow depths, strip mining 

would have recovered much more coal and would have 

been more amenable to restoration of the disturbed 

ground. The extent, duration, and time of collapse are 

complex variables related to the thickness of the mined 

coal, methods of mining, character of the roof rock, 

depth of mining, and ground water conditions. In recent years the Colorado Geological Survey has researched the problem of potential collapse but has not 

yet found completely satisfactory solutions (Myers 

and others, 1975). Near Marshall, just south of 

Boulder, the problem of collapse has been complicated 

by mine fires that have been burning for many years.

At Colorado Springs, some land above mined-out 

coal workings has been built upon successfully by 

means of bridging techniques such as grade beams 

that straddle the workings (Colorado Springs Planning 

Department, 1967, p. 7). This technique requires 

specific knowledge of the distribution of mined voids 

versus remaining pillars. Most mined areas of the 

Boulder-Weld coal field are still overlain by range or 

farmlands (Colton and Lowrie, 1973) where the economic consequences of collapse have been minimal. But 

urbanization is spreading into these areas, and 

development is proceeding cautiously. Some building 

and rezoning permits have been withheld by county 

authorities pending detailed evaluation of specific 

sites.
50

41V

ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

105°

i TELLER j 1

FIGURE 19. Map showing distribution of coal (shaded areas) in the Denver Basin, Colorado, and location of fields from which

coal has been produced (broken-line pattern). From Landis (1959).
OIL AND GAS 51

TABLE 2. Summary of Cretaceous coal occurrences in the Laramie Formation, Denver region

[Data from Landis, 1959, p. 159-166]

Field

Colorado

Springs.

BuickMatheson.

Briggsdale 

Eat on 

Wellington 

Foothills 

Boulder-WeldArea 

(km2 )

176

205

36

23 

137

233

870

Thickness of 

principal beds (m)

4.3

No data

1.7

QC o_> 

1.4-1.8

2.5-5.5

2.5-4.3

Estimated total before 

mining (million t)

367

79

80

27 

272

894

1,796

Subb i t uminous 

rank

C

C

C 

r-v-> 

C

C to B

B

Unfortunately, studies are not always definitive. 

Mining began in the Denver region in the last half of 

the 19th century and some mines have been inoperative for more than 90 years (Colton and Lowrie, 1973). 

Particularly for very old mines, maps do not always 

adequately show the distribution of mined areas or the 

locations of haulageways, shafts, or winzes. Moreover, 

from the ground surface, it may not be possible to 

locate underground openings or determine the extent 

of collapsed workings at depth.

OIL AND GAS

The Denver Basin is one of the leading oil- and gasproducing areas of the Rocky Mountain States. In Colorado, only the Piceance Creek basin in the northwestern part of the State has produced more oil, and 

only the Piceance Creek basin and the San Juan Basin 

in southwestern Colorado and adjoining states have 

produced more gas. Estimates of ultimate gas production from the Denver Basin exceed that of either of the 

above basins (Haun and others, 1976, p. 6-8). Ultimate 

production estimated by Haun and others is 

487,646,669 barrels of oil and 2,052,966,833,000 ft3 

(cubic feet) of gas. Through 1978, the Colorado part of 

the Denver Basin had produced 444.5 million barrels of 

oil and 962.7 trillion ft3 of gas (Colorado Oil and Gas 

Conservation Commission, 1978, p. 8). 1

Oil has been produced in the Denver Basin chiefly 

from areas east of the Front Range Urban Corridor, 

but Weld, Adams, Larimer, and Arapahoe Counties

1 The petroleum industry records its production figures in the inch-pound system of 

units. One barrel equals 0.159 m8. One cubic foot equals 0.02832 m".

5>7 v .-, ^>-. -

FIGURE 20. Extreme ground subsidence over an old underground 

coal mine at Marshall, near Boulder. Room-and-pillar mining pattern is clearly indicated by collapse pattern of ground. Area 

shown is about 420 m long north to south; north is at top of 

photograph. Vertical aerial photograph by R. B. Taylor and R. L. 

Parker, U.S. Geological Survey, June 1973.
52 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO
Kiowa

ELBERT

\i\\Mu%^WM^\\\\\

EXPLANATION

Oil field Gas field

::ll:!!jj!lji!!jj!li Area in which coal of Cretaceous 

and Tertiary age is present

Mountain area of Precambrian 

crystalline rocks

Outline of Front Range Urban 

Corridor

a$cc

FIGURE 21. Map showing distribution of oil and gas fields and coal in and near the Front Range Urban Corridor. Outline of corridor and mountain front (contact between Precambrian crystalline rocks on the west and younger sedimentary rock on the east) is also shown. From various 

sources in the U.S. Geological Survey and Colorado Geological Survey, and Tweto, 1976.

CO
54 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

produce oil from many wells inside the corridor area, 

some close to dense urbanization (fig. 21). Boulder 

County has a long history of modest oil and gas production, and a little oil has been produced in Jefferson 

County.

By far the largest gas field in the Denver Basin is 

Wattenberg (named from the town in southwestern 

Weld County), which lies mostly in the Front Range 

Urban Corridor between Denver and Greeley. The 

Wattenberg field is one of the great gas fields of the 

Rocky Mountain region, and it has the highest estimated ultimate gas productivity in Colorado. Haun 

and others (1976) estimated that Wattenberg ultimately will produce 1,322,579,190,000 ft3 of gas.

The scope of this report does not include a detailed 

account of production and potential development of oil 

and gas in the Front Range Urban Corridor; such a 

study was not a part of the urban-areas program of the 

U.S. Geological Survey. A voluminous literature has 

accumulated, however, particularly since the years immediately following World War II when exploration 

and development of oil and gas in the Denver Basin 

were greatly accelerated. Many papers consequently 

have been published on the stratigraphy, structure, 

petrography, paleontology, and depositional environments of the petroleum-bearing rocks and the fields in 

which they occur. Excellent papers have appeared in 

The Mountain Geologist, which is published by the 

Rocky Mountain Association of Geologists, in guidebooks of field conferences of various geological societies of the region, and in other technical journals (for 

example, Pruitt and Coffin, 1978).

The most productive reservoir rock in the Denver 

Basin is the "J" sandstone (economic usage). This 

sandstone is the uppermost member of the South 

Platte Formation of the Dakota Group in the Denver 

Basin and is the chief exploration target in most oil 

and gas fields of the basin. Petroleum geologists 

sometimes call it the Muddy Sandstone, a name more 

correctly applied to similar rocks in northern Wyoming. Production of oil and gas has also been obtained 

from many other stratigraphic intervals in the Denver 

Basin, especially from the Lyons Sandstone of Permian age, which is second only to the "J" sandstone in 

terms of total productivity, and from the Niobrara Formation and sandstone members of the Pierre Shale of 

Late Cretaceous age.

SOIL PROBLEMS 

LAND DEVELOPMENT ON EXPANSIVE SOILS

Expansive soils cause construction problems on the 

Colorado Piedmont, particularly with respect to highways and lightly loaded structures such as private 

dwellings, schools and commercial buildings, and

poured concrete slabs. Problems caused by expansive 

soil take the form of increased costs of preventive or 

precautionary construction and increased costs for 

repair of structural damage after construction buckled 

and cracked pavements, cracked or heaved masonry 

walls, heaved floors, damaged structural members, 

jammed doors and windows, and other complications 

(fig. 22).

These soils have been studied by Hart (1974) under 

an investigation jointly funded by the Colorado 

Geological Survey and the U.S. Geological Survey as 

an element of the Front Range Urban Corridor program. Many of these soils have critically high potential 

swell pressures ("critically high" denotes pressures 

greater than about 15 t per square meter; Lambe, 1960, 

p. 20). Swelling is caused by sorption of water into or 

around the clay mineral crystal lattice. The mineral 

chiefly responsible is sodium montmorillonite, a clay 

mineral commonly dispersed in marine shales, in volcanogenic terrestrial deposits, and in weathered soil 

profiles in semiarid regions (Tourtelot, 1974, p. 266, 

269). The material often called "bentonite" is nearly 

pure sodium montmorillonite. According to Tourtelot, 

pure sodium montmorillonite, rare in nature, might 

swell as much as 2,000 percent in volume. Some other 

clay minerals also have swelling properties but of less 

potential pressure.

Critically expansive soils on the Colorado Piedmont 

are most common on terrains underlain by Upper Cretaceous and Tertiary shales and mudstones, particularly in the widespread Pierre Shale and Denver 

Formation but also in several other formations. Many 

surficial deposits, such as alluvium and colluvium, 

moreover, and even eolian deposits may also have 

swelling properties, inherited from their parent formations (Hamilton and Owens, 1972a, table 2; Hart, 1974,

FIGURE 22. Building wall damaged by stairstep corner fracturing, 

Lakewood, October 1976. The center section of the wall at right 

has been uplifted relative to the corner at left. Damage of this sort 

is frequently caused by expansive clay.
SOIL PROBLEMS 55

p. 6). Because of the wide areal distribution of these 

formations on the Colorado Piedmont, expansive soil is 

one of the most prevalent and costly geologic problems 

in the Front Range Urban Corridor. Ironically, soil expansion and its consequences are little known to the 

general public.

The potential volume change of a soil sample in the 

laboratory is only one measure of its possible behavior 

under field conditions on the Colorado Piedmont. The 

availability of moisture at the field site, the freedom of 

moisture to enter into the soil, the extent of alternate 

wetting and drying, soil density, and engineering 

design are equally important. Geologic materials that 

contain high-swelling clays also are very susceptible to 

slope failure, inasmuch as swell potential, plasticity, 

and shear strength tend to be interrelated. An approach to understanding expansive soils on the Colorado Piedmont has to be based on geologic studies, soil 

sampling and testing, case histories, evaluations, and 

engineering design.

LAND DEVELOPMENT ON EOLIAN SOILS

Loess and windblown sand cover much of the Colorado Piedmont, particularly leeward (east and southeast) of the flood plains of the major streams. These 

deposits, because of their physiographic and economic 

implications, have been under detailed study by the 

U.S. Geological Survey, but a better understanding 

than we now have of their regional physical settings 

and histories is needed to permit intelligent judgments 

about their place in land use planning and development. Loessial areas are among the best agricultural 

lands in the region, because of their good tilth, 

favorable moisture retention, and general fertility. 

Areas underlain by eolian sand, on the other hand, 

mostly lack these desirable attributes; as agricultural 

lands, they are used mostly for pastures.

Engineering properties of the eolian deposits are well 

understood in a general way (Larsen and Brown, 1971, 

tables 6-7, for example) and have been intensively investigated locally by the Colorado Division of Highways, the U.S. Geological Survey (Shroba, 1977), and 

by private engineering firms, but they are not well 

documented regionally. We are familiar with the 

general areal distribution of the deposits, but we know 

little about their thickness, textural variations, or field 

performance.

Like most underconsolidated soils, loess tends to 

subside under load, particularly when wetted (Holtz 

and Gibbs, 1952, p. 15) as, for example, in a new subdivision when new lawns and foundation plantings are 

being irrigated by homeowners, or when water from 

roofs is concentrated by downspouts. Susceptibility to 

subsidence depends partly on void ratios and on cementation; it may be slight or may be great enough to 

damage footings, foundations, sewer pipes, curbings, 

and heavily loaded floor slabs (Tabor, 1969, p. 53). 

Loess also is highly vulnerable to erosion by wind and 

running water when it has been stripped of its protective vegetation cover, either by agricultural practices 

or by urban land development (Larsen and Brown, 

1971, p. 3, 16).

The loess and eolian sand of the Colorado Piedmont 

are related genetically and they grade into one another, 

but they differ markedly in physical properties and in 

response to disturbance. Eolian sand forms thin, blanketlike deposits and thick linear dunes, especially east 

of the South Platte River. Its signature on aerial 

photographs is distinctive and easily recognized (fig. 

23). The linear dunes trend mostly from northwest to 

southeast. Broad areas east and south of Greeley, east 

of and in the eastern part of Denver, and east of Colorado Springs are blanketed with sand (Scott and 

Wobus, 1973; Shroba, 1977; Colton, 1978). In this 

semiarid region, eolian sand takes and transmits water 

readily but desiccates rapidly at the ground surface 

and supports thinner vegetation than most other soils. 

It is exceedingly fragile in an ecologic sense, therefore, 

and it recovers slowly from disturbance. Some dune 

areas that reportedly were reactivated during the dust 

bowl days of the 1930's have not yet regained stability 

(R. B. Colton, oral commun., 1974). Active deflation 

blowouts southeast of Greeley are visible from aircraft. In the Englewood area old barbed wire fences are 

partly buried south of Windsor Lake (Shroba, 1977).

Most of the dune sand supports grass, which deters 

wind erosion unless it it broken up by deep plowing or 

other land use. Once disturbed, the sand is vulnerable 

to erosion by wind and running water until a protective 

cover is restored. Both deflation and deposition, therefore, are problems around new land developments.

Because of very low shear strength, eolian sand is 

unstable and, on steep-cut slopes and unsupported 

open trenches, it may collapse without warning. Lives 

of workmen have been lost in such collapses in the 

Denver area. Dune sand also tends to settle under load, 

enough at times to damage rigid structures, particularly if it is subject to vibration (Tabor, 1969, p. 54).

SOILS CONTAINING ORGANIC MATTER

Soils rich in organic matter are relatively uncommon 

on the Colorado Piedmont, in terms of total area, 

because their development has been restricted by the 

semiarid climate. They are fairly widespread, however, 

on silty, alluvial bottomlands along stream valleys and 

other geomorphic settings where drainage is sluggish 

or poor. Piney Creek Alluvium and the modern flood 

plain, for example, are settings where such soils are 

prevalent (Hunt, 1954, p. 114, 117; Gardner and
56 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 23. Inactive dune field southeast of Greeley. North is at top of photograph. Strong linear trend is parallel to direction of 

depositing winds (northwest). Crescentic dunes contain small active blowouts. Unsuited for cultivation, this area is open range. 

Cultivated fields on loess are at left. South Platte River is in upper right. Scale is indicated by grid pattern of section-line roads at 

1-mile intervals. High-altitude aerial photograph by National Aeronautics and Space Administration, October 26, 1972.

others, 1971, sheet 4; Scott, 1972d; Van Horn, 1972; 

Trimble, 1975; Shroba, 1977). Because of their compressibility and nonuniformity, organic soils are likely 

to settle under load, which can lead to structural 

damage of building foundations or road subgrades.

MOUNTAIN SOILS AND BEDROCK

The suitability of mountain areas for homesites, 

roads, underground-utility corridors, waste-disposal 

sites, and ground-water recharge depends partly on the 

thickness and character of the soil (in the engineering 

sense). Whether or not a utility trench can be dug with 

a backhoe, or whether more costly excavation procedures such as drilling and blasting are required, might 

determine the feasibility of a particular use for a given 

tract of land.

Residential expansion in the foothills of the Front 

Range, moreover, has been accompanied by problems 

of water supply and waste disposal (Hofstra and Hall,

1975a, b). The quantity of water available is small in 

many areas. Waste disposal and its effect on quality of 

water are influenced by the varied thickness of 

soil both residual and transported over crystalline 

bedrock and the varied filtration properties of mountain soils. Adequate soil thickness and percolation 

properties are essential for many uses of mountain 

tracts.

Recent work by the U.S. Geological Survey (Schmidt 

and Pierce, 1976) has outlined the distribution, 

thickness, and character of the mountain soils in the 

Front Range Urban Corridor as they relate to urbanization. This work was based on regional geomorphic 

relationships, field reconnaissance, well inventories, 

seismic profiling, and aerial-photograph interpretations. The soils include residuum and transported 

alluvium-colluvium (fig. 24). The thickest and most 

continuous deposits have developed under old, welldefined, upland erosion surfaces (pediments) that 

began to form as early as late in Eocene time (Scott,
MASS WASTING

FIGURE 24. Deeply weathered grus, Colorado Highway 67 west of Colorado Springs, formed on Pikes Peak Granite and containing fresh, 

hard core stones of granite. Easily excavated grus is widely used in the Front Range for road surfacing and earth fill. April 1972.

1975, p. 227). The characteristics of these saprolitic 

soils are related to parent material as well as to age; 

some of the thickest and most completely saprolized 

ones have developed on granitic terrains underlain 

by Pikes Peak Granite or Boulder Creek Granodiorite, 

for example (Schmidt and Pierce, 1976, p. 46). Alteration and, hence, soil formation are very intensive also 

on wide, shattered fault zones but are minimal on 

quartzose gneiss or pegmatite (Schmidt and Pierce, 

1976). The physical properties of the mountain soils, 

thus, are highly varied, and because of the growing 

pressure of mountain urbanization, the character of 

the mountain soils has increasingly concerned the 

planner and land user.

MASS WASTING

As used here, the term "mass wasting" refers to processes by which large masses of earth or rock are 

moved downslope by gravity. It excludes movements 

where water is the primary transporting medium,

although water nearly always is involved in mass 

wasting. In the Front Range area mass wasting includes many kinds of landsliding and other types of 

movement such as soil creep and solifluction (fig. 25). 

The overturning of a concrete retaining wall is an effect of soil creep.

In the last few years, growth along the Front Range 

Urban Corridor has pushed urbanization into areas 

where mass-wasting processes impinge on urban land 

use. Construction of roads and dwellings has accelerated greatly in the foothills belt, where landslides, 

mudflows, soil creep, and rockfalls have been natural 

occurrences since prehistoric time. The foothills area, 

however, is attractive for residential development 

because of its scenic setting and its broad vistas of 

mountains and plains. It is fragile in a physiographic 

sense because of its semiarid climate, steep slopes, thin 

soils, abundant plastic shales and mudstones, steep 

dips of bedding, foliation, and joints, and slow 

recovery of vegetation in areas of disturbance. Care in 

site selection and treatment in these places can minimize adverse geomorphic effects.
58 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 25. High plasticity of clay-bearing rocks promotes soil 

creep in the Laramie Formation, Jefferson County, near Interstate 70. Outcrop is topped with spoil from a nearby clay pit. 

June 1971.

Mass wasting along the Front Range is restricted 

mostly to clearly defined geomorphic settings where 

problems have a rather high element of predictability. 

In a series of pilot studies, the U.S. Geological Survey 

has published maps that show particular types and 

areas of concern including, for example, landslides, 

rockfalls, swelling soils, and flood-prone areas 

(Maberry, 1972a; Scott, 1972a, b, c; Simpson, 1973a; 

Colton and Holligan, 1977). Regional maps showing 

landslides have been prepared by Colton and others 

(1975), Colton, Holligan, and Anderson (1975a, b), and 

Crosby (1978).

Mass wasting is a response to the pull of gravity. 

Resistance is most often overcome by the introduction 

of water into soil or rock. Freezing and thawing may 

loosen and dislodge material, freeing it to slide or fall. 

Saturation by heavy or prolonged rain or by melting 

snow may induce failure, partly by reducing the shear 

resistance of the mass and partly by increasing the 

shear stress through the added weight of the water 

(fig. 26). Many small landslides followed the prolonged 

rains of May 5-6, 1973, in the Greater Denver Area 

(Hansen, 1973, p. 11-17). Earthquakes may cause instantaneous failure of material, wet or dry, both by 

reducing shear resistance and by increasing shear 

stress. A large rockfall at the foot of Cheyenne Mountain south of Colorado Springs may have been triggered by a prehistoric earthquake (Scott and Wobus, 

1973).

Mass wasting is a problem in the Front Range Urban Corridor chiefly in hilly terrains underlain by Upper Cretaceous and Tertiary shale and mudstone of the 

Dakota Group, Benton and Pierre Shales, Laramie and 

Denver Formations, and Dawson Arkose. These formations underlie broad areas in the path of urbanization. Nearly every formation in the geologic column, 

however, is involved in slope failure somewhere in the 

area. Scores of small landslides and mudflows were set 

off in the storm of May 5-6, 1973 in the formations 

cited above and also in the Lykins and Morrison Formations and weathered parts of the Precambrian 

rocks. Most of these failures were in disturbed areas 

where slopes had been oversteepened or unbalanced by 

hillside excavations such as roadcuts. Proposed excavations in hillside locations, therefore, should be 

evaluated for evidence of past slope instability, for 

evidence that the materials involved have a record of 

poor field performance, or for indications that an excavation might precipitate a slope failure.

HYDROLOGY

Urbanization has interacted with the hydrology of 

the Front Range Urban Corridor, and the effects have 

influenced the lives and fortunes of the populace. 

Urbanization has increased peak flows of streams 

following storms, decreased mean flows, and altered 

the frequency and magnitude of floods. It has affected 

channel characteristics, scour and sedimentation, 

ground-water conditions, and water quality. These 

combined effects cause ecologic changes in the streams 

and their surroundings and alter the esthetic values of 

the area what Leopold (1968, p. 1) has called the 

"hydrologic amenities . . . the impression which the 

river, its channel and its valleys, leaves with the 

observer. Of all land-use changes affecting the 

hydrology of an area, urbanization is by far the most 

forceful."

FIGURE 26. Rockfall on eastbound Interstate 70, 3.2 km west of 

Bergen Park interchange, triggered by heavy rains of May 5-6, 

1973. Oversteepened rock cut failed along joints and foliation 

planes. Photographed May 8, 1973 after preliminary cleanup had 

removed rock from lanes of traffic.
HYDROLOGY 59

Continued urban growth on the Colorado Piedmont 

has intensified hydrologic problems that have been 

recorded since pioneer days (Hansen, 1976, p. 100). A 

large part of the U.S. Geological Survey's work in the 

Front Range Urban Corridor, therefore, has been 

directed toward the hydrologic aspects of land use on 

the Colorado Piedmont. The availability of hydrologic 

data for the corridor published by the U.S. Geological 

Survey and cooperating agencies is summarized by 

Hampton, Clark, and McNutt (1974), Hampton (1975), 

and Anna (1975). Convenient sources of bibliographic 

information through 1970 have been compiled by Pearl 

(1971) and through 1972 by Chronic and Chronic 

(1974). Persons interested in specific areas or subjects 

not listed in these sources or published more recently 

should contact the U.S. Geological Survey, Denver, or 

the Colorado Department of Natural Resources for 

assistance and information.

FLOODING

Accounts dating back more than 150 years, and even 

farther in Indian legend, indicate that intense rainfall 

and severe flooding can be expected from time to time 

along all major tributaries of the South Platte and 

Arkansas Rivers in the Front Range and on the Colorado Piedmont (Follansbee and Sawyer, 1948; Jenkins, 

1961, 1963, Matthai, 1969; Ducret and Hansen, 1973; 

Hansen, 1973; McCain and Hotchkiss, 1975a, b, c). 

Early flood warnings mostly unheeded were given 

by the 19th-century Indians to the settlers, and much 

grief might have been forestalled had the warnings 

been heeded more often. Episodic flooding is perhaps 

the foremost natural hazard in the Front Range Urban 

Corridor area. Record discharges of selected streams 

are listed in table 3. These streams, and many others of 

the area, have histories of repeated catastrophic 

flooding (figs. 27 and 28).

