Geologic Map of the Estes Park 30’ x 60’ Quadrangle, 

North-Central Colorado

By James C. Cole and William A. Braddock

Pamphlet to accompany Scientific Investigations Map 3039

U.S. Department of the Interior

U.S. Geological Survey
Suggested citation: Cole, J.C., and Braddock, W.A., 2009,

Geologic map of the Estes Park 30’ x 60’ quadrangle,

north-central Colorado: U.S. Geological Survey Scientific 

Investigations Map 3039, 1 sheet, scale 1:100,000,

pamphlet, 56 p.

Multiply By To obtain

centimeters (cm) 0.3937 inches (in.)

meters (m) 3.281 feet (ft)

kilometers (km) 0.6214 miles (mi)

CONVERSION FACTORS

To convert Celsius (°C) to Fahrenheit (°F), use formula (°C x 1.8) + 32
Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................2

Geography and Geomorphology ........................................................................................................3

Compilation Sources and Methods ...................................................................................................5

Acknowledgments................................................................................................................................5

Description of Map Units..............................................................................................................................6

Alluvial Units..........................................................................................................................................6

Glacial Units...........................................................................................................................................7

Other Surficial Units .............................................................................................................................8

Post-Laramide Sedimentary Rocks ...................................................................................................8

Post-Laramide Intrusive Rocks ..........................................................................................................8

Post-Laramide Volcanic Rocks...........................................................................................................9

Laramide Sedimentary and Volcanic Rocks.....................................................................................9

Laramide Intrusive Rocks..................................................................................................................10

Pre-Cenozoic Sedimentary Rocks ...................................................................................................10

Paleozoic(?) Intrusive Rocks.............................................................................................................12

Mesoproterozoic Intrusive Rocks....................................................................................................13

Mesoproterozoic and (or) Paleoproterozoic Intrusive Rocks .....................................................14

Paleoproterozoic Intrusive Rocks....................................................................................................14

Paleoproterozoic Metamorphic Rocks ...........................................................................................15

Geologic Setting...........................................................................................................................................15

Paleoproterozoic Layered Rocks .....................................................................................................16

Paleoproterozoic Intrusive Rocks....................................................................................................18

Paleoproterozoic Metamorphism ....................................................................................................19

Mesoproterozoic Intrusive Rocks....................................................................................................22

Kimberlite .............................................................................................................................................23

Pre-Cenozoic Sedimentary Rocks ...................................................................................................25

Laramide Synorogenic Sedimentary Rocks ...................................................................................26

Laramide Intrusive Rocks..................................................................................................................27

Post-Laramide Sedimentary Rocks .................................................................................................27

Post-Laramide Intrusive and Volcanic Rocks ................................................................................29

Fluvial Gravel .......................................................................................................................................29

Quaternary Deposits ...................................................................................................................................29

Alluvial Deposits..................................................................................................................................29

Diamicton .............................................................................................................................................32

Glacial Deposits ..................................................................................................................................32

Other Surficial Deposits ....................................................................................................................33

Structure........................................................................................................................................................34

Paleoproterozoic Structures ............................................................................................................34

Mesoproterozoic Structures ............................................................................................................35

Longs Peak-St Vrain Batholith.................................................................................................35

Moose Mountain Shear Zone..................................................................................................36
iv

Laramide Orogeny...............................................................................................................................37

Eastern Margin of the Front Range.........................................................................................37

Boulder-Weld Fault Zone..........................................................................................................38

Western Margin of the Front Range.......................................................................................38

Sandstone Dikes ........................................................................................................................39

Post-Laramide Structures ................................................................................................................39

Braddock Peak Intrusive-Volcanic Complex.........................................................................40

Colorado River Fault Zone ........................................................................................................40

Block-Glide Landslides .............................................................................................................41

Late Miocene-Pliocene Uplift and River Incision...................................................................................41

Economic Geology.......................................................................................................................................44

Colorado Mineral Belt........................................................................................................................44

Coal, Oil, and Gas................................................................................................................................45

Sand and Gravel..................................................................................................................................46

Quarry Stone........................................................................................................................................46

Water ..................................................................................................................................................46

Environmental Geology...............................................................................................................................47

Landslides, Rockfalls, and Debris Flows.........................................................................................47

Expansive Soils....................................................................................................................................48

Floods ..................................................................................................................................................48

References Cited..........................................................................................................................................49

Figures

1. Map showing geographic setting of the Estes Park quadrangle ...................................................3

2. Map showing physiographic zones in the region of the Estes Park quadrangle.........................4

3. Map showing regional geologic setting of the northern Front Range.........................................17

4. Photographs and map showing Paleoproterozoic metamorphism in the

 northern Front Range ...........................................................................................................................21

5. Photographs showing intrusive relations of the Longs Peak-St Vrain batholith........................24

6. Map showing Laramide intrusions in the Colorado Mineral Belt and mining districts.............28

7. Panoramic photograph showing fluvial terrace deposits..............................................................31

8. Photograph showing glacial moraines in the Roaring Fork valley ...............................................32

9. Map showing major structural elements of the Longs Peak-St Vrain batholith ........................36

10. Sketch map showing geologic relations and geophysical anomalies in the

 Never Summer Mountains ..................................................................................................................42

11. Maps showing structural features of the Neogene Southern Rocky Mountains......................44

12. Map showing Miocene(?) high-level gravels and inferred paleodrainage of the

 Big Thompson River..............................................................................................................................45
Geologic Map of the Estes Park 30’ x 60’ Quadrangle, 

North-Central Colorado

By James C. Cole and William A. Braddock1

Abstract 

The rocks and landforms of the Estes Park 30′ × 60′

quadrangle display an exceptionally complete record of 

geologic history in the northern Front Range of Colorado. 

The Proterozoic basement rocks exposed in the core of the 

range preserve evidence of Paleoproterozoic marine sedimentation, volcanism, and regional soft-sediment deformation, 

followed by regional folding and gradational metamorphism. 

The metasedimentary rocks of the Estes Park quadrangle 

are distinct within northern Colorado for preserving the 

complete metamorphic zonation from low-grade chloritemuscovite phyllites, through middle greenschist-grade rocks 

with sequential aluminous porphyroblasts, to partially melted 

gneisses that contain high-grade cordierite and garnet in the 

non-melted residues. Regional and textural evidence shows 

that the widespread metamorphism was essentially concurrent 

with intrusion of the Boulder Creek Granodiorite and related 

magmas and with the peak of deformation in the partially 

melted high-grade rocks. The metamorphic thermal pulse 

arrived later following the peak of deformation in the physically higher, cooler, low-grade terrane.

Mesoproterozoic time was marked by intrusion of 

biotite granite in the Longs Peak-St Vrain batholith, a complex, irregular body that occupies nearly half of the core of 

the Front Range in this quadrangle. The magma was dry 

and viscous as it invaded the metamorphic rocks and caused 

wholesale plastic folding of the wall rock structure. Steep 

metamorphic foliation that resulted from the Paleoproterozoic 

deformations was bowed upward and re-oriented into flatlying attitudes as the crystal-rich magma rose buoyantly and 

spread out in the middle crust. Magma invaded the schists and 

gneisses along weak foliation planes and produced a characteristic sill-upon-sill intrusive fabric, particularly in the higher 

parts of the batholith. Broad, open arches and swales that are 

defined by the flow-aligned feldspar foliation of the granite, as 

well as by compositional banding in the intruded and included 

metamorphic rocks, formed late during batholith emplacement 

due to rising, buoyant magma and sinking, dense wall rocks. 

The Longs Peak-St Vrain batholith was intruded into crust 

that was structurally neutral or moderately extending in an 

east-northeast direction. A broad zone of mylonite, the Moose 

Mountain shear zone, formed within the batholith during the 

final stages of consolidation as a result of differential buoyancy between the magma and dense wall rock, not as a result 

of regional tectonic deformation.

Direct evidence of the late Paleozoic Ancestral Rocky 

Mountains uplift is not present in this quadrangle, but the 

erosion of those highlands is recorded by Pennsylvanian and 

younger strata that unconformably overlie the Proterozoic 

basement rocks. The Phanerozoic sediments indicate a steady 

progression of fluvial, eolian, and lacustrine environments 

throughout most of the Mesozoic Era during times of relatively slow sediment accumulation. Early Cretaceous time 

was marked by incursion of the Western Interior seaway, a 

shallow-water marine embayment that persisted throughout 

the latter part of the Mesozoic Era. Cretaceous strata consist 

of abundant shale interbedded with relatively minor fine sandstone and limestone. Sedimentation rates increased markedly 

in the latter part of this period during downwarping related to 

distant crustal loading by thrusting along the western continental margin.

Mountain building resumed in this region during latest 

Cretaceous time with onset of the Laramide orogeny. This 

deformation produced regional basement uplifts bounded by 

steep faults. The eastern margin of the Front Range is marked 

by high-angle reverse faults and drape folds in the Phanerozoic strata, and by the deep syncline of the Denver Basin. 

The western range margin is locally marked by the low-angle 

Never Summer thrust that formed in Paleocene time following 

uplift of the eastern range margin. Mafic-alkalic and alkalicalcic magmas intruded the southern part of the quadrangle 

during the Laramide orogeny in the period from about 78 to 

45 Ma; these intrusions are parts of an extensive northeasttrending province of igneous and hydrothermal activity known 

as the Colorado Mineral Belt and are host to base and precious 

metal deposits. 

Post-Laramide time was marked by a prolonged period 

of weathering, erosion, and planation of the basementrock surface, extending perhaps into late Oligocene or 

early Miocene time. Tuffaceous fluvial sediments of the 1

 Deceased, 2003
2 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Troublesome Formation, interlayered with upper Oligocene 

through middle Miocene volcanic rocks, record this element 

of the geologic history on the western side of the Front Range. 

Intrusive activity in the Never Summer Mountains during 

middle Oligocene time led to eruption of andesitic, dacitic, and 

rhyolitic lavas and welded tuffs that blanketed the landscape. 

These rocks are collectively referred to as the Braddock Peak 

intrusive-volcanic complex.

Erosion on the eastern slope of the Front Range produced 

a broad, rolling surface surrounding residual highlands and 

east-trending fluvial channels filled with coarse, boulder 

gravel. Continued degradation of the mountainous highlands 

led to aggradation of a fluvial apron westward across the 

Laramide mountain front, probably equivalent to the Miocene 

Ogallala Formation on the High Plains. Low-gradient streams 

developed broad meandering paths across this aggraded apron. 

Renewed uplift of the Front Range during Pliocene time 

caused these meandering streams to incise rapidly and deeply 

into the Proterozoic basement rocks, producing the entrenched 

meanders that are characteristic of the major east-flowing 

streams west of the mountain front today.

Significant global cooling during the Pliocene set the 

stage for glaciation during the Quaternary. In the Estes Park 

quadrangle, the renewed mountain uplift accentuated the topographic relief across the Continental Divide and established 

the altitude necessary to trigger and accumulate abundant 

snow and persistent ice. Mountain glaciers advanced and 

retreated during at least three major glacial-interglacial cycles 

during the middle and late Pleistocene in this area. Erosion 

continues on the High Plains east of the mountain front, and 

progressive incision of the drainage is recorded by at least 

five major gravel-clad terrace and pediment surfaces along the 

major fluvial channels that connect to the South Platte River 

system.

The Estes Park quadrangle has diverse resources that have 

been exploited during the last 150 years. Early hard-rock and 

placer mining developed the hydrothermal deposits related to 

Laramide intrusions along the northern part of the Colorado 

Mineral Belt. Most of these deposits were vein fillings of gold 

and silver (native, arsenides, and tellurides) within fractures 

and breccias adjacent to the intrusions. The Boulder tungsten 

district produced iron tungstate (ferberite) and scheelite 

from hydrothermal veins and sericitic alteration zones. The 

Jamestown district produced fluorite, precious metals, and 

base-metal sulfides that accumulated in nested alteration zones 

around a younger, late Laramide syenite porphyry stock.

Coal was extensively mined from Upper Cretaceous 

Laramie Formation seams in the Boulder-Weld coal field 

between the 1850’s and the 1970’s. Oil and gas are produced 

from well fields in the eastern part of the quadrangle. The 

hydrocarbons were generated during depositional burial of 

source rocks (mostly Lower Cretaceous shale) in the Denver 

Basin. Oil and gas are produced from interbedded sand 

sheets within structural traps in the Laramide folds along the 

mountain front, as well as from stratigraphic truncations and 

pinch-outs.

Sand and gravel resources are widely developed in the 

stream-bed and fluvial-terrace deposits along all the major 

streams of the area. Dimension stone is quarried extensively 

from upper Paleozoic strata near the town of Lyons, and 

decorative rock is produced from the Dakota Group at many 

localities along the eastern mountain front. Tertiary dacite 

sills are quarried along the St Vrain Creek southwest of Lyons 

for road base, riprap, and landscaping dimension stone.

Water resources have been extensively developed 

throughout the region. Early settlers built systems of 

ditches and flumes to capture spring runoff from west of the 

Continental Divide and convey it to east-flowing streams for 

agricultural use on the Colorado Piedmont. Municipalities 

built high-altitude reservoirs in several drainages to store 

winter-spring snowpack and runoff for domestic supply and 

for agricultural use during the late summer. The most farranging water development project in the area is the ColoradoBig Thompson project that was constructed between 1938 

and 1957 by the Bureau of Reclamation. It diverts as much as 

220,000 acre-feet of water from the Colorado River drainage 

through a 13-mile tunnel beneath the Continental Divide to 

the Big Thompson River drainage. The total system diverts 

and stores water, generates power, and provides flood control, 

recreation, and regulated stream flow for much of northeastern 

Colorado.

The geology of the Estes Park quadrangle presents some 

risks and hazards for certain human uses of the land. The 

mountainous terrain involves inherent hazards related to rock 

fall and debris flow. Landslides have occurred in several 

settings in the area where slopes are moderate and bedrock 

contains significant silt and clay, especially in the Troublesome Formation, glacial till in the Colorado River valley, and 

in the Laramie Formation. The Dakota Group exposed in the 

foothills-hogback belt along the eastern mountain front has 

also generated distinctive block-glide landslides, in which 

entire sections of competent strata have detached along weak 

bedding planes and moved downslope. Many of these slides 

probably moved in the early Tertiary, but some may still be 

active today.

Flooding in mountain valleys has occurred in historic 

times in most of the major canyons of the Front Range. Summer thunderstorms statistically drop much of their precipitation between about 7,000 ft and 9,500 ft elevation, which 

coincides with the upper reach of the deeply incised canyons 

within the core of the range. The 1976 Big Thompson River 

flood demonstrated that these two conditions together can lead 

to intense, concentrated runoff, highly elevated stream power, 

and intense local damage along the narrow canyon floors.

Introduction 

This report describes the geology of the Estes Park 

quadrangle in north-central Colorado. The compilation 

summarizes and integrates geologic mapping and various 

topical studies carried out between 1960 and 1990 under the 
Introduction 3

U.S. Geological Survey (USGS) study of the northeastern 

Front Range (Braddock and Cole, 1979; Hutchinson and 

Braddock, 1987), investigations of geology and land-use in 

the 1970’s under the USGS Front Range Urban Corridor 

project (Trimble, 1975; Colton, 1978; Colton and Anderson, 

1977; Hansen and Crosby, 1982), geologic mapping of Rocky 

Mountain National Park (Braddock and Cole, 1990), and 

detailed geologic mapping in northern areas of the Colorado 

Mineral Belt (Wrucke and Wilson, 1967; Gable and Madole, 

1976; Gable, 1980a; Pearson, 1980). This publication also 

draws on topical studies in geology, hydrology, and geomorphology by numerous investigators from Federal, State, 

and local agencies, as well as university research published 

through the years. 

The purpose of this compilation is to summarize the 

geologic framework of a significant portion of the Colorado 

mountain terrain in support of updated mineral-resource 

assessments, particularly in lands administered by the U.S. 

Forest Service.

Geography and Geomorphology

The Estes Park 30′ × 60′ quadrangle encompasses an area 

of 1,955 square miles, extends 57 miles east-west and 35 miles 

north-south, and includes parts of Boulder, Broomfield, Grand, 

Jackson, and Larimer Counties (fig. 1).

Approximately one-third of the land is Roosevelt, Routt, 

and Arapahoe National Forests, another one-third is Rocky 

Mountain National Park (RMNP), and a few thousand acres 

in the northwest corner lie within the Colorado State Forest. 

Wilderness areas are designated in some of the more remote, 

high alpine areas (both Forest Service and National Park 

lands), including the Never Summer, Neota, Indian Peaks, 

and Comanche Peak Wilderness areas (Pearson and others, 

1981). A large area surrounding Lake Granby is designated 

the Arapahoe National Recreation Area.

The southwestern corner of the quadrangle surrounds the 

mountain town of Granby at the confluence of the Colorado 

River and the Fraser River and is a mixture of private lands 

and public lands administered by the Bureau of Land Management (BLM).

Active and inactive mining claims cover significant acreage in and around the historic mining towns of Ward, Gold 

Hill, Wallstreet, Sunshine, and Jamestown.

The eastern one-quarter of the Estes Park quadrangle 

lies on the Colorado Piedmont (fig. 2) below about 6,000 ft 

altitude and includes the developed city centers of Boulder, 

Longmont, and Loveland, as well as interconnected suburban 

areas and significant farmland. The combined population of 

the Piedmont region is approximately 300,000 (2000 U.S. 

census) and contrasts significantly with the sparsely populated 

areas in the mountainous western regions.

Principal vehicle access in the area is provided by U.S. 

Highway 287 that passes north-south through the main cities 

in the east; U.S. Highway 34 that passes westward from Loveland, up the canyon of the Big Thompson River to Estes Park, 

and then over the Continental Divide through Rocky Mountain National Park (the highest paved-through highway in the 

nation) to Granby; Colorado State Highways 7 and 36 that 

connect mountain towns and the Piedmont; and U.S. Highway 

40 through the Granby valley (fig. 1).

The Estes Park quadrangle spans the Front Range uplift 

of the Southern Rocky Mountains (Eaton, 1986) and the 

westernmost part of the Colorado Piedmont physiographic 

province to the east (Fenneman, 1931). The Continental 

Divide traces north to south across the quadrangle along the 

glaciated crest of the Front Range where numerous mountain 

summits exceed 12,000 ft elevation. Average timberline in 

this region is about 11,000 ft, and nearly 15 percent of the land 

surface of the quadrangle lies in this non-forested climatic 

zone of tundra.

The general landforms of the Front Range can be 

described in terms of five overlapping elevation-bounded 

zones within the quadrangle (fig. 2). The easternmost zone 

consists of the gently inclined slopes and broad fluvial valleys 

of the Colorado Piedmont. The eastern margin of the Front 

Range is marked by a foothill zone consisting of hogback 

ridges formed by upturned sedimentary strata of contrasting 

hardness. The foothill zone is narrow and linear in the 

southern quarter of the quadrangle; farther north, the range 

front is segmented by northwest-trending (fault-controlled) 

valleys that break up the hogback ridges into en echelon 

segments that step progressively eastward. The middle zone 

of the Front Range uplift (elevations between about 7,500 

Figure 1. Map showing location of the Estes Park quadrangle 

and Rocky Mountain National Park (RMNP).

0 20 MILES

RMNP

CONTINENTAL D

VI DI E

25

25

Denver

Idaho

Springs

Granby

Boulder

Loveland

Fort

Collins

Lyons

40°30’

40°00’

106°00’ 105°00’

34

34

72

7

7

Walden Rustic

Estes

 Park

Longmont

40

40

14

14

70

36

36

36

287

287

287
4 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

ft and 9,000 ft) is characterized by gently rolling upland 

block of Proterozoic crystalline rocks (Steven and others, 

1997) where summits decline gently eastward but canyons 

are steep, narrow, and deeply incised. This rolling upland 

is interpreted by many to be a relic of widespread erosion 

in the middle Tertiary (Wahlstrom, 1947; Epis and Chapin, 

1975; Scott and Taylor, 1986; Steven and others, 1997). The 

highest zone is the glaciated highland terrain above 9,000 

Figure 2. Map showing physiographic zones of the Estes Park quadrangle and adjoining areas.

NEVER SUMMER

MOUNTAINS

Granby

Grand

Lake

Estes

Park

Loveland

Fort

Collins

Boulder

Denver

40°15’

40°00’

39°45’

0 15 MILES

106°00’

40°45’

41°00’

40°30’

105°45’ 105°30’ 105°15’ 105°00’ 104°45’

Continental Divide

Clear Creek

South Platte River

Colorado Piedmont

Foothill

Hogbacks

Rolling Upland

Glaciated Highland

Granby

Lowlands

Boulder Creek

Left Hand Creek

St Vrain Creek

Big Thompson River

Fraser River

Colorado River

Laramie River

Cache la Poudre River

Buckhorn Creek

LARAMIE MOUNTAINS

MUMMY

RANGE

MEDICINE

BOW

MOUNTAINS

FRONT

RANGE
Introduction 5

ft elevation that is markedly steeper than the incised upland 

zone to the east; drainage reflects glacial processes of the 

Quaternary Period and the widespread glacial deposits formed 

in the valleys. The westernmost zone of the Front Range is 

the steep, western slope that declines rapidly to about 8,000 ft 

elevation in the fault-controlled north-south valley of the upper 

Colorado River, and the Tertiary sedimentary-basin lowlands 

surrounding Granby.

Several major rivers have headwaters along the 

Continental Divide within the Estes Park quadrangle. The 

westward-flowing Colorado River heads at La Poudre Pass at 

the north end of the Kawuneechee Valley, between the Never 

Summer Mountains on the west and the Front Range on the 

east. About four miles to the south at Milner Pass, drainage 

toward the northeast forms the headwaters of the Cache la 

Poudre River, and drainage toward the southeast forms the 

headwaters of the Big Thompson River (fig. 2). These two 

rivers traverse the Front Range and join the South Platte River 

east of Loveland near Greeley, Colorado. The southern part 

of the Front Range east of the Continental Divide in the Estes 

Park quadrangle drains into the St Vrain Creek through Lyons, 

or into Boulder Creek through Boulder, and then farther east to 

the South Platte River (Crosby, 1978).

Most of the eastern slope of the Front Range is rolling, 

forested upland. Local broad valleys and grass-covered flats 

are conspicuous variations to the more typical terrain, and are 

referred to as parks (Crosby, 1978). Many of these landforms 

were preferentially occupied by early settlers because slopes 

are gentler, bedrock is more deeply weathered, and farming 

and ranching were easier. These locations are designated by 

local place names: Estes Park, Meeker Park, Allens Park, 

Hermit Park, Rattlesnake Park, and Big Elk Park, to name a 

few. These landforms typically lie at altitudes between about 

7,500 ft and 8,500 ft, where the steeper high-altitude terrain of 

the glaciated crest of the Front Range merges with the western 

margin of the uplifted, incised rolling upland, and where the 

through-flowing drainage has its gentlest gradient.

The foothills-hogback belt that marks the eastern margin 

of the Front Range uplift is a product of erosion during the 

last 5 million years (Eaton, 1987; Steven and others, 1997), 

not the result of recent mountain uplift. All drainage of the 

South Platte River system (fig. 2) has been incising more-orless continuously during that interval. The major streams east 

of the mountain front are bordered by relict river terraces that 

record several periods of stillstand and local aggradation of 

floodplains. These deposits have been correlated with 5 principal periods of floodplain and alluvial-fan aggradation during 

the last 2 million years and they are linked to cyclic changes 

in discharge related to cyclic climate change and continental 

glaciation (Madole and others, 1998).

The U.S. Board of Geographic Names acted in 2008 

to designate a previously unnamed summit (11,972 ft 

elevation) in the northwestern Never Summer Mountains as 

Braddock Peak, in recognition of W.A. (Bill) Braddock’s 

lifelong achievements in geologic mapping, interpretation 

of Rocky Mountain geology, and teaching. The feature has 

been added to the base map for the Estes Park quadrangle in 

this publication. It is the namesake for the Braddock Peak 

intrusive-volcanic complex of Oligocene age.

Compilation Sources and Methods

Geology in most of the Estes Park 30′ × 60′ quadrangle 

was mapped at 1:24,000 scale under several USGS projects 

between the late 1960’s and the 1990’s (see map). Proterozoic 

rocks in the core of the Front Range were mapped by W.A. 

Braddock and numerous graduate students from the University 

of Colorado-Boulder through the Northeastern Front Range 

Project and published at 1:24,000 scale. Braddock compiled 

the geology of Rocky Mountain National Park at 1:50,000 

scale from his and graduate students’ mapping at larger scales 

(Braddock and Cole, 1990), and prepared the first digital 

geologic map database of the same area in the mid-1990’s for 

use by National Park Service staff at RMNP.

The southern tier of 1:24,000-scale quadrangles was 

previously included in a 1:100,000-scale compilation of 

the Proterozoic crystalline rocks of the central Front Range 

(Gable, 2000). The present compilation was based on the 

same source maps and differs only slightly from Gable (2000), 

chiefly by simplification of the biotite-gneiss units.

The eastern three-eighths of the Estes Park quadrangle, 

along with additional geology to the north and east, was 

previously compiled at 1:100,000 scale by Colton (1978). 

Numerous areas, particularly within the Precambrian rocks 

of the Front Range and the complex hogback belt, were 

incorrectly labeled and portrayed on the 1978 compilation, and 

some fault relationships were omitted. For these reasons, the 

Colton (1978) compilation was not used as a primary source 

for the present work, except for the geology of portions of the 

eastern plains where most of the units are Quaternary surficial 

deposits.

Bedrock units east of the hogback belt on the Colorado 

Piedmont are largely covered by various surficial geologic 

units. Comprehensive regional study of the underlying 

Cretaceous marine deposits was completed by Scott and 

Cobban (1965, 1986) and incorporated in later geologic maps. 

Surficial geology was systematically mapped by Trimble 

(1975), Colton (1978), Colton and Anderson (1978), and by 

Madole and others (1998).

Braddock expanded the digital geologic map database for 

RMNP to include all of the Estes Park quadrangle. This work 

was completed in 1999 using commercial drafting software. 

The current digital geologic map database published with 

this report was converted to ArcInfo format by Ted Brandt 

(USGS, Denver) and modified slightly by Cole to incorporate 

additional and new information.

Acknowledgments

This geologic map compilation results from the singular 

dedication and persistence of W.A. (Bill) Braddock, involving 

hundreds of hours of meticulous digitizing over a period of 
6 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

several years. The work reflects his professional character 

and his devotion to careful, consistent geologic field mapping 

throughout his career of more than four decades. His 

geologic legacy is expressed not only by high-quality geologic 

map products like this, but also by the scores of geology 

students who learned field methods through his academic 

and professional leadership. He passed away in 2003 after 

completing the digital map compilation but before work was 

initiated on the descriptive and interpretive pamphlet. These 

latter items were completed by J.C. Cole.

Constructive comments and suggestions were incorporated based on technical reviews by Michael O’Neill, Ralph 

Shroba, and Cal Ruleman (USGS). The authors acknowledge 

and appreciate the thoroughness and thoughtfulness of their 

efforts. The final map and report are improved and refined as 

a result.

Ted Brandt converted the original digitized geologic 

map from AutoCAD files to ArcInfo coverages, audited and 

corrected quality-control issues, and constructed the digital 

databases for this project. His contributions were essential to 

completing this product.

Description of Map Units

[Map unit descriptions are summarized from descriptions 

in source maps and modified to reflect general characteristics 

of the unit across the Estes Park quadrangle. Color terms are 

descriptive of fresh rock/sediment material. Grain sizes of 

sediment particles are defined using the standard Wentworth 

size scale] 

Alluvial Units

Qa Alluvium (Holocene and upper Pleistocene)—

Stream-deposited sand, silt, and gravel 

underlying floodplains and adjacent low 

terraces along courses of major streams east 

of foothills-hogback belt of Front Range, 

and materials beneath floodplains in tillbounded glaciated valleys in mountains. 

Upper Pleistocene deposits are typically 

light gray and pale brown (clay and silt 

coatings on clasts); Holocene deposits 

are typically light pinkish gray due to 

abundance of detrital feldspar and general 

absence of clay-silt coatings (Madole and 

others, 1998). Locally includes low gravelterrace remnants and sheetwash

Qva Mountain valley alluvium (Holocene and upper 

Pleistocene)—Gravel, sand, and silt along 

courses of major and intermittent streams 

in bedrock-cut valleys, in alluvial fans 

along montane valley margins, and in 

outwash deposits downstream from terminal 

moraines of Pinedale and Bull Lake age. 

Unit underlies narrow, high-altitude valleys 

west of Continental Divide, and montane 

valleys of Big Thompson and Little 

Thompson Rivers and St Vrain and Boulder 

Creeks east of Divide

Qbr Broadway Alluvium (upper Pleistocene)—Wellsorted and well-stratified sand and gravel 

beneath terraces. Top of unit is typically 

10–25 ft above modern floodplains. Deposition was broadly coeval with Pinedale 

glaciation at 30 ka to 12 ka (Madole, 1991)

Qlv Louviers Alluvium (upper and middle 

Pleistocene)—Reddish-brown, clastsupported pebble and cobble gravel. Top 

of unit is typically 20–40 ft above modern 

floodplains. Deposition was broadly coeval 

with Bull Lake glaciation at about 300 ka to 

120 ka (Madole, 1991)

Qs Slocum Alluvium (middle Pleistocene)—Reddishbrown, clast-supported pebble and cobble 

and boulder gravel and grayish-brown to 

brown silty sand. Top of unit is typically 

110–130 ft above modern floodplains 

(Madole, 1991). Slocum also forms 

prominent pediment surface north of 

Boulder 

Qv Verdos Alluvium (middle Pleistocene)—Brown 

sand and gravel. Pebbles, cobbles, 

and boulders are weathered and partly 

decomposed. Terrace and pediment deposits 

lie about 170 ft above Little Thompson 

River. Verdos Alluvium locally contains 

Lava Creek B ash (639 ka; Madole, 1991; 

Lanphere and others, 2002)

Qrf Rocky Flats Alluvium (lower Pleistocene)—

Reddish-brown sand and clast supported 

pebble and cobble gravel and brown silty 

and clayey sand. Preserved at Table 

Mountain north of Boulder and Table 

Top Mountain north of Hygiene (Madole 

and others, 1998), where upper surface 

is typically 260–300 ft above modern 

floodplains

Qprf Pre-Rocky Flats alluvium (lower Pleistocene)—

Brown to white carbonate-cemented 

gravel and sand; only preserved on Haystack 

Mountain and Gun Barrel Hill northeast of 

Boulder. Top of gravel (eroded) is more 

than 370 ft above modern floodplains

Qgy Young gravel deposits at Granby Mesa (upper 

Pleistocene)—Gravel and sand deposits 

underlying fluvial terraces along Colorado 

and Fraser Rivers adjacent to Granby Mesa; 

tops of terraces approximately 60 ft above 

modern floodplains. Terrace deposits 
Description of Map Units 7

probably formed during Pinedale glaciation 

(Meierding, 1977; R. Shroba, written 

commun., 2006)

Qgm Intermediate gravel deposits at Granby Mesa 

(upper? and middle? Pleistocene)—Gravel 

and sand deposits underlying fluvial terraces 

along Colorado and Fraser Rivers adjacent 

to Granby Mesa; tops of terraces approximately 140 ft above modern floodplains. 

Terrace deposits probably formed during 

Bull Lake glaciation (Meierding, 1977; R. 

Shroba, written commun., 2006)

Qgo Old gravel deposits at Granby Mesa (middle? 

and lower? Pleistocene)—Gravel and sand 

deposits underlying fluvial terraces remnants 

on Granby Mesa; tops of terrace remnants 

as high as 220 ft above modern floodplains. 

Terrace deposits probably formed prior to 

Bull Lake glaciation (Meierding, 1977; R. 

Shroba, written commun., 2006)

Qfo Old alluvial-fan deposits (lower Pleistocene)—

Poorly exposed, indurated sand and gravel 

deposits on east flank of Granby Mesa that 

underlie an eroded alluvial fan landform; 

unconformably overlie tilted Mesozoic bedrock units; flanked westward by old gravel 

deposits of Granby Mesa (Qgo). Thickness 

unknown

Qpy Younger piedmont-slope alluvium (upper and 

middle Pleistocene)—Chiefly brown to 

reddish-brown sand, silty sand, and matrixsupported gravel derived from upturned 

sedimentary rocks

Qd Diamicton (upper? Pleistocene)—Bouldery 

gravel and sand deposits on the floor of 

Tahosa Valley that lie beyond known 

limits of glacial ice; poorly exposed, but 

surface displays boulders as large as 4 ft. 

Formerly interpreted to be pre-Bull Lake till 

(Richmond, 1960), but deposits are probably 

younger because they are low in landscape 

near modern drainage. Form of diamicton 

deposit near Meeker Campground suggests 

relict alluvial fan (Braddock and Cole, 1990) 

Ng Fluvial gravel (Pliocene? and Miocene)—

Unsorted, unstratified deposits of boulders 

(as large as ten ft), cobbles, and pebbles 

in a dominantly coarse sandy matrix with 

some silt; in Niwot Ridge area of Ward 

quadrangle, deposits are higher on landscape 

than highest glacial deposits of Bull Lake 

age, are much thicker (as great as 100 ft) and 

more voluminous than typical till, and are 

unlikely to have formed by glacial processes 

(Gable and Madole, 1976). Madole (1982) 

concluded these deposits formed in ancient 

valley systems that were subsequently 

disrupted by Neogene uplift of the Front 

Range. These high-level fluvial-gravel 

deposits also mark paleo-canyons of the 

Big Thompson River (north of Palisade 

Mountain) and the early Colorado River 

drainage (Gravel Mountain and north of 

Apiatan Mountain). 

Glacial Units

Qr Rock glacier deposit (Holocene and upper 

Pleistocene)—Lobate mass of rock rubble, 

silty sand, and ice that is veneered by 

angular blocks and boulders in alpine 

cirques and valleys above 10,000 ft 

elevation. Surface typically marked by 

conspicuous arcuate ridges and a steep 

lobate front (Gable and Madole, 1976)

Qo Organic-rich sediment (Holocene and upper 

Pleistocene)—Bog and marsh deposits in 

formerly glaciated terrain, locally overlain 

by glacial-outwash sediments

Qtp Till of Pinedale age (upper Pleistocene)—Subangular to subrounded boulders, cobbles, 

and pebbles in a silty sand matrix. 

Prominent, sharp-crested moraines are 

numerous, most clasts slightly weathered, 

soils thin. Contains small bodies of 

Holocene till in cirque basins. Tills in 

Arapahoe Creek drainage east of Lake 

Granby are inferred to be Pinedale age, 

but may include local remnants of older 

glacial deposits; detailed work has not 

been conducted in this drainage. Pinedale 

glaciation occurred between about 30 ka and 

12 ka in this region (Madole, 1991)

Qtb Till of Bull Lake age (upper and middle 

Pleistocene)—Subangular to subrounded 

boulders, cobbles, and pebbles in a silty 

sand matrix. Morainal form subdued, 

clasts variably weathered, and soils well 

developed. Bull Lake glaciation occurred 

between about 300 ka and 120 ka in this 

region (Madole, 1991)

Qtpb Till of pre-Bull Lake age (middle and lower? 

Pleistocene)—Weathered boulders and 

cobbles in a silty sand matrix. Deposits 

lack morainal form and have thick reddishbrown clayey soil containing decomposed 

granitic clasts. Pre-Bull Lake deposits are 

identified in Colorado River valley near 

Shadow Mountain Lake and Granby Dam 

(Meierding, 1977; Braddock and Cole, 

1990)
8 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Other Surficial Units

f Manmade fill (Holocene)—Engineered deposits 

in earthen dams in numerous locations 

throughout quadrangle. Includes large area 

of transported rock and soil surrounding a 

limestone quarry and cement manufacturing 

facility east of Lyons 

Qc Colluvium (Holocene and upper Pleistocene)—

Deposits of rock debris ranging in grain size 

from silt to boulders, formed by a variety 

of mass-wasting processes. Unit chiefly 

includes small-volume deposits in piedmont 

and marginal to montane valleys, as well as 

extensive areas near and above timberline 

where freeze-thaw action has aided mass 

wasting and downslope movement

Qac Alluvium and colluvium (Holocene and upper 

Pleistocene)—Thin deposits of silt, sand, 

and gravel in strike valleys within hogback 

physiographic zone of eastern foothills. 

Most debris is derived from resistant 

rocks in flanking strike ridges; transported 

downvalley by episodic, concentrated 

streamflow

Qlo Loess (Holocene and upper? Pleistocene)—Palebrown sandy to clayey silt deposited by 

wind. Unit chiefly confined to low-relief 

areas east of Front Range where it forms 

a thin blanketing deposit, mostly 3–10 ft 

thick. Locally may include minor deposits 

of middle Pleistocene age

Qls Landslide deposits (Holocene and upper 

Pleistocene)—Includes slumps, earth flows, 

rock-avalanche deposits, debris flows, and 

block-glide landslides. Stratigraphically 

controlled block-glide landslides are 

common on dip slopes of Dakota Group 

east of Front Range (Braddock, 1978). 

Landslides also common in glacial deposits 

on slopes of Kawuneeche Valley north of 

Lake Granby, and in Troublesome Formation 

near Granby. Several landslide deposits of 

Proterozoic metamorphic rock on and near 

Jackstraw Mountain appear to have moved 

on weak foliation planes inclined downslope 

(Braddock and Cole, 1990). Prominent 

landslide on northwest slope of Porphyry 

Peaks involves hydrothermally altered 

Oligocene subvolcanic intrusive rocks; 

jumbled blocks and irregular landform 

suggests later Pleistocene or younger age

Qta Talus deposits (Holocene and upper 

Pleistocene)—Angular blocks of rock on 

steep slopes below cliffs. Unit is chiefly 

shown in alpine and subalpine areas along 

crest of Front Range and in Never Summer 

Mountains

Post-Laramide Sedimentary Rocks

Nbt Troublesome Formation (lower Miocene and 

upper Oligocene)—Gray and orange-gray, 

tuffaceous mudstone and sandstone, volcanic 

ash beds, and minor clayey limestone 

and conglomerate; locally interbedded 

with basalt and trachyandesite (bba). 

Unit is widely exposed in southwestern 

corner of quadrangle; base is a significant 

unconformity with as much as 650 ft of 

local relief on older rocks (Braddock and 

Cole, 1990; Schroeder, 1995a). Lower 

part of Troublesome Formation is locally 

interlayered with Oligocene basalt (26.4 ± 

0.9 Ma) and latite breccia (28.4 ± 2.3 Ma; 

Izett, 1974)

bs Sedimentary rocks, undivided (Oligocene and 

Eocene?)—Small exposures of sedimentary 

rocks that interfinger with or underlie 

Oligocene volcanic deposits in northern part 

of Kawuneechee Valley

Post-Laramide Intrusive Rocks

Nbi Intrusive rocks, undivided (Miocene to 

Oligocene)—Dikes, sills, and small plugs; 

typically very fine grained and altered; 

compositions range from basalt and andesite 

to rhyolite. Intrusions are chiefly located in 

Never Summer Mountains and westward in 

Illinois River drainage

bgc Granite and rhyolite porphyry of Mount 

Cumulus stock (late Oligocene)—Massive, 

tan to orange-red, leucocratic intrusive rocks 

that grade from rhyolite porphyry (south and 

west) to coarsely crystalline granite (north 

and east); intruded at 28.2 ± 0.7 Ma (wholerock K-Ar; Marvin and others, 1974)

brp Rhyolite porphyry (late Oligocene)—White, tan, 

or light-reddish-brown porphyritic rock 

containing 20–40 percent phenocrysts of 

quartz, sanidine, oligoclase, biotite, and 

brown hornblende in an aphanitic matrix

bbi Basalt intrusion (late Oligocene)—Dark-gray to 

black, dense rock intruded into lower part 

of Troublesome Formation (Nbt) with 

interbedded basalt and trachyandesite flows 

(may be source of latter). Single body 

located about 2 mi west of northwestern 

Lake Granby shore

bgdr Granodiorite and monzonite of Mount Richthofen stock (late Oligocene)—Light- to 
Description of Map Units 9

medium-gray equigranular to slightly porphyritic granodiorite, monzonite, and quartz 

monzonite; compositional facies are all 

intergradational and all contain hornblende, 

biotite, and minor augite; intruded at 29.7 ± 

3 Ma (whole-rock K-Ar; Marvin and others, 

1974)

bti Intrusive tuff-breccia (late? Oligocene)—

Yellowish-gray to grayish-yellow-green, 

poorly sorted, weakly bedded tuff-breccia 

consisting of volcanic ash and lapilli, 

comminuted Proterozoic rocks, sparse 

siltstone and sandstone of possible Mesozoic 

age, and Oligocene igneous rocks (andesite 

porphyry and quartz latite). Forms a 

singular pipe-like body northwest of 

Bowen Mountain within Proterozoic rocks 

(Metzger, 1974); planar fabric defined by 

clast sorting dips gently inward

bqla Quartz latite of Apiatan Mountain (late 

Oligocene)—Gray to brownish-gray 

porphyry containing about 25 percent 

phenocrysts of oligoclase-andesine (as long 

as 3 cm), sanidine, and minor biotite and 

clinopyroxene in a microcrystalline groundmass rich in potassium feldspar; intruded at 

28.4 ± 2.3 Ma (whole-rock K-Ar; Marvin 

and others, 1974)

bqlx Quartz latite breccia (upper Oligocene)—Brown 

to reddish-brown monolithologic breccia; 

locally grades into quartz latite (bqla); 

breccia is probably mostly extrusive, and 

interbedded with lower part of Troublesome 

Formation (Nbt)

Post-Laramide Volcanic Rocks

br Rhyolite (upper Oligocene)—White, light-gray, 

pale-yellowish-brown, moderate-brown, 

or pale-red-purple, flow-laminated rhyolite 

with 5–10 percent phenocrysts of sanidine, 

quartz, and minor biotite; flow laminations 

are typically contorted; probably coeval 

with granite and rhyolite porphyry of Mount 

Cumulus stock (bgc). Rhyolite flows 

generally overlie rhyolite welded tuff (brw) 

north of Thunder Mountain; similar flow 

rock overlies Troublesome Formation (Nbt) 

east of the Kawuneechee Valley opposite 

mouth of Baker Gulch 

brw Rhyolite welded tuff (upper Oligocene)—Strongly 

welded, weakly layered gray, black, or 

brown crystal-rich tuff distinguished by 

brown, euhedral smoky quartz phenocrysts; 

probably coeval with granite and rhyolite 

porphyry of Mount Cumulus stock (bgc). 

Erupted at 27.7 ± 0.1 Ma (sanidine 

40Ar/39Ar; Knox, 2005) from the Braddock 

Peak intrusive-volcanic complex

bv Volcanic rocks, undivided (upper Oligocene)—

Complexly interlayered volcanic-flow rocks 

and volcanic-rich sedimentary rock in Never 

Summer Mountains and in Specimen Creek 

area north of Trail Ridge Road. Major 

rocks include rhyolite, rhyolite-welded tuff, 

heterolithic debris-flow deposits, and lesser 

andesite and volcaniclastic sandstone

bvx Volcanic breccia (upper Oligocene)—Massive 

to layered breccia containing fragments of 

older volcanic rocks, Phanerozoic sedimentary rock (as large as 12 in) and blocks of 

Proterozoic crystalline rock (as large as 35 

ft; Lee, 1917; Braddock and Cole, 1990), 

typically interbedded with thick rhyolite 

welded tuff. Character and spatial distribution suggest these deposits formed during 

eruption of rhyolite welded tuff (brw) in 

Never Summer Mountains 

bba Basalt and trachyandesite (upper Oligocene)—

Dark-gray, very fine grained rock that 

weathers to grayish brown, grayish purple, 

or moderate red. Interlayered with the 

Troublesome Formation (Nbt) in southwest 

part of map area, and with other volcanic 

rocks in northwest part; erupted at 26.4 ± 0.9 

Ma (whole-rock K-Ar; Izett, 1974)

bap Andesite porphyry (upper? Oligocene)—Darkgray to black, massive rock that contains 

abundant yellowish-white andesine phenocrysts and less common orthopyroxene, 

clinopyroxene, biotite, and hornblende in a 

partly glassy to cryptocrystalline matrix

Laramide Sedimentary and Volcanic Rocks

bc Coalmont Formation (Eocene and Paleocene)—

Fluvial sedimentary unit within North Park 

basin consisting of a lower unit of gray 

and green mudstone and an upper unit of 

fine- to coarse-grained tan arkose with 

lenses of coarse conglomerate; thickness 

exceeds 500 ft in Jack Creek drainage (top 

truncated by Never Summer thrust fault). 

Formation is partly the lateral equivalent of 

Middle Park Formation (bKm) deposited 

in Middle Park basin; arbitrary boundary 

between units is Continental Divide south of 

Illinois River. Coalmont Formation lies on 

volcaniclastic sedimentary rocks containing 

common clasts of andesite, which might be 
10 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Windy Gap Volcanic Member of Middle 

Park Formation (O’Neill, 1981) or “volcanic 

pebble member” (lower part) of Coalmont 

Formation as recognized by Kinney (1970b) 

in area immediately west

Middle Park Formation (Paleocene and Upper 

Cretaceous?)

bKm Upper part (Paleocene and Upper 

Cretaceous?)—Interbedded, light- to 

medium-brown, tan or gray, volcanic and 

(or) arkosic sandstone and siltstone; volcanic 

and (or) granitic pebble to boulder conglomerate; and red, green, and brown mudstone. 

Thickness exceeds 5,000 ft in region west of 

this quadrangle in Middle Park basin (Izett, 

1968)

Kmw Windy Gap Volcanic Member (Upper 

Cretaceous?)—Brown or greenish-brown, 

gray, or purple, massive volcanic breccia; 

interbedded volcanic conglomerate and 

sandstone; and rare, probable flows of latite 

to trachyte porphyry. Age is inferred from 

fossil leaves in overlying beds of upper part 

of Middle Park Formation (bKm) (Izett, 

1968). Thickness about 300 ft at western 

quadrangle boundary west of Granby. Beds 

of volcaniclastic conglomerate and sandstone north of Illinois River beneath Never 

Summer thrust lack volcanic breccia but 

were correlated with Windy Gap Member by 

O’Neill (1981); these andesite-clast-bearing 

beds may be lower Coalmont Formation 

(bc) (Kinney, 1970b)

Laramide Intrusive Rocks

bsy Syenite and quartz syenite (Paleocene and 

Eocene)—Light-gray, slightly pinkishgray weathering, leucocratic porphyritic 

rock with a fine-grained, dense, granular 

groundmass. Syenite in AudubonAlbion stock may be Paleocene based on 

unpublished biotite age of 65 Ma (40Ar/39Ar; 

DeWitt, written commun., 2007). Syenite 

also forms numerous small plugs and 

irregular bodies near Jamestown and Sunset 

that were intruded at 56 and 45 Ma (Gable, 

1980a; DeWitt, written commun., 2007). 

Quartz syenite grades to hornblende quartz 

monzonite in the Tuscarora stock (DeWitt, 

written commun., 2007)

bql Quartz latite and rhyodacite porphyry 

(Paleocene)—Gray-brown and greenishgray porphyritic intrusive rocks with 

phenocrysts of altered pale-yellow feldspar 

and biotite or hornblende. Porphyries 

intruded as tabular sills into tilted 

sedimentary strata in foothills-hogback belt 

between Lyons and Boulder; ages range 

between 66 Ma and 64 Ma (Hoblitt and 

Larson, 1975; DeWitt, written commun., 

2007). Similar rocks form common crosscutting dikes in the Indian Peaks area 

(Pearson, 1980) and across the Colorado 

Mineral Belt from Jamestown to Arapaho 

Pass

Klp Augite latite porphyry (Late Cretaceous)—

Medium- to dark-greenish-gray porphyry 

with phenocrysts of plagioclase and augite 

intruded into Cretaceous strata south of 

Granby. Probable age is 66.0 ± 4.2 Ma 

(fission-track; Schroeder, 1995a)

Kqm Quartz monzonite (Late Cretaceous)—Gray, 

equigranular rock that consists of feldspar, 

hornblende, biotite, and minor quartz; 

gradational to quartz syenite. Mapped in 

three small bodies in Jamestown-Ward area, 

including Tuscarora stock, Long Gulch 

stock, and an unnamed plug near northeast 

margin of Boulder Creek batholith

Ksyd Syenodiorite (Late Cretaceous)—Gray, mediumgrained rock commonly with a salt-andpepper appearance, but locally more mafic 

and darker. Syenodiorite forms central 

part of Audubon-Albion stock intruded 

at about 68 Ma (whole-rock K-Ar, Gable 

and Madole, 1976; 40Ar/39Ar, DeWitt, 

written commun., 2007); locally grades to 

syenogabbro 

Kmzd Monzodiorite (Late Cretaceous)—Gray, mediumgrained, equigranular to slightly porphyritic 

rock consisting chiefly of feldspar and about 

10–15 percent hornblende. Intruded into 

Jamestown stock between about 78 Ma and 

72 Ma (Gable, 1980a, 1984; DeWitt, written 

commun., 2007); largely unmineralized. 

Intruded into Caribou stock at about 76 Ma 

(DeWitt, written commun., 2007); largely 

unmineralized

Kmzb Monzogabbro (Late Cretaceous)—Dark-gray, 

salt-and-pepper, fine- to medium-grained, 

slightly porphyritic rock showing a crude 

flow banding with abundant mafic clots; 

forms early cumulate masses in western part 

of Audubon-Albion stock. Age is about 70 

Ma (DeWitt, written commun., 2007)

Pre-Cenozoic Sedimentary Rocks

Kl Laramie Formation (Upper Cretaceous)—Lightgray to light-yellowish-gray sandstone, 
Description of Map Units 11

sandy shale, claystone, shale, and several 

beds of economic coal in lower part. 

Laramie Formation was deposited in coastal 

plain and swamp environments, generally 

coeval with regressive Fox Hills Sandstone 

that was deposited along coastline (Roberts, 

2005). Thickness ranges between 350 ft and 

1,800 ft in the region

Kfh Fox Hills Sandstone (Upper Cretaceous)—Upper 

part of formation consists of crossbedded 

tan sandstone; grades downward into brown, 

fine-grained silty sandstone and gray shale. 

Fox Hills formed in strandline and deltafront environments during withdrawal of 

Late Cretaceous Western Interior seaway 

(Roberts, 2005); thickness variable because 

Fox Hills consists of several overlapping 

tabular sand bodies, but total thickness 

locally as great as 300 ft

Pierre Shale (Upper Cretaceous)—Formation has 

been subdivided into a number of members 

on east side of map, but is undivided in 

Never Summer Mountains where it is 

deformed. Units are well described by Scott 

and Cobban (1965; 1986); total thickness is 

as great as 6,200 ft in Denver Basin (Higley 

and Cox, 2005)

Kp Pierre Shale, undivided—Primarily dark-gray to 

black silty shale with some beds of sandstone. Adjacent to Mount Richthofen stock, 

Pierre Shale has been thermally metamorphosed to dense, hard, medium- to darkgray hornfels and recrystallized sandstone 

(O’Neill, 1981). Elsewhere, subdivided as 

follows:

Kpt Upper transition member—Friable sandstone 

and soft shaley sandstone containing large 

calcareous concretions

Kpu Upper shale member—Gray, concretionary silty 

shale

Kpr Richard Sandstone Member, Larimer 

Sandstone Member, Rocky Ridge 

Sandstone Member and two intervening 

unnamed shale members—Light-brown or 

brownish-yellow, micaceous or glauconitic 

sandstone interlayered with gray shale

Kpm Middle shale member—Claystone and sandy 

siltstone

Kph Hygiene Sandstone Member—An upper hard, 

glauconitic, ridge-forming sandstone 

separated from a lower friable sandstone by 

a shale

Kpl Lower shale member—Mostly dark-olive-gray 

bentonitic shale

Kn Niobrara Formation (Upper Cretaceous)—In 

western map area, formation consists of 

light-gray-weathering, platy, calcareous 

shale; weathers buff near top; thin, lightgray biomicrite beds near middle; thin, 

light-gray micritic limestone at base. In 

eastern map area, formation consists of two 

members (combined thickness about 320 ft):

Smoky Hill Shale Member—Very fissile calcareous shale; dark gray on fresh surfaces, 

weathers to light gray plates; distinctive 

yellowish-brown-weathering micrite at top; 

thickness about 300 ft

Fort Hays Limestone Member—Light-gray, 

thick-bedded micrite; thickness about 20 ft

Kbm Benton Shale (Upper Cretaceous) and Mowry 

Shale (Lower Cretaceous), undivided—

Combined unit of several shale, calcareous 

shale, and limestone units that lie above 

Dakota Group and below Niobrara 

Formation; exposure is typically poor. 