Of great importance to land-use applications, 

therefore, has been the portrayal of flood-prone areas 

in the corridor by maps and reports, including published synoptic maps at a scale of 1:100,000 (McCain 

and Hotchkiss, 1975a, b, c) and open file maps of 

numerous, more detailed 72-minute quadrangles at a 

scale of 1:24,000. Flood-plain information for the 

streams and gulches in the Denver metropolitan region 

also has been prepared by the U.S. Army, Corps of 

Engineers, Omaha, Neb., for the Denver Regional 

Council of Governments.

Floods on the South Platte and Arkansas Rivers and 

their Front Range tributaries generally result from intense rainfall of short duration and are characterized 

by uncommonly high velocities and short durations, 

owing to a combination of unusual physiographic and 

climatologic factors. Streams of the area are adjusted

to regimens of relatively small mean flows in channels 

with relatively steep gradients, particularly as compared with streams of similar drainage area in more 

humid, less elevated regions of the United States. Low 

relative humidity and high evapotranspiration rates 

tend to reduce infiltration and soil moisture, resulting 

in relatively sparse vegetation and humus-poor soil 

over much of the area, which further favor low infiltration and rapid runoff during intense cloudburst 

storms. Consequently, the ability of Front Range 

streams to transport sediment, rocks, and debris is increased enormously during floods.

Channel changes along most perennial streams are 

accomplished principally by occasional flows near or 

above bankfull stage (Leopold and others, 1964, p. 

82-84; Leopold, 1968, p. 7). Channels, therefore, tend 

to be adjusted to such discharges. Infrequent 

devastating floods accelerate bed and bank erosion, 

overbank scour and sedimentation, sheetflooding, and 

mass wastage.

The regional rise of the Great Plains toward the 

Rocky Mountains and the physiography of the Front 

Range itself constitute an orographic barrier that 

markedly affects the distribution and intensity of 

storms. Intense shower activity is favored by the 

upslope flow of warm, moist air from the Gulf of Mexico or from the Pacific Ocean, held in close to the mountains by easterly circulation aloft.

Historic floods in the Front Range area have been 

restricted largely to the months of May through 

September (Hansen and others, 1978, fig. 8). The time 

distribution of Front Range floods is distinctly 

bimodal, with peaks in the first week of June and the 

first week of August. Late spring and early summer 

floods often have been caused by a combination of 

rapid melting of mountain snowpacks and regional 

rainstorms. Other late spring and early summer floods, 

and nearly all late summer floods, on the other hand, 

have resulted exclusively from intense local 

thunderstorms.

McCain and Jarrett (1976, p. 39-40) believed there is 

an upper altitude limit at which severe cloudburst 

flooding is likely in the Southern Rocky Mountains, 

owing chiefly to a reduced moisture supply available to 

thunderstorms at higher altitudes. In the Front Range 

area this limit is about 2,300 m in the South Platte 

River drainage and about 2,750 m in the Arkansas 

River drainage. Past flooding in the Front Range 

seems to prove this conclusion; the upper limit of locally intense flooding accompanying the July 1976 

storm over the Big Thompson Canyon area was about 

2,550 m (McCain and others, 1979; Shroba and others, 

1979).

Astonishing precipitation rates have accompanied 

some of these storms (Follansbee and Sawyer, 1948, p.
60 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 27. Damage caused by the June 15-16, 1965, flood in Denver, Colorado. Water mark on building is at level of the top of the 10th

cinderblock. Photograph by Warren B. Hamilton.

70; Matthai, 1969, p. B3, B12; Maddox and others, 

1977). On May 30, 1935, for example, 610 mm of rain 

fell in 1 day, most of it in about 3 hours, at Elbert, 70 

km southeast of Denver (Follansbee and Sawyer, 1948, 

p. 70). On June 16, 1965, more than 350 mm fell in 

about 4 hours at Palmer Lake, 65 km south of Denver 

(Matthai, 1969, p. B14), and comparable amounts fell 

on several nearby areas. During the tragic Big Thompson Canyon storm of July 31-August 1, 1976, more 

than 150 mm of rain fell at Glen Comfort in 45 minutes, and 250 mm fell in less than 4 hours (L. R. Hoxit, 

written commun, 1977).

Downpours such as these generally are very localized and commonly trigger flash floods that may exceed normal discharge by two or three orders of 

magnitude. Matthai (1969, p. B17-B18) estimated that 

East Plum Creek, about halfway between Palmer Lake 

and Denver, increased its volume a thousandfold in 

less than 3 hours during the 1965 storm and attained a

FIGURE 28. Flooding at East Jefferson Avenue and South Emerson Street in Englewood, caused by overflow of Little Dry Creek. 

Denver Post photograph by John Prieto, May 6, 1973.
HYDROLOGY 61

TABLE 3. Record discharges of selected streams, Colorado Piedmont area, 

[Data from U.S. Geological Survey, 1969]

Stream

South Boulder Creek at Eldorado Springs 

Cache la Poudre River at Fort Collins 

Fountain Creek at Security 

Big Thompson River at mouth of canyon^ 

South Platte River at Denver 

Cherry Creek at Cherry Creek Lake 

South Platte River at Littleton 

Kiowa Creek north of Kiowa 

Bijou Creek near Wiggins (east of Roggen)-

Date

Sept

July

Aug

June

Sept

Aug

Mar.

July

July

June

June

June

May

June

June

- 2,

24,

 1,

22,

 3,

. 3,

20,

24,

31,

17,

16,

16,

31,

16,

17,

1938

1896

1888

1941

1938

1951

1904

1965

1976

1965

1965

1965

1935

1965

1965

Peak 

discharge

(ft^s) 1

7

8

8

10

11

14

21

25

31

40

59

110

110

154

466

,390

,600

,700

,500

,300

,000

,000

,000

,200

,300

,000

,000

,000

,000

,000

Discharge measurements customarily are made in ft~Ys (cubic feet per 

second). One ft3 equals 0.02832 m3 (cubic meter). 

2 McCain and others, 1979, table 3.

maximum velocity of 6.1-6.7 m/s (meters per second). 

McCain and others (1979) calculated that the Big 

Thompson River, during the 1976 storm, attained a 

velocity of nearly 8 m/s at the mouth of its canyon.

The enhanced capability of Front Range streams to 

modify the landscape at floodstage, therefore, is hardly 

surprising. Water at such velocities can move huge objects. What is suprising, even in full knowledge of the 

semiaridity of the climate, is the slow natural recovery 

of the landscape, and the effect, therefore, of flooding 

on the visual amenity (Hansen, 1976, p. 90). Many of 

the hillside and channel scars from the 1965 storms are 

little changed after 14 years (fig. 29). Gullying caused 

by a 220 mm downpour in Mount Vernon Canyon in 

the Front Range west of Denver in 1938 was still plainly visible in 1978.

The Big Thompson Canyon flood of 1976 is remembered by many Colorado citizens, partly because of its 

tragic death toll and partly because of its magnitude. 

This was the largest historic flood in the Big Thompson River basin, having exceeded any previous historic 

flood there in rate of discharge by at least a factor of 

two and in the canyon by a factor of four. Although the 

flood was exceptional, it was not unique to the Front 

Range. Many comparable floods have been recorded in

other basins along the Front Range in the past century 

or so, and in terms of total runoff, rate of discharge, 

and property damage, some of these have been much 

larger. Before 1976, the largest recorded discharge on 

the Big Thompson River at the mouth of the canyon 

was 7,600 ft3/s (cubic feet per second) on July 19,1945, 

a rate comparable to historic maxima on Bear Creek, 

Clear Creek, and South Boulder Creek. In comparison 

Buckhorn Creek, a tributary of the Big Thompson 

River, discharged 14,000 ft3/s in 1951.

Geomorphic evidence supports the certainty of past 

and projected future catastrophic flooding along Front 

Range streams. Most canyons along the Front Range 

are physiographically similar and have had parallel 

hydrogeologic histories. Nearly all Front Range canyons in the lower montane zone (upper altitude limit 

about 2,400 m) contain evidence of intense channel 

scour in recent time. Historic evidence includes 

destroyed segments of roads and railroad grades. 

Large debris fans also are common in nearly all canyons in the lower montane zone, and they are clearly 

the products of torrential runoff. Some of them have 

been modified by historic flash flooding. Morphologically they are close counterparts of fans that were formed or modified during the 1976 flooding in Big Thomp-
62 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

FIGURE 29. Debris-avalanche scar photographed in December 1972 on Hunt Mountain, 11 km south of Castle Rock, caused by torrential 

rains of June 15-16, 1965, and little changed at the time of this report (1979). Hunt Mountain is capped by Castle Rock Conglomerate, 

and scar is eroded into the Dawson Arkose.

son Canyon. Finally, the channels of most Front Range 

streams contain bouldery flood gravels similar to those 

transported and deposited by the Big Thompson flood 

of 1976.

URBANIZATION AND RUNOFF

The effects of urbanization on runoff have been 

studied by many investigators in recent years. Leopold 

(1968, p. 1) has listed four interrelated but separable effects of land-use changes that can be applied to Front 

Range streams: (1) changes in peak flow, (2) changes in 

total runoff, (3) changes in water quality, and (4) 

changes in the hydrologic amenities. His figure 1 

shows that peak discharge rises higher and arrives 

sooner after urbanization than before and that the 

volume of runoff increases proportionally. Conversely, 

infiltration and ground-water recharge decrease, thus 

reducing base flow and lowering the water table. Consequently, stream flow after urbanization is higher

during and just after storms and is lower at other 

times. Moreover, the exposure of bare ground to storm 

runoff during urban construction increases the sediment yield, which also alters runoff characteristics and 

clogs drainage channels (Guy, 1970, p. E3). Urban 

refuse, such as rubber tires, beer cans, paper cartons, 

detergents, fertilizers, and countless other wastes 

picked up during storms or carelessly dumped into 

water courses, degrades the hydrologic amenity,2 

degrades the quality of the water, and disrupts the living processes of aquatic organisms. Along Cherry 

Creek, for example, a general downstream degradation 

in water quality from Franktown through Denver has 

been documented by Costa (1978, table 14).

Human activities along the Front Range Urban Corridor have accelerated ongoing hydrogeomorphic pro-

^ Leopold (1968) defined the hydrologic amenity as the overall impression that the 

stream-and-valley system makes on the reservoir.
HYDROLOGY 63

cesses. Prolonged rains not of the cloudburst 

type on May 5-6, 1973 caused about $50 million 

damage through inundation, scour, sedimentation, 

landsliding, and mudflowage (Ducret and Hansen, 

1973; Hansen, 1973). Much of the damage from this 

storm was in areas where manmade modification of the 

terrain had altered the runoff characteristics. Constraints on the free flow of runoff were caused by 

bridges, culverts, channel constrictions and flood-plain 

encroachments. Peak flows were accentuated by total 

runoff from impervious roofs, driveways, streets, and 

parking lots. Water was retained behind many culverts 

and storm drains that were designed to pass smaller 

flows. Irrigation ditches overflowed with water intercepted from usually dry gulches. Sheetflooding was 

aggravated by the practice of stripping vegetation and 

soil from building sites and tracts during land development. Most of the many observed landslides, mudflows, and rockfalls caused by this storm were in places 

where support had been removed from previously 

stable hillslopes. The extensive damage resulting from 

this storm indicates the need for carefully planned land 

development in areas where geomorphic processes are 

active or are likely to be activated.

GROUND WATER

Many reports dealing with the ground water 

resources of different parts of the Front Range Urban 

Corridor and adjacent areas have been prepared in the 

past few years by the U.S. Geological Survey, and by 

the Colorado Department of Natural Resources. Much 

of the following information is summarized from these 

reports.

VALLEY-FILL ALLUVIUM

Important alluvial aquifers underlie the valleys of all 

the principal streams of the Colorado Piedmont (Smith 

and others, 1964; Hershey and Schneider, 1964; Hurr 

and others, 1972a, b; Bingham and Klein, 1973; Hurr 

and others, 1975; McConaghy and others, 1964; Hillier 

and Schneider, 1979a, b; Hillier and Hutchinson, 

1980a, b). The water from these aquifers is widely used 

in households and for irrigation in rural areas, either to 

supplement or to supplant surface-water sources 

(Code, 1943). The water also is used to some extent for 

municipal supplies (Hurr and others, 1975, p. 2, 4), 

although most urban areas rely mainly on surface 

water from mountain sources for municipal uses.

Valley-fill alluvium in the South Platte River valley, 

according to Hurr, Schneider, and Minges (1975, table 

1), yields 380 to 9,000 L/min (liters per minute) to irrigation wells, and concentrations of dissolved solids 

range from about 200 to 1,600 mg/L (milligrams per 

liter) (Hillier and Schneider, 1979b; Hillier and others, 

1979). The water ranges from moderately hard to very 

hard and contains excessive (more than 250 mg/L) 

sulfate concentrations. Water suitable for use as a 

drinking-water supply occurs from Kassler to Littleton 

(Hillier and others, 1979).

In addition to the tremendous ground-water reservoirs in the fills of the major stream valleys, large 

reservoirs occupy buried segments of old stream channels abandoned as a result of Quaternary drainage 

changes during the gradual evolution of the present 

drainage pattern. One such buried valley, averaging 

1.5-3 km in width, lies beneath Beebe Draw and its extensions north and south, a total extent of perhaps 

55-60 km. Beebe Draw is about 8-10 km east of the 

present valley of the South Platte River between 

Brighton and Platteville. Northward continuations of 

the buried valley pass beneath Milton and Lower 

Latham Reservoirs (Colton and Fitch, 1974). This 

buried valley apparently is an abandoned segment of 

the old Pleistocene valley of the South Platte River. It 

was abandoned when the South Platte was diverted by 

stream capture by a tributary of the Cache la Poudre 

River (Smith and others, 1964, p. 32, 33). The valley fill 

beneath Beebe Draw is as much as 24-30 m thick, and 

depth to the water table ranges from about 1 to 14 m 

below the ground surface (Hillier and Schneider, 

1979a). The water is used extensively for crop irrigation. Dissolved-solids concentrations range from about 

700 to 2,300 mg/L and locally the water contains excessive sulfate (more than 250 mg/L) concentrations 

(Hillier and Schneider, 1979b; Hillier and others, 1979).

A less well delineated buried valley trends northward through east Denver from the valley of Cherry 

Creek at about Monaco Street toward Sand Creek. 

This valley is an abandoned former course of Cherry 

Creek captured by a tributary of the South Platte 

River that is now the present Cherry Creek. As much 

as 26 m of clean, pebbly, saturated sand underlies 

1.5-6 m of eolian material (Shroba, 1977, p. 5). This 

reservoir is largely untapped.

PEDIMENT GRAVELS

A detailed study by Hurr (1976) of the hydrology of 

the alluvial gravel at and adjacent to the Rocky Flats
64 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

nuclear processing plant provides a basis for making 

inferences about the hydrologic characteristics of 

other gravel deposits that cap pediments elsewhere 

along the Colorado Piedmont. These so-called pediment gravels were laid down at various times during 

the Pleistocene Epoch by streams that deposited sediment at the foot of the mountains in response to reduced gradients and carrying capacities. The pediment 

gravels are mostly coarse and poorly sorted and contain admixtures of alluvial and eolian sand and silt. 

Overall, they diminish in grain size away from the 

mountains.

Ground water in these deposits is recharged from 

rain, snowmelt, and surface water sources. Infiltration 

rates and permeability are variable: Hurr (1976, p. 18) 

cited rates in the Rocky Flats Alluvium that ranged 

from 5-150 mm/h (millimeters per hour) in the upper 

1.5 m of soil to 99-187 mm/h in stony soil. Comparably 

wide ranges are likely in other alluviums such as the 

Verdos and Slocum. The local water tables fluctuate in 

response to seasonal variations in precipitation, 

stream flow, and irrigation, and they vary markedly 

from place to place (Hillier and Schneider, 1979a; 

Hillier and Hutchinson, 1980a). The water is mostly 

hard, because most of the deposits contain strongly 

developed Cca (calcium carbonate) soil horizons (Scott, 

1963; Colton and Fitch, 1974; Trimble and Fitch, 

1974a, b; Machette, 1977). Water is suitable for drinking in some of these deposits (Hillier and Schneider, 

1979b; Hillier and Hutchinson, 1980b).

Some pediment deposits of the area yield sufficient 

water for irrigation (for example, several hundred to 

about 2,000 liters per minute). Isolated pediment remnants high above present drainage and away from 

sources of recharge, however, are apt to be drained of 

ground water (Hershey and Schneider, 1964, p. XI4; 

Smith and others, 1964, p. 58; Hillier and Schneider, 

1979a; Hillier and Hutchinson, 1980a).

TERRACES

Gravel terraces of late Pleistocene and Holocene age 

along the major stream valleys of the Front Range Urban Corridor yield water of varied but commonly suitable quality and volume for crop irrigation. The quality of some water is suitable for municipal and industrial use also (Hershey and Schneider, 1964, p. XI4, 

X20; Smith and others, 1964, p. 59, 107; Bingham and 

Klein, 1973, table 1; Hillier and Schneider, 1979b; 

Hillier and Hutchinson, 1980b; Hillier and others, 

1979). The broad terraces of the South Platte River 

downstream from Denver, the Cache la Poudre River 

near Fort Collins and Greeley, and Fountain Creek 

downstream from Colorado Springs all are extensive

ground water reservoirs. The above-cited authors 

noted that the specific conductance and, hence, the 

dissolved solids in the water, increases generally 

downstream. Generally, the quality of water in terraces is most suitable for uses associated with urban 

development in the Colorado Springs Castle Rock 

area and least suitable in the Boulder Fort Collins Greeley area (Hillier and Schneider, 1979b; 

Hillier and Hutchinson, 1980b; Hillier and others, 

1979).

Beneath some terraces the ground water is continuous with that in the subjacent valley fill. In such 

physiographic settings, the terrace and the valley fill 

commonly are formed from the same body of gravel. 

But in some other terraces the water is perched on impervious rocks, and water issues from the terrace 

margins as seeps or springs (Smith and others, 1964 p. 

109).

EOLIAN SAND

Where saturated, eolian sand locally yields water 

containing less than 500 mg/L of dissolved solids (Hurr 

and others, 1975, table 1). Dune areas afford excellent 

infiltration for recharge because they absorb precipitation readily with minimal runoff. In some places in the 

eastern parts of the Greater Denver Area, where eolian 

sands overlie relatively impermeable rocks, marked 

rises in the water table have accompanied increased irrigation of yards in new neighborhoods. Locally, this 

rise has caused flooding in basements that previously 

were dry (Hamilton and Owens, 1972b, p. 329).

BEDROCK AQUIFERS

Water has been produced for many years from 

bedrock aquifers in the Denver Basin beneath the Colorado Piedmont (Emmons and others, 1896, p. 402). In 

recent summary description of these aquifers, Romero 

(1976, p. 101) stated that "The bedrock aquifers of the 

Denver Basin contain vast quantities of ground water 

suitable in most localities for all beneficial purposes. If 

managed with caution, the basin can supply the water 

needs of several generations." Well yields generally 

are sufficient for household, livestock, and industrial 

uses, but commonly are insufficient for crop irrigation. 

Many wells yield water under artesian conditions, but 

generally the pressure is not sufficient to cause water 

to flow from the ground (Chase, 1962; Romero, 1976, p. 

56). Large withdrawals of water over the years, moreover, have greatly lowered the artesian head in some 

areas.

The most productive artesian wells obtain water 

from the Arapahoe Formation, and the Laramie-Fox 

Hills aquifers. Some wells completed in the Arapahoe
HYDROLOGY 65

Formation yield as much as 1,500-1,800 L/min 

(Romero, 1976, p. 49). Commonly the yield is about 380 

L/min. This water is a sodium bicarbonate type and is 

the least mineralized ground water in the area, except 

for some water from sand dunes (Hurr and others, 

1975, table 1).

The Laramie-Fox Hills aquifer obtains water from 

the lower 45-60 m of the Laramie Formation and the 

underlying Fox Hills Sandstone. "Wells that fully penetrate the aquifer generally yield about 100 gal/min 

[380 L/min], and a few yield as much as 900 gal/min 

[3,400 L/rnin]. Water in the aquifer, at places, contains 

objectional amounts of methane, hydrogen sulfide, 

iron, and fluoride. In most places, the aquifer contains 

water of suitable quality for domestic use" (Romero 

and Hampton, 1972). Wells completed in sandstone 

beds in the upper part of the Laramie Formation in the 

Denver Basin generally yield only a few tens of liters 

per minute (Romero, 1976, p. 29).

In parts of the Greater Denver Area, sandstone beds 

and lenses in the Denver Formation, which overlies the 

Arapahoe Formation, provide water for household use 

and irrigation of lawns and gardens. Wells may yield a 

few tens of liters per minute. Much of the Denver Formation, however, contains too much clay to yield 

water. According to Hurr, Schneider, and Minges 

(1975, table 1), the water in the Denver Formation is a 

sodium bicarbonate type and total hardness generally 

is less than 60 mg/L.

The Dawson Arkose, which overlies and intertongues with the Denver Formation and also underlies 

parts of the Greater Denver and Colorado Springs 

Castle Rock areas, contain water in sufficient quantities and of suitable quality for uses associated with 

urban development (Romero, 1976, p. 46-51, 76). Most 

water for Castle Rock, for example, is obtained from 

the Dawson (Romero, 1976, p. 76). The principal waterbearing interval consists of coarse, micaceous, arkosic 

sandstone in the lower 120-150 m of the formation. 

The upper 150-180 m also contains water-bearing 

strata, but they are less continuous and are thinner 

than those in the lower part of the formation. Few 

wells yielding more than 400 L/s have been drilled into 

the Dawson Arkose, and its yield capacity is not well 

known. Romero (1976, p. 51) suggested that sustained 

yields of 1,500 L/min probably can be obtained in the 

south-central part of the Denver Basin.

Aquifers in older Mesozoic and Paleozoic formations 

underlying the Colorado Piedmont generally are too 

deep for economic development of water supplies except on or near the outcrop in the immediate vicinity of 

the mountain front. At points more distant from the 

mountains, moreover, suitable water supplies generally can be obtained from younger rocks at shallower 

depths.

The pre-Pennsylvanian sedimentary rocks are not 

regarded by Romero (1976, p. 28) as being principal 

aquifers in most of the Front Range Urban Corridor, 

even though they may contain water. Outcrops of 

these rocks are largely restricted to a narrow discontinuous zone between Perry Park and Colorado 

Springs at the foot of the Rampart Range and to very 

localized exposures along the Ute Pass fault (Scott and 

Wobus, 1973). Away from the mountain front, these 

rocks are too deep to be seriously considered as sources 

of ground water.

The most significant aquifers below the Laramie-Fox 

Hills aquifer are in the Fountain Formation, Lyons 

Sandstone, and Dakota Group. Wells in both the Fountain and Lyons yield 6-240 L/min, with most yields being 18-36 L/min (Romero, 1976, p. 31).

Water in the Dakota Group is obtained near the outcrop from sandstone beds in the Lytle and South 

Platte Formations. Toward the east the depth to these 

beds increases rapidly and the quality of water deteriorates to the extent that the group can no longer be 

considered as a source of water. Wells completed in the 

Dakota Group yield an average of about 60 L/min 

(Romero, 1976, p. 32-34).