Benton Shale is locally subdivided (from 

top to bottom) into the olive-gray, siltysandy Carlile Shale, dark-gray to olivegray Greenhorn Limestone, and dark-gray 

Graneros Shale. The underlying Mowry 

Shale (locally absent) is siliceous and 

weathers light gray. Combined units are 

about 350 ft thick in Granby area, and about 

530 ft thick along eastern Front Range 

margin

Kd Dakota Group or Sandstone (Lower 

Cretaceous)—In western map area, 

Dakota Sandstone consists of light-gray, 

fine-grained sandstone and interbedded shale 

in upper part, and chert-pebble conglomerate 

and sandstone in lower part; total thickness 

about 175–230 ft (Izett, 1968). In eastern 

map area, Dakota Group has generally been 

mapped as two formations:

South Platte Formation

First sandstone member—Gray, well-sorted, 

fine- to medium-grained sandstone; 40–100 

ft thick in outcrop. Unit correlates with 

Muddy Sandstone (or informal “J” sandstone) in subsurface (Higley and Cox, 2005)

Middle shale member—Dark-gray carbonaceous 

shale and siltstone; 90 ft thick

Plainview Sandstone Member—Gray to lightbrown, thinly bedded, fine-grained sandstone; about 30 ft thick

Lytle Formation—Gray to light-brown, coarsegrained to conglomeratic sandstone 

and blocky-weathering, varicolored, 

noncarbonaceous mudstone; about 120 ft 

thick. Base of Lytle Formation is a regional 

unconformity
12 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Jm Morrison Formation (Upper Jurassic)—Green, 

red, yellow, and white blocky-weathering 

claystone, siltstone, and gray micrite, and 

gray, fine- to medium-grained sandstone; 

about 300 ft thick

Jms Morrison Formation (Upper Jurassic) and 

Sundance Formation (Middle Jurassic), 

undivided—Morrison Formation is similar 

to description above (Jm). Sundance 

Formation is buff, very fine grained 

sandstone and laminated siltstone; thickness 

about 125 ft west of Front Range 

Jdmj Morrison Formation (Upper Jurassic), Sundance 

Formation (Middle Jurassic), and Jelm 

Formation (Upper Triassic), undivided—

Morrison Formation is similar to description 

above (Jm). Pine Butte Member of the 

Sundance Formation is tabular-bedded, 

fine-grained, gray to white sandstone; it 

conformably overlies Canyon Springs 

Sandstone Member of Sundance which is 

pink, orange-pink, or reddish-brown, fine- to 

medium-grained, crossbedded, calcareous 

sandstone. Sundance unconformably 

overlies Red Draw Member of Jelm 

Formation, which is orange-pink, or 

reddish-brown, fine-grained, crossbedded, 

calcareous sandstone. Combined thickness 

ranges from about 350 ft (southeast) to 420 

ft (northeast). Base of Jelm Formation is a 

regional unconformity

dPc Chugwater Formation and older units (Lower 

Triassic and Upper Permian)—Reddishbrown to orange-red shale, siltstone, and 

fine-grained sandstone; laminated to thin 

bedded. In northwestern map area, lower 

part contains light-gray to white finely 

laminated limestone (possible Forelle Limestone Member of Late Permian age). In 

southwestern map area, formation contains 

thin, lenticular limestone at base. Thickness ranges from 300 ft (southwest) to 830 

ft (northwest). Entire unit unconformably 

overlies Proterozoic basement west of Front 

Range

dPl Lykins Formation (Lower Triassic and Upper 

Permian)—In the eastern foothills-hogback 

belt

Upper part (Lower Triassic and Upper 

Permian)—Maroon or reddish-brown 

siltstone and sandstone; thin limestone near 

base; about 370 ft thick

Forelle Limestone Member (Upper Permian)—

Yellowish-brown and yellowish-gray, finely 

laminated, wavy-bedded limestone with 

stromatolite structures; about 30 ft thick

Lower part (Upper Permian)—Moderatereddish-brown silty shale and minor 

limestone; about 100 ft thick

Pl Lyons Sandstone (Lower Permian)—Mediumorange, pink, or pinkish-gray, fine- to 

medium-grained, silica-cemented, 

well-sorted, cross-stratified, quartz 

sandstone; deposited in coastal-dune 

complex; about 60 ft thick. Base of Lyons 

Sandstone is a regional unconformity

Po Owl Canyon Formation (Lower Permian)—Red 

siltstone and fine-grained, red, ripplelaminated sandstone deposited in shallowwater marine estuary environment; thickness 

as great as 250 ft (north) but formation 

pinches out south of Little Thompson River

Pi Ingleside Formation (Lower Permian)—Pink to 

light-red, fine-grained quartzose sandstone. 

Commonly well cemented with quartz or 

calcite; thin to very thick bedded, locally 

crossbedded; deposited in a shallow-water 

marine-shelf environment; thickness varies 

from 50 ft (north) to about 200 ft (south)

Phf Fountain Formation (Lower Permian and Upper 

and Middle Pennsylvanian)—Reddishbrown to purplish-gray, arkosic conglomerate; medium- to coarse-grained, feldspathic 

sandstone, trough crossbedded; dark-reddish-brown siltstone and shale; and minor 

thin limestone; deposited in alluvial fans and 

coastal-plain environments; about 1,100 ft 

thick

cg Sedimentary and metamorphic rocks (Mesozoic 

and Paleozoic), undivided—Rocks exposed 

in small fault blocks and slivers along trace 

of Never Summer thrust and as inclusions 

in granitic stock of Mount Cumulus (see 

O’Neill, 1981, for greater detail)

Paleozoic(?) Intrusive Rocks 

Dk Kimberlite (Devonian?)—Dark-gray or dark-olivegreen, altered, fragmental rock; contains 

pale-golden-yellow phlogopite, small grains 

of purplish-red pyrope garnet, ilmenite, 

and chlorite psuedomorphs of pyroxene(?); 

occurs as small dikes 1.3 mi east of Rams 

Horn Mountain, and talus blocks in Hayden 

Gorge. Early Devonian age determined 

on phlogopite from dikes (Smith, 1979). 

Hayden Gorge kimberlite may be similar or 

older, based on radiometric ages obtained 

elsewhere in the northern Front Range that 

indicate three periods of eruption at about 

350 Ma, 570 Ma, and 620–640 Ma (Lester 

and others, 2001; Lester and Farmer, 2004)
Description of Map Units 13

Mesoproterozoic Intrusive Rocks 

Proterozoic intrusive rocks in the Estes Park quadrangle 

were emplaced during two principal time periods, based on 

radiometric age determinations by several methods. Rocks 

formed during these major intrusive periods were recognized 

and formalized by Tweto (1987) as the Paleoproterozoic 

Routt Plutonic Suite (roughly 1,700 ± 25 Ma) and the 

Mesoproterozoic Berthoud Plutonic Suite (roughly 1,400 ± 

25 Ma). Batholith-sized intrusions have long been designated 

with a geographic place-name that has served as a type locality 

for a named lithodemic unit (for example, Boulder Creek 

batholith consisting primarily of Boulder Creek Granodiorite; 

or Silver Plume batholith consisting primarily of Silver Plume 

Granite). Rocks of similar composition, texture, and age have 

been recognized in separate batholithic bodies at numerous 

locations in the Colorado basement, and some have been 

correlated (by these characteristics) with the named lithodemic 

units (see Tweto, 1987). Later isotopic studies have shown 

that a few of these correlations were not valid (Premo and 

others, 2007)

The nomenclature and symbology for Proterozoic intrusive rocks in this quadrangle (and adjoining areas) are based 

on age, composition, and a local intrusive-body name, rather 

than on inferred correlation to the type of a named lithodemic 

unit.

Map-unit symbol is composed as follows: intrusion age 

(X = Paleoproterozoic 1,700 ± 25 Ma; Y = Mesoproterozoic 

1,400 ± 25 Ma); composition (g = granite; gd = granodiorite; 

d = diorite; qd = quartz diorite; gb = gabbro; j = trondhjemite); 

and intrusive body (B = Boulder Creek batholith; H = Granite 

of Hagues Peak; LP = Longs Peak-St Vrain batholith; T = 

Thompson Canyon intrusions).

Textural and mineralogic variants separately mapped 

within Longs Peak–St Vrain batholith are symbolized with an 

additional lower-case letter: s = sillimanite-bearing; x = intrusion breccia

Ygb Gabbro of the Iron Dike—Dark-gray to black 

ferrogabbro; weathers dark brown to 

orange brown with prominent limonite 

stains along joint surfaces; very fine grained 

along dike margins, and medium grained in 

centers of thicker dike segments (as wide 

as 50 ft). Intruded 1,316 ± 50 Ma (Rb-Sr 

isochron; Braddock and Peterman, 1989). 

Iron Dike forms a narrow north-northwest 

trending swarm that can be traced almost 

continuously from eastern margin of Front 

Range near Boulder, across Estes Park 

quadrangle, and beyond into Medicine Bow 

Range to near Colorado-Wyoming State 

line, over a distance of about 95 mi

Yga Biotite-muscovite alkali-feldspar granite—Gray to 

buff, fine- to medium-grained, xenomorphic 

granular granite with sparse accessory 

biotite and possible magmatic muscovite. 

Grades into pegmatite in a widespread 

intrusive complex near Drake; intruded at 

1,361 ± 30 Ma (Rb-Sr isochron; Peterman 

and others, 1968)

Ya Aplite—Cream to buff, fine- to medium-grained, 

xenomorphic-granular rock with minor 

accessory magmatic sillimanite or garnet. 

Gradational with pegmatite west of Glen 

Haven

YgLP Granite of Longs Peak batholith—Light- to 

medium-gray, grayish-orange, orangepink, or red-purple monzogranite to 

syenogranite that contains characteristic 

tabular microcline phenocrysts. Biotite is 

the principal dark mineral (5–12 percent), 

locally accompanied by magmatic 

sillimanite and (or) garnet plus minor 

accessory minerals. Equigranular matrix 

is locally fine, medium, or coarse grained 

and weakly foliated by aligned biotite 

grains. Granite intruded the Longs PeakSt Vrain batholith as a viscous magma 

that extensively deformed metamorphic 

wall-rocks and intruded as tabular sills in 

the center (highest preserved parts) of the 

batholith between rotated, flat-lying foliation 

planes in biotite gneiss (Cole, 1977; 

Braddock and Cole, 1979, 1990). Intruded 

at 1,420 ± 25 Ma (Rb-Sr isochron; Silver 

Plume Granite of Peterman and others, 

1968). Secondary muscovite (non-oriented, 

porphyroblastic) is locally prominent, 

particularly along the northeastern margin 

of the batholith and in small satellite 

intrusions; formed by subsolidus hydration 

of sillimanite + potassium feldspar or 

breakdown of primary biotite to muscovite + 

magnetite.

YgLPx Intrusion breccia—Large irregular area (about 

0.5 sq mi) east of Stones Peak consisting 

of intrusive matrix of biotite-rich granite of 

Longs Peak (YgLP) containing about 75 

percent rounded to angular fragments of 

biotite gneiss, pegmatite, amphibolite, and 

medium-grained porphyritic granite; similar 

breccia occurs in small (unmapped) areas 

near Sky Pond and just north of Hidden 

Valley parking area. Probably formed 

during late-stage, explosive exsolution of 

water vapor during crystallization of granite 

of Longs Peak (Cole, 1977)

YgLPs Garnet-sillimanite granite—Light-grayish-pink 

or grayish-yellow, medium- to coarsegrained, xenomorphic-granular syenogranite 

with accessory magmatic garnet, sillimanite, 

and minor biotite (less than 5 percent total). 
14 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Mapped in small area west of Copeland 

Mountain, but also occurs in smaller 

(unmapped) masses northeastward to 

Bluebird Lake, and at Mt Lady Washington 

(Cole, 1977; Braddock and Cole, 1990)

Yd Mafic dikes and plugs—Black to dark- or lightgray, fine- to medium-grained diabasic 

rocks of basaltic or andesitic composition 

that form a north- to northwest-trending 

dike swarm throughout much of northern 

Front Range (Hepp, 1966). Plagioclase 

forms phenocrysts in most dikes and 

matrix consists of finer-grained feldspar, 

hornblende, pyroxene, and magnetite, or 

secondary minerals related to metamorphism 

or hydration alteration. The dike swarm cuts 

1,430 Ma-old Sherman Granite (Zielinski 

and others, 1981) and is cut and deformed 

by 1,420 Ma-old Silver Plume-type granite 

(Cole, 1977). Unit also includes small plugs 

of gabbro, quartz diorite, and granodiorite in 

Glen Haven area (Bucknam and Braddock, 

1989). Dismembered, lensoid pieces of 

these dikes are contained in rotated biotite 

gneisses in upper, flat-lying parts of Longs 

Peak-St Vrain batholith, attesting to extreme 

plastic deformation that accompanied 

emplacement of viscous granite magma 

(Cole, 1977)

YgH Granite of Hagues Peak—Tan, coarse-grained 

to very coarse grained porphyritic biotite 

granite. Contains conspicuous flow-aligned 

phenocrysts of microcline 2–8 cm long; 10–

15 percent biotite. Granite of Hagues Peak 

is crosscut by granite of Longs Peak (YgLP) 

and yields a whole-rock Rb-Sr isochron age 

of 1,480 Ma (C.E. Hedge, written commun., 

1977, cited in Braddock and Cole, 1990)

Yqd Quartz diorite—Fine- to medium-grained, 

medium- to dark-gray, locally gneissic 

rock with accessory hornblende and (or) 

biotite (20–30 percent total). Scattered 

porphyroblasts of microcline are common. 

Rock forms small masses within granite 

of Hagues Peak (YgH), along Highway 34 

west of Estes Park, near Mt Cairns, and at 

Pawnee Lake; probably intruded near same 

time as granite of Hagues Peak

Mesoproterozoic and (or) Paleoproterozoic 

Intrusive Rocks 

YXp Pegmatite—White, medium-grained to very coarse 

grained pegmatite with accessory biotite, 

muscovite, garnet, and (or) tourmaline. 

Most pegmatites probably intruded along 

with granite of Longs Peak (YgLP) but 

some are deformed and spatially related to 

older Boulder Creek Granodiorite (XgdB); 

reliable field criteria for distinguishing pegmatite ages have not been established

Paleoproterozoic Intrusive Rocks 

XgdB Boulder Creek Granodiorite—Light- to mediumgray, medium- to coarse-grained, weakly 

to strongly foliated granodiorite and 

monzogranite containing 5–15 percent 

biotite and 0–5 percent hornblende; 

phenocrysts of microcline (1–4 cm long) 

common (Gable, 1980b). Age of Boulder 

Creek Granodiorite in the Boulder Creek 

batholith was determined to be 1,664 ± 

40 Ma (Rb-Sr isochron; Peterman and 

others, 1968); U-Pb zircon analyses 

suggest true age of intrusion was 1714 

± 5 Ma (Premo and Fanning, 2000) and 

that some monzogranitic phases may be 

significantly younger (ca. 1410 Ma; Premo, 

written commun., 2004). Similar rock was 

emplaced north of Lake Granby, and east of 

Granby in the Strawberry Creek area, but is 

separately identified as unit Xgd

Xgd Granodiorite—Light- to medium-gray, medium- to 

coarse-grained, weakly to strongly foliated 

granodiorite and monzogranite containing 

biotite and variable amounts of hornblende 

(10–15 percent total). Rocks are physically 

and mineralogically similar to Boulder 

Creek Granodiorite but were emplaced in 

bodies separate from the Boulder Creek 

batholith

Xg Biotite monzogranite—Light- to medium-gray, 

fine- to medium-grained, massive to gneissic 

rock that weathers buff to orangish buff. 

Contains accessory biotite (5–10 percent) 

and secondary muscovite. Forms small 

bodies within Boulder Creek batholith

XjT Trondhjemite of Thompson Canyon—Lightgray, very fine grained and porphyritic or 

medium-grained and equigranular, biotitebearing (less than five percent) leucocratic 

trondhjemite; forms thick tabular intrusions 

that are largely concordant to relict bedding 

in enclosing metasedimentary rocks; 

some locales display weak igneous biotite 

foliation parallel to trends of younger cross 

folds in the region (Braddock and Cole, 

1979). Age of emplacement is 1,726 ± 15 

Ma (Barovich, 1986)
Geologic Setting 15

Paleoproterozoic Metamorphic Rocks 

Biotite-bearing schist and gneiss units (Xb, Xbq, Xbk, 

Xbp) are metasedimentary rocks; hornblende gneiss units 

(Xh) and granitic-gneiss units (Xf, Xfb, Xfcq) were probably 

volcanic rocks prior to metamorphism. The biotite-bearing 

schists vary in metamorphic grade and minerology as shown 

by the metamorphic-mineral zone boundaries. Pelitic rocks 

south of the Moose Mountain shear zone are migmatitic and 

are metamorphosed to a higher grade than the M metamorphic 

zone boundary. The age of metamorphism is 1,713 ± 30 Ma 

(Rb-Sr isochron; Peterman and others, 1968) and 1,715–1,690 

Ma (U-Pb zircon; Premo and others, 2007), similar to the 

emplacement age for the Boulder Creek Granodiorite (Premo 

and Fanning, 2000). The primary age of interbedded volcanic 

rocks ranges between about 1,790 Ma–1,760 Ma (U-Pb zircon; 

Premo and others, 2007), consistent with model Sm-Nd ages 

of about 1,800 Ma for formation of the continental crust in 

northern Colorado (DePaolo, 1981)

Xb Biotite schist and gneiss—Conspicuously banded 

rock marked by alternating layers of 

contrasting composition that probably reflect 

original sedimentary layering, as well as 

effects of metamorphic segregation and 

partial melting. Unit is typical of highest 

grade metamorphic terranes, where coarse 

sillimanite is common and partial-melt 

textures are widespread. Dark-gray to black, 

medium- to coarse-grained layers are rich 

in biotite, sillimanite, and magnetite, and 

locally contain high-grade cordierite and 

(or) garnet (Gable and Sims, 1969; Cole, 

1977, 2004b). Irregular quartzofeldspathic 

layers, bordered by selvages rich in biotite, 

sillimanite, and oxide minerals, formed 

in place during partial melting at peak of 

metamorphism. This unit is equivalent 

to units Xbq and Xbk, combined, which 

could not be mapped separately in higher 

metamorphic-grade terranes (Cole, 1977; 

Braddock and Cole, 1990)

Xbq Quartzofeldspathic mica schist—Mica-poor 

schist interbedded with quartzofeldspathic 

metasandstone. Contains interbedded 

knotted mica schist and sparse, thin beds of 

granule-sized metaconglomerate. Locally 

displays crossbedding. Porphyroblastic 

minerals preferentially located in mica-rich 

layers

Xbk Knotted mica schist—Mica-rich biotite-muscovite 

schist or muscovite-chlorite phyllite 

that displays 0.1–0.5 in porphyroblasts 

or clots of metamorphic porphyroblast 

minerals that exhibit a lumpy or knotted 

outcrop appearance. Contains interbedded 

quartzofeldspathic mica schist and sparse, 

thin beds of granule-sized metaconglomerate

Xbp Porphyroblastic biotite schist—Muscovite- or 

sillimanite-rich knotted mica schist that is 

characterized by large biotite porphyroblasts 

(typically 0.3 in diameter)

Xbf Biotite gneiss and granitic gneiss, interlayered

Xbh Biotite gneiss and hornblende gneiss, interlayered

Xh Hornblende gneiss and amphibolite—Dark-gray, 

greenish-gray, fine- to medium-grained, 

weakly to strongly layered hornblendeplagioclase gneiss and hornblende-biotite 

schist, locally interlayered with massive 

amphibolite. Contains thin layers and pods 

of white to light-green calc-silicate gneiss

Xf Granitic gneiss—Light- to medium-gray, fine- to 

coarse-grained, weakly to strongly foliated 

rock composed primarily of quartz, oligoclase, and biotite. Microcline is generally 

present but amount varies from sparse to 

abundant; hornblende, garnet, and rare 

sillimanite locally present. Rocks have 

been described as gneissic granite, gneissic 

alaskite, gneissic quartz monzonite, or 

gneissic granodiorite. Chiefly occurs in a 

large irregular mass south and southeast 

of Grand Lake, and as folded conformable 

sheets within metasedimentary rocks near 

Thunderbolt Peak, west of the Indian Peaks 

Wilderness Area

Xfb Biotite granofels—Light- to medium-gray, fine- to 

medium-grained, granoblastic to weakly 

schistose rock, commonly contains garnet 

and sillimanite; locally interlayered with 

minor biotite gneiss (Xb) and amphibolite 

(Xh). Unit only occurs in southern Never 

Summer Mountains (O’Neill, 1981)

Xfcq Granitic gneiss, calc-silicate gneiss, and quartzite, 

interlayered—Highly variable assemblage 

of interlayered gneisses composed of 

layers, an inch to several feet thick, of some 

or all of the following rocks: quartzite, 

marble, calc-silicate gneiss, microclinerich gneiss, amphibolite, granitic gneiss, 

and amphibole-microcline-quartz gneiss. 

These interlayered gneisses are contained 

within more widespread biotite gneisses. 

Unit only occurs southwest of Meadow 

Creek Reservoir along southern border of 

quadrangle and near Glen Haven

Geologic Setting

The area covered by the Estes Park 30′ × 60′ quadrangle 

displays a diverse assemblage of rocks and surficial units 
16 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

that range in age from Paleoproterozoic to Quaternary (fig. 

3). The geology records several major rock-forming events 

and deformations that are typical of the Southern Rocky 

Mountains of Colorado.

The core of the Front Range consists of igneous and 

metamorphic rocks that formed in the Paleoproterozoic 

between about 1,770 and 1,680 Ma. They were subsequently 

intruded and locally intensely deformed again during 

emplacement of widespread biotite granite at about 1,400 

Ma. The east flank of the Front Range is marked by ridges 

of upturned sedimentary rock in the foothills-hogback belt, 

where tilting reflects Late Cretaceous-Paleogene (Laramide) 

mountain uplift. The sediments in these strata were initially 

deposited as early as the Pennsylvanian Period in response 

to the uplift and erosion of the older Ancestral Rocky 

Mountains. The oldest sediments were deposited adjacent 

to these now-eroded mountains by braided fluvial systems, 

and were succeeded by eolian, deltaic, marine, lagoonal, and 

other fluvial deposits that accumulated slowly through Jurassic 

time. By Early Cretaceous time, the dominant depositional 

environment was a fluvial delta system and lagoons on the 

margin of a vast inland sea; subsidence led to marine incursion 

and deposition of very thick shales and local limestones and 

sands. All of these Phanerozoic rocks underlie the hogback 

belt and the Colorado Piedmont areas of the eastern part of the 

Estes Park quadrangle.

A major regional tectonic event known as the Laramide 

orogeny impacted this area beginning in latest Cretaceous 

and led to the initial uplift of the Front Range in its modern 

configuration (Tweto, 1975). The eastern margin of the range 

rose on west-dipping, high-angle reverse faults (many are 

blind at the present level of erosion) and secondary eastdipping reverse faults that border en echelon tilted blocks 

of the Proterozoic basement. The western margin of the 

Laramide Front Range is marked by more complex, eastdipping thrusts that place Proterozoic crystalline rocks above 

steep and overturned Phanerozoic sedimentary rocks as 

young as Paleocene, exposed in the westernmost part of the 

quadrangle (O’Neill, 1981).

The Late Cretaceous was also marked by intrusive 

activity and hydrothermal mineralization within the Colorado 

Mineral Belt, which affected the Jamestown, Gold Hill, 

Ward, and the Audubon-Albion stock areas in this quadrangle 

(Lovering and Goddard, 1950). 

The Laramide orogeny, probably assisted by intrusions 

and thermal effects of the Colorado Mineral Belt, led to 

withdrawal of the inland sea and exposed the Late Cretaceous 

Front Range to erosion. Fluvial deposits accumulated in 

flanking basins on both the east and west margins of the 

Laramide highlands. Continued erosion and deep weathering 

during the hot, humid climate of the Eocene and Oligocene 

Epochs reduced the height of the Laramide uplifts and 

produced residual ridge crests and large areas of subdued 

rolling topography that are reflected today in broad areas with 

nearly concordant hilltop elevations west of the foothillshogback belt.

Renewed intrusion and volcanism during the Oligocene 

Epoch is recorded by stocks, flows, and ash-flow tuffs 

of the Braddock Peak intrusive-volcanic complex near 

the northwestern corner of the quadrangle. This younger 

magmatic activity was accompanied by high-angle normal 

faulting, both of which might have been manifestations of 

broad regional uplift related to crustal thinning and rise of 

aesthenospheric mantle (Eaton, 1986, 1987) and graben 

faulting in the northern extents of the Rio Grande rift. 

Regional uplift led to erosion across the Front Range and 

deposition of blanketing upper Miocene and lower Pliocene 

gravels and sands east and north of this quadrangle (Steven 

and others, 1997).

The Pliocene and younger history of this area is 

dominated by renewed uplift and erosion (Steven and others, 

1997), accompanied by significant climate change to cooler 

and wetter conditions (Fleming, 1994; Chapin and Kelley, 

1997; Bluemle and others, 2001; Williams and Cole, 2007). 

The drainage system of the South Platte River to the east has 

gradually eroded through the blanketing Miocene gravels 

and excavated the hogback ridges flanking the Front Range. 

Streams in the lower parts of the mountain block have been 

deeply incised into the Proterozoic crystalline core of the 

range.

Climate cooling that began during the Pliocene 

Epoch continued to the full glacial-interglacial conditions 

that characterize the Pleistocene and Holocene (Bluemle 

and others, 2001). Two major phases of mountain-valley 

glaciation are well recorded by high-altitude deposits within 

the Estes Park quadrangle (Madole and others, 1998). The 

spectacular alpine scenery along the Continental Divide 

reflects both glacial sculpting in cirques and downvalley 

transport of rubble by the tongues of ice.

Paleoproterozoic Layered Rocks

Proterozoic igneous and metamorphic rocks exposed in 

the Front Range uplift include the oldest rocks in this part of 

the North American cratonic basement. The metamorphic 

schists and gneisses were chiefly derived from marine 

sediments that were deposited south of the older (Archean) 

craton that is recognized just north of the Colorado-Wyoming 

State line (Reed and others, 1987; Aleinikoff and others, 

1993; Reed, 1993). These metasediments, and interlayered 

metavolcanic rocks (erupted at about 1,760 Ma to 1,780 Ma), 

were progressively metamorphosed at about 1,715 Ma to 

1,700 Ma (Premo and others, 2007) during the time they were 

being folded and intruded by calc-alkaline granodiorite and 

granite (Routt Plutonic Suite of Tweto, 1987). 

The Estes Park quadrangle preserves an unusually 

complete record of this progressive regional metamorphism 

(from chlorite-sericite phyllite to sillimanite-biotite schist 

to partially melted biotite gneisses with residual high-grade 

cordierite and (or) garnet). Metasedimentary rocks in the 

Front Range in general contain sillimanite and show evidence 
Geologic Setting 17

Figure 3. Map showing regional geologic setting of the northern Front Range, Colorado.

40°30’

41°00’

40°00’

106°00’

0 10 20 KILOMETERS

105°00’

Granby

Boulder

Golden

Fort

Collins

Estes

Park

Y

Y

Y

Y K

K

K

Xq K

Y

Y

Y

X

X

X

X

X

Y

Y

Y

Y

Y

Y

Y

Y

Y

Xv

Xv Xv

Xv

Xv

Xv

Xv

Xv

Xv

Xv

Xv

Xv

Xv

Xv Xv

Xv

Xs

Xs

Xs

Xs

Xs

Xs

Xs

Xs

Xs

Xs

Xs

Xs

Xs Xs

Xs

Xs

Xs

Xs

Xs Xs

Xs

Xs

Xs

Xs

Xs

T

T

T T

T

Xs

Xv

K

K

K

K

K

K

X

X

X

X

X

X

X

X

M M� �

M�

M�

M�

M�

M�

M�

M�

Mid-Tertiary intrusion

Cretaceous

Pennsylvanian–Jurassic

Mesoproterozoic intrusion

Paleoproterozoic intrusion

Quartzite

Meta-sediment

Meta-volcanic

Mafic dike

Fault

Thrust fault

Mylonite zone

Concealed fault

EXPLANATION

Laramide intrusion

Tertiary and Quaternary

Y

T

K

X

Xv

Xq

Xs

M�
18 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

of variable degrees of partial melting (Reed and others, 1987). 

In the area of the Big Thompson River canyon east of Drake, 

sillimanite-free assemblages are preserved and the grade 

of metamorphism decreases eastward to greenschist-facies 

phyllites (Braddock and Cole, 1979).

The most common metamorphic unit in the quadrangle 

is a metasedimentary rock consisting chiefly of quartz, 

feldspar, and micas. Original sedimentary structures preserved 

in low-grade rocks near the mouth of the Big Thompson 

River canyon (Braddock, Calvert, and others, 1970) include 

graded bedding, crossbedding, scour-and-fill structures, 

thin conglomerate lenses, and current-bedding lineations 

(Braddock, 1970). These structures, and the generally finegrained nature of the parent sediments, are consistent with 

submarine-fan depositional environments. The two most 

common rock types are distinguished from each other by 

the proportions of original clay/sand: the mica-rich variety 

contains abundant mica and was probably derived from shale 

(Xbk); the quartz-feldspar-rich variety contains less than about 

15 percent mica and was probably derived from sandstone and 

siltstone (Xbq). Thin, persistent beds of very mica-rich rock 

distinguished by conspicuous porphyroblasts of biotite (Xbp) 

are mapped within the metasedimentary sequence north of the 

Big Thompson River canyon.

The distinctions among these varieties of metasedimentary rock diminish in the central and western parts of the 

quadrangle due to widespread partial melting during extreme 

metamorphism. For this reason, all metasedimentary rocks 

west of the longitude of Estes Park (approximately) are 

mapped as biotite gneiss (Xb).

Mafic schists and gneisses are locally prominent in 

the Paleoproterozoic metamorphic terrane of the Front Range. 

They generally form layers that are conformable with the 

surrounding metasedimentary rocks, are chemically similar 

to basalt and andesite, contain local relict phenocrysts, and 

are interpreted to have originated from volcanic flows. They 

typically occur as hornblende schist, hornblende gneiss, or 

amphibolite (Xh). Epidote-bearing calc-silicate schist and 

gneiss are rare but these distinctive rock types are locally 

present in this part of the Front Range. They commonly 

occur in association with the mafic metavolcanic rocks and 

may have formed from sediments eroded from them. 

Quartzofeldspathic, leucocratic gneisses are conspicuous 

elements of the metamorphic terrane in parts of the Estes Park 

quadrangle, especially north of Lake Granby, east of Monarch 

Lake, and in the southern Never Summer Mountains. These 

gneisses typically display compositional banding marked by 

differing proportions of quartz, feldspar, and biotite or by 

variations in grainsize, and their bulk compositions are similar 

to granite, granodiorite, or tonalite. These felsic gneisses are 

commonly interlayered with mafic metavolcanic gneisses 

(O’Neill, 1981; Schroeder, 1995b) or form conformable 

layers within metasedimentary units (Pearson, 1980), and are 

interpreted to have formed from felsic volcanic rocks. Similar 

granitic gneisses in the area north of the Cache la Poudre River 

(also interlayered with hornblende gneiss) contain magmatic 

zircons that are among the oldest in northern Colorado, 

consistent with primary volcanic origin of these granitic 

gneisses at about 1,770 ± 10 Ma (Premo and others, 2007). 

Paleoproterozoic Intrusive Rocks

Proterozoic intrusive rocks are common in the Estes Park 

quadrangle and consist of calc-alkaline granitoids ranging 

from monzogranite to tonalite to trondhjemite in composition. 

These rocks belong to the regional Routt Plutonic Suite of 

Tweto (1987). Their contacts with enclosing metamorphic 

rocks are broadly conformable to compositional layering 

and commonly lobate, suggesting expansive deformation 

of the wall rocks during emplacement. Some contacts are 

locally sharply discordant, but these intrusive rocks also form 

crescent-shaped masses (phacoids) in the cores of wall-rock 

folds that suggest emplacement during regional deformation 

(Braddock and Cole, 1979).

The most widespread unit is the Boulder Creek Granodiorite (XgdB), which was named from its type area in the 

Boulder Creek batholith that extends into the south-central 

part of this quadrangle (Gable, 1980b). The Boulder Creek 

is typically a foliated granodiorite containing biotite and (or) 

hornblende that shows concentrations of mafic minerals in 

schlieren bands, aplitic-pegmatitic zones, and local feldspar 

porphyry textures. Gable (1980b) was able to identify irregular masses within the Boulder Creek batholith consisting of 

tonalitic, granodioritic, and monzogranitic compositions, but 

all such internal distinctions are gradational. Internal dikes are 

not common, but generally consist of leucocratic granitic and 

aplitic rocks or pegmatite. The Boulder Creek batholith was 

emplaced at 1,713 ± 4 Ma, based on the statistical weightedmean of values determined from individual zircon grains 

(Premo and Fanning, 2000).

Rock similar to the type Boulder Creek Granodiorite 

(XgdB) has been mapped in three other principal locations in 

the quadrangle, designated as Paleoproterozoic granodiorite 

(Xgd). Pearson (1980) mapped a substantial pluton of 

foliated granodiorite east and west of Arapahoe Pass along 

the southern border of the quadrangle. This body is largely 

conformable to compositional banding in the enclosing 

biotite gneisses and contains large concordant sheets of the 

same. Schroeder (1995b) mapped a large, equant intrusive 

body in the Strawberry Lake quadrangle that shows generally 

conformable contacts with biotite gneiss around its margins 

and steep internal foliation; phases of this pluton are 

conspicuously porphyritic with microcline phenocrysts as long 

as 16 cm. Complex amoeboid bodies of Boulder Creek-type 

granodiorite were mapped in the Glen Haven quadrangle in 

the vicinity of Crosier Mountain by Bucknam and Braddock 

(1989); they are intimately intruded by, and surrounded by 

extensive bodies of pegmatite-aplite that were probably 

emplaced at Boulder Creek time.

Biotite monzogranite and syenogranite (Xg) have been 

mapped in several small areas of the quadrangle that differ 
Geologic Setting 19

from the Boulder Creek in composition but probably belong 

to the Paleoproterozoic Routt Plutonic Suite (Tweto, 1987). 

Many of these are small masses within the Boulder Creek 

batholith, referred to as Twin Spruce monzogranite (“quartz 

monzonite” of Gable, 1980b). Detailed zircon geochronology 

by Premo and Fanning (2000), however, has demonstrated that 

some monzogranite within the Boulder Creek batholith was 

emplaced much later during the Berthoud Plutonic Suite at 

about 1,400 Ma.

Distinctive fine-grained biotite trondhjemite (XjT) 

intrudes the metasedimentary section in the vicinity of 

the Big Thompson River canyon. This rock typically 

forms continuous, concordant sheets that can be traced for 

miles along the strike of sedimentary bedding (Braddock, 

Nutalaya, and others, 1970). Several stubby sills coalesce 

at Palisade Mountain, east of Drake, to form a cylindrical 

plug; surrounding sedimentary bedding is deformed outward 

by expansive emplacement of the trondhjemite. Braddock 

and Cole (1979) inferred that the trondhjemite magmas were 

emplaced during the waning stages of regional folding because 

they crosscut an older set of regional folds but contain aligned 

biotite in the direction of axial surfaces of the youngest set 

of regional folds. These observations were confirmed in 

more detailed work by Barovich (1986), who also determined 

the trondhjemite intruded at 1,726 ± 15 Ma by U-Pb zircon 

methods.

Paleoproterozoic Metamorphism

Metamorphic rocks of the Estes Park quadrangle record 

evidence of two regional metamorphic events. The first 

is only preserved in the lowest grade phyllites and schists 

and is indicated by aligned sericite, chlorite, and muscovite 

that probably grew from detrital clay minerals. This first 

metamorphism seems to have accompanied and followed 

the first regional deformation during which major tight folds 

formed within the thick pile of saturated marine sediments 

(Braddock, 1970; Braddock and Cole, 1979). Original clay 

minerals were aligned parallel to axial-plane cleavage of firstgeneration folds, a process that involved laminar intrusion of 

clay-rich seams through more granular sand-silt beds during 

soft-sediment deformation (Braddock, 1970). Subsequent 

recrystallization of these clay minerals during the earliest 

metamorphism produced sericite, chlorite, and muscovite 

whose fabric alignment mimicked the alignment of clay 

minerals.

 The second episode of metamorphism produced the 

pervasive prograde mineral assemblages within the basement 

rocks of the Front Range and largely coincided with intrusion 

of the Boulder Creek Granodiorite (XgdB) at about 1,715 

Ma (Peterman and others, 1968; Braddock and Cole, 1979; 

Hutchinson and Braddock, 1987). Metamorphic grade ranges 

from greenschist near the mouth of the Big Thompson River 

canyon (fig. 4A) to upper-amphibolite/partial-melting west 

of Drake. This higher metamorphic grade is typical of most 

areas in the Front Range. The lower-grade assemblages are 

preserved in an east-west swath that marks a fossil thermal 

“trough” outlined by nested isograds defined by the appearance/disappearance of zonal minerals (and mineral assemblages) from east to northwest, west, and southwest (fig. 

4A; Braddock and Cole, 1979; Nesse, 1984). From low- to 

high-grade, pelitic rocks display porphyroblasts of chlorite, 

chloritoid, biotite, garnet, staurolite, cordierite, andalusite, and 

potassium feldspar that formed through sequential reactions 

(figs. 4B–E). Sillimanite first appears in pelites at about the 

position that low-grade garnet and cordierite disappear, and 

staurolite and andalusite disappear a short distance up-grade 

from the sillimanite-in line. All prograde muscovite was 

consumed at temperatures below the onset of melting by the 

reaction muscovite + quartz = potassium feldspar + sillimanite 

+ water (Nesse, 1984).

Partial melting (summarized from Cole, 1977, 2004b) 

occurred in the pelitic and quartzofeldspathic metasediments 

that consisted of varying proportions of quartz, plagioclase, 

potassium feldspar, biotite, sillimanite, and magnetite. The 

low-melting fraction of quartz and feldspars melted first (in 

the presence of water) and segregated into seams and lenses 

(leucosomes), generally parallel to compositional banding in 

the host rock. These leucosomes are typically bordered by 

dark-colored selvages (melanosomes) that contain enriched 

concentrations of the non-melted fraction of the rock (chiefly 

biotite, sillimanite, and magnetite). This distinctive texture 

of leucosome-melanosome segregations within the nonmelted host rock is the characteristic of migmatite, and 

its first appearance has been systematically mapped in the 

Estes Park quadrangle, forming the outermost reactionisograd of the nested zonal isograds along the course of Big 

Thompson Canyon (Braddock and Cole, 1979). The fact that 

the migmatite-in isograd lies generally parallel to the other 

mineral-reaction isograds, and in the direction of inferred 

higher temperatures and pressures, is strong evidence that the 

leucosomes formed in place by partial melting and were not 

injected from some external magmatic source (Nesse, 1984).

Progressive partial melting is indicated by the local 

appearance of new porphyroblastic cordierite and (or) garnet 

in more strongly melted rocks (fig. 4D). These minerals 

originated through incongruent melting of biotite, a process 

that added iron and aluminum to the partial melt and produced 

magnesium-enriched cordierite or garnet (or both) in the 

residue, depending on bulk composition of the rock (Cole, 

1977). This high-grade cordierite, and garnet to a lesser 

degree, replaces sillimanite and biotite (Gable and Sims, 1969; 

Cole, 1977, 2004b).

Regional metamorphism took place during the protracted 

orogenic event marked by the intrusion of Boulder Creek 

Granodiorite (XgdB) and by the formation of kilometer-scale 

folds of two contrasting trends (Braddock and Cole, 1979; 

Hutchinson and Braddock, 1987). The thermal maximum is 

interpreted to have coincided with the peak of deformation 

in the highest-grade rocks. The conclusion is supported by 

1) partial-melt leucosome masses that are phacolithic in fold 
20 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

hinges and elongate parallel to fold axes (fig. 4E), 2) coarse 

sillimanite and biotite aligned with fold axes (fig. 4C), and 3) 

some potassium feldspar porphyroblasts containing helicitic 

inclusions of quartz and magnetite that suggest rotation during 

growth. This conclusion has not yet (2007) been confirmed by 

high-resolution geochronology. 

The evidence from the lower-grade terranes is quite 

different. Mineral textures here indicate the thermal peak of 

metamorphic recrystallization traversed the rocks following 

the conclusion of folding. All porphyroblastic minerals, and 

especially andalusite, cordierite, and staurolite, are irregular 

and amoeboid and overprint the traces of small folds defined 

by shingled muscovite, biotite, quartz, and feldspars (Cole, 

1977, 2004b; Braddock and Cole, 1979).

Interpreted conditions at the peak of metamorphism 

have been estimated by several authors. Cole (1977, 2004b) 

concluded that peak conditions were about 5.5 kb, 

675°–725° C, and undersaturated with vapor on the basis 

of regional relations and experimentally calibrated phase 

equilibria. Nesse (1984) used mineral compositions and phase 

equilibria to conclude that the muscovite + quartz = potassium 

feldspar + sillimanite + water reaction occurred at 3–4 kb 

and about 650° C under water-saturated conditions (higher 

pressures would have prevailed under water-undersaturated 

conditions; Cole, 1977). Both authors conclude that pressures 

greater than about 6 kb could not have been attained because 

no kyanite has ever been reported in this region.

Detailed mineralogical and chemical studies in a few 

scattered localities in the Front Range have led to conclusions 

widely different from the above. Munn and Tracy (1992) 

and Munn and others (1993) reported thermobarometric 

calculations based on garnet, biotite, and hornblende 

chemistry that suggested pressures near 7 kb north of the Estes 

Park quadrangle. Selverstone and others (1995, 1997) assert 

pressures of 8 kb to 10 kb based on unpublished chemistry of 

garnet and plagioclase in staurolite-bearing schists near the 

Big Thompson River, but do not explain how such extreme 

conditions could have been attained in the absence of partial 

melting or without formation of kyanite. We discount these 

conclusions and suggest that either the chemical analyses 

were obtained from non-equilibrium assemblages (Cole, 

2004b) or that the underlying chemistry-temperature/pressure 

calibrations are not applicable to these rocks.

Retrograde metamorphism is manifest by three distinct 

kinds of mineral transformations that depend on the extent of 

prograde metamorphism. The overall patterns of retrograde 

assemblages indicate that they were not influenced by 

intrusions of much younger plutons at about 1,400 Ma 

(Cole, 2004b). In the lower-grade schists, aluminous 

porphyroblasts are replaced by very fine grained sericite and 

chlorite, accompanied by tourmaline. Retrograde muscovite 

is common in medium- and high-grade metasedimentary 

rocks where it forms irregular, non-oriented porphyroblasts 

that replace oriented sillimanite and biotite; tourmaline 

and topaz also occur in this assemblage (Cole, 1977). This 

porphyroblastic retrograde muscovite is mostly confined to 

rocks near and below the migmatite-in isograd and seems 

to indicate it formed with water that was released from 

crystallizing leucosome melts that reacted with potassium 

feldspar and sillimanite (Cole, 1977). The highest-grade 

metasediments show only limited retrograde reactions, 

probably because most of the water released from crystallizing 

partial melts had already migrated to higher, cooler areas of 

the orogen. Small amounts of wormy andalusite are present in 

some migmatites, particularly those with abundant high-grade 

cordierite indicating high degrees of partial melting (Gable 

and Sims, 1969; Cole, 1977). The origin of this late andalusite 

is not clear, but textural evidence suggests it is related to 

breakdown of biotite to andalusite + magnetite under very dry 

conditions (Cole, 1977).

Selverstone and others (1995, 1997) and Shaw and others 

(1999b) have asserted that the metamorphic assemblages 

in this area reflect the combined effects of regional 

metamorphism at about 1,710 Ma and again at about 1,400 

Ma. These statements appear to be based on ambiguous 

textural relations involving staurolite, and on numerous 

1,400 Ma ages determined by Ar–Ar analysis from prograde 

minerals and similar U-Pb results for monazite (see also Shaw 

and others, 1999a). It has been known for decades that most 

single-mineral isotopic systems in the Colorado basement 

were reset due to generalized heating that accompanied 

intrusion of widespread granitic rocks of the Berthoud 

Figure 4 (facing page). Photographs and map showing features 

of Paleoproterozoic metamorphism in the northern Front Range 

(C, D, E photographs, J.C. Cole, 1972–1974). A, Sketch map 

showing distribution of metamorphic-mineral isograds in pelitic 

rocks, marked by the first appearance (+) or disappearance (-) of 

andalusite (A), biotite (Bi), cordierite (Cd), garnet (G), potassium 

feldspar (K), sillimanite (S), or staurolite (St). Line marking 

appearance of partial-melt textures (+melt) indicates the western 

and northern extent of unmelted (low-grade) pelitic rocks. Garnet 

and cordierite in restite phase of partially melted gneisses (dot 

and square symbols) originated through incongruent melting 

of biotite and are compositionally and texturally distinct from 

garnet and cordierite in the lower-grade terrane to the east and 

southeast. B, Photomicrograph showing matrix cordierite (Cd), 

quartz (colorless), and muscovite (Ms) with porphyroblastic 

staurolite (St) in low-grade schist (W.A. Braddock photograph, 

1970). C, Photomicrograph showing coarse, oriented prisms of 

sillimanite (Si) intergrown with polygonal cordierite (Cd) that 

replaces fine sillimanite needles and remnant biotite (Bi) in 

partially melted biotite gneiss. D, Photomicrograph showing 

polygonal cordierite (Cd) and irregular garnet (G) replacing 

biotite (Bi) and sillimanite (Si) in partially melted biotite gneiss. E, 

Outcrop photograph showing partial-melt textures (light-colored 

quartz-feldspar leucosomes surrounded by dark melanosomes 

containing unmelted biotite, sillimanite, and magnetite) in partially 

melted biotite gneiss. Leucosomes form rod-shaped bodies 

elongate parallel to fold axis due to growth during deformation. 

Lens cap about 2 in diameter.
 

Geologic Setting 21

Garnet

Cordierite

Minerals in partial

melt restites

+G

+St

+A

+S

-St

+Melt +K

+S

+S

-A

+K

+K

+K

+K

+Melt

+Melt

+Melt

−A, −Cd, −G

-A

-Cd

-G

-Cd

-G

Biotite is the only (Fe,Mg)

 phase in this zone

Big Thompson

Canyon

105°30’

40°30’

Estes

 Park

A

Ms 1 mm

Cd St

Cd

St

St

Cd

B

E

Si

Si

Cd

Cd

Cd Bi

1 mm

C

Si

Cd

Cd

Cd

Cd

G

Bi

Bi

1 mm

D

Moose Mountain shear zone
22 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Plutonic Suite (for example, Peterman and others, 1968). 

However, the textural and regional field evidence described 

above clearly shows that the sequential mineral assemblages 

preserved in the metasedimentary rocks formed during a single 

major Paleoproterozoic prograde event. The age recorded by 

closure of isotopic systems within individual minerals does not 

date the time of growth of the particular minerals.

Mesoproterozoic Intrusive Rocks

Younger Proterozoic intrusive rocks were emplaced 

between about 1,480–1,380 Ma (Berthoud Plutonic Suite 

of Tweto, 1987) and are widespread in the Estes Park quadrangle. These include a regional swarm of northwest-trending 

mafic porphyry dikes, batholithic masses of distinctive Silver 

Plume-type granite, smaller bodies of granodiorite and quartz 

diorite, and pegmatite. In addition, an unusually long, narrow 

ferrogabbro dike swarm known as the Iron Dike (Ygb) was 

intruded across the entire quadrangle and beyond at about 

1,315 Ma (Braddock and Peterman, 1989).

The oldest magmas of the Berthoud Plutonic Suite are 

relatively sparse in this area and are more mafic than the 

abundant, younger magmas. The granite of Hagues Peak 

(YgH; north-central border of the quadrangle) is a biotite 

monzogranite to granodiorite that is distinguished by large 

microcline phenocrysts and as much as 15 percent biotite, both 

of which are flow-aligned; it was intruded at about 1,480 Ma 

(Braddock and Cole, 1990). Small masses of undated biotitehornblende quartz diorite are present in the area west of Estes 

Park and are cross-cut by the granite of Hagues Peak (YgH) 

and by widespread Silver Plume-type granite.

Mafic porphyry dikes form a persistent and widespread 

swarm that trends north-northwest through the metamorphic 

rocks of the Front Range. These distinctive dikes are typically 

1–5 m thick and 100 m to several kilometers long and are 

especially common in the areas west of Loveland and north 

of this quadrangle. They consist of basalt and mafic andesite 

with conspicuous plagioclase phenocrysts in a fine-grained 

groundmass with diabasic texture (Hepp, 1966; Kellogg, 

1973). These dikes crosscut older batholiths of the Berthoud 

Plutonic Suite (Sherman Granite) and are intruded and 

deformed by younger batholiths of Silver Plume-type granite 

(Eggler, 1968; Peterman and others, 1968; Cole, 1977). Their 

emplacement age is constrained by the ages of these batholith 

granites between about 1,415 Ma and 1,405 Ma. 

Silver Plume Granite is a recognized lithodemic-unit 

name (Lovering and Goddard, 1950; Tweto, 1987) for widespread intrusive rocks in the Front Range that form irregular 

batholiths, simple ovoid plutons, and dikes. The name is 

based on the type area at Silver Plume, Colorado (60 km 

south of Estes Park), where characteristic mineralogical and 

textural traits are displayed (Tweto, 1987). Throughout the 

Front Range (but outside the type area of the Silver Plume 

batholith), granites that possess Silver Plume characteristics 

have been correlated with the type rock-unit and the name 

Silver Plume Granite has been used in order to express 

the general similarities in age, texture, and composition. 

However, each batholith of Silver Plume-type granitic 

rocks conveys its own history and it is likely that each arose 

at somewhat different times under somewhat individual 

circumstances.

In the interest of clarity, and in concert with the compilers 

of adjacent 30′ × 60′ quadrangles, we elected to restrict use of 

the term Silver Plume Granite to the Silver Plume batholith in 

its type area. We describe correlated rock masses elsewhere in 

the northern Front Range as Silver Plume-type granites. Each 

individual intrusive complex is designated according to the 

historically established name of the pluton or batholith where 

it intruded. In this quadrangle, granite of Longs Peak (YgLP) 

denotes the Silver Plume-type granitic rocks within the Longs 

Peak-St Vrain batholith.

The granite of Longs Peak (YgLP) is fairly homogeneous 

in composition and mineralogy, but shows local textural and 

grain size variations. The most typical rock types consist 

of biotite monzogranite and biotite syenogranite, some of 

which contain accessory sillimanite and (or) garnet. The most 

common variety of granite of Longs Peak (YgLP) is coarsely 

porphyritic and contains conspicuous tabular phenocrysts of 

microcline, typically flow oriented (fig. 5A), that are 1–3 cm 

long and set in a fine- to medium-grained matrix of quartz, 

feldspars, biotite, and accessory minerals. This trachytic 

feldspar foliation is a mappable feature within the granite 

and defines internal structure formed by flow patterns of the 

magma as it intruded (Cole, 1977). Less common varieties 

are finer grained, less porphyritic, more leucocratic, and some 

grade into aplite-pegmatite bodies. 

Anderson and Thomas (1985) studied the chemistry, 

mineralogy, and isotopic characteristics of Silver Plume-type 

granites in this area. They noted the presence of muscovite 

in some samples and interpreted it to be a primary, magmatic 

phase based (presumably) on the fact that biotite and 

muscovite are “intergrown.” We conclude otherwise that the 

muscovite is a subsolidus mineral formed by breakdown of 

magmatic sillimanite and potassium feldspar (to muscovite 

+ quartz), and by the breakdown of biotite to muscovite and 

magnetite (Cole, 1977; Braddock and Cole, 1990). Our 

conclusion is based on textural and spatial criteria. The 

magmatic biotite is generally flow-aligned parallel to feldspars 

in the rock, but subsolidus muscovite is everywhere randomly 

oriented. In addition, this late muscovite is preferentially 

located near the external margins of the major batholith and is 

essentially absent from the center (Cole, 1977).

Almost all of the Silver Plume-type granite in the Estes 

Park quadrangle was emplaced within the structurally complex 

Longs Peak-St Vrain batholith (Boos and Boos, 1934; Cole, 

1977). This body extends from the Left Hand Creek drainage 

on the south to just beyond the northern quadrangle boundary, 

and from the Colorado River on the west to beyond the unconformity with the Phanerozoic sediments on the east. The body 

is irregular in three-dimensional form but spans approximately 
30 mi by 35 mi and comprises nearly half of the Proterozoic 

rocks exposed in the quadrangle.

External contacts of the Longs Peak-St Vrain batholith 

are discordant to wall rock foliation in detail, but broadly 

conformable. Outer contacts tend to be relatively steep 

and lobate. Trachytic foliation within the granite defines 

local antiformal patterns that are separated by synformal 

screens of metamorphic rock (fig. 5C, 10 following). Cole 

(1977) attributed these structural patterns to gravity-induced 

sinking of denser metasedimentary rocks between buoyant, 

rising lobes of the viscous magma near the time of pluton 

consolidation.

Contacts with wall rock in the upper most parts of 

the Longs Peak-St Vrain batholith dip very gently and are 

strongly concordant with wall rock foliation (figs. 5C–E). The 

intrusion geometry is well displayed in the glaciated valleys 

on both sides of the Continental Divide, where numerous 

flat-lying sheets of the light-colored granite are separated by 

conformable tabular masses of dark-colored biotite gneiss. 

Cole (1977) argued that these contact relations indicate 

the magma was nearly as viscous as the wall rock during 

intrusion, and that both of them deformed together as the 

buoyant magma rose into the middle crust (fig. 5B). The 

flat-lying foliation of the metamorphic wall rocks is the result 

of bodily rotation (from pre-existing near-vertical attitudes) 

during emplacement of the granite. Remnants of the mafic 

porphyry dikes that were regionally emplaced as sub-vertical 

dike swarms just prior to the Longs Peak-St Vrain batholith 

are preserved only as dismembered, flattened tabular slabs 

within the enclosing biotite gneisses. Reconnaissance 

paleomagnetic investigation of these dismembered dike 

segments showed that the formerly vertical dikes had 

experienced large angular rotations in the deformed contact 

zone of the batholith (Cole, 1977).