MOUNTAIN LAND DEVELOPMENT AND 

GROUND-WATER SUPPLIES

Many mountain residents rely on untreated well 

water for domestic supplies. Most of this water is obtained from fractures in the bedrock; the crystalline 

bedrock itself is virtually impervious. Vertical and 

lateral movement of water in the fractures is dependent on the size of openings, interconnections, and 

number of fractures extending to the land surface 

(Hofstra and Hall, 1975b).

Most mountain households also have individual 

septic-tanks and leach-field waste-disposal systems. 

The suitability of the mountain terrain for on-site 

waste disposal varies with the extent, thickness, and 

character of the soil overlying the bedrock (Schmidt 

and Pierce, 1976). Consequently, ground-water contamination is a serious problem in some mountain 

areas. Findings from a hydrologic and water-quality 

survey of western Jefferson County, Colo. (Hofstra 

and Hall, 1975a, b), which probably are representative 

of ground-water conditions in other urbanizing mountainous areas in the Front Range Urban Corridor, 

were: (1) ground-water contamination was common 

where one homeowner's leach field was upgradient 

from his own or his neighbor's well; (2) during wet 

seasons, especially on sloping ground, the water table
66 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

rises locally to the ground surface, and inadequately 

treated waste water issues from the ground; (3) in some 

mountainous parts of Jefferson County, as many as 20 

percent of the wells are contaminated with fecalcoliform bacteria; and (4) many wells contain nitrate, 

phosphorus, and chloride in excess of natural concentrations (Biesecker and others, 1973, p. 422).

LAKES

The hydrologic characteristics of lakes in the Front 

Range Urban Corridor are inventoried on maps by 

Ficke and Danielson (1973), Danielson (1975), and 

Adams (1976). These characteristics include surface 

area, shoreline length, specific conductance, pH, 

transparency of the water, and algal and chlorophyll 

concentrations. For a semiarid region, the Front Range 

Urban Corridor contains a surprising number of small 

lakes. Some are natural water bodies that have been 

enlarged by damming. Many are owned by private individuals, partnerships, or irrigation and ditch companies. Some lakes are owned by municipalities and 

are used for domestic water supplies. Some, such as 

Barr Lake near Brighton, are wildlife refuges. Many of 

the lakes now have multiple uses, and with increasing 

urbanization, uses are changing. Lakes enhance the 

value of adjacent property as they become centers of 

real-estate development and water-related recreation. 

The lake inventories, therefore, should be helpful in 

planning for the best future uses of these increasingly 

valuable resources.

SEISMICITY

The swarm of earthquakes that followed the injection of waste water into the deep disposal well at the 

Rocky Mountain Arsenal the "Derby earthquakes" 

(Evans, 1966) generated wide concern about potential 

earthquake hazards along the Front Range Urban Corridor. Algermissen's seismic risk map of the United 

States (Algermissen, 1969) places Colorado in category 

1 minor damage, based on a statistical evaluation of 

past earthquake magnitudes, intensities, and strain 

releases. Frequency of occurrence was not a consideration in assigning ratings to the various risk zones of 

Algermissen's map, but Algermissen pointed out that 

one approach to the problem of determining relative 

seismicity of various regions is to plot earthquake 

magnitude versus frequency. If this approach were 

used, the relative seismicity of Colorado would have to 

be judged as low. Algermissen also noted that 

historical records do provide guidelines to relative 

seismicity, although the historic record for the United 

States is too short to adequately estimate recurrence 

rates for many areas. Geologic evidence of past earthquake occurrences, therefore, offers useful clues as to 

the probability of future earthquake activity that cannot be judged from historic occurrences alone.

Except for the Derby earthquakes related to the 

Rocky Mountain Arsenal disposal well, few earthquakes larger than MM (modified Mercalli intensity) 

III have been experienced in the Front Range Urban 

Corridor area in the 100 years of record. (Several of the 

Derby earthquakes exceeded MM IV, and the strongest attained MM VII and a Richter magnitude of 5.3; 

Hadsell, 1968; Major and Simon, 1968; National Oceanic and Atmospheric Administration, 1977, reported 

by Kirkham and Rogers, 1978, appendix 3). In 1882 an 

earthquake of MM VII was felt in Denver and in a 

wide surrounding area, including southern Wyoming, 

(Hadsell, 1968), but no natural occurrence of that intensity has happened since. Except for the artificially 

triggered Derby shocks, historic earthquake activity 

in the Front Range area has been infrequent and mainly of low intensity.

Some faults in the Front Range area displace 

deposits of Quaternary age with offsets that indicate 

earthquakes of Richter magnitudes 5 to 6 or more (fig. 

30). Few earthquakes smaller than Richter magnitude 

6.5-7 cause surface faulting, although smaller shocks 

occasionally cause ground ruptures through landsliding. Shallow-focused earthquakes of magnitudes 

less than 6 have caused surface faulting in California; 

an example is the Parkfield-Cholame earthquake of 

1966. Also, ground breakage can be caused by tectonic 

creep without perceptible earthquakes. Scott (1970) 

has identified eight faults in east-central Colorado that 

have displaced Quaternary deposits. Three of these in 

the Front Range area have displaced gravels correlated by Scott with the Verdos Alluvium, which has 

an age of about 0.6-0.7 m.y., equivalent to the Yarmouth Interstadial. A movement of the Golden fault 

had been bracketed in time between the Yarmouth and 

Sangamon(?) Interstadials (Kirkham, 1977, p. 692). 

Because no offset has been found in the Front Range 

area in deposits younger than Sangamon(?) age (about 

125,000 years ago), the frequency of earthquakes 

strong enough to cause ground breakage along the 

Front Range has been very low.

Despite the low frequency of past events, the possibility of large earthquakes in the future cannot be discounted. Many Front Range faults have very long histories of recurrent movements, some dating back to 

Precambrian time. The large total displacements of 

some of these faults in Neogene (Miocene and Pliocene) 

time (Epis and Chapin, 1975; Taylor, 1975; Scott, 

1975) and the relatively short geologic time since last 

movement compared to the total period of activity suggest that further Holocene movements are probable.
SURFICIAL DEPOSITS AND ROCK UNITS IN THE GREATER DENVER AREA 67

FIGURE 30. Map showing traces of known Quaternary faults in the 

Front Range and adjacent areas. Bar and ball are on downthrown 

side. Dotted fault is deeply buried and inferred. From Kirkham 

and Rogers, 1978.

Matthews (1973, p. Ill) has even suggested a reevaluation of Colorado's seismic-risk classification and a 

building code that embodies an earthquake-resistant 

design, arguing that 100 years of record is insufficient 

to establish a realistic measurement of earthquake 

recurrence in the Front Range area.

To learn more about the distribution of potentially 

active faults in the State, the Colorado Geological 

Survey has restudied Colorado seismicity (Kirkham 

and Rogers, 1978) in a project jointly funded by the 

U.S. Geological Survey as an element of the Front 

Range Urban Corridor program, with the objective of 

applying the information gained to a reappraisal of 

seismic risk and to an evaluation of the risk impact on 

land-use planning. R. M. Kirkham and W. P. Rogers 

(written commun., 1978) believe that the seismic-risk 

potential of Colorado in general and of the Front 

Range Urban Corridor in particular is greater than the 

historic record of seismicity indicates. The recurrence 

intervals of some geologically indicated seismic events 

have been much longer than the historic record.

PHYSICAL PROPERTIES AND

PERFORMANCE CHARACTERISTICS OF

SURFICIAL DEPOSITS AND ROCK UNITS

IN THE GREATER DENVER AREA

By RALPH R. SHROBA

INTRODUCTION

The Front Range Urban Corridor encompasses a 

wide array of surficial deposits and rock units of contrasting physical properties and performance characteristics that influence the suitability of the land for 

varied urban development. General information pertaining to the properties and characteristics of the 

materials at and near the surface therefore is useful for 

making preliminary site evaluations and land-use decisions. This section focuses on the surficial deposits and 

rock units in the Greater Denver Area, although the 

properties of these units are similar in the BoulderFort Collins-Greeley area and in the Colorado SpringsCastle Rock area (fig. 1). The Greater Denver Area was 

selected for study because of the diversity of geologic 

units, availability of geologic and geotechnical data, 

and rapid rate of urban growth. Much of the information presented in this section was compiled from geologic maps and reports by the U.S. Geological Survey 

(Bryant and others, 1973; Colton, 1978; Colton and 

Fitch, 1974; Gardner and others, 1971; Lindvall, 1978, 

1979; Maberry, 1972b, 1973; Maberry and Lindvall, 

1972, 1974, 1977; Machette, 1977; Machette and 

others, 1976; Miller and Bryant, 1976; Robinson and 

Lee, 1974a, 1974b; Robinson and others, 1974; Scott, 

1962, 1963a, 1963b, 1972b, 1972d; Scott and others, 

1966; Shroba, 1980; Simpson, 1973b, 1973c; Trimble 

and Fitch, 1974a, 1974b; Trimble and Machette, 

1979a, 1979b; Van Horn, 1968, 1972, 1976; and Wells, 

1967), county soil surveys by the Soil Conservation
68 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

Service (Larsen and Brown, 1971; and Sampson and 

Baber, 1974), and test data collected by the author. 

Other sources are cited where appropriate.

Surficial deposits of Quaternary age, including 

eolian deposits, alluviums, slope deposits, and manmade deposits (fig. 31), cover much of the Colorado 

Piedmont where they form a variety of landforms, 

which are major components of the landscape. These 

deposits are less extensive in the hogback belt and the 

Front Range (fig. 2) where they are interspersed with 

large areas of bedrock and are limited mostly to valley 

bottoms and gentle slopes.

The major factors that influence the physical properties and performance characteristics of the surficial 

deposits in the Greater Denver Area are grain size3 

(table 4), amount of compaction, and degree of weathering. The first two factors are related mainly to the 

grain size of the sediments in the source area, distance 

of transport, and mode and environment of deposition. 

The degree of weathering tends to increase with the 

age of the deposit. In general, the Louviers Alluvium 

and deposits of equivalent age and younger (fig. 

31) which include much of the eolian deposits, floodplain and terrace alluviums, and slope deposits are 

only slightly modified by weathering processes to a 

depth of about a meter. Most manmade deposits are 

less than 100 years old and are unweathered. Surficial 

deposits older than the Louviers Alluvium which include pediment alluviums, some terrace alluviums, and 

some slope deposits however, are more weathered 

than younger deposits and are characterized by prominent horizons of clay accumulation (Bt horizons) and 

horizons of carbonate enrichment (Cca and K horizons) 

that have formed in the upper 1-3 m of these older surficial deposits.

In this section, surficial deposits are grouped, 

according to mode and environment of deposition, as 

follows: eolian deposits, flood-plain and terrace 

alluviums, pediment alluviums, slope deposits, and 

manmade deposits.

EOLIAN DEPOSITS

GENERAL DESCRIPTION

Loess and eolian sand (fig. 31) are by far the most 

widespread surficial deposits east of the mountain 

front. Wind-deposited silt and sand cover extensive 

areas south and east of the major streams (fig. 1). 

Much of this material was blown from alluvium exposed along valley floors. Some of the loess, however,

3 In this section, grain-size nomenclature and size classes for grain-size analyses conform to the Wentworth system. In addition, the results of several standard engineering 

tests are presented in this section. These tests were conducted according to procedures 

established by the American Society for Testing Materials, which specify grain-size limits 

that correspond with those of the Unified Soil Classification System.

was deflated from shallow basins formed in finegrained sedimentary rocks near the eastern margin of 

the hogback belt (Scott, 1962; Colton, 1977). Minor 

amounts of eolian sand were derived from sandstone 

outcrops and sandy pediment alluviums. Eolian sand 

is most abundant east of the South Platte River and 

forms large, generally stable dune fields northeast of 

Denver. Loess is most widespread west of the South 

Platte River and to the south and east of the areas 

mantled by eolian sand. The distribution of eolian 

deposits, the orientation of sand dunes and deflation 

basins, and the general southeastward decrease in 

grain size especially of eolian sand all strongly suggest sediment transport by northwesterly winds. 

Eolian deposits are generally less than 5 m thick and 

tend to thin in a down-wind direction. In some areas, 

loess is as much as 7 m thick; locally, large parabolic 

sand dunes have thicknesses in excess of 15 m.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Grain-size analyses of eolian material from southeast 

Denver indicate that most loess is 70-80 percent silt 

and clay slightly more silt than clay 20-30 percent 

sand, and about 1 percent granules; eolian sand is commonly 20-30 percent silt and clay about twice as 

much silt as clay 70-80 percent sand, and about 1 

percent granules (table 5). Eolian deposits northeast of 

Denver tend to be slightly more sandy than those in 

southeast Denver.

Eolian sediments in southeast Denver have a wide 

range of Atterberg limits. Loess generally has a liquid 

limit of 40-45 and a plasticity index of about 20, 

whereas eolian sand has a liquid limit of about 25 and 

usually is nonplastic. According to the Unified Soil 

Classification (U.S. Bureau of Reclamation, 1974); 

(table 6 and fig. 32, this report), most loess is classified 

as CL and most eolian sand as SM, SP, and SC (table 

7).

Eolian materials beneath heavy structures are 

susceptible to excessive compaction and differential 

settlement, especially if they are overwatered. Loess 

that has a dry density of less than 1,400 kg/m3 

(kilograms per cubic meter) and eolian sand that has 

dry density of less than 1,700 kg/m3 decrease the most 

in volume. Test data for eolian sand show a strong inverse relationship between dry density and consolidation; that is, when loaded, the lower the dry density the 

greater the amount of consolidation. This relationship, 

however, is not as well defined for loess. Other factors 

such as the amount and type of clay-size material and 

the shape and orientation of silt- and clay-size particles 

may influence the amount of consolidation of loess
SURFICIAL DEPOSITS AND ROCK UNITS IN THE GREATER DENVER AREA 69

Eolian 

deposits

Flood-plain and 

terrace alluviums Pediment alluviums Slope deposits

Manmade 

deposits

FIGURE 31. Surficial Quaternary deposits in the Greater Denver Area and their approximate ages. Modified in part from Trimble and

Machette (1979b).

under a heavy load. Loess and eolian sand commonly 

consolidate about 1 percent when wetted and loaded at 

25-50 kN/m2 (kilonewtons per square meter); however, 

consolidation of 3-5 percent is not uncommon (table 8).

Eolian sand and sandy loess are nonexpansive when 

wetted and loaded at 25- 50 kN/m2. Some clayey loess, 

however, expands in excess of 3 percent and exerts 

swelling pressures of more than 150 kN/m2 . Average 

values for loess in southeast Denver are about 1 percent and 25 kN/m2, respectively (table 8). The swelling 

pressure of loess tends to increase as density increases. 

Clayey Bt horizons, about 10-70 cm thick, in the upper 

part of the loess tend to have higher swelling potentials than the underlying less clayey loess.

The shear strength of eolian material tends to vary 

with the grain size and moisture content. Loess in

southeast Denver has an unconfined compressive 

strength of about 100-700 kN/m2 , and averages about 

300 kN/m2 (table 8). Eolian sand has much lower values 

of about 30-45 kN/m2 and 35 kN/m2, respectively. Test 

data indicate that a threefold increase in water content 

of loess can result in as much as a tenfold reduction in 

unconfined compressive strength. In contrast, 

measurements for eolian sand show that shear 

strength decreases with reduced moisture content for 

samples that range from moist to air dry.

Loess and eolian sand commonly have the following 

performance characteristics: Excavation is easy with 

hand tools or light power equipment. Foundation conditions are only fair, owing to considerable frost heaving in the upper meter of the deposit and susceptibility 

to excessive compaction and differential settlement.
70 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

TABLE 4. Grain-size scale, showing size classes of the Wentworth and Unified Soil Classification systems

[Modified from U.S. Bureau of Reclamation (1974) and Schwochow and others (1974). Values in millimeters. ..... not specified. Values are boundaries between size

Clay Silt Sand Gravel

Granule Pebble Cobble Boulder

Wentworth 

system 0.0039

Unified Soil 

Classification

s y s t em 

0.0625

.074 4.76

64

76

256

305

Particles less than 0.074 mm in diameter are referred to as silt and clay or fines. 

"Particles from 4.76 to 76 mm in diameter are referred to as gravel.

60

50

40

t 30 

o

20

10

0

CL

CL-ML

-V-^MLorOL

ML J--'

0 10 20 30 40 50 60 70 80 90 100 

LIQUID LIMIT

FIGURE 32. Plasticity chart for the classification of fine-grained 

materials (50 percent or more by weight smaller than 0.74 mm in 

diameter) according to the Unified Soil Classification (table 6). 

Modified from U.S. Bureau of Reclamation (1974, fig. 7). 

Classification is based on a material's plasticity index versus its 

liquid limit. The "A" line is an empirical boundary that generally 

separates inorganic clays of low to high plasticity in the area 

above the line from inorganic silts and organic silts and clays of 

low to high plasticity below the line. The vertical line at a liquid 

limit of 50 separates inorganic silts and organic silts and silty 

clays of low plasticity and inorganic clays of low to medium 

plasticity in the area to the left of the line from inorganic silts and 

clays of high plasticity and organic clays of medium to high 

plasticity to the right of the line.

Permeability is usually high in eolian sand and 

moderate in loess. Vertical cuts are unstable and will 

slump to form slopes of about 10-25°.

FLOOD-PLAIN AND TERRACE ALLUVIUMS

GENERAL DESCRIPTION

The major streams east of the hogback belt (fig. 2) 

are bordered by extensive flood-plain and terrace

deposits that are generally less than 6 m above stream 

level and are made up of post-Piney Creek alluvium 

and Piney Creek Alluvium (fig. 31). These alluviums 

are of Holocene age and are primarily sand to silty 

sand and sandy silt, respectively. Discontinuous terraces of Broadway and Louviers Alluviums (fig. 31), of 

late Pleistocene age, are typically 7-20 m above 

stream level. Broadway Alluvium is most widespread 

along the east side of the South Platte River, whereas 

Louviers Alluvium is most extensive along eastwardflowing tributaries. These alluviums are mostly pebbly 

sands containing minor amounts of pebble to cobble 

gravel. Locally, they are mantled by eolian deposits 

that tend to thicken toward the valley margin.

Stream alluviums in the Denver area tend to be 

thickest along the South Platte River and are progressively thinner along increasingly smaller 

tributaries. Piney Creek Alluvium and post-Piney 

Creek alluvium are less than 6 m thick and commonly 

range from about 1.5 to 4.5 m in thickness. Along major streams these alluviums overlie older gravelly 

alluvium. Along small intermittent streams postPiney Creek alluvium is usually less than 1.5 m thick. 

Broadway Alluvium is commonly about 7.5 m thick 

and is locally as much as 9 m thick along the South 

Platte River in downtown Denver. The Louviers Alluvium is one of the thickest surficial deposits in the 

Denver area. In the valley of the South Platte it is as 

much as 21 m thick; locally the base of the Louviers is 

as much as 12 m below present stream level.

The grain size of flood-plain and terrace alluviums 

east of the mountain front is related to several factors 

including: (1) nature of the bedrock and surficial 

deposits in the source area, (2) age of the alluvium, (3) 

position within the alluvial sequence, (4) stream size or 

competence, (5) environment of deposition, and (6) distance of transport. Late Pleistocene alluviums along
SURFICIAL DEPOSITS AND ROCK UNITS IN THE GREATER DENVER AREA 71

TABLE 5. Grain-size distributions of surficial deposits in the Denver area

[Values are approximate and are rounded to the nearest 5 percent. <, less than; «, less than or equal to; 5 , range in size of gravel in samples tested]

Surficial deposit Location

of samples

tested

Grain-size distribution 

(percent by weight)

Silt 

and 

clay Sand Gravel

Gran- Peb- Cob- Boulule ble ble der

 Southeast Denver 70-80 20-30

 Southeast Denver 20-30 70-80

Post-Piney Creek Along major streams <60 30-90

Loess 

Eolian sandAlluvium.

Piney CreekAlluvium.

in the Denver 

area.

Along the South 

Platte River 

and Cherry Creek.

Broadway Alluvium, Along the South 

main body. Platte River.

Broadway Alluvium, Along the South 

fine-grained Platte River, 

layers.

Louviers Alluvium Along major streams

in the Denver 

area.

Slocum, Verdos, Between the hogand Rocky Flats back belt and 

Alluviums, upper the South 

parts. Platte River.

Slocum, Verdos, 

and Rocky Flats 

Alluviums, lower 

parts.

Colluvium 

Between the hogback belt and 

the South 

Platte River.

East of the hogback belt.

40

0-5

h <5- 

I <5-

 <30-

20-70 30-65

0-10 60-80 10-40-

55

30-60 35-70-

30-60 25-45 h <10 K5-25-)

10-15 30-40

60-70 20-30

 10- -30-401 5-10-

the South Platte River and its major eastward-flowing 

tributaries consist chiefly of coarse detritus from the 

crystalline rocks of the Front Range, whereas 

alluviums of equivalent age along major westwardflowing tributaries are slightly finer grained and were 

produced by the erosion of sedimentary rocks, mostly 

sandstone and conglomerate. Younger, valley-bottom 

alluviums are more locally derived and probably represent the reworking of older surficial deposits. For example, the fine-grained, humic character of the upper 

part of the Piney Creek Alluvium suggests that this 

material was eroded from soils formed in silty and 

clayey surficial deposits, mainly loess and colluvium 

consisting of loess mixed with disintegrated siltstone 

and claystone. In addition, (1) late Pleistocene 

alluviums are more gravelly and less silty than 

Holocene alluviums; (2) the upper parts of most 

deposits are finer grained than the lower parts, owing 

to the deposition of overbank and (or) eolian sediment

in the upper parts and the accumulation of minor 

amounts of pedogenic clay; and (3) alluviums of equivalent age are coarsest in the vicinity of the mountain 

front and in the central part of the valley of the South 

Platte River, are slightly finer grained near the valley 

sides (especially below confluences with minor 

streams), and are progressively finer grained in increasingly smaller tributary valleys.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Field observations and test data indicate that 

alluviums of different ages have different grain-size 

distributions. The grain size of post-Piney Creek 

alluvium is highly variable. Along main streams it is a 

silty sand, sand, or pebbly sand interbedded with pebble gravel and sandy silt. Mainstream post-Piney 

Creek alluvium is 30-90 percent sand and contains as
72 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

TABLE 6. Unified Soil Classification1 chart

[Modified from U.S. Bureau of Reclamation (1974, fig. 7)]

Major 

categories

Group 

symbol

Common field description

Coarse-grained soils

Gravels 3 GW Well-graded (engineering sense) gravels and

sandy gravels.

OP Poorly graded gravels and sandy gravels. 

GM Silty gravels and poorly graded silty, sandy

gravels. 

GC Clayey gravels and poorly graded clayey, sandy

gravels.

Sands SW Well-graded sands and gravelly sands.

SP Poorly graded sands and gravelly sands.

SM Silty sands and poorly graded silty sands.

SC Clayey sands and poorly graded clayey sands.

Fine-grained soils

Slightly plastic ML Inorganic silts, very fine sands, and silty or 

silts and clays. clayey fine sands of low plasticity.

CL Inorganic clays and gravell, sandy, or silty

clays of low to medium plasticity. 

OL Organic silts and silty clays of low 

plasticity.

Highly plastic 7 MR Inorganic silts, 

silts and clays.

CH Inorganic clays of high plasticity.

OH Organic clays of medium to high plasticity.

Highly organic soils

Pt Peat and other organic-rich sediments.

The Unified Soil Classification is a system that groups

unconsolidated materials according to their engineering properties. The 

first letter of the group symbol indicates grain size and inorganic or 

organic character of the material. The second letter indicates 

gradation (engineering sense) and plasticity of the material. Material 

that is at or near the boundary between two groups is given a dual 

classification, such as GP-GC.

More than 50 percent by weight is larger than 0.74 mm in diamter.

More than 50 percent by weight of the coarse fraction (greater than 

0.74 mm) is larger than 4.76 mm in diamter.

Fifty percent or more by weight of the coarse fraction is 0.74 to 

4.76 mm 

in diameter.

Fifty percent or more by weight is smaller than 0.74 mm in diameter. 

6Liquid limit is less than 50.

Liquid limit is 50 or more.

Q

Contains abundant fibrous organic matter.

much as 60 percent silt and clay, or as much as 30 percent granule to cobble gravel (table 5). This wide range 

is probably a result of the erosion of nearby surficial 

deposits and a relatively short distance of transport.

Piney Creek Alluvium along the South Platte River 

and its major tributaries consists of two parts that are 

physically different. The upper part is commonly a 

humic, slightly calcareous, sandy silt that is interlayered with silty sand and a few lenses of silty clay. 

The lower part is chiefly sand and some layers of pebble gravel. Grain-size data indicate that Piney Creek 

Alluvium along the South Platte River and Cherry 

Creek is made up of 20-70 percent silt and clay, 30-65 

percent sand, and 0-15 percent granule to pebble 

gravel. Piney Creek Alluvium along Cherry Creek is 

more gravelly and less silty than along the South 

Platte River. In small tributary valleys that have 

slopes mantled by loess and fine-grained colluvium,

Piney Creek Alluvium is as much as 80 percent silt and 

clay. In areas of extensive eolian sand, Piney Creek 

Alluvium is sandy.

Broadway Alluvium along the South Platte is mostly pebbly sand interbedded with sandy silt. At the 

mountain front it is composed of cobble gravel. Broadway Alluvium along minor streams is sand to sandy 

silt. Grain-size data show that much of the Broadway 

Alluvium in the valley of the South Platte River is 

very sandy and is made up of 0-10 percent silt and 

clay, 60-80 percent sand, and 10-40 percent granule to 

cobble gravel. Fine-grained layers average about 40 

percent silt and clay, 55 percent sand, and 5 percent 

granule to pebble gravel.

Louviers Alluvium along major streams is typically 

a pebbly sand interbedded with pebble to cobble gravel 

and lenses of silty sand to clayey silt. It is bouldery 

along perennial streams near the mountain front and is 

sandy to clayey along intermittent streams. Grain-size 

analyses for Louviers Alluvium along the South Platte 

River, Bear Creek, Clear Creek, and Cherry Creek indicate that the alluvium is very gravelly and is composed of 0-5 percent silt and clay, 30-60 percent sand, 

and 35-70 percent granule to boulder gravel.

Differences in the plasticity of flood-plain and terrace alluviums are in part related to their wide range in 

grain size. Limited test data indicate that Piney Creek 

Alluvium has a liquid limit of 30-45 and a plasticity index of 10-25; post-Piney Creek alluvium has slightly 

lower values (table 7). Most of the Broadway and 

Louviers Alluviums are nonplastic. Silty layers in the 

Broadway Alluvium have liquid limits of 20-40 and 

are nonplastic or have plasticity indices of 20 or less. 

According to the Unified Soil Classification (U.S. 

Bureau of Reclamation, 1974), typical group symbols 

for the above alluviums, in order of decreasing silt and 

clay and increasing gravel, are: CL, SM, and SC for 

Piney Creek Alluvium; SC and SW for post-Piney 

Creek alluvium; SC, SM, and CL for fine-grained layers 

in Broadway Alluvium; SP and SM for coarse-grained 

Broadway Alluvium; and SW, GP, and GW for 

Louviers Alluvium.

Bearing capacities and volume-change characteristics of the stream alluviums depend mostly on grain 

size and amount of organic matter. Structures built on 

Piney Creek Alluvium and post-Piney Creek alluvium 

tend to have more foundation problems than those on 

older, coarser-grained alluviums. Piney Creek 

Alluvium along major streams and fine-grained alluviums of various ages along minor streams commonly 

contain lenses of clayey silt and silty clay. These 

materials are highly compressible and may cause differential compaction under a light to moderate load.
SURFICIAL DEPOSITS AND ROCK UNITS IN THE GREATER DENVER AREA 73

TABLE l. Atterberg limits and typical Unified Soil Classification group symbols for surficial deposits in the

Denver area

[Values are approximate and are rounded to the nearest 5 percent. NP, nonplastic; <, less than or equal to; < , less than; . . . , little or no data]

Surficial 

deposit

Location 

of samples 

tested

Atterberg limits

Liquid 

limit

Plasticity 

index

Unified Soil

Classification

groupsymbols

Loe s s 

Eolian sand 

Post-Piney Creek 

alluvium. 

Piney Creek- - 

Alluvium.

Broadway Alluvium,

main body. 

Broadway Alluvium,

fine-grained

layers. 

Louviers Alluvium

Slocum, Verdos, 

and Rocky 

Flats alluviums, 

upper parts.

Slocum, Verdos, 

and Rocky 

Flats Alluviums, 

lower parts.

Colluvium 

Engineered fills 

and embankments.

Southeast Denver 40-45 

Southeast Denver 25 

Along major streams in <40

the Denver area. 

Along the South Platte 30-45

River and Cherry Creek.

Along the South Platte 

River. 

Along the South Platte 

River.

20-40

Along major streams in 

the Denver area. 

Between the hogback belt 20-50

and the South Platte

River.

Between the hogback belt 15-45 

and the South Platte 

River.

Southeast Denver- 45-50

Building sites and low-lying <50 

areas in the Denver area.

20 

NP 

<20

10-25

NP 

<20

NP 

10-20

CL

SM, SP, SC 

SC, SW

CL, SM, SC

SP, SM 

RC, SM, CL

SW, GP, GW 

SM, SC, CL

_<20 GP-GC, GC, SM, SC

20-30 CL

<30 SP, SM, CL

Organic-rich layers in the upper part of the Piney 

Creek Alluvium are also highly compressible. Some 

deposits of post-Piney Creek alluvium contain layers of 

well-sorted sand that may be prone to consolidation. In 

contrast, much of the Broadway and Louviers Alluviums along major streams are coarse sand and gravel 

that can support moderately heavy loads and withstand forces of as much as 600 kN/m2 (table 8). Gravelly, sandy, and most silty alluviums are nonexpansive. 

However, clayey alluvium, especially Piney Creek 

Alluvium derived from expansive claystone, commonly shows a few percent increase in volume when 

wetted and exerts swelling pressures of as much as 

about 200 kN/m2 when wetted and loaded at 25-50 

kN/m2 (table 8).

Flood-plain and terrace alluviums commonly have 

the following performance characteristics: Most 

alluviums are easily excavated with power equipment. 

Foundation conditions are generally good in sand and 

gravel but are poor to very poor in clayey and humic 

materials. Permeability usually varies from high in 

sand and gravel to low in clayey alluvium. Vertical 

cuts are unstable and will slump or ravel to form slopes 

of about 10-25°.

PEDIMENT ALLUVIUMS 

GENERAL DESCRIPTION

East of the hogback belt, the South Platte River and 

its major eastward-flowing tributaries (fig. 1) are 

flanked by a series of gently sloping surfaces capped 

by thin deposits of gravelly pediment alluvium4 of 

Pleistocene age. East of the South Platte River, 

alluvium of similar age is buried beneath widespread 

eolian deposits. Three and locally four ages of pediment alluvium are recognized in the Greater Denver 

Area (fig. 31). From youngest to oldest, the three major 

alluviums and their approximate range in elevation 

above stream level are Slocum Alluvium (25-35 m), 

Verdos Alluvium (60-75 m), and Rocky Flats 

Alluvium (110 m). The Nussbaum Alluvium, the oldest 

pediment alluvium, is of very limited extent and occurs 

at elevations of about 15-25 m above the level of the 

Rocky Flats Alluvium. Slocum Alluvium covers large 

areas in southwest Denver and locally forms terraces 

along some of the major eastward-flowing streams.

4 As used in this section, the term "pediment alluvium" refers to deposits of waterlaid 

sediment, regardless of thickness, that rest on an erosional, pedimentlike surface cut on the 

underlying bedrock.
74 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

TABLE 8. Volume change, swelling pressure, and unconfined compressive strength of surficial deposits in the Denver area

[Values are approximate; those for volume change are rounded to the nearest 0.5 percent; those for swelling pressure and unconfined compressive strength are rounded to 

the nearest 5 kN/m2 if less than 100 kN/m2 or to the nearest 50 kN/m2 if greater than 100 kN/m2. . . . , little or no data; <, less than or equal to; NE, nonexpansive]

Surficial deposit Location

of samples

tested

Piney Creek Alluvium Along major streams in the Denver

area.

Broadway Alluvium, main body Along the South Platte River 

Louviers Alluvium Along major streams in the Denver

area.

Slocum, Verdos, and Rocky Between the hogback belt and the

Flats Alluviums, upper parts. South Platte River.

Slocum, Verdos, and Rocky Between the hogback belt and the

Flats Alluviums, lower parts. South Platte River.

Colluvium East of the hogback belt 

Volume

Consolidation

(percent)

1

.. .

...

...

1

Change

Swell 2

(percent)

1

. . .

. . .

...

0-4

Swelling

pressure^

(kN/m2 )

25

0

<200

NE

NE

<200

<50

0-250

Unconfined

compressive

(kN/m2 )

35

<600

<600

...

...

strength^

Percent decrease in volume that occurs when undisturbed material loaded at 25-50 is wetted.

2Percent increase in volume due to expansion, that occurs after consolidation when material with free 

access to water is loaded at 25-50 kN/m2 for 24 hours.

Additional weight per unit area required to return the expanded material to its original volume. 

Maximum axial load required to deform a sample when lateral pressure is equal to atmospheric pressure.

The Verdos and Rocky Flats Alluviums are more common in west Denver and in the area northwest of 

Denver.

Much of the pediment alluvium is slightly silty, 

sandy pebble gravel that was deposited by small 

streams that headed in the hogback belt. These 

deposits commonly contain layers of clayey to sandy 

silt and cobble to boulder gravel. Pediment alluvium 

locally varies from sand and gravel to pebbly silt and 

clay. Deposits of bouldery cobble gravel are common 

along the mountain front; most alluviums tend to be 

coarse grained to the west and fine grained to the east. 

Terrace alluviums along major streams near the mountain front contain the least amount of silt and clay and 

the greatest amount of coarse gravel, whereas pediment alluviums of equivalent age, laid down by intermittent streams that head in the sedimentary terrane 

of the hogback belt, tend to be the most clayey and 

least gravelly. The upper parts of most deposits are 

slightly pebbly, sandy silt; this material may be overbank alluvium and (or) old loess.

The fine-grained material in the upper part and the 

upper part of the underlying gravelly alluvium have 

been modified to a considerable extent by soil development. Most of these soils have well-developed, clayey 

Bt horizons that are about 0.5-1 m thick and overlie 

calcium carbonate-enriched Cca or K horizons that extend to depths of as much as 2-3 m. Many of the granitic and gneissic stones in these soils are disintegrated.

The thickness of pediment alluviums generally 

averages 3-5 m and commonly ranges from less than 1 

to as much as 12 m. Buried stream channels beneath 

Rocky Flats (fig. 1), however, contain alluvial fills as 

much as 37 m thick (Hurr, 1976). Pediment deposits 

tend to thin from the hogback belt eastward toward 

the South Platte River.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Analyses of the pediment alluviums show that they 

have similar grain-size distributions. The lower parts 

typically are 10-15 percent silt and clay, 30-40 percent 

sand, 10 percent granules, 30-40 percent pebbles, and 

5-10 percent cobbles and boulders (table 5). This 

material is commonly interbedded with clayey layers 

made up of 50 percent or more silt and clay, and gravelly alluvium that contains 20-35 percent cobbles and 

boulders. Locally, much of the lower parts of the 

alluviums range from clayey material composed of as 

much as 60 percent silt and clay to bouldery material 

containing as much as 40 percent cobbles and 

boulders. The upper 1-1.5 m of pediment deposits contain 30-60 percent silt and clay, 25-45 percent sand, 

10 percent or less granules, and 5-25 percent pebbles. 

Weathering causes some of this three- to fourfold increase in silt and clay and the marked decrease in 

gravel in the upper part of the alluvium, relative to the 

alluvium at a depth of 2-3 m.

Pediment alluviums have a considerable range in 

plasticity, owing primarily to the wide variation in 

grain size. Test data indicate that the lower parts of 

Slocum, Verdos, and Rocky Flats Alluviums commonly have liquid limits of 15-45 and are nonplastic or 

have plasticity indices of 20 or less (table 7). The finegrained upper parts have slightly higher values that 

frequently range from 20-50 for the liquid limit and 

10-20 for the plasticity index. Liquid limits of as much 

as 60 and plasticity indices as high as 30 have been 

measured for material from the upper 1-1.5 m and for 

material from fine-grained layers in the lower parts of 

the alluviums. The upper parts of the pediment alluviums are usually classified, according to the Unified 

Soils Classification (U.S. Bureau of Reclamation,
SURFICIAL DEPOSITS AND ROCK UNITS IN THE GREATER DENVER AREA 75

1974), as SM, SC, and CL, whereas the lower parts are 

more variable and are commonly designated as GPGC, GC, SM, and SC.

Pediment alluviums have different bearing 

strengths, depending on the relative abundance of clay 

and gravel. The upper parts and fine-grained layers in 

the lower parts probably have bearing capacities 

slightly greater than those of loess. Coarse-grained 

layers in the lower parts can support greater loads 

than can the fine-grained upper parts and probably 

have bearing capacities equal to or slightly greater 

than those of Broadway and Louviers Alluviums.

The swelling potential of pediment alluviums varies 

with the amount of clay and tends to decrease with 

depth. Clayey Bt horizons in the fine-grained upper 

parts generally produce the highest swelling pressures. 

Very clayey Bt horizons may exert swelling pressures 

of as much as 200 kN/m2, whereas moderately clayey 

Bt horizons probably have swelling pressures of less 

than 50 kN/m2 (table 8). The underlying horizon of carbonate enrichment is less expansive than the Bt horizon, owing to the abundance of nonexpansive calcium 

carbonate, lower clay content, and the stabilizing effect of calcium ions on the montmorillonitic clays. The 

upper 2-3 m of pediment alluviums contain as much as 

40-95 percent calcium carbonate in the less than 

2-mm fraction (Machette and others, 1976). Because 

the lower parts of pediment alluviums tend to be more 

sandy and gravelly than the upper parts, they probably have swelling pressures of less than 50 kN/m2 .

Pediment alluviums commonly have the following 

performance characteristics: Excavation is easy to 

moderately easy with light power equipment; however, 

scattered large boulders within a few kilometers of the 

hogback belt, and indurated K horizons cemented by 

calcium carbonate, may require the use of heavy power 

equipment. Foundation conditions are generally good 

in the sandy and gravelly lower parts of the alluviums 

but are poor in layers of clayey alluvium and clayey Bt 

horizons, owing to moderate swelling pressures and 

lower bearing capacities. Permeability is usually high 

in sand and gravel but commonly is low in clayey silt 

and in carbonate-cemented horizons. Vertical cuts are 

unstable and will slump or ravel to form slopes of 

about 15-25°.

SLOPE DEPOSITS

GENERAL DESCRIPTION

Slopes in the Greater Denver Area are mantled by 

thin surficial deposits that have moved under the influence of gravity or running water. These deposits include colluvium (mostly slope wash and sheetwash), 

landslide deposits, talus, and alluvial-fan deposits (fig. 

31). Colluvium is widespread on slopes steeper than 2°.

Most landslides are on steep slopes near the mountain 

front. Landslides on sedimentary rocks are usually on 

slopes steeper than 8°, whereas those on crystalline 

rocks are on slopes steeper than 15°. Landslide 

deposits are most common along the flanks of steepsided landforms, such as Rocky Flats, Green Mountain, and North and South Table Mountains (fig. 1), all 

of which are made up primarily of fine-grained sedimentary rocks of low shear strength capped by gravel 

or lava flows. Landslide deposits are present but less 

common on the east-facing slope of the hogback of the 

Dakota Group, dipslopes on the Fountain Formation 

and Lyons Sandstone, steep canyon sides, and on 

crystalline rocks along the mountain front. Talus is 

limited to canyon sides steeper than 15°. Alluvial-fan 

deposits accumulate mostly along minor streams 

where a decrease of gradient diminishes transport 

capacity. Near the mountain front, alluvial-fan 

deposits are on slopes of about 10-20°. Farther east, 

along the margins of flood plains of major streams, 

they are on slopes of about 3-5°.

Slope deposits tend to reflect the grain size of the 

surficial deposits and rock units from which they are 

derived farther upslope. They rarely are more than a 

few meters thick. On the piedmont, colluvium is chiefly 

pebbly, sandy, silt and clay derived from loess, pediment alluvium, and fine-grained sedimentary rocks. 

Along the eastern margin of the Front Range and in 

the hogback belt, however, colluvium commonly is cobbly to bouldery and contains angular rock fragments 

as much as a meter across. Limited grain-size data indicate that colluvium east of the hogback belt is made 

up of about 60-70 percent silt and clay, 20-30 percent 

sand, less than 10 percent granules and pebbles and, 

locally, a small amount of cobbles (table 5). Most colluvium is less than 3 m thick and commonly less than 

1.5m.

Landslide deposits are composed of locally derived 

material. Some have moved in historic time, and some 

are still moving. Depending on the source, landslide 

deposits range in composition from bouldery debris in 

a fine-grained matrix to silty and clayey material with 

a few scattered stones. Most landslide deposits are less 

than 6 m thick.

Talus consists mostly of angular, bouldery rubble 

that has fallen or rolled down slopes underlain by 

crystalline rocks. Loose, unstable talus has large voids 

and a sparse lichen cover on surface boulders, whereas 

stable talus generally has a matrix of sand or sandy 

silt and a nearly continuous lichen cover on surface 

boulders. Talus is commonly less than 6 m thick.

Alluvial-fan deposits are heterogeneous masses of 

waterlaid material that are characterized by sharp 

lateral and vertical changes in grain size. Deposits
76 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

commonly range from bouldery sand and gravel along 

the mountain front to sand, sandy silt, and silty clay 

along the margins of flood plains east of the hogback 

belt. Most alluvial-fan deposits are less than 4.5 m 

thick.

Most slope deposits are of Holocene or latest 

Pleistocene age and, therefore, are only slightly 

modified by soil development. The upper meter of 

many of these deposits shows little evidence of stone 

weathering and only minor accumulation of clay or 

calcium carbonate.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Atterberg limits for slope deposits are similar to 

those of the original surficial deposits and rock units 

prior to downslope movement. Limited data suggest 

that fine-grained colluvium in southeast Denver, derived from loess and the Denver Formation, has liquid 

limits of 45-50 and plasticity indices of 20-30 (table 7). 

This material is usually classified as CL, according to 

the Unified Soil Classification (U.S. Bureau of 

Reclamation, 1974).

The volume-change characteristics and bearing 

capacity of colluvium are related to those of the source 

materials. Fine-grained colluvium undergoes moderate 

volume change when loaded and wetted. Structures 

founded on this material may be susceptible to damage 

owing to differential compaction under heavy loads 

and uneven heaving under light loads. When wetted 

and loaded at 25-50 kN/m2, fine-grained colluvium 

decreases about 1 percent in volume, and then increases 0-4 percent (table 8), depending on the original 

bulk density, moisture content, and amount and type 

of clay. Swelling pressures produced by colluvium are 

highly variable and differ with the composition of the 

source material. Values commonly range from zero for 

sandy material to about 250 kN/m2 for colluvium derived from expansive rock units. Values for percent 

swell and swelling pressure of fine-grained colluvium 

in southeast Denver average about 1.5 percent and 60 

kN/m2, respectively. In the Pueblo area, colluvium and 

other surficial deposits have swelling pressures that 

are about one-half those of the source material (Scott, 

1969). Colluvium usually has a lower bearing capacity 

than that of the source material prior to downslope 

movement. Colluvium and other materials on steep 

slopes may fail by landsliding if overloaded or 

saturated, or if lateral support is removed.

The bearing capacity and volume-change characteristics of landslide deposits, talus, and alluvial-fan 

deposits are not well known, but they must range widely depending on the grain size of the transported 

material as well as the type of deposit and distance of

transport. Landslide deposits derived from finegrained materials tend to have lower bearing capacities than those of the original, undisturbed material 

and have similar or slightly lower swelling pressures. 

The ability of talus to support loads depends on steepness of slope, size of debris, angularity, amount of 

block-to-block contact, and amount and type of interstitial material. Because of the very small amount of 

interstitial clay, talus is nonexpansive. The bearing 

capacity and shrink-swell characteristics of alluvialfan deposits change with the grain size of the sediment; gravelly alluvial fans probably are comparable 

to pediment alluviums, whereas alluvial fans derived 

from eolian sediments would probably have values 

similar to those of their eolian sources.

Slope deposits commonly have the following performance characteristics: Excavation is generally easy 

with power equipment, except for those deposits near 

the mountain front that contain large boulders. Foundation conditions are commonly very poor to fair for 

landslide deposits, poor for talus, poor to fair for colluvium and silty alluvial-fan deposits, and good for 

sandy and gravelly alluvial-fan deposits. Silty and 

clayey slope deposits are prone to excessive expansion 

and contraction, differential compaction, and frost 

heaving. Permeability is usually high in talus and 

sandy and gravelly alluvial-fan deposits and generally 

is low in silty alluvial-fan deposits and landslide 

deposits and colluvium derived from fine-grained 

materials. Slope stability is generally poor, owing to 

landslide potential, except on alluvial fans with low 

surface gradients.

MANMADE DEPOSITS

GENERAL DESCRIPTION

Manmade deposits are common features in the urban 

environment and include engineered fills, sanitary 

landfills, highway and railroad embankments, earth 

dams, trash dumps, and spoil piles (fig. 31). These 

deposits and man-modified land regraded for residential, commercial, and industrial development cover 

many tens of square kilometers of the Greater Denver 

Area. Artificial fills are especially common around 

buildings and along segments of roads and railroads in 

low-lying areas, in reclaimed gravel pits, and along irrigation ditches.

Manmade deposits are composed of varying amounts 

of surficial material and refuse, and they differ 

significantly in degree of compaction. They range in 

thickness from less than 1 m to as much as 45 m. Embankments and engineered fills are unmodified by 

weathering processes, whereas most landfills and 

dumps are in various stages of decomposition.
SEDIMENTARY ROCK UNITS 77

Engineered fills and embankments vary widely in composition but consist primarily of poorly sorted but 

well-compacted mixtures of silt, sand, and pebbles that 

contain minor amounts of cobbles and clay. Landfills 

are made up mostly of poorly compacted industrial and 

residential wastes interlayered with surficial material. 