The Silver Plume-type granites are representative of a 

suite of similar granitic intrusive rocks that were emplaced 

in the North American craton at about 1,400 ± 50 Ma along 

a broad belt between southern California and Labrador. 

These granites are compositionally similar, chiefly biotite 

monzogranite or syenogranite, and are either metaluminous 

or slightly peraluminous (Anderson, 1983). Most have initial 

strontium-isotope ratios that are similar to the calculated 

1,400-Ma mantle and thus do not seem to have assimilated 

large amounts of more-radiogenic wall rock. Many share 

the characteristic porphyry texture with the Silver Plume or 

the rapakivi texture of mantled feldspar phenocrysts. They 

generally have simple crosscutting relations with wall-rock 

structures (in regional context), and Anderson (1983) referred 

to them as “anorogenic” granites due to the general absence of 

evidence for coeval regional deformation. These granites do 

not seem to have formed through melting above a descending 

oceanic-crustal slab because they lack the compositional 

diversity typical of that environment, few basalts are 

associated with them, and no supportive plate-tectonic context 

has been identified for this part of North America at 1,400 

Ma (Anderson and Cullers, 1999). We believe that their 

widespread occurrence, compositional homogeneity, and 

primitive isotopic character indicate they formed as a result 

of fundamental changes in mantle structure (possibly related 

to mineralogical phase changes) at 1,400 Ma (Cole, 1977; 

Anderson and Bender, 1989).

Selverstone and others (1997, 2000) have argued instead 

that the regional stress field during Silver Plume Granite 

emplacement was compressional with a shortening direction 

oriented north-northwest. We concur with Cole (1977) and 

Anderson and Cullers (1999) that regional stress was neutral 

or slightly extensional during intrusion. The evidence from 

this area (and the contrasting interpretations) are presented 

below in the section on Mesoproterozoic structure.

The youngest Proterozoic rock in the Estes Park 

quadrangle is the Iron Dike (Ygb), a distinctive intrusion 

swarm that consists of iron-rich augite ferrogabbro. Its 

chemistry was studied in some detail by Wahlstrom (1956) 

and Cole (1977) described its form, contact relations, and 

mineralogy. Braddock and Peterman (1989) reported an 

emplacement age of 1,316 ± 50 Ma based on a mineral-wholerock Rb-Sr isochron. The Iron Dike consists of a swarm of 

north-northwest trending sub-vertical dikes that can be traced 

from just west of Boulder through the Gold Hill area to Allens 

Park, and then through Rocky Mountain National Park and 

Trail Ridge to the northern quadrangle boundary at Long Draw 

Reservoir. Further mapping to the north shows that this swarm 

continues along the east side of the Medicine Bow Mountains 

as far as the Wyoming-Colorado State line (Pearson and 

others, 1982; Braddock and Cole, 1990). The combined 

length of the swarm is approximately 95 mi. Thickness 

ranges from a few feet or less in some of the thinner dikes to 

as much as 130 ft. Contacts with the Proterozoic wall rock 

are typically chilled within a few inches of the margin (Cole, 

1977). The Iron Dike (Ygb) was hot enough during intrusion 

to cause local grain-contact melting in the Silver Plume-type 

granite (Braddock and Peterman, 1989).

Kimberlite

Three small bodies of kimberlite intrusive rock (Dk) 

are present in the Estes Park quadrangle. Other kimberlite 

pipes and dikes have been mapped and explored in the 

northern Front Range between the Colorado-Wyoming State 

line and Boulder (Chronic and others, 1969; Smith, 1979; 

Eggler and Braddock, 1988; Kellogg, 1973). They typically 

contain abundant phlogopitic biotite, pyrope garnet, chromian 

diopside, and widespread carbonate and sheet-silicate 

alteration products. Some pipes in the State-line area have 

yielded small quantities of industrial-grade diamond, but none 

has been recovered from the Estes Park quadrangle. 

Fossiliferous Cambrian through Silurian limestone blocks 

are contained in some of the kimberlite pipes in the State-line 

area, and initial fission-track studies supported a Devonian 

age of emplacement (Naeser and McCallum, 1977). More 

recent work by Lester and others (2001) shows that at least 

Geologic Setting 23
24 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

A B

C

E

D
Geologic Setting 25

the kimberlites near Boulder and in the Cache la Poudre River 

drainage (north of this quadrangle) were emplaced at about 

700 Ma to 600 Ma during the Neoproterozoic. Phlogopite 

from the kimberlite dike located about 2 mi south of Estes 

Park produced an Rb-Sr isochron age of 404 ± 18 Ma (Early 

Devonian; Smith, 1979).

Pre-Cenozoic Sedimentary Rocks

Paleozoic and Mesozoic sedimentary rocks are exposed 

on both the east and west flanks of the Front Range uplift in 

the Estes Park quadrangle. Excellent summaries of detailed 

investigations of the Phanerozoic units are available for 

this area, including Lee (1927), Scott and Cobban (1965, 

1986), and Weimer (1996). These papers and many of the 

source geologic maps form the basis for the following short 

summations.

The oldest sediments deposited on the eroded 

and weathered Proterozoic crystalline rocks are fluvial 

sandstones that reflect uplift and erosion during the Ancestral 

Rocky Mountains orogeny in Pennsylvanian time. The 

Pennsylvanian and Permian Fountain Formation (Phf) 

(arkosic conglomerate and sandstone; 1,100 ft thick) records 

the initial deposition along the eastern flank of the uplift. 

The Fountain Formation is overlain by Permian Ingleside 

Formation (Pi) (planar-bedded sandstone; 50 to 200 ft thick), 

Owl Canyon Formation (Po) (red siltstone; pinches out south 

of Little Thompson River; maximum 250 ft), and Lyons 

Formation (Pl) (eolian crossbedded sandstone and fluvial 

sand; 60 ft thick). The Lyons is overlain by Permian and 

Triassic Lykins Formation (dPl) that comprises about 550 ft 

of red siltstone and fine sandstone, with limestone and gypsum 

beds (Forelle Limestone Member and other units) near the 

base. The Upper Triassic Jelm Formation (in unit Jdmj) 

overlies the Lykins and consists of about 100 ft of crossbedded 

fine sandstone.

The west flank of the modern Front Range remained 

positive through Pennsylvanian and Early Permian time. The 

oldest sediments deposited there (northern Never Summer 

Mountains and Granby valley areas) are the Chugwater 

Formation (dPc) of Permian and Triassic age (including 

Forelle Limestone Member and probable upper Satanka 

Shale), roughly equivalent to the Lykins Formation (dPl) on 

the east flank. 

By Middle Jurassic time, the Ancestral Rocky Mountains 

uplift had been eroded to sea level in this area. Fine-grained, 

crossbedded sandstone and laminated siltstone of the Sundance Formation (in unit Jms), probably equivalent to the 

Canyon Springs Sandstone Member, is preserved on both the 

east (about 15 ft thick) and west (about 125 ft thick) flanks of 

the Front Range. The Upper Jurassic section is represented 

by about 300 ft of Morrison Formation (Jm), consisting of 

variegated fluvial claystone, siltstone, and thin sandstones on 

both sides of the Front Range.

The vast, shallow Western Interior seaway flooded 

the center of North America beginning in Early Cretaceous 

time. The marginal-marine Dakota Group (Kd) of sandstone, 

shale, and sparse limestone comprises the Lower Cretaceous 

section across the whole of the Estes Park quadrangle. The 

disconformable base of the Dakota Group is overlain by 

pebbly sandstones of the Lytle Formation (about 60 ft thick) 

representing deposition on a channeled floodplain. The 

overlying South Platte Formation contains the transgressive 

Plainview Sandstone Member at its base (30 ft thick), deepwater shales equivalent to the Skull Creek Shale (about 90 ft 

thick), and beach and tidal-flat sands equivalent to the Muddy 

(or “J”) Sandstone at the top (40 to 100 ft thick).

Combined subsidence and sea-level rise near the end 

of Early Cretaceous time led to the return of full marine 

conditions that persisted throughout most of the Late 

Cretaceous. Shale, thin-bedded limestone, and sparse sands 

accumulated in the Western Interior seaway across this 

entire area and are known (from bottom to top) as the Lower 

Cretaceous Mowry Shale (10 ft thick) and the overlying units 

of the Benton Shale (Kbm), consisting of Upper Cretaceous 

Graneros Shale (160 ft thick), Greenhorn Limestone (260 ft 

thick), and Carlile Shale (80 ft thick). The overlying Niobrara 

Formation (Kn) consists of the Fort Hays Limestone Member 

(15 ft thick) and the Smoky Hill Shale Member (65 ft thick).

Increased rates of subsidence are recorded by the 

Upper Cretaceous Pierre Shale (Kp) across the Estes 

Park quadrangle. Subsurface data show that the Pierre is 

approximately 5,500 ft thick in North Park (northwest of 

Figure 5 (previous page). Photographs showing internal structures and intrusive relations of the Longs Peak-St Vrain batholith (J.C. Cole photographs, 1972–1975). A, Outcrop photograph 

showing typical texture of granite of Longs Peak (YgLP) marked 

by strong flow-alignment of feldspar phenocrysts. Lens cap 

about 2 in diameter. B, Outcrop photograph showing sills of lightcolored granite of Longs Peak (YgLP) that were folded along with 

the enclosing, banded biotite gneisses. Continuity of structure 

indicates folding took place during intrusion of granite. Hammer 

handle at left is about 1 ft long. C, Photograph of the west face of 

Longs Peak, Keyboard of the Winds, and Pagoda Mountain showing banded structure of intrusive granite sheets (YgLP, gray) 

separated by tabular sheets of biotite gneiss (Xb, dark). Arches 

and swales outlined by the biotite-gneiss layers show apparent 

folding that reflects rising, buoyant magma and sinking, dense 

biotite gneiss during intrusion of the Longs Peak-St Vrain batholith. D, Photograph looking west from Mt Meeker ridge toward the 

east face of Longs Peak (summit at left) showing detailed pattern 

of numerous sills of granite (YgLP, gray) intruded along foliation 

planes in biotite gneiss (Xb, dark). E, Photograph of steep rock 

wall on south ridge of Pagoda Mountain showing tabular structure formed by subhorizontal sheets of granite (YgLP, light and 

medium gray) that intrude each other and intrude biotite gneiss 

(Xb, dark). The arch in the lower-middle part of the image was 

formed by a finger-like extension near the margin of a tabular 

sheet of granite (light gray, lower part of image). 
26 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

this quadrangle) and 6,800 ft thick in the deepest parts of the 

Denver Basin east of Boulder, all of which was deposited 

in about 14 million years (Obradovich and Cobban, 1975; 

Obradovich, 1993). The isopach patterns of Late Cretaceous 

deposition suggest the accelerated subsidence may have 

been caused by crustal flexure, a possible harbinger of the 

Laramide Front Range uplift. From base to top, the Pierre 

Shale (Kp) is compiled on this map as the lower shale member 

(Kpl); Hygiene Sandstone Member (Kph); middle shale 

member (Kpm); combined Rocky Ridge Sandstone, Larimer 

Sandstone, and Richard Sandstone Members (Kpr); upper 

shale member (Kpu); and upper transition member (Kpt) 

(Scott and Cobban, 1965, 1986).

 The Western Interior seaway began to recede in Late 

Cretaceous, and its withdrawal in this area is marked by the 

strandline marine deposits and coeval, interfingering, and 

younger deposits of coastal plain, coal-forming swamp, and 

delta-plain environments (Roberts, 2005). The shore-line 

sands are the Fox Hills Sandstone (Kfh), which consists of 

shingled sand bodies representing repeated minor transgressions and regressions along the seaward edge of a large delta 

complex (Davis and Weimer, 1976; Weimer, 1996); aggregate 

thickness is as great as 330 ft in the Erie quadrangle (Colton 

and Anderson, 1977; Roberts, 2005). The overlying coastalplain deposits comprise the Laramie Formation (Kl) that 

contains numerous seams of commercially extracted coal.

Laramide Synorogenic Sedimentary Rocks

The landscape of much of the Rocky Mountains area 

changed dramatically as the Western Interior seaway receded 

near the end of Cretaceous time. The outlines of the modern 

mountain ranges in this part of Colorado were established 

when subduction-related buckling initiated uplift on mountainscale crustal blocks (Dickinson and Snyder, 1978). The 

deformation (tilting and uplift) began at somewhat variable 

times in different places (Kluth and Nelson, 1988). The end 

of marine deposition and the transition to coastal and subaerial 

environments is generally taken to mark the onset of the 

Laramide orogeny (Tweto, 1975; Kluth and Nelson, 1988; 

Raynolds, 2002).

 As the mountain blocks rose, erosion stripped some 

of the Cretaceous and older cover and shed synorogenic 

sediments into adjoining foreland basins, especially east 

of the Front Range (Tweto, 1975; Weimer and LeRoy, 

1987; Raynolds, 2004). Coastal plain, swamp, and delta 

plain deposits constitute the subaerial facies of the Laramie 

Formation (Kl) that consists of sandstone, claystone, 

carbonaceous shale, and several economic coal seams in the 

lower several hundred feet; the Laramie is chiefly claystone in 

the upper several hundred feet (Roberts, 2005). The Laramie 

Formation was deposited over about the same time interval as 

the recessional shoreline deposits of the Fox Hills Sandstone 

(Kfh). An erosional unconformity marks the top of the 

Laramie-Fox Hills sequence in many areas, separating it from 

alluvial channel deposits (arkosic conglomerate and sandstone 

of the Arapahoe Formation) exposed east and south of this 

quadrangle (Weimer and LeRoy, 1987; Raynolds and Johnson, 

2003).

Cobble conglomerate and arkosic sediment derived 

from eroded Proterozoic basement rocks mark the pulse of 

significant Laramide mountain uplift. These synorogenic 

sediments accumulated to the east in the Denver Basin 

(Arapahoe Formation and younger Tertiary deposits; 

Raynolds, 2004) almost continuously from about 70 Ma to 64 

Ma (Raynolds and Johnson, 2003). Stacked, coarse alluvialfan deposits and intervening carbonaceous siltstones in this 

sequence indicate that uplift was episodic. In the western and 

southern parts of the Denver Basin, sediment accumulation 

ceased after about 64 Ma and a deep-weathering profile (and 

paleosol) was formed prior to deposition of another coarsegrained alluvial fan after about 54 Ma (Raynolds and Johnson, 

2003).

The Middle Park Formation and partly equivalent 

Coalmont Formation (bc) are the synorogenic deposits 

preserved on the western flank of the Front Range uplift. 

They resemble the synorogenic deposits of the Denver Basin 

but seem to be significantly younger. The local basal unit 

of the Middle Park is the distinctive Windy Gap Volcanic 

Member (Kmw) (Izett, 1968) that consists of hundreds of 

feet of andesitic volcanic breccia, which originated from 

contemporary volcanic domes farther west in the Rabbit Ears 

Range (Izett, 1968). The Windy Gap was not directly dated 

but was tentatively assigned to the Upper Cretaceous? on the 

basis of Late Cretaceous pollen and spores in a single sample 

from overlying beds (Izett and others, 1963; Izett, 1968). 

All other samples of the Middle Park Formation contained 

Paleocene flora. In further contrast with the Denver Basin, 

both the Laramie-Fox Hills sequence and much of the upper 

Pierre Shale were eroded prior to deposition of the Middle 

Park and Coalmont Formations in this area (Izett, 1968; 

Tweto, 1975).

Most of the Middle Park Formation (bKm, above 

the Windy Gap Volcanic Member) consists of a lower unit 

of coarse volcaniclastic conglomerate and sandstone that 

grades into an upper unit of arkosic sediment with abundant 

Proterozoic clasts (Kinney, 1970b). The transition in clast 

composition reflects the initial stripping of volcanic deposits 

off the uplifting mountain blocks, followed by erosion of the 

Proterozoic crystalline core. Local beds of coal are reported 

in the Middle Park Formation. Thickness of the upper part is 

highly variable owing to Laramide erosion prior to deposition 

as well as to time-transgressive onset of sedimentation. The 

formation is more than 6,000 ft thick in the Middle Park Basin 

to the west (Izett, 1968). The Coalmont Formation (bc) is 

similar to the Middle Park and includes a lower unit rich in 

volcanic clasts and a thick upper unit that is more arkosic; 

total thickness may exceed 9,000 ft in the North Park basin 

(Hail, 1968). Pollen in the Coalmont Formation is exclusively 

Paleocene and Eocene (Hail, 1968).
Geologic Setting 27

Laramide Intrusive Rocks

The Laramide orogeny was accompanied by widespread 

alkalic intrusive activity in two main pulses between about 

78–68 Ma and 62–45 Ma (fig. 6). These intrusions probably 

fed volcanic edifices on the rising mountain blocks, but 

concurrent and later erosion stripped most of those volcanic 

deposits (Tweto, 1975). The intrusive rocks are part of a 

broad, northeast-trending tract of early Tertiary igneous 

activity that became the locus of ore deposition known as 

the Colorado Mineral Belt (Lovering and Goddard, 1950; 

Tweto, 1975). The northern margin of the Colorado Mineral 

Belt in this quadrangle includes the mineralized intrusions 

at Jamestown, Ward and Sunset, and in the Audubon-Albion 

stock west of Ward (Lovering and Goddard, 1950; Pearson, 

1980).

The principal magmas associated with the Laramide 

uplift are of two principal compositional types. Maficalkalic rocks include alkali gabbro, monzogabbro (Kmzb), 

monzonite, syenodiorite (Ksyd), monzodiorite (Kmzd), and 

shonkinite. Alkali-calcic rocks include syenite (bsy), quartz 

monzonite (Kqm), and latite to rhyodacite (bql). Some 

intrusive centers were invaded by both types of magma over 

periods of 5 to 15 million years. The early intrusions (about 

78–68 Ma) include both mafic-alkalic and alkali-calcic types. 

The younger intrusions (about 54–45 Ma) are mostly syenite 

and quartz syenite (bsy) that post-date mineralization. 

Volumetrically minor sills intruded into Phanerozoic sedimentary rocks in the foothills-hogback belt at about 66–64 Ma 

are primarily rhyodacite and quartz latite porphyries (bql) 

(DeWitt, unpublished data; written commun., 2007).

Source reports (Gable and Madole, 1976; Gable, 1980a) 

describe many of the plutonic intrusive rocks as quartz 

monzonite or granodiorite, leading to the impression that calcalkalic magmas were more widespread in this region during 

Laramide activity. Modern whole-rock chemical data show 

that mafic-alkalic magmas were the predominant types, and 

that most of the rocks are either alkali gabbro, syenodiorite, 

monzodiorite, or monzonite (DeWitt, written commun., 2007).

The Jamestown stock (fig. 6) consists of hornblende 

monzodiorite and minor hornblende-biotite quartz monzonite 

intruded at about 78–72 Ma (Gable, 1980a; Cunningham and 

others, 1994). A second intrusion near the end of Laramide 

activity consists of syenite and quartz syenite porphyry that 

was emplaced at about 56 Ma (Gable, 1980a; DeWitt, written 

commun., 2007).

Several intrusions in the Gold Hill-Sunset area (fig. 6) 

consist of leucocratic biotite syenite and biotite quartz syenite, 

including the Sugarloaf stock, Sunset stock, Burnt Mountain 

stock, Bald Mountain stock, and the Grassy Mountain stock. 

The Sunset stock yielded a K-Ar age of about 54 Ma (Gable, 

1980a), whereas Sugarloaf Mountain and Bald Mountain 

appear to be distinctly younger (about 45 Ma) and postmineralization (DeWitt, written commun., 2007).

The Audubon-Albion stock (fig. 6) is an irregular composite body of mafic-alkalic phases that are more and more 

siliceous eastward (Mathews, 1970). Alkali gabbro with small 

pods of pyroxenite on the west are intruded by syenodiorite 

and monzonite on the east. These rocks are intruded in turn 

by gray nepheline syenite and pink quartz syenite along the 

eastern margin of the composite stock. Hornblendes in the 

sequence range from 70 Ma to 65 Ma (progressively younger 

with increasing silica content), and biotite from the syenites 

indicate thermal closure at about 65 Ma (Gable and Madole, 

1976; DeWitt, written commun., 2007). 

The northern end of the Caribou stock (fig. 6) is exposed 

along the southern quadrangle boundary, south of the 

Audubon-Albion stock. Most rocks in this part of the Caribou 

stock are monzodiorite, associated with mafic pyroxenites. 

Hornblende ages are about 76 Ma (DeWitt, written commun., 

2007).

An unusual Laramide dike intrudes the Pierre Shale (Kp) 

in the eastern part of Boulder near to the settlers’ village of 

Valmont. This dike consists of augite shonkinite, a variety of 

nepheline-bearing alkali gabbro, similar to early mafic flows 

interbedded with the Denver Formation at Table Mountain 

near Golden, Colorado (about 25 km south; Weimer, 1996). 

The Valmont dike yielded a whole-rock Rb-Sr age of 64 

Ma (Simmons and Hedge, 1978), also similar to the Table 

Mountain flows (Obradovich, 2002).

Several Laramide calc-alkaline magmas were intruded 

along the foothills-hogback belt just prior to uplift and tilting 

of the edge of the Front Range. These bodies form thick sills 

that intrude the Fountain (Phf) or Lyons (Pl) Formation just 

west of Boulder (Flagstaff Mountain sill), near the mouth of 

Left Hand Creek, and along the South St Vrain Creek just 

west of Lyons. These rocks are described as dacite (Braddock 

and others, 1988), but are chemically classified as quartz latite 

and rhyodacite (DeWitt, written commun., 2007). All of these 

sills yield K-Ar and fission-track ages between 66 Ma and 

62 Ma (Hoblitt and Larson, 1975; corrected to revised decay 

constants).

Post-Laramide Sedimentary Rocks

Erosion characterized most of the Eocene Epoch and continued through much of the Oligocene in this region (Tweto, 

1975; Epis and Chapin, 1975). Sedimentation resumed in the 

Granby valley area during the latest Oligocene with deposition 

of the basal Troublesome Formation (Nbt) in fault-bounded 

basins.

The base of the Troublesome Formation (Nbt) overlies 

an erosional unconformity that truncates folded beds of the 

Middle Park Formation and older units (Izett, 1974, 1975; 

Schroeder, 1995a). Initiation of deposition is locally well 

constrained by interbedded basalt flows (26.4 ± 0.9 Ma; Izett, 

1974) and by interbedded laharic breccia derived from a latite 

plug (28.4 ± 2.3 Ma; Izett, 1974). The Troublesome consists 

of tuffaceous mudstone and sandstone with local conglomerate. Upper parts of the formation may be as young as 11 Ma 

(Izett, 1975).
Figure 6. Sketch map showing Laramide intrusions in the Colorado Mineral Belt and mining districts in the Estes Park quadrangle.

Caribou

district

Boulder tungsten

district

Ward

district

Jamestown

district

Gold Hill-Sunshine

district

Ksyd

Kmzb

Kmzd

Kmzd

Kqm

Kqm

Long Gulch

stock

Hoosier

fault

Maxwel

Rogers fault

Livingston

fault

Audubon-Albion stock

Monzogabbro 70 Ma

Syenodiorite 68 Ma

Syenite 65 Ma

Porphyry Mtn stock

56 Ma

Jamestown stock

Monzodiorite 78–72 Ma

Burnt Mtn 

stock

Caribou stock

Monzodiorite 76 Ma

Sunset stock

 54 Ma

Grassy Mtn

stock

Bald Mtn

stock 45 Ma Sugarloaf stock 45 Ma

Tuscarora

stock 

40°00’

Nederland

105°30’

Dike

Fault

0 3 MILES EXPLANATION

�sy

�sy

�sy

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Quaternary Deposits 29

Small areas of tuffaceous sandstone, mudstone, and 

conglomerate are present in the upper parts of the modern 

Colorado River drainage and underlie Oligocene volcanic 

rocks erupted from sources in the Never Summer Mountains. 

Most of these sparse remnants lie within paleovalleys, and 

some Proterozoic rock clasts in the conglomerate beds are as 

large as 3 ft in diameter (Braddock and Cole, 1990).

Post-Laramide Intrusive and Volcanic Rocks

Renewed magmatic activity is recorded throughout 

much of central and southern Colorado during the Oligocene 

Epoch, typified by intermediate calc-alkalic or differentiated 

high-silica magmas (Steven, 1975). The major centers of 

activity in the Estes Park quadrangle were the Braddock Peak 

intrusive-volcanic complex in the Never Summer Mountains 

and the Granby valley. The following descriptions are largely 

summarized from Izett (1974), O’Neill (1981), Braddock and 

Cole (1990), and Cole and others (2008).

The uplifted block of the Never Summer Mountains 

exposes the Braddock Peak intrusive-volcanic complex, including several intrusive bodies emplaced during late 

Oligocene, and surrounding areas that preserve some of 

the volcanic ejecta erupted from these magma chambers. 

Andesite porphyry (bap) at Tepee Mountain erupted in late 

Oligocene and produced andesite and basalt flows and laharic 

breccias that are preserved in a tilted fault block inclined 

eastward toward Little Yellowstone Pass, as well as in a 

paleovalley to the north. 

The andesitic rocks are intruded by granodiorite and 

monzonite of the Mount Richthofen stock (bgdr), which was 

emplaced at 29.7 ± 3 Ma (Marvin and others, 1974). Farther 

south, the Mount Richthofen stock is intruded by granite and 

granite porphyry of the Mount Cumulus stock (bgc), which 

were emplaced at 28.2 ± 0.7 Ma (Marvin and others, 1974). 

Both of these intrusive centers are thought to have been the 

sources of rhyolitic welded tuff (brw), and varied volcanic 

rocks that are preserved eastward in the tilted fault block in the 

Little Yellowstone area. Rhyolite ash-flow tuff and flow rock 

are preserved at Specimen Mountain, along the Continental 

Divide between Iron Mountain and Thunder Mountain, and at 

Seven Utes Mountain in the northwestern quadrangle corner. 

These rhyolites are geochemically similar to the granite of 

Mount Cumulus (bgc), are highly differentiated (strong 

europium anomalies), and are among the youngest volcanic 

rocks of the Braddock Peak intrusive-volcanic complex (Knox, 

2005; Cole and others, 2008). They most likely erupted from 

the Mount Cumulus stock or adjacent areas marked by a 

pronounced negative gravity anomaly (see fig. 10, following; 

Cole and others, 2008).

The area surrounding Apiatan Mountain in the Trail 

Mountain quadrangle was also a center of intrusive and 

volcanic activity in late Oligocene time (Izett, 1974). Quartz 

latite porphyry (bqla), erupted at about 28.4 ± 2.3 Ma and 

produced laharic breccia (bqlx) that was interbedded with 

the lower Troublesome Formation (Nbt). Rhyolite porphyry 

intruded in the vicinity of Porphyry Peaks and rhyolite flows 

are locally preserved. Basaltic and trachyandesitic flows, and 

a few intrusive plugs, are common in this area, and the flows 

are interlayered with the Troublesome Formation at several 

stratigraphic levels.