Dump fills and soil piles typically consist of uncompacted or very poorly compacted surficial material, 

demolition debris, and other poorly sorted materials 

that are interspersed with large voids.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Most engineered fills and embankments have 

plasticity and volume-change characteristics that are 

similar to those of the original surficial materials from 

which they were derived. They usually have low liquid 

limits and plasticity indices, generally less than 50 and 

less than 30 respectively (table 7), and they are nonexpansive. Engineered fills and embankments are commonly classified as SP, SM, or CL, according to the 

Unified Soil Classification (U.S. Bureau of Reclamation, 1974). The physical properties and Unified Soil 

Classification of nonengineered fills are highly variable 

and depend primarily on the type and amount of surficial material that was placed in the fill.

Manmade deposits commonly have the following 

performance characteristics: Excavation with power 

equipment is easy in embankments, earth dams, and 

engineered fills and is easy to difficult in landfills, 

trash dumps, and spoil piles. Foundation conditions 

are very poor to poor on landfills, trash dumps, and 

spoil piles and are generally good for light structures 

on engineered fills. Uncompacted fills settle unevenly 

and compact slowly or rapidly with time. Permeability 

is highly variable and is probably high in trash dumps, 

moderate to high in other uncompacted fills, and low in 

engineered fills. When placed in excavations in permeable material, dump fills and landfills are potential 

sources of ground water pollution. In addition, unvented landfills are sources of methane and other 

potentially hazardous gases that are produced by the 

microbial decomposition of organic materials. Slope 

stability is poor for uncompacted fills and generally 

good for engineered fills.

SEDIMENTARY ROCK UNITS

Sedimentary rocks mainly claystone, siltstone, 

shale, and sandstone, and lesser amounts of conglomerate and limestone make up most of the bedrock east 

of the Front Range (table 1). Other sedimentary units 

include minor beds of gypsum and gypsiferous siltstone in the hogback belt south of Morrison, thin

seams of subbituminous coal along the eastern margin 

of the hogbacks, and thick lignite deposits east of 

Denver (fig. 1). Sediments in the piedmont differ from 

those in the hogback belt in that the former tend to be 

more expansive and less well indurated.

To a large extent, the physical properties and performance characteristics of the sedimentary rocks in 

the Greater Denver Area are related to: (1) grain size, 

(2) mineralogy of the clay-size particles, (3) degree of 

lithification, including the amount of compaction and 

cementation, (4) amount and type of any water-soluble 

minerals, and (5) relative abundance and orientation of 

fractures and bedding planes. Much of the sedimentary rock at or near the surface is only slightly 

weathered. The limited extent and shallow depth of 

weathering (a few meters or less) is probably due in 

part to the degree of lithification and low content of 

relatively unstable minerals. Weathering generally 

results in a reduction in density and bearing capacity, 

owing to the removal of unstable minerals and (or) an 

increase in the amount of voids or fractures. Weathering also tends to increase the swelling pressures of 

clayey rocks.

In this section, sedimentary rock units are grouped, 

according to potential volume change and degree of induration, into the following categories: (1) slightly to 

highly expansive and weakly indurated units, (2) moderately to highly expansive and weakly indurated 

units, (3) nonexpansive and weakly to moderately indurated units, (4) nonexpansive and nonindurated 

units, and (5) nonexpansive to slightly expansive and 

moderately to strongly indurated units.

SLIGHTLY TO HIGHLY EXPANSIVE AND 

WEAKLY INDURATED UNITS

GENERAL DESCRIPTION

The Graneros Shale, Greenhorn Limestone, Carlile 

Shale, Smoky Hill Shale Member of the Niobrara Formation (table 1), and the lower and upper shale units 

and the transition unit of the Pierre Shale (table 1; fig. 

33) are made up primarily of fine-grained, shaly rocks 

that are slightly to highly expansive and weakly indurated. These rock units are exposed in a narrow zone 

along the western margin of the piedmont just east of 

the hogbacks (fig. 2).

The rock units consist mainly of noncalcareous to 

very calcareous, overconsolidated, silty claystone and 

very clayey to sandy siltstone and shale. All of the 

rock units, except the lower shale unit of the Pierre, 

contain a small amount of clayey to silty sandstone. 

Several thin beds of dense limestone are present in the 

upper part of the Greenhorn Limestone; chalky limestone occurs near the top and in the lower one-half of
78 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

Fox Hills Sandstone

0)

<jj 

h_

Transition unit

Upper shale unit

Sandstone unit

Lower shale unit

Niobrara Formation

Figure 33. Generalized lithologic subdivisions of the 

Pierre Shale west of Denver. Modified from Van Horn 

(1976).

the Smoky Hill. Individual beds are several centimeters to about 10 m thick and are laterally persistent. 

Layers of highly expansive bentonite about 0.5-30 cm 

thick are present locally. Zones of hard limestone and 

ironstone concretions, as much as a half meter in 

diameter, are common in the Pierre. Most sandstones 

and siltstones are cemented by clay, and locally by 

calcium carbonate and iron oxide. Calcium montmorillonite and mixed-layer illite-montmorillonite are the 

chief clay minerals in the transition unit of the Pierre; 

the other rock units contain mixed-layer illite-montmorillonite, montmorillonite, illite, kaolinite, and a 

minor amount of chlorite. Silt and sand grains are 

mostly quartz, feldspar, and calcite. (For additional information, see the lithologic descriptions in the section 

on "Descriptive outline of rock units").

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Few data are available for the grain-size distributions, Atterberg limits, and Unified Soil Classification 

group symbols for the various fine-grained rock types 

in the above units. One grain-size analysis for siltstone 

of the Graneros and one for shale of the Smoky Hill indicate that these rocks are composed of clay- and siltsize particles. The siltstone is 55 percent clay and 45

percent silt, whereas the shale is 70 percent clay, 25 

percent silt, and 5 percent sand (table 9). Both have a 

liquid limit of 30 and a plasticity index of 10. Their 

Unified Soil Classification group symbol (U.S. Bureau 

of Reclamation, 1974) is CL (table 10). As opposed to 

the small amount of data available for the Greater 

Denver Area, a considerable amount of test data has 

been summarized for clayey rocks of Late Cretaceous 

age in the Pueblo area (fig. 1, this report; Scott, 1969). 

Near Pueblo, shales of the Graneros, Greenhorn, and 

Carlile are about 60-65 percent clay, 35 percent silt, 

and 0-5 percent sand. Shales of the Niobrara are 

slightly less clayey and are about 55 percent clay and 

45 percent silt. Shales of the Pierre consist of about 60 

percent clay and 40 percent silt. The Atterberg limits 

for the above shales are related primarily to their 

grain-size distributions. Both the liquid limit and 

plasticity index tend to increase as the clay content increases. Shales of the Graneros, Greenhorn, and Carlile 

have liquid limits of about 35-45 and plasticity indices of 10-15. Shales of the Niobrara have lower 

values owing to their slightly lower clay content. The 

liquid limits and plasticity indices of these sediments 

are about 30-35 and 5-10, respectively. Values for the 

shales of the Pierre are more variable than those for 

the above units. They range from about 30-55 for the 

liquid limits and 10-25 for the plasticity indices. 

Shales of the Graneros, Greenhorn, Carlile, and Smoky 

Hill are usually classified as ML, according to the 

Unified Soil Classification (U.S. Bureau of Reclamation, 1974), whereas similar rock types of the Pierre are 

commonly classified as CL, ML, and CH (Scott, 1969).

The various rock units and the different rock types 

within these units have a wide range in swelling 

pressures and bearing capacities. Swelling pressures 

for the fine-grained rock types commonly range from 

less than about 75 kN/m2 for the upper two-thirds of 

the lower shale unit of the Pierre to about 75-350 

kN/m2 for the lower one-third of the lower shale unit of 

the Pierre and the other rock units. Some of the 

Graneros, however, is very expansive; bentonite at the 

top of the formation and siliceous shale at the base 

have swelling pressures of 520 and 560 kN/m2, respectively (table 11). Sandstone and dense limestone have 

higher bearing capacities than claystone, siltstone, 

shale, or chalky limestone.

Slightly to highly expansive and weakly indurated 

sedimentary units commonly have the following performance characteristics: Weathered and unweathered 

rock are usually easily excavated with power equipment. Foundation conditions are generally poor on 

siltstone and claystone, owing primarily to high swelling pressures and large volume change. Beds of dense 

limestone and thick sandstone beds provide more suit-
SEDIMENTARY ROCK UNITS 79

TABLE 9. Grain-size distributions of weakly to stongly indurated sedimentary rock units and weathered igneous and metamorphic rock

units in the Greater Denver Area

[Values are approximate and area rounded to the nearest 5 percent. 5 , range in size of gravel in samples tested; <, less than; .... little or no data]

Rock units

Grain-size distribution 

(percent by weight)

Silt and Clay Sand Gravel

Granule Pebble Cobble

- 100

- 95

- 10-35

- <15

- 5-20

- 5

- 100

- 90

- 5

0 

5 

60-80

80-90

30 

75 

25

65 

35-50

95 

75

0 

10 

0 

40 

35

 i r i i

1 1 1 1

 1

 1

 1 

 1 

1

Grain-size data for decomposed and naturally disaggregated material less than 64 mm in diameter.

able foundation conditions. Permeability commonly 

ranges from negligible in claystone and siltstone to low 

to moderate in fractured sandstone and limestone. 

Trafficability on clayey rock is generally poor. Undisturbed slopes and revegetated cuts, less than several meters in height, on gentle slopes tend to be 

stable, whereas unvegetated slopes are prone to gullying and deflation, and steep, unvegetated cuts greater 

than several meters in height may fail.

MODERATELY TO HIGHLY EXPANSIVE AND

WEAKLY INDURATED UNITS

GENERAL DESCRIPTION

The sandstone unit of the Pierre Shale (fig. 33), the 

Fox Hills Sandstone, the Laramie, Arapahoe, and 

Denver Formations, the Dawson Arkose, and the 

Green Mountain Conglomerate (table 1) are made up of 

varying amounts of fine-grained to very coarse grained 

sediments that are moderately to highly expansive and 

weakly indurated. These rock units occur in the piedmont, along the eastern margin of the hogback belt 

(fig. 2).

The sandstone unit of the Pierre and the Fox Hills, 

Laramie, Arapahoe, and Denver consist mostly of interbedded silty to sandy claystone, clayey to sandy

siltstone, and silty to pebbly sandstone. Conglomerate 

is generally absent from the Laramie and older rock 

units, but it makes up a minor amount of the Arapahoe 

and Denver. Thin coal beds are locally present in the 

Denver (Landis, 1959) and in the lower 60 m of the 

Laramie (Van Horn, 1976). In areas away from the 

mountain front, the Denver contains thick lignite beds 

(Soister, 1974). In comparison, the Dawson and Green 

Mountain are composed primarily of conglomerate and 

conglomeratic sandstone and lesser amounts of micaceous sandstone and sandy siltstone and claystone. 

Conglomerate is most abundant in the lower half of the 

Dawson and in the lower and upper one-third of the 

Green Mountain. Pebble- to cobble-size clasts are common, and boulders longer than a meter are locally present. Beds of sandy ironstone, less than a half meter 

thick, occur in the upper part of the Dawson (Scott and 

Varnes, 1967).

The thickness and lateral continuity of individual 

beds differ among the various formations. Individual 

beds in the Laramie and Fox Hills are several centimeters to a few tens of meters thick and are laterally 

persistent, whereas beds in the younger rock units are 

a few centimeters to several tens of meters thick, tend 

to be lenticular, and pinch out within a few meters to 

less than a kilometer. Zones containing iron- or car-
80 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

TABLE 10. Atterberg limits and typical Unified Soil Classification 

group symbols for weakly to moderately indurated sedimentary 

rock units and weathered igneous and metamorphic rock units in 

the Greater Denver Area

[Values are approximate and are rounded to the nearest 5 percent. < , less 

than; . . . , little or no data; NP, nonplastic; P, slightly plastic]

Rock units Atterberg limits Unified Soil

Liquid Plasticity Classification

limit index 8r °up symbols

Siltstone of the Graneros Shale 30 10 

Smoky Hill Shale Member of the 30 10

Niobrara Formation.

Claystone of the Denver Formation 50-RO 30-50 

Siltstone of the Denver Formation 35-55 15-30

Sandstone of the Denver Formation 35 <10 

Claystone of the Dawson Arkose 40-65 10-25 

Siltstone of the Dawson Arkose ... ...

Sandstone of the Dawson Arkose 20-50 <20 

Conglomerate and Sandstone of the 

Castle Rock Conglomerate. ... NP-P

CL 

CL

CH 

CL

SM 

MH 

ML 

SC

GP,GW,SP,SW

Siltstone of the Lykins Formation 25

Siltstone of the Morrison Formation- 25

Claystone of the Laramie Formation 55

Weathered gneiss and schist* 20-30

Weathered granitic rocks* 20-30

<5

5

20

NP

NP

ML

ML

MH

GW

GW

Atterberg limits and Unified Soil Classification for decomposed 

and naturally disaggregated material.

bonate-cemented concretions, as much as 30 cm in 

diameter, are locally present in sandy beds in the 

Laramie, Fox Hills, and Arapahoe.

The degree of induration of the coarse-grained rocks 

varies somewhat and is related to the type of interstitial material. Most sandstone and conglomerate are 

weakly indurated and loosely bonded by clay or calcium carbonate; more strongly indurated rocks are 

cemented by iron oxide and silica. The clay-size fraction of the claystones is composed primarily of montmorillonite, mixed-layer illite-montmorillonite, and (or) 

kaolinite. Montmorillonite is the chief clay mineral in 

the Denver, whereas illite and montmorillonite or 

mixed-layer illite-montmorillonite are common in the 

Arapahoe and the sandstone unit of the Pierre. Kaolinite and montmorillonite or mixed-layer illite-montmorillonite are the principle clay minerals in the 

Laramie, Fox Hills, and Dawson. (For additional information, see the lithologic descriptions in the section on 

"Descriptive outline of rock units."

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Few test data are available for the different 

lithologies of the Fox Hills, Laramie, and Arapahoe. 

Limited grain-size data suggest that sandstone of the 

Fox Hills contains more silt and clay than sandstone of 

the Laramie, and that sandstone of the Arapahoe has a

TABLE 11. Swelling pressure and unconfined compressive strength 

of nonexpansive to highly expansive and weakly to strongly indurated sedimentary rock units in the Greater Denver Area

[Values are approximate and are rounded to the nearest 5 kN/m2 for swelling pressure and to the nearest 50 kN/m2 for unconfined compressive 

strength. < , less than; . . . , little or no data; NE, nonexpansive]

Rock units

Swelling 

pressure 

(kN/m2 )

Unconfined

compressive

strength

Claystone and siltstone of the upper twothirds of the lower shale unit of the 

Pierre Shale 

Claystone and siltstone of the Graneros 

Shale, Greenhorn Limestone, Carlile 

Shale, Smoky Hill Shale Member of the 

Niobara Formation, lower one-third of 

the lower shale unit, upper shale unit, 

and transition unit of the Pierre ShaleBentonite of the Graneros Shale 

Siliceous shale at base of the Graneros Shale 

Claystone of the sandstone unit of the Pierre

Shale 

Claystone of the Fox Hills Sandstone and the

Laramie Format ion 

Claystone of the Arapahoe Formation 

Claystone of the Dawson Arkose 

Sandstone of the Dawson Arkose 

Claystone of the Denver FormationSiltstone of the Denver Formation 

Sandstone of the Denver FormationConglomerate and sandstone of the Castle

Rock Conglomerate 

Claystone the Ralston Creek, Morrison, Lytle, 

and South Platte Formations 

Siltstone of the Lykins FormationSiltstone of the Morrison Formation 

75-350 

520

560 

60-295

15-345 

130-360

140-930 

<120 

<50

NE

<75 

30 

40

200-400

250-1,000

500-1,000

<350

greater range in composition than sandstone of the 

Fox Hills or the Laramie. The grain-size distribution 

for sandstone of the Fox Hills is about 20-40 percent 

silt and clay and 60-80 percent sand; for sandstone of 

the Laramie about 10-20 percent silt and clay and 

80-90 percent sand; and for sandstone of the Arapahoe 

about 10-35 percent silt and clay, 65-80 percent sand, 

0-10 percent granules, and 0-5 percent pebbles (table 

9, this report; McConaghy and others, 1964).

Grain-size data for equivalent lithologies of the 

Denver and Dawson indicate that they differ slightly 

in grain-size distribution. Claystone of the Denver is 

commonly composed of 85-95 percent silt and clay and 

5-15 percent sand, whereas the same rock type in the 

Dawson is slightly more sandy and averages about 75 

percent silt and clay and 25 percent sand (table 9). 

Generally, clay-size particles are slightly more abundant than silt-size particles. In the Denver, siltstone is 

more sandy than claystone and consists of about 70 

percent silt and clay and 30 percent sand. Sandstone of 

the Denver averages about 20 percent silt and clay, 75 

percent sand, and 5 percent granule to pebble gravel; 

sandstone of the Dawson is slightly finer grained and 

is composed of about 20-35 percent silt and clay, 65 

percent sand, and 0-15 percent granule to pebble 

gravel.
SEDIMENTARY ROCK UNITS 81

The plasticity characteristics of the various rock 

types of the Denver and Dawson are related chiefly to 

the relative abundance of silt- and clay-size particles. 

Liquid limits and plasticity indices for the Denver 

commonly range from about 50-80 and 30-50 respectively for claystone, 35-55 and 15-30 for siltstone, and 

35 and less than 10 for sandstone (table 10). In comparison, claystone of the Dawson usually has a liquid 

limit of 40-65 and a plasticity index of 10-25, whereas 

sandstone has a liquid limit of 20-50 and a plasticity 

index of less than 20. Claystone, siltstone, and sandstone of the Denver are usually classified as CH, CL, 

and SM, respectively, according to the Unified Soil 

Classification (U.S. Bureau of Reclamation, 1974); 

equivalent rock types of the Dawson are commonly 

designated as MH, ML, and SC.

Values for swelling pressure, percent swell, and bearing capacity correlate with rock type, although values 

for individual rock types vary for different formations. 

For example, swelling pressures for claystone commonly range from about 60 kN/m2 for the sandstone 

unit of the Pierre, to 60-295 for the Fox Hills and 

Laramie, 15-345 for the Arapahoe, and 130-360 kN/m2 

for the Dawson, to as much as 140-930 kN/m2 for the 

Denver (table 11). Siltstone and sandstone of the 

Denver have swelling pressures that are about onefifth and one-twentieth, respectively, that of claystone 

of the same formation. Sandstone of the Dawson is 

more clayey and more expansive then the sandstone of 

the Denver. Sandstone of the Dawson commonly exerts expansive forces of about one-half to one-third 

those of the sandy claystone of the Dawson. Percent 

swell for sediments of the Denver, under loads of 25-50 

kN/m2, corresponds with swelling pressure and averages about 0.5 percent for sandstone, 1 percent for siltstone, and 4 percent for claystone. Values of as much 

as 10 percent, for percent swell, have been reported for 

claystone of the Denver. Test results indicate that 

claystone and siltstone of the Denver commonly have 

unconfined compressive strengths of about 250-1,000 

kN/m2 (table 11). Sandstone and weathered siltstone 

and claystone of the Denver have values of about onehalf to one-fifth as much. Arkosic sandstone of the 

Dawson has an unconfined compressive strength of 

about 200-400 kN/m2, which is generally higher than 

that of sandstone of the Denver.

The performance characteristics of moderately to 

highly expansive and weakly indurated sedimentary 

units differ considerably with rock type. Most claystone and siltstone are easy to excavate with power 

equipment. These rock types tend to have poor foundation conditions and trafficability, low to very low 

permeability, high susceptibility to frost heave and

erosion, and relatively unstable natural and manmade 

slopes. In contrast, most sandstone and conglomerate 

are moderately difficult to excavate with power equipment. They generally have good foundation conditions 

and trafficability, low to moderate permeability, low 

susceptibility to frost heave and erosion, and relatively 

stable natural and manmade slopes. Locally, the 

Denver and Arapahoe contain sulfates and other salts 

which are deleterious to certain kinds of cement and 

may cause corrosion of unprotected steel pipes.

NONEXPANSIVE AND WEAKLY TO 

MODERATELY INDURATED UNITS

GENERAL DESCRIPTION

The Castle Rock Conglomerate (table 1) consists 

mostly of weakly to moderately cemented conglomerate and sandstone that are nonexpansive. The Castle 

Rock underlies part of the large interstream area east 

of the hogback belt between Denver and Colorado 

Springs (fig. 1). The lower part of the Castle Rock is a 

very coarse conglomerate interbedded with arkosic 

sandstone. Some of the clasts are as much as a meter in 

diameter. The upper part is coarse arkosic sandstone 

interstratified with thin beds of pebble conglomerate 

(Romero, 1976). Siltstone and claystone are present in 

the eastern part of the outcrop area. Individual beds 

are lenticular and vary in thickness over short distances. Much of the formation is weakly to moderately 

cemented by iron oxide, clay, and locally by calcium 

carbonate; parts of it are well indurated and cemented 

by iron oxide and minor amounts of silica. Conglomerate consists chiefly of rock fragments, quartz, and 

feldspar, whereas sandstone is made up mostly of 

quartz, feldspar, and some mica. (For additional information, see the lithologic description in the section on 

"Descriptive outline of rock units.")

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

The physical properties of the Castle Rock are 

strongly influenced by the low content of silt and clay 

and the degree of lithification. Limited test data indicate that the conglomerate is less than 15 percent 

silt and clay, 30-50 percent sand, and about 40-70 percent granule to boulder gravel (table 9). The matrix is 

fine grained and is nonplastic to slightly plastic (table 

10). Much of the formation would probably be classified, according to the Unified Soil Classification (U.S. 

Bureau of Reclamation, 1974), as poorly to well-graded 

sand and gravel (GP, GW, SP, and SW). Sandstone 

and conglomerate of the Castle Rock are nonexpansive 

(table 11) and probably have bearing capacities that
82 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

are greater than or equal to those of the arkosic sandstone and conglomerate of the Dawson.

The Castle Rock commonly has the following performance characteristics: The conglomerate and most 

of the sandstone are moderately difficult to difficult to 

excavate with power equipment and may require blasting. Both rock types have good foundation conditions 

and trafficability because of the adequate bearing 

capacity and low clay content. Permeability usually 

ranges from low in the unfractured rock to moderate to 

high along fractures. The formation has low susceptibility to frost heave and erosion. It generally forms 

stable natural and manmade slopes; however, rockfalls 

and debris slides may form locally where the formation 

caps steep-sided mesas.

NONEXPANSIVE AND NONINDURATED UNITS 

GENERAL DESCRIPTION

High-level gravel deposits, of Pliocene to Oligocene 

age, cap some of the ridges and spurs west of the 

mountain front, along Clear Creek and near the communities of Evergreen and Pine (fig. 1). These deposits 

are about 300 m above stream level. They are composed of rounded to subangular pebbles, cobbles, and 

boulders as much as 6 m in diameter in a sandy matrix 

(Bryant, 1974; Peterson, 1964; Sheridan and others, 

1972). The major rock types are granite, gneiss, quartz, 

quartzite, migmatite, and porphyry. The thickness of 

these deposits has not been determined, although 

those in the vicinity of Pine are estimated to be about 

8-30 m thick (Peterson, 1964).

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

The physical properties and performance 

characteristics of high-level gravel deposits are not 

known. They may be somewhat similar to those of 

bouldery pediment alluvium along the eastern margin 

of the hogback belt.

NONEXPANSIVE TO SLIGHTLY EXPANSIVE AND 

MODERATELY TO STRONGLY INDURATED

UNITS 

GENERAL DESCRIPTION

The Fountain Formation, Lyons Sandstone, the 

Lykins, Ralston Creek, and Morrison Formations, the 

Lytle and South Platte Formations of the Dakota 

Group, and the Fort Hays Limestone Member of the 

Niobrara Formation (table 1) are made up of varying 

amounts of sandstone and conglomerate or limestone, 

and fine-grained shaly rocks that are nonexpansive to 

slightly expansive and moderately to strongly indurated. These rock units are exposed in the hogback belt 

(fig. 2) and along its eastern flank.

The Fountain is composed of conglomeratic sandstone, conglomerate, and some micaceous siltstone and 

fine-grained sandstone. The Lyons is mostly fine- to 

medium-grained sandstone, and some conglomerate 

and siltstone. The Lykins is predominantly calcareous, 

clayey to sandy siltstone and has some silty limestone 

and a few beds of silty sandstone. The Ralston Creek 

and Morrison are made up of varicolored siltstone, 

silty to sandy claystone, some sandstone, and minor 

amounts of limestone. Siltstone and claystone of the 

Ralston Creek are calcareous. South of Morrison (fig. 

1), the Ralston Creek contains beds of gypsum and 

gypsiferous siltstone. The Lytle and South Platte consist of sandstone, clayey siltstone, clayey to sandy 

claystone, and some conglomeratic sandstone and conglomerate. The Fort Hays is primarily a dense clayey 

limestone separated by thin beds of calcareous claystone and highly expansive bentonite.

The thickness and lateral continuity of individual 

beds differs among the various formations. Individual 

beds of the Lykins and Fort Hays are generally a few 

centimeters to about a meter thick and are laterally 

persistent; those of the Fountain, Lyons, Lytle, and 

South Platte are usually about several centimeters to 

10 m thick and are discontinuous; whereas beds of the 

Ralston Creek and Morrison are about 10 cm to 10 m 

thick and are laterally persistent or discontinuous.

The degree of induration of the various rock types 

varies somewhat with the amount and type of interstitial material. Limestones of the Lykins and Fort 

Hays are generally hard, dense, and cemented by 

calcium carbonate. Sandstone, conglomeratic sandstone, and conglomerate of the Fountain, Lyons, Lytle, 

and South Platte are dense, hard, and cemented by 

silica and iron oxide, but locally they are friable where 

cemented by calcium carbonate and iron oxide. Siltstone and sandstone of the Lykins, Ralston Creek, and 

Morrison are firm to hard and are cemented by calcium 

carbonate, clay, and lesser amounts of iron oxide and 

silica Claystone in the various formations is firm and 

is bonded by clay minerals. Gypsum in the Ralston 

Creek is soft to firm. Illite, mixed-layer illite-montmorillonite, and kaolinite are the principal clay minerals in 

clay stones of the Ralston Creek, Morrison, Lytle, and 

South Platte. The Lyons is made up mostly of quartz 

sand. Sandstone and conglomerate of the Fountain are 

composed of quartz, feldspar, and minor amounts of 

rock fragments. Quartz sand and chert pebbles are the 

chief constituents of the sandstone and conglomeratic 

sandstone of the Dakota Group. (For additional information, see the lithologic descriptions in the section on 

"Descriptive outline of rock units.")
IGNEOUS AND METAMORPHIC ROCK UNIT 83

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

The grain-size distributions and Atterberg limits of 

the various rock types of the above units are not well 

known. Limited grain-size data suggest that the sandstones of the Lyons, Lytle, and South Platte are composed of very fine to fine sand that is very slightly silty 

to silty and clayey. These rocks consist of about 5-25 

percent silt and clay and 75-95 percent sand (table 9, 

this report; McConaghy and others, 1964). Test data 

for one sample each of siltstone of the Lykins and the 

Morrison, and claystone of the Laramie indicate that 

these lithologies differ considerably in clay content 

and plasticity. Siltstone of the Lykins is 30 percent 

clay and 70 percent silt, whereas siltstone of the Morrison is 60 percent clay, 30 percent silt, and 10 percent 

sand. The claystone contains more clay than the siltstones and is composed of 80 percent clay and 20 percent silt (table 9). The siltstones and claystone also 

have different Atterberg limits and Unified Soil Classification group symbols. The siltstones have a liquid 

limit of 25 and a plasticity index of 5 or less. Their Unified Soil Classification group symbol is ML. In comparison, the claystone is considerably more plastic. It 

has a liquid limit of 55 and a plasticity index of 20. Its 

Unified Soil Classification (U.S. Bureau of Reclamation, 1974) group symbol is MH (table 10).

Rocks of Early Cretaceous age and older tend to be 

less expansive and have higher bearing capacities than 

younger rocks of similar grain size. Claystone of the 

Ralston Creek, Morrison, Lytle, and South Platte produce swelling pressures of less than 75 kN/m2, compared with about 75-930 kN/m2 for much of the 

younger claystone in the piedmont. Siltstones of the 

Ralston Creek, Morrison, Lytle, and South Platte are 

less expansive than the claystones of these formations. 

Siltstones of the Lykins and Morrison have swelling 

pressures of about 30 and 40 kN/m2, respectively (table 

11). Swelling pressures exerted by thin beds of claystone and bentonite of the Fort Hays, of Late Cretaceous age, are higher than those of most claystone of 

Early Cretaceous age and older. Swelling pressures of 

the fine-grained rocks of the Fort Hays Limestone 

Member are probably comparable with those of similar 

rock types of the Smoky Hill Shale Member. Massive 

beds of silica-cemented sandstone and conglomerate, 

and thick beds of dense limestone have high bearing 

capacities and can support heavier loads than thinner, 

less well indurated, or fractured rock of similar 

lithology.

Nonexpansive to slightly expansive and moderately 

to strongly indurated sedimentary units commonly 

have the following performance characteristics: 

Claystone, siltstone, chalky limestone, and gypsum are

easy to moderately difficult to excavate with power 

equipment, whereas most sandstone, conglomerate, 

and dense limestone are difficult to excavate and may 

require blasting. Foundation conditions are generally 

good except in areas underlain by water-soluble 

material such as gypsum or gypsiferous siltstone, or 

thin interlayered beds having different bearing capacities that may be prone to differential settlement. Permeability is highly variable and commonly ranges 

from very low to low for most claystone and siltstone 

to moderate to high for sandstone and conglomerate, 

in areas away from fractured zones and outcrops. Fractured sandstone and conglomerate, especially in outcrop areas, are highly permeable. Limestone usually 

has low to moderate permeability. Clayey bedrock generally has poor trafficability and is more susceptible to 

frost heave and erosion than sandstone and conglomerate. Slope stability is usually good on gentle slopes 

and in undisturbed areas where the bedding is inclined 

into the slope. Dipslopes and areas where planes of 

weakness dip into steep cuts are less stable and may 

produce rock slides and rockfalls. Special precautions 

against corrosion are needed to protect steel pipe and 

concrete that are in contact with gypsum or gypsiferous sediments.

IGNEOUS AND METAMORPHIC ROCK

UNITS 

GENERAL DESCRIPTION

Igneous and metamorphic rocks make up the 

bedrock of the Front Range, but they are of very 

limited extent in the piedmont and hogback belt (fig. 

2). In the Front Range west of Denver, the major rock 

types are gneiss, schist, and migmatite, along with 

lesser amounts of plutonic rocks (granite to granodiorite), and minor amounts of quartzite and felsic to 

mafic dike rocks (table 1). On the piedmont, igneous 

rocks make up a minor part of the bedrock and include 

the shoshonite (Trimble and Machette, 1979b) or monzonite (Van Horn, 1976) dikes near Ralston Reservoir, 

the shoshonite (Trimble and Machette, 1979b) or latite 

(Van Horn, 1976) lava flows on North and South Table 

Mountains, and the Wall Mountain Tuff a rhyolitic 

ash flow which caps many of the buttes south of Castle Rock (fig. 1). In the hogback belt, igneous rocks are 

rare and consist a few scattered sills and irregular 

bodies of rhyodacite northwest of Denver. The only exposure of metamorphic rocks in the area east of the 

Front Range is in the Deer Creek anticline (fig. 1), 

south of Morrison.

WEATHERING

The physical properties and performance
84 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

characteristics of the igneous and metamorphic rocks 

in the Greater Denver Area are related primarily to the 

degree of weathering and, in areas adjacent to fault 

zones or Laramide intrusive bodies, to the amount of 

recrystallization, cementation, shearing, and alteration. In areas of deeply weathered rock, such as the 

gently rolling uplands in the mountains west of 

Denver, fresh, moderately hard to hard crystalline rock 

containing relatively few fractures is covered by thick 

regolith. The regolith can be subdivided on the basis of 

weathering into two parts, which commonly consist of 

the underlying slightly to moderately weathered 

rock characterized by numerous fractures, abundant 

yellowish iron oxide staining on rocks and mineral 

grains, reduced shearing strength, and lower bulk density and the overlying highly weathered rock or 

saprolite. The saprolite is a product of prolonged 

physical and chemical weathering and has properties 

that contrast markedly with those of the underlying 

weathered and fresh rock. Locally, the regolith has 

been partly or completely removed by erosion or is 

buried by younger surficial deposits. The thickness, 

degree of weathering, and other properties of the 

regolith often vary over short distances. Some of the 

major factors that influence its development in crystalline rocks are (1) mineralogy, especially the relative 

abundance of biotite, (2) steepness of surface topography, (3) relative abundance and orientation of fractures, (4) amount of shearing and (or) hydrothermal alteration (Schmidt and Pierce, 1976), (5) degree of development and orientation of foliation, and (6) length of 

time that the rock has been exposed to weathering processes. The regolith tends to be thick and extensively 

weathered in areas where the rock was particularly 

susceptible to granular disintegration and decomposition, has been relatively unaffected by erosional 

processes that could have removed the weathering products and exposed fresh rock, and has been subjected 

to weathering processes for many millions of years.

In general, crystalline rocks in areas of gently sloping terrain or sheared rocks along fault zones or intrusive bodies are more highly weathered or hydrothermally altered than similar rocks in areas of steeply 

sloping terrain or in areas away from fault zones or intrusive bodies. In areas of gently sloping terrain, away 

from fault zones or intrusive bodies, strongly foliated 

rock with numerous, closely spaced, nearly vertical 

fractures and abundant biotite is much more weathered than nonfoliated, massive rock that is low in 

biotite. In addition, rocks composed primarily of 

quartz and potassium feldspar tend to be less weathered than those high in biotite and plagioclase. Comparison of the weathered materials formed from crystalline rocks of varying composition suggest the following sequence of relative stability for the common 

rock-forming minerals in the near-surface environment: quartz> potassium feldspar> plagioclase>hornblende> biotite (least stable). Besides differences in the amount of weathering among different 

crystalline rocks, the depth of weathering can vary 

considerably, even within the same rock body. Typically, it is shallowest in massive rock and deepest along 

closely spaced, open fractures. Deep weathering is 

common in coarse-grained granitic rock, especially the 

Pikes Peak Granite, where locally it extends to depths 

of greater than 50 m (Blair, 1975).

In this section, crystalline rocks unaffected by 

faulting and igneous intrusion are grouped, on the 

basis of weathering characteristics, into the following 

categories: (1) units commonly overlain by thick, 

slightly to highly weathered regolith, (2) units commonly overlain by thin, slightly to moderately weathered regolith, and (3) units commonly overlain by thin 

and discontinuous, unweathered to slightly weathered 

regolith; a fourth category units affected by faulting 

and igneous intrusion includes those crystalline 

rocks that are cut by faults and Laramide intrusive 

rocks.

UNITS COMMONLY OVERLAIN BY THICK, 

SLIGHTLY TO HIGHLY WEATHERED REGOLITH

GENERAL DESCRIPTION

Gneiss, schist, migmatite, granite, quartz monzonite, granodiorite, and amphibolite are widespread in 

the mountains west of Denver. These rocks are of Precambrian age and are composed of various amounts of 

potassium feldspar, plagioclase, quartz, mica (mainly 

biotite), and hornblende. Commonly, they are mantled 

by thick regolith, which grades from saprolite in the 

upper part to moderately to slightly weathered rock in 

the lower part. The regolith consists of varying 

amounts of weathered rock fragments in a sandy 

matrix that ranges from clean, coarse sand to clayey, 

fine to coarse sand. At stable sites, the saprolite usually extends deeper than 3 m, and fresh bedrock commonly occurs at depths of less than 30 m. The fresh 

bedrock is nonfoliated to strongly foliated, nonlayered 

to layered, massive to slightly fractured, and moderately hard to very hard. It tends to have fewer fractures and much less iron oxide staining than the 

overlying weathered rock.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Available test data indicate that regolith derived 

from gneiss, schist, and granitic rocks has fairly 

similar grain-size distributions and plasticity
IGNEOUS AND METAMORPHIC ROCK UNIT 85

characteristics. Particles of weathered gneiss and 

schist, less than 64 mm in size, are composed of about 

10 percent silt and clay, 40 percent sand, 15 percent 

granules, and 35 percent pebbles (table 9). In comparison, the same size fraction for weathered granitic 

rocks consists of about 5 percent silt and clay, 35 percent sand, 25 percent granules, and 35 percent pebbles. 

The amount of cobble- and boulder-size rock fragments 

tends to increase with depth; they commonly make up 

about one-half or less of the upper several meters of the 

regolith. Weathered material, less than 0.4 mm in size, 

from gneiss, schist, and granitic rocks is nonexpansive, has a liquid limit of about 20-30, and is nonplastic (table 10). Much of the regolith derived from the 

above rock types would probably be classified, according to the Unified Soil Classification (U.S. Bureau of 

Reclamation, 1974), as well-graded gravel (GW).

Gneissic, schistose, granitic, and amphibolitic rocks 

and the overlying regolith commonly have the following performance characteristics: The regolith is usually easy to moderately easy to excavate with power 

equipment, but the underlying unweathered bedrock is 

moderately difficult to difficult to excavate and usually requires blasting. Foundation conditions are generally excellent on the unweathered bedrock; however, 

conditions range from good on weathered gneiss, migmatite, and granitic rocks to fair on weathered amphibolite and schist. Permeability is commonly moderate to 

high in the saprolite and along fractures in the weathered rock and is relatively low in the unweathered bedrock. Regolith on steep slopes and in areas where the 

vegetation has been disturbed or removed is susceptible to sheet erosion and gullying. Gentle slopes 

generally are stable; debris slides and rockslides may 

occur locally on steep slopes or in areas where the rock 

is intensely fractured or highly weathered, or the foliation parallels the slope.

UNITS COMMONLY OVERLAIN BY THIN,

SLIGHTLY TO MODERATELY WEATHERED

REGOLITH

GENERAL DESCRIPTION

Shoshonite, rhyolite, and rhyodacite, of Paleocene 

and Oligocene age, are very minor components of the 

bedrock in the piedmont and hogback belt, whereas 

quartz monzonite, monzonite, latite, and diabase, of 

Late Cretaceous to Eocene(?) age, make up a small 

amount of the bedrock in the Front Range. These 

volcanic and shallow-intrusive rocks are composed of 

varying amounts of felsic and mafic minerals: rhyolite, 

quartz monzonite, and rhyodacite consist primarily of 

quartz, potassium feldspar, plagioclase, and (or) 

biotite; monzonite and latite are made up mostly of

potassium feldspar, plagioclase, biotite, and (or) hornblende; and diabase and shoshonite (potassium-rich 

basalt) are formed mainly of plagioclase and augite. 

These rock types are commonly mantled by regolith 

that is considerably thinner and much less weathered 

than that developed on granitic and gneissic rocks. The 

upper part of the regolith commonly consists of slightly to moderately weathered rock and little or no saprolite. The lower part is only slightly weathered, mostly 

along fractures. The underlying fresh bedrock is typically nonfoliated, massive to slightly fractured, and 

hard to very hard. These rock units are tabular bodies 

that range in thickness from less than 1 m to about 600 

m for intrusive rocks and about 5 to 70 m for volcanic 

rocks.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

The grain-size distribution and plasticity 

characteristics of regolith derived from lava flows, 

tuff, and dike rocks are not well known. The regolith 

formed from these rocks probably contains more granule- to boulder-size material and less silt and clay, and 

probably has a lower liquid limit and is less plastic 

than the weathered mantle on gneiss, schist, and granitic rocks.

Volcanic and shallow-intrusive rocks and the overlying regolith commonly have the following performance 

characteristics: The regolith is moderately easy to excavate with power equipment, but the underlying fresh 

bedrock is moderately difficult to difficult to excavate 

and locally requires blasting. Foundation conditions 

are excellent on the unweathered rock and generally 

are good on the regolith. Permeability tends to be low 

to moderate along fractures in the intrusive rocks and 

moderate to high in the regolith and along fractures in 

the lava flows and tuff. Slope stability is generally 

good on the intrusive rocks, but poor to fair for volcanic rocks, which commonly cap steep-sided buttes 

and mesas and are prone to debris sliding and rockfall.

UNITS COMMONLY OVERLAIN BY THIN,

DISCONTINUOUS, UNWEATHERED TO

SLIGHTLY WEATHERED REGOLITH

GENERAL DESCRIPTION

Precambrian pegmatite and quartzite are associated 

with other crystalline rocks of the Front Range. Pegmatite dikes are common features in areas underlain 

by granitic, schistose, and gneissic rocks. Quartzite, 

which includes conglomeratic quartzite and some conglomerate, is much less common except in the area adjacent to the lower part of Coal Creek Canyon (fig. 1). 

Both pegmatite and quartzite are rich in quartz and
86 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

commonly contain little or no biotite, although biotitic 

pegmatite occurs locally. Most pegmatites are composed primarily of quartz, potassium feldspar, and 

muscovite, whereas, quartzite is made up of quartz and 

a minor amount of muscovite. Because of their stable 

mineralogic compositions, quartzite and most pegmatites are very resistant to weathering and often form 

unweathered outcrops. Regolith, where present, is thin 

and patchy and consists of angular rock fragments of 

varying sizes in a sandy matrix that is very low in silt 

and clay. The regolith shows little or no evidence of 

chemical weathering and is usually less than about 1-2 

m thick. The underlying bedrock is fresh, massive, and 

very hard; the quartzite is foliated and layered, 

whereas the pegmatite is not. These rock types range 

in thickness from about 10 cm to 70 m for dikes and 

lenticular bodies of pegmatite to as much as about 

1,000 m for quartzite.

PHYSICAL PROPERTIES AND PEFORMANCE 

CHARACTERISTICS

Very little is known about the grain-size distribution 

and plasticity characteristics of regolith formed from 

pegmatite and quartzite. They may be somewhat similar to those for regolith derived from volcanic and shallow-intrusive rocks.

Pegmatite and quartzite and the overlying regolith 

commonly have the following performance characteristics: The regolith is moderately easy to excavate with 

power equipment, but the fresh bedrock is very difficult to excavate and usually requires blasting. Foundation conditions are excellent on the bedrock and 

generally are good on the regolith. However, conditions may be fair to poor on loose regolith that has 

large voids and a low amount of stone-to-stone contact. 

Permeability tends to be very high in the regolith and 

low along the tight fractures in the bedrock. Slope 

stability is usually good, although rockfalls may 

develop locally on steep slopes and in highly fractured 

rock.

UNITS AFFECTED BY FAULTING AND IGNEOUS 

INTRUSION

GENERAL DESCRIPTION

Crystalline rocks along and within fault zones and 

adjacent to intrusive bodies of Late Cretaceous to 

early Tertiary age commonly have properties that differ greatly from those of the same rock mass beyond 

the influence of tectonic and (or) igneous activity. 

Faults ranging from single fractures to broad shear 

zones hundreds of meters wide are common structural 

features in the Front Range west of Denver (Tweto and

Sims, 1963). Individual faults vary considerably in 

length and width and can end gradually or terminate 

abruptly against unfractured rock. Material within 

fault zones is highly variable and ranges from 

recrystallized breccia to massive blocks of unbroken 

rock bounded by masses of highly sheared and altered 

rock fragments in a matrix of sandy clay. Fault zones 

composed of sheared and altered rock usually form 

saddles or valleys, whereas fault zones that are 

silicified or recrystallized tend to be more resistant 

than the adjacent rock and locally form narrow, linear 

ridges. Veins, dikes, and stocks are commonly emplaced along faults and other zones of weakness. Rocks 

adjacent to these bodies also show differing amounts 

of tectonic deformation and chemical alteration. In 

general, alteration near faults and along contacts with 

intrusive rocks is less extensive in granitic rocks than 

in metamorphic rocks that have high contents of 

biotite, calcic plagioclase, and (or) hornblende.

PHYSICAL PROPERTIES AND PERFORMANCE 

CHARACTERISTICS

Owing to the considerable range in the degree of 

recrystallization, cementation, shearing, and alteration, faulted and intruded rocks commonly have a wide 

array of physical properties and performance characteristics. These properties and characteristics vary 

from those of well-indurated, recrystallized fault breccias, which are similar to unweathered granitic and 

gneissic rocks, to those of highly sheared and altered 

metasedimentary rocks, which are similar to saprolite.

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90 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

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scale 1:24,000.

___1973b, Map showing earth materials that may compact and 

cause settlement in the Golden quadrangle, Jefferson County, 

Colorado: U.S. Geological Survey Miscellaneous Investigations 

Series Map I-761-D. 

_1973c, Map showing man-modified land and man-made

deposits in the Golden quadrangle, Jefferson County, Colorado: 

U.S. Geological Survey Miscellaneous Investigations Series 

Map I-761-E.

Smith, R. O., Schneider, P. A., Jr., and Petri, L. R., 1964, Groundwater resources of the South Platte River basin in western 

Adams and southwestern Weld Counties, Colorado: U.S. 

Geological Survey Water-Supply Paper 1658, 132 p.
92 ENVIRONMENTAL GEOLOGY, FRONT RANGE URBAN CORRIDOR, COLORADO

Soister, P. E., 1965a, Geologic map of the Fort Lupton quadrangle, 

Weld and Adams Counties, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-397, scale 1:24,000.

___1965b, Geologic map of the Hudson quadrangle, Weld and 

Adams Counties, Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ-398, scale 1:24,000.

___1965c, Geologic map of the Platteville quadrangle, Weld 

County, Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ-399, scale 1:24,000.

___1967, Relation of Nussbaum Alluvium (Pleistocene) to the 

Ogallala Formation (Pliocene) and to the Platte-Arkansas 

divide, southern Denver Basin, Colorado, in U.S. Geological 

Survey research 1967: U.S. Geological Survey Professional 

Paper 575-D, p. D39-D46.

___1968a, Geologic map of the Corral Bluffs quadrangle, El Paso 

County, Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ-783, scale 1:24,000.