Fluvial Gravel

Scattered relics of bouldery, very poorly sorted, coarsegrained gravel, conglomerate, and sandstone deposits are 

preserved in several localities in the quadrangle. Some of 

these were investigated and reported by Madole (1982), and 

a regional map of similar or related deposits was compiled by 

Scott and Taylor (1986). 

Wahlstrom (1947) examined some of these bouldery 

deposits (Ng) in the Niwot Ridge area west of Ward and 

concluded they were formed by glacial processes. He 

correlated these deposits, lying at altitudes as high as 12,300 

ft, with similar bouldery deposits downvalley to the east 

at altitudes below 8,000 ft. Despite the similarity of these 

deposits to glacial till, Madole (1982) showed that a glacial 

origin was highly unlikely because the high-altitude gravels 

are higher than glacial ice is known to have amassed, and 

the low-altitude gravels are preserved far downvalley from 

the known limits of the younger, and more extensive, glacial 

deposits. 

These bouldery gravels and conglomerates most probably 

were transported in paleovalleys that had developed during 

incision related to Miocene uplift (Scott and Taylor, 1986; 

Steven and others, 1997). One such paleodrainage (probable 

prior course of the Big Thompson River) is indicated by 

remnants of boulder gravel stranded on the mature rolling 

landscape north of Palisade Mountain (see fig. 12, following). 

Remnants of boulder gravel that cap ridges in the Bowen 

Mountain quadrangle at Gravel Mountain and between Blue 

Ridge and Apiatan Mountain appear to have formed in 

south-flowing drainage that pre-dates the modern Colorado 

River. 

Quaternary Deposits

Quaternary deposits in the Estes Park quadrangle 

primarily reflect the climatic fluctuations of the last 1.8 

million years related to the glacial-interglacial cycles. They 

comprise glacial deposits at high altitudes, stream deposits at 

lower altitudes beyond the mouths of mountain canyons, and 

various widespread thin deposits related to mass wasting and 

eolian processes. 

Alluvial Deposits

The Quaternary geomorphic history of the Front Range 

reflects persistent erosion that has taken place since early 

Pliocene time. East of the mountain front, the South Platte 

River drainage cut through the Miocene sedimentary apron 

(Ogallala Formation) and exposed the tilted resistant layers 

in the hogback belt; upstream, river valleys became deeply 
30 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

entrenched in the mountain block (Steven and others, 1997). 

Progressive, probably episodic lowering of base-level on 

the piedmont has led to incomplete preservation of various 

surficial deposits in the eastern parts of the quadrangle. 

Similar processes have acted in the Colorado River drainage 

west of the Continental Divide. The main Quaternary fluvial 

units in the area are valley-floor alluvium and numerous 

terrace and pediment deposits that record fluvial and alluvial 

deposition beyond the mountain front (fig. 7).

Considerable study during the last five decades has been 

devoted to the fluvial-terrace and pediment deposits on the east 

side of the Front Range and their record of episodic drainage 

incision. Scott (1960, 1963a) described five prominent 

alluvial surfaces along the South Platte River drainage southwest of Denver, the geomorphology of the associated deposits 

and their soil and weathering characteristics, and established 

a nomenclature scheme. Subsequent work has largely 

validated these observations, allowed correlation of terrace and 

pediment deposits elsewhere along the mountain front (Colton, 

1978), and provided better control on the ages of terrace and 

pediment formation (summarized by Madole, 1991). 

Regional correlation of terrace and pediment deposits is 

challenging, particularly among older units, because numerical 

age criteria are few, relative dating methods are inexact, and 

local geology exerts considerable control on drainage evolution (Madole, 1991). For example, correlation based on terrace height above modern floodplain level can be unreliable, 

especially along the western margin of the Colorado Piedmont. 

Many older streams that flowed on gravel beds were pirated by 

younger streams that cut rapidly headward on erodable Pierre 

Shale (Kp). The disparity in stream level would be different in 

the non-pirated valley where the younger stream also flowed 

on a gravel-clad floodplain. Figures stated below for terrace 

heights represent average values in locations several miles east 

of the mountain front.

The oldest well-preserved deposit is the Rocky Flats 

Alluvium (Qrf). It includes an old alluvial fan that caps the 

Table Mountain mesa between Boulder and Lyons, as well 

as the elongate ridge (Table Top Mountain) north of Hygiene 

that marks the old course of the Little Thompson River 

during the early Pleistocene (Madole and others, 1998). The 

tops of these old gravel deposits are generally 260–300 feet 

above modern stream channels in this area. The Rocky Flats 

Alluvium is interpreted to be early Pleistocene based on its 

relation to younger, dated Verdos Alluvium (Qv). 

Two small areas in the quadrangle preserve fluvial gravel 

at a higher position relative to the Rocky Flats terrace, and 

they are designated pre-Rocky Flats alluvium (Qprf). Their 

numerical age is unknown, but early Pleistocene is inferred 

(Scott, 1963a). These include gravel at the top of Gun Barrel 

Hill south of Niwot (at least 370 ft above Boulder Creek) and 

Haystack Mountain west of Niwot (360 ft above Left Hand 

Creek).

The next youngest fluvial deposit below the Rocky Flats 

is designated the Verdos Alluvium (Qv), which caps a few 

ridges along the Little Thompson River. Its upper surface lies 

about 170 ft above modern drainage channels, or about 100 

ft below the Rocky Flats Alluvium (Qrf). The Verdos locally 

contains the Lava Creek B volcanic ash (639 ka; Izett and 

others, 1992; Lanphere and others, 2002) and is established as 

early middle Pleistocene (Madole, 1991).

The Slocum Alluvium (Qs) is widespread for several 

miles east of the hogback belt along the eastern margin of 

the Front Range. It forms a prominent pediment deposit in 

north Boulder, a terrace deposit along the Left Hand Creek 

drainage east to Niwot, and widespread pediment deposits on 

slopes surrounding Loveland. The Slocum terrace lies about 

110–130 ft above modern drainage; its age may be as great as 

240 ka, but it is not well defined (Madole, 1991).

The Louviers Alluvium (Qlv) forms a conspicuous terrace along reaches of Boulder Creek, Left Hand Creek, and St 

Vrain Creek, but it is largely absent from the Big Thompson 

drainage that had active glaciers in its headwaters. The top of 

the Louviers lies about 20–40 feet above modern streams. The 

Louviers Alluvium is interpreted to be lowland glaciofluvial 

debris derived from montane glacial deposits of Bull Lake 

age (Qtb) (roughly 200–130 ka), because of similarities in 

soil characteristics and clast weathering (Madole and Shroba, 

1979; Madole, 1991). Much of the Louviers was probably 

deposited during late Bull Lake time.

The Broadway Alluvium (Qbr) forms a prominent 

terrace along Boulder Creek, South Boulder Creek, St Vrain 

Creek, and the Big Thompson River, 10 to 25 ft above modern 

stream beds. Characteristics of soil development and clast 

weathering are similar to glacial deposits of Pinedale age 

(Qtp). The Broadway is interpreted to have been deposited 

late in Pinedale time (30–12 ka; Madole and Shroba, 1979; 

Madole, 1991).

Alluvium in the modern drainageways is designated on 

this map as two distinct units based on geographic position 

in the mountain or plains regions. Piedmont streams east of 

the foothills-hogback belt locally preserve one or more low 

terraces, all of which are probably Holocene based on their 

position relative to Broadway Alluvium (Qbr). Mountainvalley alluvial deposits (Qva) are coarser and less structured 

due to steeper stream gradients and the availability of coarse 

side-stream detritus; they are probably both Holocene and late 

Pleistocene in age.

Granby Mesa lies between the confluence of the 

Colorado River and the Fraser River in the southwestern part 

of the quadrangle and is capped by coarse gravel and sand 

deposits (Qgv, Qgm, Qgo) that overlie eroded Tertiary 

and Mesozoic rocks (Schroeder, 1995a). Meierding (1977) 

correlated parts of these terrace deposits with the Pinedale, 

Bull Lake, and pre-Bull Lake glacial-interglacial cycles, 

based on their relative heights above the modern rivers. 

Geomorphic relations suggest Granby Mesa consists of a 

relatively old west-sloping alluvial-fan deposit (Qfo), a high 

terrace deposit (Qgo) about 220 feet above the Colorado 

River, an intermediate terrace deposit (Qgm) (Granby town 

site) at about 140 feet, and a low terrace deposit (Qgy) about 

40 to 60 feet above both rivers. Other relics of terrace gravels 
Figure 7. Panoramic photograph and drawing showing fluvial terraces, alluvial fans, and high-level erosion surfaces viewed northwestward Cole photograph, 2006).

Longs

Peak

Twin Sisters Mummy Range

Left Ha

(lined w

Louviers Alluvium

Rocky Flats Alluvium

Haystack Mountain

(pre-Rocky Flats alluvium)
32 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Figure 8. Oblique aerial photograph viewed westward over 

Moraine Park (MP) showing paths of major glacial streams 

(black dotted lines) that flowed from Forest Canyon Pass (FCP) 

and cirque basins along the Continental Divide (snow-capped 

skyline). South (left) and north (right) lateral moraines of the 

Pinedale glaciation are indicated by dashed white lines. Remnant of older Bull Lake lateral moraine is indicated by dotted 

white line west of Rocky Mountain National Park headquarters 

complex (PHQ) at middle right. Width of view is about 3 miles 

at PHQ (J.C. Cole photograph, 1974).

preserved along the course of the Colorado River north and 

south of Lake Granby (Braddock and Cole, 1990) were also 

assigned relative ages based on elevation relations.

Diamicton

Several small areas within the quadrangle are underlain 

by poorly sorted bouldery deposits (Qd) that do not appear to 

be confined to paleochannels and thus differ from the fluvial 

gravel deposits described before. All lie at low positions in 

the modern landscape, which suggests they are relatively 

young, unlike the older fluvial gravels. These younger 

deposits are located in the Tahosa Valley east of Longs Peak. 

Richmond (1960) interpreted one of them to be pre-Bull Lake 

till (Qtpb), but a non-glacial origin seems more consistent 

with its location beyond the known limits of glacial ice. The 

southernmost deposit appears to be a remnant of a bouldery 

alluvial fan (Braddock and Cole, 1990) that formed on the 

eastern slope of Mount Meeker.

Glacial Deposits

The high mountain country of the Estes Park quadrangle 

records the history of Pleistocene and Holocene glaciation in 

considerable detail. The prominent glacial cirques along the 

Continental Divide, the classic U-shaped valley cross sections 

formed by glacial action, and the characteristic forms of 

lateral and terminal moraine ridges all attest to the extent of 

glaciation and are major scenic attractions in Rocky Mountain 

National Park (fig. 8).

Early work by Lee (1917, 1922) and Richmond (1960) 

identified the main elements of the glacial landscape here 

and established the evidence for multiple glacial advances 

and retreats. Subsequent detailed work by Madole (1969), 

Shroba (1977), Madole and Shroba (1979), and Birkeland and 

others (1987) has added further information about the glacial 

stratigraphy, soil characteristics, and ages of glacial events. 

Meierding (1977) mapped many of the glacial deposits on 

the west side of the quadrangle. A comprehensive regional 

compilation by Madole and others (1998) provides a succinct 

summary of much of the relevant information on Front Range 

glaciation. The following paragraphs are summarized from 

these sources.

Three principal ages of Pleistocene glacial activity 

are recognized in the Front Range. Pre-Bull Lake glacial 

deposits (Qtpb) chiefly consist of scattered remnants of 

deeply weathered till that predate late middle Pleistocene. 

Deposits of the Bull Lake glaciation (Qtb) consist of till 

and glaciofluvial sediment that formed during one or more 

advances and retreats in late middle and late Pleistocene time 

(roughly 300–120 ka). The Pinedale glaciation occurred 

during the late Pleistocene (about 30 ka to 12 ka) and 

produced the most extensive and best preserved moraines 

and glaciofluvial sediments (Qtp) in the Front Range. 

Glacial ice in this area was largely confined to valleys, and 

the intervening ridgelines appear to have remained exposed 

(ice-free) during most of the Pleistocene Epoch. Each of the 

three major glacial periods seems to have been similar in terms 

of ice accumulation; the lowest down-valley occurrence of 

undisputed till is at about 8,200 ft elevation, regardless of age. 

Pre-Bull Lake till (Qtpb) is preserved along Green Ridge 

between Shadow Mountain Lake and Lake Granby. Small 

remnants are also identified at Harvey Island in Lake Granby 

and near the Granby Dam spillway. These deposits are all 

deeply weathered. Surface boulders on subdued moraines are 

strongly weathered and subsurface biotite-bearing boulders are 

disintegrated.

MP

PHQ

FCP
Quaternary Deposits 33

Till and gravel deposits of Bull Lake age (Qtb) are 

preserved beyond the Pinedale terminal moraines in almost all 

of the major montane valleys east and west of the Continental 

Divide. The ridge form of the moraine crests is typically 

subdued but still discernible, about a quarter of the moraine 

boulders are decomposed, and soils are developed to depths 

greater than 3 ft. Multiple moraine-crest ridges suggest 

multiple advances and retreats during the Bull Lake glaciation.

Till and glaciofluvial sediments of Pinedale age 

(Qtp) are widespread in the Estes Park quadrangle. Their 

youthful age is indicated by sharp moraine crests, thin soils, 

and fresh glacial boulders (some retain glacial polish and 

striations). Pinedale terminal moraines form the downstream 

impoundment for outwash sediments that cover flat-floored 

valleys (for example, Horseshoe Park, Moraine Park, and 

Kawuneechee Valley) that have only recently begun to incise. 

Kettles and kettle lakes are common in areas of Pinedale 

drift. Well preserved multiple moraines show that two or 

three advances and retreats occurred during the entire Pinedale 

glaciation; parallel lateral moraines and nested terminal 

moraines are conspicuous in many of the major glacial valleys.

Till in the southwestern part of the Estes Park quadrangle 

has not been studied in sufficient detail to systematically 

distinguish Pinedale-age from possible Bull Lake-age deposits 

(Pearson, 1980; Schroeder, 1995b). Most of this till appears 

to be Pinedale-age based on topographic characteristics 

and on work by Meierding (1977) around Lake Granby and 

drainages to the north. Till in the Arapaho Creek drainage 

east of Monarch Lake and south on Strawberry Bench is likely 

Pinedale-age because older deposits probably would have 

been disturbed and redistributed by the thick Pinedale glacial 

stream in this valley. Till deposits mapped in the west-central 

part of the quadrangle in the headwaters of Trail Creek below 

Gravel Mountain and west of Apiatan Mountain show sharp 

lateral moraine ridges that suggest Pinedale age, but till on 

the high-altitude slopes west of Porphyry Peaks shows less 

primary morphology and may be partly older.

Two additional minor glacial units are shown locally 

on the map. Rock-glacier deposits (Qr) are present in the 

cirque-basin areas along the Continental Divide and in the 

Never Summer Mountains at altitudes greater than about 

10,000 ft. Most of these lobate, ice-cored masses of talus 

boulders have flowed downvalley during Holocene time, but 

some accumulation may date to late Pleistocene (Braddock 

and Cole, 1990). Organic-rich sedimentary deposits (Qo) 

(bog peat and carbonaceous silt) are shown in several areas 

underlain by glacial outwash and reworked, fine-grained 

glacial material. These deposits are preserved chiefly in the 

Roaring Fork valley east of Longs Peak, and west of Ward in 

the valleys of South St Vrain Creek and North Boulder Creek. 

Detailed records of late Pleistocene and Holocene glacial 

environments have been constructed from study of these lake 

and bog materials (Madole, 1976, 1980).

Other Surficial Deposits

Other surficial units of various origins are widespread in 

parts of the Estes Park quadrangle, but they have only been 

compiled in areas where they are especially thick, extensive, 

or important as indicators of geologic conditions or processes.

Colluvium (Qc) is widespread on moderate slopes adjacent to steeper terrain, for example in the foothills-hogback 

belt. Colluvium is also shown in high-altitude areas on the 

map where persistent frost action causes bedrock to fracture 

and move slowly downslope under the action of gravity.

Talus deposits (Qta) are largely confined to the bases 

of steep slopes in the high mountainous terrain. Talus cones 

and aprons are mapped most commonly at the toes of steep 

bedrock slopes that were scoured out by glacial ice.

Landslides are present in the Estes Park quadrangle in 

several contrasting geologic settings; some are old and seem 

to be relatively immobile at present while others remain active 

today. Several high-altitude landslide deposits (Qls) involve 

masses of Proterozoic crystalline rock, particularly south of 

Forest Canyon Pass and Trail Ridge Road on the west side of 

the Continental Divide. A large mass of biotite gneiss on the 

northwest-facing slope of Jackstraw Mountain has moved in 

the direction of dip of the gneissic foliation, and displays the 

typical hummocky topography of an active slide (Braddock 

and Cole, 1990; Raup, 1996). Numerous small, shovel-shaped 

slides have been identified within glacial till on the west side 

of the Kawuneechee Valley north of Grand Lake, and on the 

north shore of Lake Granby. The Troublesome Formation 

(Nbt) is also host to numerous small slides, probably due 

to its high content of silt and altered volcanic clays. Altered 

volcanic rocks at Porphyry Peaks have slumped westward in 

a large complex landslide mass that covers about 0.7 square 

mi. over Pierre Shale (Kp). Landslides are also common 

within the foothills-hogback belt. These masses involve loose 

jumbles of clay, silt, and sand with blocks as large as several 

meters that have moved down the dip slope of the Dakota 

Group (Kd), Fountain Formation (Phf), and Lyons Sandstone 

(Pl), in order of decreasing frequency. 

Some landslide masses within the Dakota Group (Kd) 

involve bedding-plane slip beneath the first sandstone member of the South Platte Formation that allowed large blocks 

to glide downdip and override eroded younger formations to 

the east. Some of these block-glide landslides are known to 

be Oligocene in age because they are overlain by Miocene 

Ogalalla Formation north of this quadrangle (Braddock, 1978). 

This distinctive class of landslides is described in greater detail 

in the following Structure section.

Eolian deposits (Qlo), consisting of sandy loess and silty 

loess, are common east of the foothills-hogback belt where 

they form blanketing sheets on low-relief surfaces. Thin 

loess (commonly 3–10 ft) is widespread and is extensively 

developed for agricultural use. These eolian deposits are 

inactive today; they accumulated downwind of alluvium and 
34 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

decomposed Cretaceous shale during high-wind conditions 

that accompanied late Pleistocene and Holocene glaciations.

Structure

The geology of the Estes Park quadrangle records a 

complex structural history spanning from the Paleoproterozoic 

to the late Cenozoic (see map). The earliest deformation 

is recorded by the metamorphic rocks of the Front Range, 

followed by structures that formed during intrusion of the 

Mesoproterozoic granitic rocks and subsequent faulting under 

deep-crustal conditions. Phanerozoic deformation related 

to the Ancestral Rocky Mountains orogeny is not explicitly 

recorded in the quadrangle, but is inferred from depositional 

patterns of sediments shed from the uplifted basement 

blocks. The Laramide orogeny, beginning in Late Cretaceous 

time, exerted a major influence on the Front Range region 

by bringing Proterozoic crystalline rocks up from depth 

to the same altitude as Late Cretaceous marine sediments. 

Subsequent uplift and block faulting, during and following 

volcanic activity in the western part of the quadrangle, has 

added structural complexity to the region and influenced the 

evolution of modern drainage. The most recent uplift of the 

central Front Range, in conjunction with climate change in 

the Pliocene, has produced most of the modern landscape and 

stripped much of the earlier Tertiary cover from the foothills 

and plains regions.

Paleoproterozoic Structures

Braddock (1970) estimated that more than 40,000 ft 

of marine sediment accumulated in this region south of 

the Wyoming Archean craton during the Paleoproterozoic. 

These sediments were chiefly marine shales and sandstones 

deposited by submarine turbidity currents from distant 

sources. Graded bedding and crossbedding preserved in areas 

of low metamorphic grade record the facing direction of the 

beds. These sedimentary structures, especially in the Drake 

quadrangle (Braddock, Nutalaya, and others, 1970) and areas 

to the north, define the axial trace of a major east-northeasttrending syncline that is overturned to the north. All beds 

dip southward, but those on the south limb of the fold are 

overturned. The syncline interpretation is further supported by 

the symmetrical outcrop of distinctive units of porphyroblastic 

biotite schist (Xbp) on either side of the syncline axis, and by 

the symmetrical position of the contact between knotted mica 

schist (Xbk) and quartzofeldspathic mica schist (Xbq). Nesse 

and Braddock (1989) found evidence for a parallel syncline 

north of the Estes Park quadrangle in a position that implies a 

fold wavelength greater than 9 mi; these first-generation folds 

can be traced for distances of more than 22 mi in outcrop.

The major first-generation folds appear to have formed 

during early diagenetic lithification while the sediments were 

still saturated with water. Braddock (1970) found that the 

axial zones of outcrop-scale first-generation folds showed 

evidence of shale injection across folded beds of sand. Layersilicate minerals (detrital clays) were strongly aligned by 

mechanical sorting during this wet-sediment injection parallel 

to axial planes in the hinge zones of folds, whereas the same 

minerals remained parallel to overall bedding in the long 

limbs of these folds. Braddock (1970) and Braddock and Cole 

(1979) inferred that an early, low-grade metamorphic event 

overprinted these first-generation folds because metamorphic 

sericite and chlorite replaced the original detrital clay minerals and preserved the mechanical orientations formed during 

wet-sediment deformation. Nothing more is known of this 

earliest metamorphic event due to overprinting by the subsequent (dominant) regional metamorphism. The early event 

likely resulted from stratigraphic burial and structural loading 

during folding.

Second- and third-generation folds are widely developed 

in the area and they consist of antiforms and synforms with 

wavelengths of generally less than a mile (summarized from 

Braddock and Cole, 1979; Hutchinson and Braddock, 1987). 

The two fold sets are thought to be closely related in time 

and space because of their physical similarities. Secondgeneration folds (and the crenulation cleavages related to 

them) trend chiefly to the east or northeast; third-generation 

folds and cleavages trend northwest or north. Otherwise, both 

fold sets produce structures of similar scale and geometry, 

both produce axial-plane cleavage, both show steep fold 

axes, and both were active during intrusion of Boulder Creek 

Granodiorite (XgdB) or trondhjemite of Thompson Canyon 

(XjT) at about 1,715 ± 10 Ma. Mile-scale folds related to 

the second event are present in the Gold Hill quadrangle 

(Gable, 1980a) and west of the Continental Divide southeast 

of Granby (Pearson, 1980); second-generation cleavage and 

small folds are pervasive north of Highway 36 (Lyons to Estes 

Park). Third-generation folds are well developed along the 

Big Thompson River canyon in a northwest-trending zone 

near Drake, and related crenulation cleavages are widespread. 

This trend is known to be the younger of the two because it 

consistently overprints second-generation structures, although 

both fold trends probably represent essentially conjugate shear 

directions that formed during a protracted contractional event.

This deformation that produced the crossing sets of second- and third-generation folds may have been a continuation 

of the contraction implied by the major first-generation 

wet-sediment folding. The overturned syncline indicates contraction oriented broadly north-south, and the same orientation 

is suggested by the younger northeast and northwest (conjugate) folds.

All of this folding took place before the thermal peak of 

metamorphic recrystallization passed through these rocks at 

sillimanite-grade or lower conditions (summarized from Cole, 

1977, 2004b). The index porphyroblastic minerals of biotite, 

garnet, cordierite, staurolite, and andalusite consistently 

overprint the crenulated fabric displayed by the matrix micas 

(sericite, chlorite, muscovite, and biotite). The porphyroblasts 
Structure 35

are irregular and their internal inclusions of quartz and 

feldspars show no helicitic patterns that would indicate 

rotation during growth. The matrix micas are shingled around 

the hinges of crenulation folds, rather than kinked by the folds, 

indicating the micas recrystallized after folding.

Higher-grade metamorphic rocks (above the first 

occurrence of sillimanite) show evidence of mineral 

growth and recrystallization more nearly coincident with 

peak deformation (Cole, 1977, 2004b). Both biotite and 

(especially) sillimanite are weakly to strongly aligned parallel 

to hinges of the late folds. Migmatite leucosomes form 

phacolithic masses in the hinge zones of late folds indicating 

anatectic melt was accumulating and migrating into fold cores 

during deformation (fig. 4).

The difference in timing between peak deformation and 

peak recrystallization in low- vs. high-grade metamorphic 

rocks is not contradictory. Rather, it is an expectable effect of 

the low thermal-conductivity of rocks and reflects the time-lag 

for transmission of heat through the deforming orogen. The 

deformation was probably broadly synchronous throughout the 

region, but the thermal maximum arrived later in the lowergrade terranes as it migrated upward and outward from the 

core of the orogen after folding had ceased.

Mesoproterozoic Structures

Mesoproterozoic time is chiefly marked in Colorado by 

emplacement of plutons and batholiths of the Silver Plume 

Granite and equivalents at about 1,400 Ma (Braddock and 

Cole, 1979). These granites are representative of widespread 

similar intrusions that have been identified across the North 

American continent between Labrador and southern California 

(Anderson, 1983). On the basis of similarities in composition, 

isotopic characteristics, and petrology, these extensive 1,400 

Ma granites are interpreted as products of continent-wide 

melting of the lower crust, possibly related to fundamental 

changes in the underlying mantle at that time (Cole, 1977; 

Anderson, 1983; Anderson and Bender, 1989). This magmatic 

event cannot be sensibly related to any known or inferred contemporaneous oceanic arc-trench system nearby, and the lack 

of magmatic chemical diversity further distinguishes the 1,400 

Ma granites from the magmatic suites typical of magmatic 

arcs (Anderson and Cullers, 1999). 

The Mesoproterozoic granites have been described as 

“anorogenic” because most of the plutons are simple, elliptical 

structures that sharply crosscut the country rock and were 

emplaced in the absence of regional deformation (Anderson, 

1983). In this part of the Colorado Front Range, the regional 

tectonic setting during emplacement is interpreted to be nearly 

neutral or slightly extensional, the latter inferred from the 

widespread swarm of north-northwest-trending mafic dikes 

that were intruded during the Silver Plume event (Cole, 1997; 

Anderson and Cullers, 1999). Contrary interpretations are 

discussed at the end of this section.

Longs Peak-St Vrain Batholith

The principal body of Silver Plume-type granite in the 

Estes Park quadrangle is the Longs Peak-St Vrain batholith 

(fig. 9; Boos and Boos, 1934; Cole, 1977), which extends 

from the eastern mountain front across the Continental Divide 

to the Colorado River valley. The batholith spans nearly 30 

mi by 35 mi, is well exposed at elevations between 6,000 ft 

to over 14,000 ft, and emplacement-related structures are 

exceptionally well displayed in the alpine glaciated cirques 

along the Continental Divide. The following description 

of deformation related to emplacement of the batholith is 

summarized from Cole (1977).