_____1968b, Geologic map of the Hanover NW quadrangle, El Paso 

County, Colorado: U.S. Geological Survey Geologic Quadrangle 

Map GQ-725, scale 1:24,000.

___1972, Preliminary geologic map and lignite deposits of the 

Strasburg NW quadrangle, Arapahoe and Adams Counties, 

Colorado: U.S. Geological Survey Open-file report, scale 

1:24,000, 3 sheets.

_____1974, A preliminary report on a zone containing thick lignite 

beds, Denver Basin, Colorado: U.S. Geological Survey OpenFile Report 74-27, 46 p. 

.1978, Stratigraphy of uppermost Cretaceous and lower Tertiary rocks of the Denver Basin, in J. W. Pruit, and P. E. Coffin, 

eds., Energy resources of the Denver Basin: Denver, Rocky 

Mountain Association of Geologists, p. 223- 229.

Soule, J. M., 1974, Gravel resources, urbanization, and future land 

use, Front Range Urban Corridor, Colorado, with a section on 

Calculation of gravel reserves, by H. R. Fitch: U.S. Geological 

Survey Open-File Report 74-178, 29 p.

Spencer, F. D., 1961, Bedrock geology of the Louisville quadrangle, 

Colorado: U.S. Geological Survey Geologic Quadrangle Map 

GQ-151, scale 1:24,000.

Story, J. A., and Howell, D. H., 1963, Gypsum deposits of the 

northern Colorado Front Range, in Guidebook to the geology of 

the northern Denver basin and adjacent uplifts: Rocky Mountain Association of Geologists, 14th Field Conference, 1963, p. 

258-263.

Tabor, G. F., Jr., 1969, Problem soils in Colorado, a summary, in 

The Governor's conference on environmental geology: Colorado 

Geological Survey Special Publication 1, p. 53-54.

Taylor, R. B., 1975, Neogene tectonism in south-central Colorado, in 

Bruce Curtis, ed., Cenozoic history of the Southern Rocky 

Mountains: Geological Society of America Memoir 144, p. 

211-226.

Tourtelot, H. A., 1974, Geologic origin and distribution of swelling 

clays: Association of Engineering Geologists Bulletin, v. 11, no. 

4, p. 259-275.

Trewartha, G. T., 1961, The Earth's problem climates: Madison, 

Wis., University of Wisconsin Press, 334 p.

Trimble, D. E., 1975, Geologic map of the Niwot quadrangle, 

Boulder County, Colorado: U.S. Geological Survey Geologic 

Quadrangle Map GQ-1229, scale 1:24,000.

Trimble, D. E., and Fitch, H. R., 1974a, Map showing potential 

sources of gravel and crushed-rock aggregate in the Greater 

Denver area, Front Range Urban Corridor, Colorado: U.S. 

Geological Survey Miscellaneous Investigations Series Map 

I-856-A, scale 1:100,000.

_____1974b, Map showing potential sources of gravel and crushedrock aggregate in the Colorado Springs-Castle Rock area, Front 

Range Urban Corridor, Colorado: U.S. Geological Survey 

Miscellaneous Investigations Series Map I-857-A, scale 

1:100,000.

Trimble, D. E., and Machette, M. N., 1979a, Geology of the Colorado Springs-Castle Rock area, Front Range Urban Corridor, 

Colorado: U.S. Geological Survey Miscellaneous Investigations 

Series Map I-857-F, scale 1:100,000.

___1979b, Geology of the Greater Denver area, Front Range Urban Corridor, Colorado: U.S. Geological Survey Miscellaneous 

Investigations Series Map I-856-H, scale 1:100,000.

Tweto, Ogden, 1975, Laramide (Late Cretaceous-early Tertiary) 

orogeny in the Southern Rocky Mountains, in B. F. Curtis, ed., 

Cenozoic history of the Southern Rocky Mountains: Geological 

Society of America Memoir 144, p. 1-44.

___1976, Preliminary geologic map of Colorado: U.S. Geological 

Survey Miscellaneous Field Studies Map MF-788, scale 

1:500,000, 2 sheets. 

_1977, Tectonic history of west-central Colorado, in H. K.

Veal, ed., Exploration frontiers of the central and southern 

Rockies: Rocky Mountain Association of Geologists, Symposium, Aspen, Colo., 1977, p. 11-22.

Tweto, Odgen, and Sims, P. K., 1963, Precambrian ancestry of the 

Colorado mineral belt: Geological Society of America Bulletin, 

v. 74, no. 8, p. 991-1014.

U.S. Bureau of Mines, 1965, Mineral facts and problems: U.S. 

Bureau of Mines Bulletin 630, 1118 p.

U.S. Bureau of Reclamation, 1974, Earth manual (2nd ed.): 

Washington, D.C., U.S. Government Printing Office, 810 p.

U.S. Geological Survey, 1964, Mineral and water resources of Colorado: U.S. 88th Congress, 2d session, Senate Committee on Interior and Insular Affairs, Committee Print, 302 p.

___1969, Surface water records, pt. 1 of Water Resources data 

for Colorado: [Washington, D.C.] U.S. Geological Survey, 394 p.

U.S. Public Health Service, 1962, Drinking water standards: U.S. 

Public Health Service Publication 956, 61 p.

Van Horn, Richard, 1968, Preliminary surficial geologic map and 

materials test data of the Golden quadrangle, Jefferson County, Colorado: U.S. Geological Survey Open-File report, 3 sheets.

___1972, Surficial and bedrock geologic map of the Golden 

quadrangle, Jefferson County, Colorado: U.S. Geological 

Survey Miscellaneous Investigations Series Map I-761-A, 

scale 1:24,000. 

.1976, Geology of the Golden quadrangle, Colorado: U.S.

Geological Survey Professional Paper 872, 116 p. 

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Bureau of Mines Report of Investigations 5553, 156 p. 

Vanderwilt, J. W., and others, 1947, Mineral resources of Colorado:

Denver, Colorado Mineral Resources Board, 547 p.

Waage, K. M., 1952, Clay deposits of the Denver-Golden area, Colorado: Colorado Scientific Society Proceedings, v. 15, no. 9, p. 

373-390.

___1961, Stratigraphy and refractory clayrocks of the Dakota 

Group along the Northern Front Range, Colorado: U.S. 

Geological Survey Bulletin 1102, 154 p.

Walker, T. R., and Harms, J. C., 1972, Eolian origin of flagstone 

beds, Lyons Sandstone (Permian), type area, Boulder County, 

Colorado: Mountain Geologist, v. 9, nos. 2-3, p. 279-288.

Weimer, R. J., 1973, A guide to uppermost Cretaceous stratigraphy, 

central Front Range, Colorado Deltaic sedimentation, growth 

faulting and early Laramide crustal movement: Mountain 

Geologist, v. 10, no. 3, p. 53-97.
REFERENCES 93

___1976, Cretaceous stratigraphy, tectonics, and energy 

resources, western Denver basin, in R. C. Epis, and R. J. 

Weimer, eds., Studies in Colorado field geology: Professional 

Contributions of Colorado School of Mines, no. 8, p. 180-227.

Weimer, R. J., and Land, C. B., Jr., 1972a, Lyons Formation (Permian), Jefferson County, Colorado A fluvial deposit: Mountain Geologist, v. 9, nos. 2-3, p. 289-297.

___1972b, Field guide to Dakota Group (Cretaceous) stratigraphy, Golden-Morrison area, Colorado: Mountain Geologist, 

v. 9, nos. 2-3, p. 241-267.

Wells, J. D., 1967, Geology of the Eldorado Springs quadrangle, 

Boulder and Jefferson Counties, Colorado: U.S. Geological 

Survey Bulletin 1221-D, p. D1-D85.

Williamson, D. R., 1963, Colorado gypsum and anhydrite: Colorado 

School of Mines Mineral Industries Bulletin, v. 6, no. 2, 20 p.

Wilson, R. F., 1975, Eastern Colorado, in E. D. McKee, E. J. Crosby, 

and others, Paleotectonic investigations of the Pennsylvanian 

System in the United States: U.S. Geological Survey Professional Paper 853, pt. 1, p. 245-264.

Withington, C. F., 1964, Gypsum and anhydrite, in Mineral and 

water resources of Colorado: U.S. 88th Congress, 2d session, 

Senate Committee on Interior and Insular Affairs, Committee 

Print, p. 190-197.

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Mineral and water resources of Colorado: U.S. 88th Congress, 

2d session, Senate Committee on Interior and Insular Affairs, 

Committe Print, p. 181-183.

INDEX

[Italic page numbers indicate major references]

Page

Acoustical-insulation products .............................. 40

Adams County, lignite ................................... 13, 49

oil .............................................................. 51

Aggregate .................................................... 40, 42

Agricultural land ................................. 2, 41, 47, 55

Agricultural soil conditioner ..................... 41, 42, 44

Air Force Academy ........................................... 43

Air quality ........................................................ 11

Alluvial deposits ............................... 12, 68, 74, 75

foundation conditions .................................... 76

Altona, gravel ................................................... 35

Amphibolite, physical properties ..................... 84, 85

ancestral Front Range highland ..................... 18, 28

ancestral Rocky Mountains ............................. 15, 18

Ancient river gravels ......................................... 11

Anhydrite ..................................................... 19, 41

Anticline ........................................................... 28

A. P. Greene Refractories Co. ............................ 32

Apishapa highland ................................... 18, 19, 27

Arapahoe County, lignite .................................... 13

on .............................................................. 51

Arapahoe Formation ..................... 13, 24, 25, 79, 80

clay ....................................................... 38, 40

expandable shale .......................................... 40

ground water ............................................... 64

Arkansas bauxite ............................................... 37

Arkansas River .......................................... 3, 8, 36

flooding ....................................................... 59

Artesian wells ................................................... 64

Artifical fill ................................. 31, 46, 68, 76, 77

Ash flows ............................................... 13, 25, 83

Athletic fields .................................................... 47

Ault ............................................................................... 10

Austin Bluffs ..................................................... 11

B

Backfilling .................................................... 45, 48

Ball clay ........................................................... 37

Barr Lake ......................................................... 66

Batholiths, Southern Rocky Mountains ................. 28

Bear Creek ................................................... 6, 72

flooding ....................................................... 61

gravel .................................................... 34, 36

Bedrock ....................................................... 10, 56

ground water ............................................... 64

Beebe Draw ................................................. 10, 37

gravel ......................................................... 35

ground water ............................................... 63

sand dunes .................................................. 35

Bench-and-valley uplands ............................... 10, 26

Benton Shale .......................................... 13, 23, 58

clay ....................................................... 38, 39

expandable shale .......................................... 40

Bentonite ............................................... 13, 78, 83

Bergen Shale Member ........................................ 14

Big Thompson Canyon, flooding ..................... 59, 61

rainfall ........................................................ 60

Big Thompson River, gravel ............................... 34

Bijou Creek .... ........................................ ............ 38, 40

Blasting ................................ 42, 56, 82, 83, 85, 86

Blowouts ...................................................... 10, 55

Page

Boulder .................................................. 10, 11, 13

expandable shale .......................................... 41

gravel ......................................................... 35

Sundance Formation ..................................... 21

Valmont dike ............................................... 13

Boulder County, clay sources ......................... 38, 39

oil .............................................................. 54

Boulder County Road Department ....................... 35

Boulder Creek ........................................ 34, 35, 47

gravel ......................................................... 35

Boulder Creek Granodiorite ........................... 15, 28

economic minerals ........................................ 42

saprolitic soil ............................................... 57

Boulder Fort Collins Greeley area ........................ 3

Boulder-Weld coal field ....................................... 49

Brick ................................................................ 37

Pierre Shale ................................................ 39

Brighton ............................................................ 10

gravel ......................................................... 34

ground water ............................................... 63

sand dunes .................................................. 26

wildlife refuge ............................................. 66

Broadway Alluvium ........................... 12, 26, 70, 72

building problems ......................................... 73

gravel ......................................................... 33

Buckhorn Creek ............................................ 34, 61

Building stone, Wall Mountain Tuff ..................... 42

See also Dimension stone.

Buildings damage, colluvium ............................... 76

eolian soils ............................................. 55, 68

loess ...................................................... 55, 68

organic soils ................................................ 56

structural ............................................... 54, 83

Bull Lake glaciation, gravel ................................ 33

Buttes .................................................... 10, 11, 85

Cache la Poudre River, gravel terraces .......... 34, 64

Calhan area, clay source ............................... 38, 40

lignite ......................................................... 49

Caliche ......................................................... 36, 37

Canon City ........................................................ 27

Canyon Springs Member ................................ 14, 21

Carlile Shale ..................................... 13, 23, 77, 78

Castle Rock.................................................. 11, 29

ash flows .................................................... 25

building stone .............................................. 44

clay source .................................................. 40

coal ............................................................ 49

gravel terraces ............................................ 64

ground water ............................................... 65

igneous rocks ............................................... 42

Wall Mountain Tuff ...................................... 83

Castle Rock Conglomerate ............ 13, 25, 26, 37, 81

gravel ........................................................ 36

Cement plants ................................................... 44

Cement retardant ............................................... 41

Cherry Creek .................................................... 11

Castle Rock Conglomerate ............................ 37

gravel ......................................................... 36

ground water ............................................... 63

Louviers Alluvium ........................................ 72

water quality ............................................... 62

Page 

Cherry Creek Reservoir ..................................... 31

Cherry Creek Shopping Center ........................... 45

Cheyenne Mountain, rockfall ............................... 58

Chinook winds ................................................... 12

Cities. See Towns.

Clay ............................................................ 31, 37

expandable shale .......................................... 40

mining costs ................................................ 38

soil horizons :............................................... 68

sources ................................................... 38, 39

Clay pits, reclamation ........................................ 47

Clear Creek ...................................................... 26

flooding ....................................................... 61

gravel .............................................. 34, 35, 82

Louviers Alluvium ........................................ 72

reclamation .................................................. 47

Clear Creek Canyon ........................................... 26

Climate ............................................................. 11

Coal ................................................ 23, 48, 77, 79

Castle Rock................................................. 49

Dawson Arkose ............................................ 13

Laramie Formation ...................................... 38

gasification .................................................. 49

liquifaction ................................................... 49

maps ........................................................... 49

subbituminous .............................................. 49

See also Lignite.

Coal Creek Canyon ....................................... 36, 85

Coal Creek Quartzite, economic minerals ........ 35, 42

Coal Creek, clay ................................................ 39

gravel ......................................................... 35

Coal mines .................................................... 2, 49

collapsing ............................................... 49, 51

faulting ....................................................... 31

fires............................................................ 49

Colluvium ......................................... 12, 71, 75, 76

building damage ........................................... 76

Colorado Legislature, land reclamation ................. 45

Colorado mineral belt .............................. 13, 24, 26

Colorado Piedmont ....................................... 3, 6, 8

economic minerals ........................................ 32

lakes ........................................................... 47

Colorado Springs ............................................ 9, 11

building stone .............................................. 44

building techniques ....................................... 49

clay ....................................................... 39, 40

coal field ..................................................... 49

Dawson Arkose ............................................ 25

gravel .............................................. 34, 36, 64

gypsum source ............................................. 41

Lyons Sandstone .......................................... 21

Pierre Shale, clay ........................................ 39

limestone ................................................ 43, 44

sand ...................................................... 12, 55

Colorado Springs-Castle Rock area ....................... 3

Concrete aggregate ................................. 36, 41, 43

Conglomerate ..................................................... 37

Coniferous forest ................................................ 11

Construction problems ........................................ 54

Coors Porcelain Co. ........................................... 32

Crushed stone .............................................. 31, 45

Crystalline rock ................................................. 6

95
96 INDEX

D

Dacono .............................................................. 31

Dakota Group ................................... 14, 22, 23, 82

building stone ......................................... 44, 45

clay source ....................................... 14, 38, 39

ground water ............................................... 65

landslides .................................................... 75

mass wasting ............................................... 58

oil and gas .................................................. 54

sand ...................................................... 44, 45

Dam construction ............................................... 44

Dawson Arkose ...................... 13, 24, 25, 79, 80, 81

building stone .............................................. 44

clay ....................................................... 38, 40

conglomerate ................................................ 37

gravel ......................................................... 36

ground water ............................................... 65

mass wasting ............................................... 58

Debris slides ................................................ 82, 85

Deer Creek anticline .......................................... 83

Denver .. 3, 8, 10, 24, 27, 32, 37-40, 42, 45, 49, 59,

65, 66, 68, 69, 70, 73, 74, 81, 84

clay products ............................................... 37

colluvium ..................................................... 76

heat island .................................................. 12

sand dunes .................................................. 55

Denver Basin ......................................... 26, 27, 29

faults .......................................................... 31

gas ............................................................. 51

oil .............................................................. 51

Denver Brick and Pipe Co. ................................ 32

Denver Coliseum ................................................ 45

Denver Formation .................. 13, 24, 76, 79, 80, 81

clay ....................................................... 54, 80

Green Mountain ........................................... 25

ground water ............................................... 65

lignite ......................................................... 49

mass wasting ............................................... 58

montmorillonite ............................................ 80

Deposits, manmade ............................................ 76

Derby earthquakes ............................................. 66

Dikes ........................................................... 28, 86

Dimension stone ........................... 31, 42, 43, 44, 45

Dinosaur bones .................................................. 13

Disposal sites .................................................... 48

Domestic water supplies ..................................... 66

Douglas County ................................ 26, 37, 38, 39

clay sources ............................................ 38, 39

limestone quarry .......................................... 44

Driveway surfacing ............................................ 43

Dunes .............................................. 10, 12, 21, 68

E

Earth dams ....................................................... 76

Earth fill ..................................................... 31, 76

Earthquakes ......................................................' 66

East Plum Creek ............................................... n

flooding ....................................................... 60

Eaton-Ault area ................................................. 10

Economic minerals ............................................. 31

Boulder Creek Granodiorite .......................... 42

clay ...................................... 14, 31, 37, 33, 39

coal .......................... 13, 23, 38, 48, 49, 77, 79

Coal Creek Quartzite ................................... 42

Colorado mineral belt ................................... 24

Colorado Springs, sand ................................. 26

crushed stone .............................................. 31

dimension stone .................... 31, 42, 43, 44, 45

earth fill ............................................... 31, 75

expandable shale .......................................... 40

feldspar ....................................................... 42

fluorite ...................................................... 42

Fort Hays Limestone Member ...................... 44

Glennon Limestone Member .......................... 43

gravel .................. 10, 31, 32, 33, 34, 36, 47, 64

gypsum ...................................... 19, 41, 77, 32

igneous rocks .......................................... 41 42

Ingleside Formation ..................................... 43

Economic minerals Continued

Kassler ....................................................... 45

kerogen ....................................................... 44

Leadville Limestone ..................................... 43

limestone .......................................... 31, 42, 43

Lyons Sandstone .......................................... 45

Manitou Limestone ....................................... 43

metamorphic rocks .................................. 41, 42

mica ........................................................... 42

Niobrara Formation ...................................... 44

quartz ......................................................... 42

sand .............. 10, 12, 26, 31, 32, 34, 39, 47, 55

sandstone .................................................... 44

shale, expandable .................................... 40, 41

silica ...................................................... 44, 45

Smokey Hill Shale ....................................... 44

supply ......................................................... 32

Table Mountain Shoshonite ........................... 42

vein rock .................................................... 42

Wall Mountain Tuff ...................................... tf

Williams Canyon Limestone .......................... 43

El Paso County, clay sources ............................. 38

Dawson Arkose ............................................ 37

lignite ......................................................... 49

Elbert, rainfall ................................................... 60

Elbert County, clay sources ................................ 38

England, ball clay .............................................. 37

Englewood, sand dunes ...................................... 55

Environmental Lime Co. .................................... 32

Environmental stress ...................................... 2, 45

Eolian sand ........................................ 2, 64, 68, 69

Eolian sediments ................................................ 10

Eolian soils ....................................................... 55

Erie, gravel ...................................................... 35

Erosion surface .................................................. 56

Erosion, extent .................................................. 68

flooding ....................................................... 59

loess ........................................................... 55

metamorphic rocks ....................................... 84

Evergreen ........................................................... 6

gravel deposits ............................................ 82

Expandable shale .......................................... 40, 41

Expansive clay .................................................... 2

Expansive soils .................................................. 54

Falcon Limestone Member .................................. 14

Fault zones, soil formation ................................. 57

weathering .................................................. 84

Faulting, coal mines ........................................... 31

Dacono ........................................................ 31

Firestone ..................................................... 31

Fox Hills Sandstone ..................................... 31

Frederick .................................................... 31

Laramie Formation ...................................... 31

Pierre Shale ................................................ 31

Rampart Range fault blocks ......................... 25

Rocky Flats ................................................ 31

Faults ......................................... 23, 26, 29, 66, 86

Denver Basin .............................................. 31

Golden ................................................... 28, 29

Jarre Creek ................................................. 28

Ralston shear zone ....................................... 31

Rampart Range ...................................... 28, 29

Ute Pass................................................ 26, 28

Feldspar ............................................................ 42

Fires, coal mine ................................................ 49

Firestone ........................................................... 31

Flagstone .......................................................... 45

Flash floods ....................................................... 60

Flatirons, The ................................................... 21

Flood plains ............................................. 9, 47, 55

alluviums ..................................................... 70

Flooding ....................................................... 47, 59

Florida, kaolinite ................................................ 37

Fluorite ............................................................. 42

Foehn winds ...................................................... 12

Foothill ridges ..................................................... 6

Forelle Limestone Member ................................. 14

Forest, Palmer divide ........................................ 11

Formations ........................................................ 12

See also individually named units. 

Fort Carson Military Reservation, Niobrara

Formation ....................................... 44

Fort Collins .................................................. 10, 29

cement plant ............................................... 44

gravel .................................................... 34, 64

Fort Hays Limestone Member ........... 23, 44, 82, 83

Fort Lupton area, gravel ................................... 34

Fossil plants ...................................................... 13

Foundation conditions .................... 3, 13, 77, 83, 85

Fountain Creek .......................................... 8, 9, 11

clay, overburden .......................................... 38

gravel .................................................... 36, 64

gypsum source ............................................. 41

temperature inversion ................................... 12

Fountain Formation ................................. 14, 19, 82

building stone .............................................. 44

ground water ............................................... 65

landslides .................................................... 75

Four-mile Canyon Creek ...................................... 14

Fox Hills aquifer ............................................... 29

Fox Hills Sandstone .......................... 13, 23, 79, 80

building stone .............................................. 44

faulting ....................................................... 31

ground water .......................................... 64, 65

Franktown, water quality ................................... 62

Frederick .......................................................... 31

Front Range ....................................... 3, 6, 27, 28

Front Range highland ................................... 19, 23

Front Range uplift ........................................ 26, 31

Frost heaving ......................................... 69, 76, 83

G

Garden of the Gods ...................................... 21, 41

Gas........................................................ 46, 51, 77

Dakota Group .............................................. 54

Denver Basin .............................................. 31

South Platte Formation ................................ 54

Wattenberg field .......................................... 54

Gasification, coal ................................................ 49

Geographic area ................................................... 3

Glaciers ............................................................. 26

Glen Comfort, rainfall ........................................ 60

Glen Eyrie Shale Member .................................. 14

Glen Eyrie, gypsum source ................................. 41

Glennon Limestone Member ........................... 14, 43

Gneiss .............................................. 42, 83, 84, 85

Golden .............................................................. 10

clay ........................................... 13, 37, 38, 40

expandable shale .......................................... 41

igneous rocks .................................... 13, 25, 42

Golden fault ............................................ 28, 29, 66

Golden-Morrison area, crossbedded sandstone ....... 21

Golf courses ...................................................... 47

Grading............................................................. 76

Graneros Shale ............................ 13, 14, 22, 77, 78

clay ............................................................ 39

Granite .................................................. 83, 84, 85

Granodiorite ............................................ 83, 84, 85

Grassland .......................................................... 11

Gravel................................... 10, 12, 26, 31, 32, 82

ancient river................................................ 11

Broadway Alluvium ...................................... 33

Bull Lake glaciation ..................................... 33

Colorado Springs .......................................... 34

Louviers Alluvium ........................................ 33

Pinedale glaciation ........................................ 33

Pleistocene deposits ...................................... 36

Riley Mound ................................................ 37

ground water .......................................... 47, 64

legislation .................................................... 32

sources ........................................................ 34

Gravel pits .................................................... 2, 47

reclaimed .................................... 33, 45, 47, 76
INDEX 97

Page

Great Plains .................................................. 3, 25

Greater Denver Area ........................................... 3

gravel ......................................................... 32

sand ........................................................... 32

Greeley .............................................. 9, 10, 11, 29

gravel .................................................... 34, 64

sand dunes ............................................. 26, 55

Green Mountain, Denver Formation ..................... 25

landslides .................................................... 75

Green Mountain Conglomerate ............ 13, 25, 37, 79

Greenhorn Limestone ........................ 13, 22, 77, 78

Ground. See Soils.