The Longs Peak-St Vrain batholith was intruded 

into biotite gneisses at a time of neutral crustal stress, but 

the rising magmas profoundly deformed the surrounding 

rocks in the process. The simplest evidence of this is that 

compositional layering in the regional metamorphic rocks 

is characteristically steeply inclined in areas distant from 

the batholith (for example, the Big Thompson Canyon), but 

layering is sub-horizontal in the upper parts of the batholith 

along the Continental Divide (fig. 9). The rotation involved 

in this change of orientation occurred as the dry, viscous 

granite magmas rose plastically through the lower crust; the 

wall-rock gneisses also deformed plastically because they 

were nearly as hot and fluid as the intruding magmas. These 

structures related to emplacement of the Longs Peak-St Vrain 

batholith are designated fourth-generation folds in the regional 

scheme (Braddock and Cole, 1979). Small-scale folds of 

the fourth generation show crenulation of metamorphic 

biotite and sillimanite knots, as distinct from the pre-existing 

second- and third-generation folds. Recognizable older folds 

(second or third generation) are preserved between Estes Park 

and the Twin Sisters ridge area, but they were flattened and 

significantly rotated during emplacement of the granite.

The granite exploited the inherent weakness of 

the foliation direction in metamorphic rocks to intrude 

innumerable tabular sills, and the granite sheets and enclosing 

gneisses deformed and folded together, as shown by outcrop 

exposures and map-scale patterns. The trachytic flow-foliation 

within the granite defines the same patterns of folds as defined 

by compositional layering in the enclosing gneisses.

The external border of the Longs Peak-St Vrain batholith 

consists of several large lobate, bulbous domes separated by 

tight synformal screens of folded biotite gneiss (fig. 9). This 

pattern is particularly notable in the part of the batholith north 

of the Longs Peak area. These domes reflect gravity-driven 

deformation caused by the buoyant rise of the intruding 

magmas, and attendant sinking of the denser metamorphic 

wall rock. The tight synforms between the lobate domes 

reflect squeezing and crowding along the margins of the 

growing, rising domes. Broad, open swales and arches are 

evident in the shapes of tabular metamorphic-rock inclusions 

in the upper parts of the batholith, well displayed in the 

glaciated terrain. These structures also reflect the effects 
36 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

of magma buoyancy relative to sinking country rock, not 

contraction or layer-parallel shortening.

Moose Mountain Shear Zone

The granite of Longs Peak (YgLP) and some of its local 

country rock are deformed in a broad zone of mylonite and 

protomylonite northwest of Lyons called the Moose Mountain shear zone (Braddock and Cole, 1979; Punongbayan and 

others, 1989). Cataclastic fabric within the zone trends eastnortheast and dips steeply southward, and a steeply inclined 

streaking lineation is locally expressed by elongate clusters of 

minerals. The band of cataclasis varies in width from about 

3,000 ft to 4,000 ft and the margins are gradational into undeformed rock. The Moose Mountain shear zone has been traced 

approximately 13 mi along strike from the eastern mountain 

front to the canyon of North St Vrain Creek east of Allens 

Park, where it feathers out and disappears. No structural zone 

of similar character was found west of Highway 7 along strike, 

nor was any mapped farther west (Gable and Madole, 1976; 

Pearson, 1980; Schroeder, 1995b).

The sense of offset across the Moose Mountain shear 

zone is inferred from regional and local relationships. Pelitic 

rocks north of the zone and east of Highway 36 are nonmigmatitic (non-melted) and the zone truncates several 

amphibolite-grade mineral-zone boundaries (Punongbayan 

and others, 1989). All pelites south of the Moose Mountain 

zone are strongly migmatitic and must have melted at greater 

depth (higher pressure-temperature conditions), implying that 

the southern block has risen relative to the north. The steep 

mineral-streaking lineation within the mylonites is consistent 

with a reverse sense of displacement, as is the fact that mapunit contacts do not show major lateral (wrench) offset.

Cataclastic foliation along most of the trace of the 

Moose Mountain shear zone dips more than 60° S. Magmatic 

trachytic foliation within the granite of Longs Peak (YgLP) 

in this area has a similar northeasterly strike but chiefly dips 

northward. Abundant tabular microcline grains in the granite, 

which define the trachytic foliation, become progressively 

Figure 9. Map showing major structural elements of the Longs Peak-St Vrain batholith. Inset outcrop photograph 

shows detail within the Moose Mountain shear zone where magmatic flow-foliation of tabular feldspar phenocrysts 

(F) is re-oriented and overprinted by mylonitic shear planes (M). Pencil is about 4 in long.

U

D

55 55

50

50

75 40

65

40 80

55

40

45

75 70

60 50

40

70

55

55

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55

45

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45

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55

25

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45 60

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50 Flow foliation and

wall-rock foliation

Silver Plume Granite

Synform

Antiform

EXPLANATION

Moose

Mountain

shear zone

(M)

(F)

0 XX KILOMETERS
Structure 37

rounded as mylonite fabric becomes more pervasive in the 

rock, and feldspars are eventually rotated into parallelism 

with the enclosing cataclastic foliation. Selverstone and 

others (2000) cited various mineral textures and experimental 

deformation studies to conclude that the mylonite fabric 

probably developed while the granite was intruding. Cole 

(1977) also noted widespread slip surfaces farther west within 

the Longs Peak-St Vrain batholith that dislocate the trachytic 

foliation, aplite-filled “joints” within the granite, and some 

late dikes that locally show internal protomylonite fabric. 

He interpreted these features as local adjustments to volume 

reduction that occurred as a result of magma crystallization. 

The fact that the Moose Mountain shear zone feathers out to 

the southwest and disappears within the batholith is consistent 

with the notion that shearing ceased before magmatic 

crystallization was complete. In any event, the deformation in 

the Moose Mountain shear zone is older than the crosscutting 

Iron Dike (Ygb) (1,317 ± 50 Ma; Braddock and Peterman, 

1989; Braddock and Cole, 1990).

Selverstone and others (1997, 2000) have interpreted 

the Moose Mountain shear zone as evidence of a regional 

contractional tectonic setting during the time of intrusion 

of Silver Plume-type granites. They point to the reverse 

slip-sense across the zone, the gradation between magmatic 

and cataclastic textures, and regional structural elements as 

evidence of north-northwest-directed compression during 

emplacement of the granite. However, the mafic dikes they 

site as indicators of extension direction are, first of all, older 

than the Longs Peak-St Vrain batholith and, second, mute 

about whether the regional compression axis was horizontal 

or vertical. Second, their interpretation of fourth-generation 

folds in and around the batholith as regional contractional 

structures ignores their diverse orientations (fig. 9) and 

abundant evidence that they resulted from gravity-driven 

vertical movements. 

The simplest explanation of the Moose Mountain 

shear zone, in light of all the evidence for conditions during 

emplacement of the Longs Peak-St Vrain batholith, is 

that it formed during buoyant rise of the southern block 

(dominantly granite) relative to the northern block (dominantly 

metasedimentary rocks) while the magma was still partially 

molten. A similar interpretation is consistent with the facts in 

other sheared border zones of 1,400 Ma granites elsewhere in 

the region that have been proffered as examples of “regional 

compressional tectonics” (Selverstone and others, 2000, and 

included references).

Laramide Orogeny

Structures related to the Laramide orogeny are common 

in the Estes Park quadrangle. They include tilted basement 

blocks, drape folds, and reverse faults along the eastern 

mountain front, as well as thrusts and footwall folds on the 

western margin of the range (see map). Structures in these 

two areas are described separately below because they are 

kinematically distinct and because they differ substantially 

in age.

Eastern Margin of the Front Range

The most significant structure formed in the Laramide 

event lies covered beneath surficial deposits of the Colorado 

Piedmont on the eastern side of the quadrangle. The great 

asymmetric synclinal Denver Basin is the downwarped 

counterpart to the mountain uplift, and the Proterozoic 

basement rocks are depressed more than 13,000 ft along 

its axis (Higley and Cox, 2005). The deepest part of the 

basin coincides with that part of the mountain front where 

the Proterozoic crystalline rocks are thrust directly over 

the overturned Phanerozoic sedimentary rocks and the 

thrust-loading is greatest (Erslev, 1993). The basin is most 

asymmetric where the western edge is defined by such a 

thrust; the western limb is generally steep to overturned, 

whereas the eastern limb dips very gently westward (Haun, 

1968). The thrust front is nearly continuous from Colorado 

Springs northward to about Coal Creek just south of the Estes 

Park quadrangle. The main thrust is blind northward through 

Boulder. The local “Boulder thrust” places steeply tilted 

Fountain Formation (Phf) eastward over strongly overturned 

Cretaceous strata at the southern quadrangle boundary, but 

the fault cuts upsection in the hanging wall to the north and 

to the south, loses throw, and dies out within a few kilometers 

(Wrucke and Wilson, 1967; Wells, 1967).

From Boulder north to Left Hand Creek (about 12 km), 

the range front is marked by a simple homocline, moderate 

dips, and very limited folding in the Cretaceous and older 

strata. This style continues northward to Lyons, as the trend 

of the range front changes to north-northeast and the general 

inclination of beds declines to 20° or less. The axis of the 

Denver Basin also swings northeastward and migrates nearly 

20 miles to the east beyond the quadrangle boundary (Haun, 

1968). 

At Lyons, and northward to the Wyoming State 

line, the range front steps progressively northeastward on 

northwest-trending faults (Braddock, Calvert, and others, 

1970; Matthews, 1976; Braddock, Houston, and others, 1988; 

Braddock, Nutalaya, and others, 1988). These faults are eastdipping, high-angle reverse faults that raise the Proterozoic 

basement as much as 3,500 ft on the east (Denver Basin) side 

(see map). Sedimentary strata are primarily folded over the 

crests of these crystalline-block uplifts. Parallel anticlines and 

synclines east of the range front reflect reverse faults in the 

buried basement rocks. The west limb of the Denver Basin 

dips moderately overall in this region and it is folded along 

several of these blind, reverse-fault basement uplifts, but the 

overall basin is far more symmetrical and shallower at the 

latitude of Loveland than it is at Boulder (Haun, 1968; Erslev, 

1993; Weimer, 1996).

Uplift of the eastern margin of the Front Range in the 

Estes Park quadrangle is inferred to be Late Cretaceous 
38 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

to Early Paleocene, based on evidence south of this area. 

The synorogenic sediments in the Denver Basin show that 

uplift began in Late Cretaceous time after about 70 Ma and 

continued more-or-less continuously until about 64 Ma 

(Raynolds and Johnson, 2003; Raynolds, 2003). This timing 

is consistent with the geologic relations in the Estes Park 

quadrangle.

Boulder-Weld Fault Zone

Upper Cretaceous rocks are broken by an anastomosing 

set of high-angle, northeast-trending faults in the southeast 

corner of the quadrangle known as the Boulder-Weld fault 

zone. These faults affect the upper Pierre Shale (Kp), the 

Fox Hills Sandstone (Kfh), and the overlying fluvial and 

coal-bearing strata of the Laramie Formation (Kl). The 

faults are well-documented through extensive subsurface 

mapping in coal mines and through surface mapping, and fault 

patterns have been inferred from widespread gas-well records 

(Spencer, 1961; Colton and Lowrie, 1973; Weimer, 1996; 

summarized in Roberts and others, 2001). This fault swarm 

displaces the Fox Hills Sandstone by as much as 500 ft along 

select strands (200 ft is more typical), but the net effect across 

several horst ridges and graben sags is essentially neutral; the 

overall southeasterly dip of strata is maintained across the 

Boulder-Weld fault zone from north to south, and the fault 

system does not significantly offset the Dakota Group (Kd) 

(Haun, 1968). The Boulder-Weld fault zone also appears to 

die out to the west and is not detected in the foothill-hogback 

zone adjacent to the Proterozoic core of the Front Range 

(Wells, 1967; Weimer, 1996).

Most of the faults in the Boulder-Weld zone were initially 

mapped and interpreted as north-dipping high-angle reverse 

faults that placed the Fox Hills (Kfh) above the Laramie 

Formation (Kl) (for example, Spencer, 1961). This interpretation is consistent with the synclinal bending of Laramie beds 

in the graben blocks and some arching of strata in the horst 

blocks, and with the presence of dip-slip slickensides on fault 

surfaces (Spencer, 1961). Kittleson (2004) also used extensive 

well records to demonstrate that the deformation is confined to 

the upper Pierre Shale (Kp) and younger units and is not seen 

in structure-contoured data for identifiable horizons deeper in 

the Pierre. He interpreted the Boulder-Weld fault zone to have 

formed by gravity-driven sliding along a surface of detachment within the upper Pierre Shale, causing imbrication of the 

section in the downdip direction toward the axis of the Denver 

Basin. In his interpretation, the Boulder-Weld fault zone is a 

consequence of the Late Cretaceous uplift of the Front Range 

and downwarping of the Denver Basin axis.

Weimer (1996, summary paper) interpreted structurecontour data quite differently and believes the Boulder-Weld 

fault zone is dominantly strike-slip. Tweto and Sims (1963) 

and Warner (1978) noted the apparent coincidence of a 

Proterozoic fault system (Idaho Springs-Ralston shear zone) 

with the Laramide Colorado Mineral Belt as indication of 

some persistent crustal zone of weakness. Weimer (1996) 

expanded on that observation and pointed out that the BoulderWeld fault zone in the Denver Basin also aligns with these 

features in the mountain block, suggesting some connection 

among the three. The evidence for the Weimer strike-slip 

interpretation primarily rests on inferred differential offsets, 

or of thickness anomalies, in stratigraphic units ranging in 

age between 97 Ma and 62 Ma. His interpretation implies no 

fewer than eight distinct periods of wrench-fault displacement 

over about 35 million years, even though no fault dislocation 

appears in Lower Cretaceous or older strata. These apparent 

anomalies were interpreted from drill-hole records and 

generally amount to less than a few tens of feet over areas of 

many square miles. Kittleson (2004) noted that “correlations” 

of horizons within the Laramie-Fox Hills Formations, in 

particular, cannot be made with confidence because of inherent 

variations in thickness and facies formed during withdrawal of 

the Late Cretaceous Western Interior seaway. 

We believe the data are insufficient to support the 

wrench-fault model of Weimer (1996), and prefer the 

interpretation by Kittleson (2004) that these stratigraphically 

and geographically localized faults arose from gravity-driven 

sliding within the upper Pierre Shale (Kp). Erslev (1993) 

similarly concluded this zone demonstrates “southeastdirected gravity detachments” in the Denver Basin. The 

absence of structural dislocation along the trend between the 

Boulder-Weld fault zone and the Idaho Springs-Ralston shear 

zone in the foothills-hogback belt is a strong indicator that the 

two structures arose independent of each other, regardless of 

coincidental location and trend.

Western Margin of the Front Range

The Never Summer thrust (O’Neill, 1981) marks the 

western structural margin of the Front Range uplift. This 

thrust places broken Proterozoic rocks westward over Upper 

Cretaceous Pierre Shale (Kp) and Paleocene and Eocene 

Coalmont Formation (bc) (partial lateral equivalent of the 

Middle Park Formation), which are deformed in west-vergent 

overturned footwall folds (see map). The Never Summer 

thrust is particularly well exposed because it was domed 

upward by intrusion of the subvolcanic stocks during the 

Oligocene and now lies thousands of feet higher than the 

level at which it was emplaced. The structural overhang of 

Proterozoic rocks on this thrust may be about 6 mi, based 

on its exposed position east and west of the Never Summer 

Mountains. The thrust can be traced southward through the 

Bowen Mountain quadrangle (Braddock and Cole, 1990; 

Braddock, unpub. mapping, 1985), but it is obscured by 

Oligocene volcanic and intrusive rocks near Apiatan Mountain 

(bqla, br) (Izett, 1974) and by Neogene deposits of the 

Granby basin (Schroeder, 1995a). Westward-vergent folding 

is displayed by the Windy Gap Volcanic Member (Kmw) of 

the Paleocene and Upper Cretaceous Middle Park Formation 

in the southwestern corner of the quadrangle.

Erslev (1993) and Erslev and Selvig (1997) summarize 

their interpretation that the Never Summer thrust is a 
Structure 39

back-thrust that is secondary to a dominant east-vergent structure that underlies the Front Range uplift. However, uplift of 

the two flanks of the range are distinct in age and may not be 

directly related. The Never Summer thrust overrides Paleocene beds of the synorogenic Coalmont Formation (bc) and 

probably late Paleocene beds of the Middle Park Formation 

(bKm). These formations rest on eroded Pierre Shale (not on 

the Laramie-Fox Hills couplet), indicating that erosion was 

continuing in Late Cretaceous-early Paleocene time while 

synorogenic sediments were accumulating in the Denver 

Basin (Raynolds, 2003). Therefore, the Never Summer thrust 

may have formed under different stress conditions operating 

perhaps 10 million years later than initial uplift of the eastern 

margin of the Front Range.

Sandstone Dikes

Pearson (1980) reported lenticular and dike-like bodies 

of cemented, quartzose sandstone within the Proterozoic 

crystalline rocks along the trace of the west-northwesttrending Arapahoe Pass fault zone in the south-central part 

of the Estes Park quadrangle (see map). Schroeder (1995b) 

reported similar bodies in a north-trending fault zone farther 

west in the Strawberry Lake quadrangle. The following 

description is summarized from Pearson (1980), and from a 

more recent assessment by Dockal (2005).

The sandstone dikes are exposed for a distance of about 

2.5 miles along the Arapahoe Pass fault zone and over a 

vertical elevation range between about 10,800 ft and 11,900 

ft. They form discontinuous bodies that are elongate along 

the trend of the Arapahoe Pass fault and range in width from 

a foot or so to about 60 ft. The rock is internally brecciated, 

the Proterozoic wall rock is sheared, and all rocks are mineralized by secondary hematite, silica, and dispersed lead, zinc, 

and silver minerals. The sandstone within the dikes is mostly 

white (secondary tan, gray, or reddish-brown coloration) and 

consists almost exclusively of well-rounded quartz grains with 

a few percent feldspar. The dikes contain angular fragments of 

granitic wall rock, feldspar, and mica.

Similar sandstone dikes were described southwest 

of Denver by Scott (1963b) and a regional summary 

and petrographic study was reported by Harms (1965). 

Controversy remains regarding the origin and age of these 

distinctive structures. Both Scott (1963b) and Harms (1965) 

concluded that the sand most likely infiltrated fractures in 

the Proterozoic basement as a granular slurry from sources 

at higher structural levels. The uniformity and maturity of 

the rounded sand grains and heavy-mineral suites suggested 

the most likely source was the Upper Cambrian Sawatch 

Quartzite (Scott, 1963b). However, the Sawatch was removed 

from most of this area by erosion during the late Paleozoic 

Ancestral Rocky Mountains orogeny. Harms (1965) pointed 

to the close correspondence between the trend and location of 

sandstone dikes and the (flexed) leading edge of the main eastvergent uplift of the Rocky Mountain front between Colorado 

Springs and south Denver, indicating a Laramide age for the 

structures. He also noted that red-brown siltstone chips were 

entrained in the sandstone slurry in several locations and that 

such materials were only common in the Fountain Formation 

(Phf) in this region. Thus, the source of the sand may be 

higher in the Phanerozoic section than the Pennsylvanian.

The sandstone dikes in the Arapahoe Pass fault zone and 

west in the Strawberry Lake quadrangle offer no additional 

clues to the origin of these features. Their position on the 

landscape is nonetheless strong evidence that open fractures 

extended deep into the Proterozoic crystalline basement at 

time(s) in the past, and are indications of localized, significant 

high-level extension (fracturing) within the uplifted Front 

Range basement block. We believe that the preponderance 

of evidence favors the Laramide age for emplacement. The 

source of the sand could be the mature eolian quartzites of 

the Lyons Sandstone (Pl), which were loosely cemented with 

calcite in the principal area of the sandstone dikes.

Post-Laramide Structures

The deformation history of the Front Range area 

following the Laramide orogeny is mostly contained in 

deposits preserved outside the Estes Park quadrangle. The 

Laramide orogeny concluded in the area of the Estes Park 

quadrangle with emplacement of the Never Summer thrust. 

The underlying beds of Middle Park and Coalmont Formations 

were regionally folded and locally overturned in the thrust 

footwall.

The late Eocene Epoch was marked by reduced uplift; 

accelerated weathering due to intensely warm, moist climate; 

and widespread erosion throughout the Southern Rocky 

Mountain region (Epis and Chapin, 1975). The result of 

these factors was the formation of a topographically subdued 

landscape over large areas characterized by rolling hills and 

broad valleys (Epis and Chapin, 1975; Steven and others, 

1997) between relics of mountainous terrain. This late Eocene 

eroded landscape is well preserved southwest of Denver 

beneath ash flows of the Wall Mountain Tuff (35 Ma), but no 

marker deposits of this age are present in the northern Front 

Range. Scott and Taylor (1986) inferred that much of the 

east-central part of the Front Range block in the Estes Park 

quadrangle, lying between about 8,000 ft and 9,000 ft elevation, represents relics of the Eocene landscape, but it is equally 

or more likely that this area of broadly concordant hilltop summits formed during Oligocene-early Miocene time (Evanoff, 

1990; Steven and others, 1997).

Oligocene sediments and volcanic strata are only present in a few areas of the Estes Park quadrangle, most having 

been removed by subsequent erosion. The upper Oligocene 

and lower Miocene Troublesome Formation (Nbt) consists 

of tuffaceous siltstone, sandstone, and conglomerate that were 

deposited unconformably over Middle Park Formation in the 

Granby area. The Troublesome appears to have filled basins 
40 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

and half-graben that were bounded by north-south or northwest-trending high-angle faults. 

Braddock Peak Intrusive-Volcanic Complex

Oligocene deposits exposed in the northern Never 

Summer Mountains record some of the history of the 

Braddock Peak intrusive-volcanic complex (O’Neill, 1981; 

Braddock and Cole, 1990; Cole and others, 2008). The 

granodiorite of the Mount Richthofen stock (bgdr) (29.7 ± 3 

Ma) and the granite of the Mount Cumulus stock (bgc) (28.2 

± 0.7 Ma) are the intrusive roots of volcanoes that erupted here 

in the late Oligocene. Thick deposits of rhyolite ash, welded 

ash-flow tuff, and rhyolite flows are preserved at Specimen 

Mountain, Thunder Mountain, Seven Utes Mountain, and 

upland areas north of the quadrangle (fig. 10). Interbedded 

volcaniclastic sediments and possible cooling breaks in the 

tuff sequences attest to a considerable span of time marked by 

variable volcanic activity. Somewhat older andesitic flows and 

breccias are interbedded with rhyolite flows, volcaniclastic 

sediments, and ash-flow tuffs that are preserved in a faultbounded half-graben in the Little Yellowstone area (Colorado 

River headwaters). Tilting and abnormal thickness in this and 

other areas of the Braddock Peak complex indicate extensional 

faulting accompanied volcanic activity.

Emplacement of the stocks in the center of the range 

caused deformation of the Never Summer thrust. The thrust is 

generally flat-lying or gently inclined to the east, except where 

it was bowed up across the apex of the Mount Richthofen 

stock (Corbett, 1968; O’Neill, 1981; Braddock and Cole, 

1990). Between Mount Cindy and Bearpaws Peaks, the 

thrust is gently inclined to the west where it cuts upsection 

through Coalmont Formation in the footwall (O’Neill, 1981), 

indicating post-thrust tilting. 

The rhyolitic ash-flow tuffs and flows at Thunder 

Mountain and areas north comprise two major eruptive 

deposits and possibly more. The total thickness of rhyolite 

locally exceeds 1,000 ft but declines notably toward the north 

(Cole and others, 2008).

The rhyolitic ash-flow tuffs and flows provide interesting 

information about the timing and amount of faulting that took 

place during eruption. The lower unit is a crystal-rich ashflow tuff that contains conspicuous phenocrysts of euhedral, 

brown quartz. It is many hundreds of feet thick in tilted 

sections at Little Yellowstone, Seven Utes Mountain, and 

below Thunder Mountain, as well as at Gould Mountain just 

north of the quadrangle (fig. 10). From Thunder Mountain 

north, this ash-flow tuff thins rapidly due to erosion and is 

overlain by subhorizontal rhyolite flows that also thin northward (Cole and others, 2008). The correlation of unusually 

thick sections with tilted beds suggests that normal faulting 

occurred during eruption. Local lenses of very coarse breccia that contain clasts (as large as 30 ft in diameter) of various 

volcanic rocks, Proterozoic gneiss, and Phanerozoic sediment are interbedded with rhyolite welded tuff at Seven Utes 

Mountain and Little Yellowstone (Lee, 1917; O’Neill, 1981). 

The angular nature of these breccia deposits is consistent with 

rapid deposition possibly due to local fault displacement.

Both of the rhyolite units are highly silicic and 

differentiated (Knox, 2005) and most similar in composition to 

the Mount Cumulus stock. The ages of eruptive and intrusive 

rocks are similar within analytic error (Knox, 2005). The 

Mount Cumulus stock would seem to be a logical source for 

the extrusive rhyolites but no caldera structure is apparent 

in its vicinity. Gravity data show a conspicuous oval low 

anomaly that includes the Mount Cumulus stock and areas 

to the west underlain by a blind granite porphyry (proved 

by drilling; Pearson and others, 1981), a tuff-breccia ring 

intrusion (Metzger, 1974), and thermally metamorphosed 

Coalmont Formation. Taken together, this evidence indicates 

a more widespread, low-density granite body at shallow depth 

that probably fed the rhyolite eruptions (Cole and others, 

2008).

Colorado River Fault Zone

 The upper Colorado River drainage north of Grand 

Lake is a conspicuously linear feature that trends nearly due 

north for a distance of about 17 miles to the site of abandoned 

Lulu City. The glaciated floor of the Kawuneechee Valley 

exposes widespread brecciation, silicification, and iron-oxide 

mineralization in shattered Proterozoic rocks that indicate 

a substantial brittle fault zone (Braddock and Cole, 1990). 

The fault zone has only been mapped in the lower parts of 

the valley and it does not continue northward through the 

Oligocene volcanic rocks of the Braddock Peak intrusivevolcanic complex that cap Thunder Mountain, Lulu Mountain, 

and Iron Mountain (north of this quadrangle). Rather, the fault 

zone appears to turn northwestward and splay into smaller 

breccia zones that feather out. Some of the northwest splays 

offset granodiorite of the Mount Richthofen stock (bgdr), 

which would indicate activity younger than 29.7 Ma if the 

faults are connected. The fault zone may be displaced by 

younger movement on the northwest-trending fault zone that 

traverses Specimen Mountain, the Little Yellowstone area, 

Thunder Pass, and the headwaters of the Michigan River. 

The Colorado River fault zone also lies on strike with 

the Laramie River valley to the north, and Chapin (1983) has 

speculated on regional considerations that this long, combined 

lineament might have been a Laramide-age wrench fault zone. 