Ground heaving ................................................. 54

Ground water ............................................... 47, 63

coal mining .................................................. 49

pollution potential ................................... 46, 65

See also Hydrology.

Gypsum ............................................ 19, 41, 77, 82

H

Hording Sandstone ........................................ 14, 18

Harriman Shale Member ..................................... 14

High Plains ..................................................... 3, 8

Highway embankments ....................................... 76

History, geologic ................................................ 15

Hogback belt ............................................ 6, 23, 75

economic minerals ........................................ 32

Holocene alluvium ............ ................................. 26

Hydraulic gradient, changes ................................ 47

Hydrologie amenities .......................................... 62

Hydrology ......................................................... 58

Hygiene Sandstone Member ................................ 13

I

Igneous activity ............................................ 23, 28

Igneous rocks ........................ 13, 25, 41, 42, 83, 85

Industrial wastes, limestone ................................ 43

Ingleside Formation ................................ 14, 20, 43

Interdrainage ridges ........................................... 11

Intrusive masses ........................................... 13, 23

Ironstone concretions .......................................... 13

Irrigation ditches ............................................... 76

"J" sandstone, oil and gas .................................. 54

Jarre Creek fault ............................................... 28

Jefferson County ........................................... 65, 66

clay sources ............................................ 38, 39

gas ............................................................. 54

oil .............................................................. 54

Jelm Formation ............................................ 14, 21

Jimmy Camp Creek ........................................... 11

Julesburg .......................................................... 29

K

Kaolinite ...................................................... 37, 39

Kassler, building stone ....................................... 45

ground water ............................................... 63

lignite ......................................................... 13

sand quarry ................................................. 45

Kiowa-Bijou Creeks, clay source .................... 38, 40

La Salle area, gravel ......................................... 34

Lakes .................................................... 45, 47, 66

Land development, grading ................................. 76

water runoff ................................................ 62

Land reclamation ............................................... 45

Land use, physical properties ............................. 67

Land-use planning ................................................ 2

Page

Landfills ............................................................ 76

gas problems ............................................... 46

maps ........................................................... 46

Landforms ........................................................... 6

Landscaping rock ............................................... 42

Landslide deposits ......................................... 12, 75

foundation conditions .................................... 76

Landslides ......................... 8, 55, 57, 58, 63, 66, 75

See also Mass wasting.

Laporte, cement plant ........................................ 44

Laramide mineral belt ........................................ 24

Laramide mountains ........................................... 15

Laramide orogeny ........................ 15, 21, 23, 24, 28

Laramie Formation ................ 13, 23, 24, 79, 80, 83

clay source ............................................. 38, 39

coal ....................................................... 38, 49

faulting ....................................................... 13

ground water .......................................... 64, 65

mass wasting ............................................... 58

sand ........................................................... 39

subsidence ................................................... 49

Laramie Mountains ............................................. 27

Laramie aquifer ................................................. 29

Larimer County, gypsum source ......................... 41

oil .............................................................. 51

Laramie-Fox Hills aquifer .................................. 49

Larimer Sandstone Member ................................ 13

Larkspur ........................................................... 36

late Eocene erosion surface ................................ 25

Latite ............................................................... 85

Lava flows ................................................... 13, 25

Laws .................................................................. 2

Leach-field waste-disposal systems ....................... 65

Leachates .......................................................... 46

Leadville Limestone ................................ 14, 18, 43

Left Hand Creek ............................................... 34

gravel ......................................................... 35

Liggett, gravel .................................................. 35

Lignite ............................................. 48, 49, 77, 79

Lime ponds, byproducts ..................................... 43

Limestone .................................................... 31, 42

byproducts ................................................... 43

Liquifaction, coal ................................................ 49

Little Fountain Creek ........................................ 42

Little Thompson River, gravel ....................... 34, 37

Littleton, ground water ...................................... 63

Loess .......................... 2, 12, 55, 68, 69, 71, 74, 76

building damage ........................................... 68

ground water ............................................... 69

Longmont .......................................................... 37

brick kilns ................................................... 37

gravel ......................................................... 34

Louviers Alluvium ............................. 12, 26, 70, 72

building problems ......................................... 73

gravel ......................................................... 33

weathering .................................................. 68

Louviers, clay source ......................................... 40

Loveland ........................................................... 29

gravel ......................................................... 34

gypsum source ............................................. 41

Low rolling uplands ........................................... 10

Lower Latham Reservoir ............................... 35, 63

Lykins Formation ................................... 14, 21, 82

gypsum ....................................................... 41

limestone ..................................................... 43

mass wasting ............................................... 58

Lyons ............................................................... 21

Niobrara Formation ...................................... 44

cement plant ............................................... 44

gravel ......................................................... 34

igneous rocks .......................................... 13, 42

Lyons Sandstone ............................... 14, 20, 82, 83

building stone ......................................... 44, 45

ground water ............................................... 63

landslides .................................................... 75

oil and gas .................................................. 54

Lytle Formation ............................... 14, 22, 82, 83

ground water ............................................... 65

M Page

McNichols Arena ................................................ 45

Mafic dikes ........................................................ 83

Manitou Limestone .................................. 14, 18, 43

Manitou Springs ................................................. 15

limestone ..................................................... 43

Manmade deposits ........................ 31, 46, 68, 76, 77

Mantled lowlands ............................................... 10

Maps .............................................................. 3, 6

coal ............................................................ 49

earthquake risk ............................................ 66

faults .......................................................... 67

flooding ....................................................... 59

lakes ........................................................... 66

landfills ....................................................... 46

landslide ...................................................... 58

mass wasting ............................................... 58

rock units physical properties ....................... 67

Marshall, coal mine fires .................................... 49

Mass wasting ................................................ 57, 59

Mesa............................................................... 10

Mesas ............................................................... 11

Metallurgical processes ....................................... 42

Metamorphic rocks .................................. 15, 41, 83

economic minerals ........................................ 42

Methane gas ...................................................... 46

Mica ................................................................. 42

Migmatite ............................................... 83, 84, 85

Mile High Stadium ............................................. 45

Milton Reservoir, ground water .......................... 63

Mineral extraction ................................................ 3

Mineral resources. See Economic minerals.

Minerals, relative stability .................................. 84

Mining, surface effects ....................................... 49

See also Economic minerals. 

Missouri fire clay ............................................... 37

Monocline .......................................................... 28

Montmorillonite ....................................... 13, 54, 80

Monument ......................................................... 36

Monument Creek ............................................... 11

gravel ......................................................... 36

Monuments ........................................................ 42

Monzonite .......................................................... 85

Morrison, Arapahoe Formation, clay .................... 40

gypsum .................................................. 41, 82

Morrison Formation ........................... 14, 21, 82, 83

mass wasting ............................................... 58

Mortar ......................................................... 42, 44

Mount Vernon Canyon, flooding .......................... 61

Mountain building .............................................. 15

Mountain land, ground water .............................. 65

Muddy Sandstone, oil and gas ............................ 54

Mudflows ...................................................... 58, 63

N

Niobrara Formation ...................... 13, 23, 43, 77, 78

economic minerals ........................................ 44

oil and gas .................................................. 54

North Carolina, kaolinite .................................... 37

North Table Mountain ....................... 10, 13, 25, 83

economic minerals ........................................ 42

landslides .................................................... 75

Nussbaum alluvium ................................. 12, 26, 73

0

oa .................................................................... 51

Dakota Group .............................................. 54

Denver Basin .............................................. 31

"J" sandstone .............................................. 54

Smoky Hill Shale ......................................... 44

South Platte Formation ................................ 54

Wattenburg field .......................................... 54

Open pits, reclamation ....................... 33, 45, 47, 76

Open space ........................................................ 47

Organic loam ....................................................... 2

Orogenies .......................................................... 15

Orographic effect ............................................... 12

Overbank scour .................................................. 59
98 INDEX

Palmer Lake ................................................ 36, 60

Palmer Park ...................................................... 11

Palmer divide ...................................... 8, 9, 11, 12

defined ........................................................ 36

Park development .............................................. 45

Parker, clay source ............................................ 40

Dawson Arkose ............................................ 37

Parking lots ...................................................... 47

Parks. See Open space.

Paving blocks .................................................... 42

Pediment alluviums ....................................... 73, 74

Pediment gravels .......................................... 36, 63

Pediments ......................................................... 56

Peerless Dolomite ................................... 14, 18, 43

Pegmatite ..................................................... 15, 85

Perry Park ........................................................ 21

ground water ............................................... 65

gypsum source ............................................. 41

limestone ..................................................... 43

Petroleum. See Oil.

See also Gas.

Piceanee Creek basin, oil.................................... 51

Piedmont lowlands ............................................... 9

Piedmont uplands ............................................... 10

Pierre Shale .................... 13, 23, 24, 31, 77, 78, 79

clay ................................................. 38, 39, 54

expandable shale .......................................... 40

faulting ....................................................... 31

mass wasting ............................................... 58

Pikes Peak batholith .......................................... 28

gravel source ............................................... 34

Pikes Peak Granite ................................. 14, 15, 24

clay source .................................................. 40

economic minerals ........................................ 42

gravel .................................................... 36, 37

saprolitic soil ............................................... 57

weathering .................................................. 84

Pine, gravel deposits .......................................... 82

Pine Butte Member ...................................... 14, 21

Pinedale glaciation, gravel .................................. 33

Piney Creek Alluvium ....................... 12, 26, 70, 72

building problems ......................................... 72

soil ............................................................. 55

Pit ponds, potential uses ................... 33, 45, 47, 76

Plaster mill, Loveland ........................................ 41

Plaster of parts ................................................. 41

Plasterboard ...................................................... 41

Platteville area, gravel ....................................... 34

ground water ............................................... 63

Pleistocene time ................................................. 26

Pluin Creek, Castle Rock Conglomerate ............... 37

gravel ......................................................... 36

Pollution....................................... 2, 11, 45, 62, 65

Porcelains .......................................................... 37

Portland, cement plant ....................................... 44

Portland cement............................................ 42, 44

post-Piney Creek alluvium ....................... 12, 70, 71

building problems ......................................... 72

Pottery clay ...................................................... 37

pre-Rocky Flats Alluvium ................................... 12

Precambrian time ............................................... 15

Precast construction panels ................................. 42

Precipitation ...................................................... 12

Pseudokarst topography ...................................... 49

Pueblo, Colo. ..................................................... 27

clay products .......................................... 37, 39

colluvium ..................................................... 76

Pyrite, Smokey Hill Shale .................................. 44

Q

Quarry reclamation ............................ 33, 45, 47, 76

Quartz .............................................................. 42

Quartz monzonite .................................... 13, 84, 85

Quartzite ........................................................... 85

Quicklime .......................................................... 43

R

Rain ................................................ 12, 59, 60, 62

damage ....................................................... 63

Ralston Creek Formation ................... 14, 21, 82, 83

gypsum .................................................. 41, 82

Ralston dike ................................................. 13, 42

Ralston Reservoir ......................................... 13, 21

igneous rocks .................................... 13, 25, 83

Ralston shear zone ............................................. 31

Rampart Range ................................ 15, 18, 25, 26

ground water ............................................... 65

Rampart Range fault ......................................... 28

Rampart summit ................................................ 25

Reclamation, open pits ...................... 33, 45, 47, 76

Red Rocks Park ............................................ 6, 21

Redstone Creek, gravel ...................................... 34

References ......................................................... 86

Regolith ............................................................ 84

Rhyodacite ........................................................ 85

Rhyolite ............................................................ 85

Richard Sandstone Member ................................. 13

Riley Mound, gravel .......................................... 37

Riprap ........................................ 31, 42, 43, 44, 45

Road fill ........................................................... 44

Road metal .................................................. 42, 43

Robinson Brick and Tile Co. .............................. 32

Rock formations ............................................ 12, 67

Rockfalls ...................................... 8, 57, 82, 85, 86

Rockslides ......................................................... 85

Rocky Flats ...................................................... 10

expandable shale .......................................... 41

faulting ....................................................... 31

gravel ......................................................... 36

ground water ............................................... 63

landslides .................................................... 75

Rocky Flats Alluvium ....................... 12, 26, 73, 74

buried stream channels ................................. 74

ground water ............................................... 64

Rocky Mountain Arsenal, earthquakes ................. 66

Rocky Ridge Sandstone Member ......................... 13

Roofing granules ................................................ 42

Roxborough Park ............................................... 21

Runoff .............................................................. 62

St. Vrain Creek ................................................ 35

gravel ......................................................... 34

reclamation .................................................. 47

San Juan Basin, gas .......................................... 51

Sand ........................................... 10, 12, 31, 32, 55

Laramie Formation ...................................... 39

ground water ............................................... 47

legislation .................................................... 32

sources ........................................................ 34

Sand Creek, ground water ................................. 63

Sand dunes ................................. 21, 26, 35, 55, 68

ground water ............................................... 64

Sandstone .......................................................... 44

Sandstone bluffs ................................................. 11

Sangamon Interstadial, faulting ........................... 66

Saprolite ........................................................... 84

Satanka Formation ........................................ 14, 21

Sawatch Sandstone ........................................ 14, 18

building stone .............................................. 44

Schist .................................................... 83, 84, 85

Seas ...................................................... 15, 18, 21

Sedimentary rocks .............................................. 77

Seismicity ...................................................... 3, 66

Selenite, Smokey Hill Shale Member ................... 13

Septic tanks ...................................................... 65

Sequential land use ............................................ 45

Sewage treatment, limestone ............................... 43

Sewer pipe ........................................................ 37

Shale, expandable ......................................... 40, 41

Sheetflooding ................................................ 59, 63

Page

Sheetwash ......................................................... 75

Sherman pluton ................................................. 28

Shopping centers, land reclamation ...................... 45

Shoshonite ......................................................... 85

Silica sand ......................................................... 45

Sills .................................................................. 28

Silurian rocks, Wyoming-Colorado border ............. 15

Silver Plume Granite, gravel source .................... 34

Silver Plume Quartz Monzonite ................ 14, 15, 42

gravel ......................................................... 35

Silver Plume pluton ........................................... 28

Site rehabilitation ............................................... 45

Slaked lime ....................................................... 43

Slocum Alluvium ............................... 12, 26, 73, 74

ground water ............................................... 64

Slope deposits ............................................... 68, 75

Slope failure ...................................................... 55

Slope stability ...................................................... 3

Slope wash ........................................................ 75

Smoky Hill Shale Member ............................. 13, 78

economic minerals ........................................ 44

expansion .................................................... 77

Snow ................................................................ 12

Soil chemistry ...................................................... 2

Soil creep .......................................................... 57

Soil development ................................................ 74

Soil horizons ........................................... 68, 74, 75

Cca ............................................................. 64

Soil problems ..................................................... 54

Soil stabilizers ................................................... 43

Soils ............................................ 3, 10, 54, 55, 57

eolian, land development............................... 55

mountain ..................................................... 56

residuum ..................................................... 56

saprolitic ..................................................... 57

transported alluvium-colluvium ....................... 56

Solifluction ......................................................... 57

South Boulder Creek, flooding ............................ 61

gravel ......................................................... 35

South Park ........................................................ 25

South Platte-Arkansas Rivers divide, clay source . 40

South Platte Formation ..................... 14, 22, 82, 83

building stone .............................................. 45

ground water ............................................... 65

oil and gas .................................................. 54

South Platte River .......................... 3, 8, 12, 36, 42

alluviums ................................................ 70, 72

clay, overburden .......................................... 38

flooding ....................................................... 59

former channel ............................................. 10

gravel .............................................. 34, 36, 64

ground water ............................................... 63

South Platte valley ............................................ 10

South Table Mountain ....................... 10, 13, 25, 83

economic minerals ........................................ 42

landslides .................................................... 75

Southern Rocky Mountains ................... 3, 6, 26, 28

flooding ....................................................... 59

Spoil piles ......................................................... 76

Springs, gravel terraces ..................................... 64

Steep-sided buttes .............................................. 85

Stocks ............................................................... 86

Stone quarries, reclamation ................................. 47

Strain Shale Member ......................................... 14

Stream channels, buried .......................... 35, 63, 74

Streams, erosion ................................................ 26

Streets, on map ................................................... 6

Strip mining ...................................................... 49

Structure, Front Range ...................................... 28

geologic ....................................................... 26

Subbituminous coal ............................................. 49

Subsidence .......................................... 2, 46, 48, 49

Sugar refining .............................................. 42, 43

Sundance Formation ...................................... 14, 21

Surficial deposits, physical properties ................... 67

Swell pressures .................................................. 54

Swelling ground, Denver Formation .................... 13
INDEX 99

T Page

Table Mountain .................................................. 10

gravel .................................................... 35, 37

Table Mountain Shoshonite ................................. 25

Talus ........................................................... 12, 75

foundation conditions .................................... 76

Teepee buttes, Pierre Shale ................................ 13

Temperature inversions ....................................... 12

Tennessee, ball clay ........................................... 37

Terrace alluviums ............................................... 70

Terraces ........................................................ 9, 26

ground water ............................................... 64

Terry Sandstone Member ................................... 13

Thermal-insulation products ................................. 40

Tile .................................................................. 37

Tonalite ............................................................. 15

Topographic map .................................................. 3

Towns, Altona ................................................... 35

Ault............................................................ 10

Boulder ..................... 10, 11, 13, 21, 35, 41, 64

Brighton ................................ 10, 26, 34, 63, 66

Calhan ............................................. 38, 40, 49

Castle Rock . 3, 11, 25, 40, 42, 44, 49, 64, 65, 83 

Colorado Springs .......... 3, 9, 11, 12, 21, 25, 34,

36, 39, 40, 41, 43, 44, 49, 55, 64, 81 

Dacono ........................................................ 31

Denver .. 3, 8, 10, 12, 24, 27, 32, 37-40, 42, 45, 49,

55, 59, 65, 66, 68, 69, 70, 73, 74, 76,

81,84

Eaton ......................................................... 10

Elbert ......................................................... 60

Englewood ................................................... 55

Erie ............................................................ 35

Evergreen ............................................... 6, 82

Firestone ..................................................... 31

Fort Collins ........................... 10, 29, 34, 44, 64

Fort Lupton ................................................ 34

Franktown ................................................... 62

Frederick .................................................... 31

Glen Comfort............................................... 60

Golden ................. 10, 13, 25, 37, 38, 40, 41, 42

Greeley ................. 9, 10, 11, 26, 29, 34, 55, 64

Julesburg .................................................... 29

Kassler ............................................ 13, 45, 63

Laporte ....................................................... 44

Towns Continued Page

Larkspur ..................................................... 36

La Salle ...................................................... 34

Liggett ....................................................... 35

Littleton ...................................................... 63

Longmont ............................................... 34, 37

Louviers ...................................................... 40

Loveland .......................................... 29, 34, 41

Lyons ............................. 13, 20, 21, 34, 42, 44

Manitou Springs ...................................... 15, 43

Marshall ...................................................... 49

Monument ................................................... 36

Morrison .......................................... 40, 41, 82

Palmer Lake .......................................... 36, 60

Parker ................................................... 37, 40

Pine ............................................................ 82

PlatteviDe ............................................... 34, 63

Portland ...................................................... 44

Pueblo ....................................... 27, 37, 39, 76

Valmont ................................................. 37, 42

Watkins ...................................................... 49

Wheat Ridge ............................................... 47

Windsor ...................................................... 34

Trash dumps ..................................................... 76

Turkey Creek, gravel ......................................... 36

U

U.S. Air Force Academy ................................... 10

Uncompahgre highland ........................................ 18

Unified Soil Classification ................................... 77

Upland erosion surfaces ...................................... 56

Upland terraces, gravel ...................................... 36

Urban mineral resources ..................................... 31

Urban planning ................................................... 2

Urbanization ......................................- . 62

Ute Pass, building stone .................................... 44

Ute Pass fault .............................................. 26, 28

ground water ............................................... 65

Valley-fill alluvium, water ................................... 63

Valmont dike ..................................................... 13

Page

Valmont, brick kilns ........................................... 37

igneous rocks ............................................... 42

Van Howe Ceramic Supply Co. .......................... 32

Vein rocks ................................................... 42, 86

Verdos Alluvium ............................... 12, 26, 73, 74

faulting ....................................................... 66

ground water ............................................... 64

Visual blight ...................................................... 45

Volcanic vents, Wyoming-Colorado border ............ 15

Volcanoes ............................................... 24, 25, 28

W

Wall Mountain Tuff........................... 13, 25, 26, 83

economic minerals ........................................ 1$

gravel ......................................................... 36

Waste disposal ......................... 3, 46, 48, 56, 65, 76

Water, ground ........................................ 58, 63, 78

pollution ........................................................ 2

runoff ......................................................... 62

supply, foothills ........................................... 56

table ........................................................... 33

See also Ground water.

Watkins, lignite ................................................. 49

Wattenberg field, gas ......................................... 54

Weathering ............................................. 68, 84, 85

fault zones .................................................. 84

Weld County, gas .............................................. 54

oil .............................................................. 51

Wet Monument Creek ........................................ 37

Wheat Ridge, reclamation ................................... 47

Wildlife refuges ................................................. 66

WiDiams Canyon Limestone ..................... 14, 18, 43

Wind-blown sand ................................................ 10

Winds .......................................................... 12, 26

Windsor Lake, sand dunes ................................. 55

Windsor, gravel ................................................. 34

Windy Hill Member ........................................... 14

Wyoming State line ........................................... 27

Wyoming ........................................................... 29

Wyoming-Colorado border, volcanic vents ............. 15

Yarmouth Interstadial, faulting

* U.S. GOVERNMENT PRINTING OFFICE: 676-041 1982