No local evidence supports this speculation. The Laramie 

River valley is controlled by Laramide-age reverse faults that 

place Proterozoic rocks in the Front Range and Medicine 

Bow Mountains over synclinal Mesozoic rocks in this valley 

(Beckwith, 1942). The northwest-trending, Mesoproterozoic 

Iron Dike (Ygb) does not appear to be offset across the 

Laramie River valley (Pearson and others, 1982).

 Other north-trending faults are present in the Never 

Summer Mountains and all are mapped as normal-offset structures (O’Neill, 1981). They primarily displace the Oligocene 

intrusive rocks in the center of the range upward relative to 

the surrounding country rocks; it is possible the faults reflect 
Late Miocene–Pliocene Uplift and River Incision 41

late adjustments to the emplacement of the low-density stocks 

during volcanic activity.

Block-Glide Landslides

The east-dipping strata of the foothills-hogback belt have 

been substantially disrupted by large landslide complexes, 

exclusively within strata of the Dakota Group (Kd). Detailed 

mapping by Braddock and Eicher (1962) and Braddock 

(1978) showed that these landslides are characterized by 

gravity-driven detachment of parts of the Dakota section 

along stratigraphic zones of weakness, causing the detached 

block to slide over the downslope strata and override the 

contemporaneous land surface. The detached block was 

folded in the process of sliding over ridges in the underlying, 

intact strata. The block-glide landslide complex north of 

Boulder involves only the first sandstone member of the South 

Platte Formation, but most others farther north to Carter Lake 

Reservoir involve nearly the entire Dakota Group (Braddock, 

1978; Braddock, Nutalaya, and others, 1988).

Similar landslide complexes have been mapped as far 

north as the Wyoming State line (Braddock and Eicher, 1962; 

Courtright and Braddock, 1989). Evidence there indicates 

that some of the block-glide landslides formed during 

Oligocene time because the slide complex locally overlies 

basal conglomerate of the White River Group and is overlain 

by Miocene Ogallala Formation (Braddock, 1978). On 

geomorphic grounds, it appears that many other block-glide 

landslides formed during the Pleistocene because the elevation 

at which the detached block broke and overrode the land 

surface coincides with one of the major pediment surfaces that 

formed during early Pleistocene erosion. Braddock (1978) 

concluded that the landslides were triggered by erosional 

removal of unconsolidated material at the east base of the 

hogback slope, probably during prolonged wet periods. 

Late Miocene–Pliocene Uplift and 

River Incision

Following the late Oligocene and early Miocene volcanic 

eruptions from the Braddock Peak intrusive-volcanic complex 

in the Never Summer Mountains, the Miocene Epoch 

appears to have been largely a time of continued erosion and 

landscape smoothing across the Front Range (Scott, 1975; 

Steven and others, 1997). No deposits of early Miocene 

age are preserved in the mountains or on the eastern flank 

of the uplift (Courtright and Braddock, 1989; Steven and 

others, 1997). West of the Front Range, siltstone, sandstone, 

and conglomerate of the Troublesome Formation (Nbt) 

accumulated in the Granby basin, starting in the late Oligocene 

and continuing into late Miocene (to about 11 Ma; Izett, 1974, 

1975).

Middle Miocene time marked the onset of significant 

renewed uplift of the Front Range in this area (Steven and 

others, 1997; Bolyard, 1997). Eaton (1986, 1987) assembled 

regional evidence to argue that this younger uplift established 

the main topographic elements of the modern Southern Rocky 

Mountains province between Casper, Wyo. on the north 

and El Paso, Tex. on the south (fig. 11). The uplift is subcontinental in scale and most likely reflects thermal rise of a 

welt of aesthenospheric mantle beneath the region and related 

lithospheric thinning (Alvarado Ridge of Eaton, 1986). The 

axis of this broad ridge is marked by the location of the Rio 

Grande rift, a north-south trending series of normal faults and 

deep local basins that formed concurrent with (and directly 

resulting from) the regional uplift (Eaton, 1986; Chapin and 

Cather, 1994).

In the area of the Estes Park quadrangle, this uplift is 

suggested by the transport of coarse boulder gravel deposits 

(Ng) in east- and northeast-trending paleocanyons of the early 

Big Thompson River and South St Vrain Creek canyons (Scott 

and Taylor, 1986), and late Oligocene and Miocene southwestflowing drainage of the early Colorado River system (Izett, 

1968, 1974; Braddock, unpub. mapping, 1985). These gravel 

deposits that mark the positions of old canyons define fairly 

straight drainages that suggest the stream gradients were 

moderate to steep across the Proterozoic crystalline rocks. 

Steven and others (1997) noted that these Miocene drainages 

cross the pre-existing Oligocene drainages of the White River 

Group at a high angle and indicate regional post-Oligocene 

tilting of the Front Range block toward the northeast (that 

is, uplift toward the southwest). Miocene sand, gravel, and 

conglomerate of the Ogallala Formation (about 17 Ma to 6 

Ma; Izett, 1975) record the middle to late Miocene uplift and 

erosion of the modern Front Range and Laramie Range. The 

fact that the Oligocene White River Group is eroded south of 

Fort Collins but the Ogallala persists southward is consistent 

with northeastward tilting of the range margin (Steven and 

others, 1997) during Ogallala time.

The change in general tilt direction appears to be 

somewhat more complex than described above, based on 

geomorphic relations in the Estes Park quadrangle and farther 

north. The Miocene paleovalleys in the northern Front 

Range are depicted (Scott and Taylor, 1986) as discharging 

directly to the east or northeast across the foothills-hogback 

belt. However, the paleovalley of the Big Thompson River 

apparently did not continue straight northeastward from 

the high-level gravel remnants north of Palisade Mountain 

because a continuous ridge of Proterozoic rock stands at 

higher elevation than those gravel deposits (fig. 12; Braddock, 

Nutalaya, and others, 1970). Rather, it appears that the Big 

Thompson drainage was deflected southeastward toward the 

modern canyon mouth, as indicated by the easternmost and 

lowest boulder gravel deposit preserved on the side of the 

modern Cedar Creek valley. Very minor faults in the valley 

floor seem insufficient for major post-Miocene uplift of the 

Green Ridge block, and so the Green Ridge prominence 

probably existed during the Miocene Big Thompson 
42 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

40°30’

40°20’

106°00’

Q

K

K

JC

UI

KV

SM

LY

SU

MRF

NST

NST

NFT

CmP

BP

CRF

MC

K

Ng

Ng

K K

K

Q

Q

Q

Q

Q

?

CP

NP

OM

CL

105°50’

0 5 MILES
Late Miocene–Pliocene Uplift and River Incision 43

Figure 10 (above and previous page). Structural sketch map of 

Oligocene-Miocene volcanic and tectonic features related to the 

Braddock Peak intrusive-volcanic complex in the northern Never 

Summer Mountains. Geologic features shown: oval gravity low 

(dashed, hachured line enclosing buried granite porphyry at Jack 

Creek (JC) and Mount Cumulus (MC) stock); tuff-breccia ring 

in Upper Illinois River (UI) drainage; Colorado River fault (CRF); 

Michigan River fault (MRF); North Fork thrust (NFT); Never 

Summer thrust (NST). Geographic features shown: Braddock 

Peak (BP); Chambers Lake (CL); Cameron Pass (CmP); Clark 

Peak (CP); Kawuneechee Valley (KV); Little Yellowstone (LY); 

North Park (NP); Owl Mountain (OM); Specimen Mountain (SM); 

Seven Utes Mountain (SU).

courses show little regard for structural weaknesses in the 

basement. The meanders in these river courses indicate they 

were established in low-gradient drainages on top of the 

Ogallala Formation, probably with broad floodplains, and 

then subsequently superimposed on the Proterozoic crystalline 

rocks during later uplift (Bolyard, 1997; Steven and others, 

1997; Cole, 2004a). The fact that entrenched meanders are 

preserved several miles west of the foothills-hogback belt 

suggests that the Ogallala cover probably extended at least that 

far west as well.

Erosion has dominated Front Range geomorphology 

since the end of Ogallala Formation deposition in late 

Miocene (Steven and others, 1997; Leonard, 2002). The 

main drainages tied to the South Platte River have excavated 

almost all of the Oligocene and Miocene deposits east of the 

Continental Divide in this area, and the prominent hogback 

ridges of the foothills belt were exhumed from overlying 

Cenozoic cover (Steven and others, 1997). This erosion was 

certainly triggered in part by renewed uplift of the Front 

Range block. Cross-sectional forms of present canyons 

through the Proterozoic basement rocks show that the most 

recent deepening has occurred rapidly, leading to very steep 

inner-canyon walls (Bolyard, 1997; Steven and others, 1997). 

The entrenched meanders show that the rapid incision (post5 Ma) did not allow time for streams to adjust to contrasts in 

canyon-wall fabric or rock types.

The accelerated erosion during Pliocene time may also 

reflect the marked change to cooler, wetter climate following 

the Miocene (Molnar and England, 1990; Crowley and North, 

1991; Krantz, 1991; Bluemle and others, 2001). However, 

most studies conclude that climate-induced accelerated stream 

power alone is insufficient to explain the incision. McMillan 

and others (2002) examined the Ogallala Formation along the 

Colorado-Wyoming State line and concluded that the eastward 

tilt of beds is greater than it could have been during deposition, 

implying both tectonic uplift and isostatic uplift related to 

removal of overburden from the range block. Leonard (2002) 

examined the Pliocene and Quaternary incision of the North 

and South Platte River drainages and concluded, as well, that 

both tectonic uplift and isostatic adjustment to erosion were 

required to account for the gradients of modern streams and 

post-depositional warping of the Ogallala.

Pliocene and younger faults that might have 

accommodated this tectonic uplift of the Front Range core 

have not been positively identified. Steven and others 

(1997) suggest that the Pliocene uplift of the Southern 

Rocky Mountains involved diffuse adjustments in irregular 

fault-bounded blocks within the main ranges. One fault 

that may have accommodated some of this late uplift strikes 

northwest through Sunshine, Jamestown, and Allens Park, 

along the eastern base of Longs Peak, and continues over 

the Continental Divide at the head of Forest Canyon. Northtrending fault zones in the upper Colorado River valley are 

also likely candidates for Pliocene-Quaternary displacement.

paleodrainage. It was probably never covered by the Ogallala 

Formation deposits. Similar relationships are evident along 

the old course of Buckhorn Creek just north of this area 

(Braddock and others, 1989).

Deposition of the Ogallala Formation continued until 

latest Miocene time and appears to have led to fluvial 

aggradation west of the hogback belt in this area. The surface 

of terminal Ogallala deposition must have had a gentler slope 

than the middle Miocene streams that flowed in the fairly 

straight paleocanyons. This conclusion is deduced from the 

present meandering courses of the canyons of the lower Big 

Thompson River, Little Thompson River, North and South St 

Vrain Creeks, and Boulder Creek (Steven and others, 1997; 

Cole, 2004a). These streams today flow through steep-sided 

canyons cut in granitic and metamorphic rocks, where stream 

Quaternary alluvium

Volcanic rocks, undivided (upper Oligocene)

Granodiorite of the Mount Richthofen stock

Granite and rhyolite porphyry of the Mount Cumulus

 stock (late Oligocene)

Coalmont Formation (Eocene and Paleocene)

Upper Cretaceous Pierre Shale

Mesozoic (pre-Pierre Shale) and Paleozoic rocks

Proterozoic rocks

Thrust fault—Sawteeth on upper plate, dashed where concealed

High-angle fault—Bar and ball on downthrown block, dashed

 where concealed

Gravity gradient—Hachures point to low-density rock

Magnetic gradient—Hachures point to low-susceptibility rock

Bedding direction

EXPLANATION

Fluvial gravel (Pliocene? and Miocene)

Rhyolite tuff and flows (upper Oligocene)

Q

K

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Ng

�v

�r

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44 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Economic Geology

The geologic resources of the Estes Park quadrangle 

have afforded wide opportunities for commercial benefit 

and economic growth over the years. The mountain regions 

contain deposits of gold, silver, and tungsten that were 

exploited in the late 1800’s and early 1900’s. The foothillshogback zone has been extensively quarried for various stone 

products over the years. The Denver Basin area has been 

explored and developed for substantial resources of coal, oil, 

and natural gas. The valleys of the major rivers and streams 

have all produced substantial quantities of sand and gravel 

for building materials, and will continue to do so in the 

foreseeable future. Above all, water has been the most heavily 

developed natural resource of the region.

Colorado Mineral Belt

Early Tertiary intrusive rocks emplaced during and 

following the Laramide orogeny provided heat sources, 

hydrothermal fluids, and hosts for several significant mineral 

deposits in the southern part of the Estes Park quadrangle. 

These mineralized areas constitute the northernmost extent 

of the Colorado Mineral Belt in this region, as defined and 

cataloged by Lovering and Goddard (1950). Most of the 

following summary is based on their report, modified on the 

basis of new information about rock composition and age 

(DeWitt, written commun., 2007).

The Boulder County tungsten district (fig. 6) is defined 

by numerous east-trending veins that cut the Boulder Creek 

Granodiorite (XgdB) in a narrow zone just north of Middle 

Boulder Creek several miles west of Boulder (Lovering and 

Tweto, 1953). The veins were assayed repeatedly for gold 

and silver in the late 1800’s, but they were not developed 

because they lacked enrichment in these elements. The 

principal ore mineral is ferberite, the iron-rich variety of the 

wolframite tungstate series. It forms large, dark, metallic 

crystals intergrown with quartz, minor scheelite, and sulfides, 

and the rock adjacent to the veins shows considerable sericitic 

alteration. Ferberite was finally identified in 1899 after 

commercial uses of tungsten were identified, and numerous 

mines were developed throughout the district. Production 

grew to a peak in World War I due to the need for hightungsten steels in armament, and continued at a low level 

through World War II. The tungsten district is not clearly 

related to Laramide intrusive rocks, but mafic monzonite 

Figure 11. Sketch maps showing structural features of the Neogene Southern Rocky Mountains. A, Contrast between 

trends of Laramide folds and block uplifts versus Neogene uplift of the Southern Rocky Mountains (after Eaton, 1986, 

1987; Tikoff and Maxson, 2001). B, Distribution of Neogene sediments eroded from the uplifted axis of the Southern Rocky 

Mountains.

EXPLANATION

A

Area of younger uplift during the

 Miocene and Pliocene

Trends of Laramide block uplifts

 and folds

Direction of compression

OKLAHOMA

KANSAS

SOUTH DAKOTA

NEBRASKA

TEXAS

WYOMING

COLORADO

NEW

MEXICO

UTAH

ARIZONA

Casper

Estes Park

Denver

Albuquerque

FLORISSANT

FOSSIL BEDS

NATIONAL

MONUMENT

0 50 100 MILES

41 ̊

35 ̊

110 ̊ 105 ̊ 100 ̊

OKLAHOMA

KANSAS

SOUTH DAKOTA

NEBRASKA

TEXAS

WYOMING

COLORADO

NEW MEXICO

UTAH

ARIZONA

Casper

Estes Park

Denver

Albuquerque

RIO GRANDE RIFT

FLORISSANT

FOSSIL BEDS

NATIONAL

MONUMENT

0 50 100 MILES

41 ̊

35 ̊

110 ̊ 105 ̊ 100 ̊

EXPLANATION

B

Sediments eroded from younger

 (Miocene-Pliocene) uplift of southern

 Rocky Mountains (15–5 million years ago)

Area of younger uplift during the Miocene

 and Pliocene
Economic Geology 45

porphyries (Late Cretaceous) are spatially associated with this 

mineralization nearby south of the Estes Park quadrangle.

The other mine districts of the Colorado Mineral Belt 

(fig. 6) in the southern part of this quadrangle are similar 

and are clearly related to hydrothermal action surrounding 

the Laramide intrusive rocks. The principal ores are native 

gold and silver, precious-metal tellurides, minor arsenides 

and antimonides, and some base-metal sulfides. The 

principal gangue minerals are quartz, ankerite, fluorite, and 

barite. Productive veins are most common in fracture zones 

and silicified faults within Proterozoic rock adjacent to the 

Laramide intrusions.

The Ward district was discovered first in 1861, and the 

nearby Sunset district soon thereafter. Ore shoots occur in 

fractured basement rock near intrusions of syenite, monzonite 

porphyry, and quartz latite.

Gold Hill (and surrounding areas) was discovered in 

1872 and is known for veins rich in gold telluride minerals, as 

well as pyritic gold. Ore is preferentially located in and near 

several large, northwest-trending, silicified breccia zones that 

traverse the area in proximity to abundant Laramide porphyry 

dikes. 

The Jamestown district is hosted by a large stock of Late 

Cretaceous monzodiorite (Kmzd) that is, for the most part, 

only slightly mineralized. Fluids generated by a younger 

syenite porphyry (bsy) (about 56 Ma) produced mineralization and zoned patterns of alteration and ore distribution 

typical of copper-molybdenum porphyry deposits worldwide. 

Principal value in the district is associated with gold telluride 

veins and with silver in base-metal sulfides. The Jamestown 

district also produced significant quantities of fluorite that 

were deposited in the brecciated collapse zone on the margin 

of the syenite porphyry.

Coal, Oil, and Gas

The marine deposits of the Cretaceous Western Interior 

seaway contain abundant carbonaceous debris that served 

as source rocks for significant oil and gas deposits of the 

Figure 12. Map showing Miocene(?) high-level gravel deposits and inferred paleodrainage of the Big Thompson River.

7200

7000

6800

6600

6400

40°15’

40°28’

105°20’ 105°15’

0 1 MILE

Drake

6400 6400

7200

7600

7200

6800

6400

6000 6000

6800

6000

5600

6400

6000

6000

6400

5600

6000

6400

6800

6800

6800

6400

6400

6800

6800

7200

6000

6400

Palisade

Mountain

Alexander

Mountain

Cedar

Park

Thompson Canyon

fault

Big

Thompson River

Sulzer Gulch

Cedar Creek

The Narrows

Ridge line

Bedrock fault

West base of Fountain Formation

Contours in paleovalley of Big Thompson River

High-level gravel deposit

7200

EXPLANATION

Green Ridge
46 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

region. Chemically distinctive, but relatively minor, oil is 

also produced from the Permian Lyons Sandstone (Pl) near 

Berthoud and Loveland. All oil and gas resources in the 

Denver Basin are described in detail and summarized by 

Higley and Cox (2005), the major source for the following 

summary.

More than 245 million barrels of oil and more than 2.2 

trillion cubic feet of natural gas have been produced through 

2000 from wells in the Denver Basin since early development 

began in the late 1800’s. The principal source rocks are in 

the middle part of the South Platte Formation (upper Dakota 

Group) as well as the shaly formations between the Dakota 

Group (Kd) and the Niobrara Formation (Kn). Hydrocarbon 

production is greatest from sandstone bodies within the 

Dakota Group, at the base of the Niobrara Formation, and 

within the Upper Cretaceous Pierre Shale (Kp). The high 

rates of subsidence and sedimentation within the Denver 

Basin during Late Cretaceous time led to burial, heating, and 

generation of oil and gas.

Most of the oil and gas production in the Denver Basin 

comes from the Muddy Sandstone (upper Dakota Group; 

equivalent to the first sandstone member of the South Platte 

Formation), also known in the petroleum literature as the 

“J” sandstone. It is well confined above and below by shale 

formations (both significant source rocks) and its high 

permeability enhances production. Traps are a combination 

of stratigraphic pinch-outs, up-dip stratigraphic truncations, 

and numerous anticlinal structural traps within the Laramide 

folds adjacent to the Front Range uplift. Oil-producing wells 

are concentrated in the southeastern corner of the Estes Park 

quadrangle in what is known as the Spindle field (mostly in 

Weld County). The much larger Wattenberg field produces 

mainly natural gas, and extends far to the east and northeast 

beyond the quadrangle.

Coal is contained in several economic seams within the 

Laramie Formation (Kl), a collection of floodplain and swamp 

deposits that accumulated on the landward side of the regressional Fox Hills Sandstone (Kfh) during the withdrawal of the 

Late Cretaceous Western Interior seaway. These coals in the 

lower part of the Laramie Formation were mined at the surface 

and in the shallow subsurface within the Boulder-Weld coal 

field as early as the 1850’s, and sporadically until the 1970’s 

(Roberts and others, 2001). The following summary is based 

largely on the comprehensive summary of Roberts (2005).

Approximately 130 million tons of subbituminousrank coal have been produced from the Denver Basin area 

during about 120 years of mining. Nearly all of the coal is 

extracted from the lower 300 ft of the Laramie Formation and 

is contained in seams that range from 1 to 20 ft thick. Most 

mines operated from surface excavations down to depths of 

400 ft or more (Roberts and others, 2001). The principal area 

of production within the Estes Park quadrangle is the BoulderWeld coal field, and it historically produced more than 80 

percent of the coal from the entire Denver Basin. Mining 

ceased in 1979 in this area due to competing uses for the land 

surface (chiefly housing and commercial centers), and because 

adequate alternate sources were available in the region. Coalbed methane has not been produced in this area as of 2007, but 

increasing demand and market price may lead to exploration in 

the near future. Estimates of unmined coal and lignite within 

3,000 ft of the land surface are substantial.

Sand and Gravel

Quaternary floodplain and fluvial-terrace deposits of sand 

and gravel have been widely developed within the quadrangle 

for construction materials (Schwochow and others, 1974). 

The youngest terraces (Holocene, Broadway Alluvium (Qbr), 

and some Slocum Alluvium (Qs)) have been most thoroughly 

developed because the clasts are freshest and post-depositional 

soils are relatively thin. Sand and gravel quarries are common 

along the floodplains of the Big Thompson River, St Vrain 

Creek, and Boulder Creek, as well as along the Colorado and 

Fraser Rivers west of the Continental Divide.

Quarry Stone

Numerous quarries have been opened in the foothillshogback belt east of the Front Range to exploit resources of 

dimension stone and flagstone for building materials. The 

Lyons Sandstone (Pl) is extensively quarried in and around the 

town of Lyons because the crossbedded, eolian sandstone is 

strongly cemented with silica and naturally cleaves into slabs 

along bedding-plane surfaces.

Sandstone of the Dakota Group (Kd) is also quarried 

in numerous locations along the range front. It is less 

dimensional than the Lyons Sandstone, but the Dakota hosts 

abundant lichen that produce an aesthetically pleasing product 

referred to as “moss rock.”

Sills of Laramide-age dacite and quartz latite crop out in 

the Paleozoic sedimentary rocks within the foothills-hogback 

belt between Lyons and Boulder (Wrucke and Wilson, 1967; 

Braddock, Houston, and others, 1988). One of the largest 

sills has been extensively quarried southwest of Lyons where 

it is exposed along the St Vrain Creek. This high-density rock 

is used chiefly for road base, riprap, and landscape dimension 

stone.

Water

The earliest settlers in this region recognized that water 

was a critical commodity to develop and control. For many, 

the long, slow journey west of the Mississippi River must have 

provided ample opportunity to observe that average annual 

rainfall diminished westward and that the native vegetation 

became progressively more meager closer to the foot of the 

Front Range. Their first views of the snow-capped peaks of 

the Continental Divide probably stimulated speculation about 

means to capture meltwater from the snowpack and hold it in 

reserve for the summer growing season. 
Environmental Geology 47

Some of the earliest development projects in this region 

were ditch systems constructed by settlers during the post1849 California gold-rush migration. Every major stream that 

issues from the mountain front is diverted at numerous levels 

in privately-owned ditches that convey the spring and summer 

flow to agricultural lands on the Colorado Piedmont. Rights 

to divert streamflow for beneficial use follow the distinctly 

western U.S. legal doctrine of prior appropriation, simply 

expressed as “first in time, first in right”. That is, users are 

afforded shares of streamflow based on priority of use, and the 

earliest water developers are vested with “senior” water rights 

in western streams. One of the significant legal cases that 

confirmed this system in the Rocky Mountains was settled by 

the 1882 Colorado Supreme Court in a dispute over diversion 

of the St Vrain Creek (Coffin vs. Left Hand Ditch Company; 

see Wilkinson, 1992, p. 231–235).

Further water development began in the high mountain 

valleys during the late 1800’s and early 1900’s. These projects 

constructed dams, reservoirs, and conveyances for the purpose of storing winter snowpack so that meltwater could be 

released during the middle and late summer for irrigation on 

the piedmont (Cole, 2004a). Many projects included ditches 

and tunnels in the headwaters of the Colorado River designed 

to capture and divert Pacific-bound waters to the eastern side 

of the Continental Divide. The Grand Ditch that traverses the 

eastern face of the Never Summer Mountains is one of the 

earliest water-diversion and storage projects that remains in 

use today. Several small reservoirs were constructed by irrigation companies and piedmont municipalities prior to establishment of Rocky Mountain National Park in 1915. All of these 

structures have since been removed (Sandbeach Lake, Pear 

Reservoir, Bluebird Lake, and Lawn Lake) following Park 

Service acquisition of the water rights. Numerous other small 

reservoirs remain in the east-flowing drainages of the Front 

Range to provide water supply to local communities.

By far, the largest trans-mountain water-diversion 

infrastructure of the region is the Colorado-Big Thompson 

(C-BT) Project of the Federal Bureau of Reclamation. It was 

constructed in stages between 1938 and 1957 to meet numerous needs in northern Colorado (Cole, 2004a). It collects and 

diverts water from the Colorado River headwaters west of the 

Continental Divide and transmits it through the 13-mile-long 

Alva B. Adams tunnel beneath Rocky Mountain National Park 

to a distribution system along the Big Thompson River drainage on the eastern slope. The C-BT Project spans 150 miles of 

waterways and includes 12 reservoirs, 35 miles of tunnels and 

siphons, and 6 hydroelectric power plants (Cole, 2004a). The 

project provides municipal water for 30 communities east of 

the Continental Divide and irrigation water for approximately 

615,000 acres along the Big Thompson, Cache la Poudre, and 

South Platte River drainages.

The C-BT Project provides reliable water supply, regulated streamflow through seasons and climate changes, environmentally clean hydroelectric power, recreation, additional 

lakes and canals, agricultural production, economic development, flood control/mitigation, and increased discharges to the 

South Platte River system and the Missouri-Mississippi River 

drainage. The trade-offs for this trans-mountain water diversion are decreased flow in the Colorado River headwaters; 

decreased discharge to the Gulf of California; reductions in 

wetlands in the upper Colorado River drainage; visual impacts 

of impoundment structures, power plants, and power lines; 

evaporation losses from reservoirs; and reduction in peak 

flood-flows of dammed rivers (Cole, 2004a).

Environmental Geology

Geology is one of the underlying issues that warrants 

consideration when any part of the landscape is evaluated for 

a particular use. The geology of the Estes Park quadrangle is 

both diverse and complex and so the geologic issues that come 

into play are equally diverse. Different aspects of geology and 

topography are important in the relatively populated areas of 

the Colorado Piedmont in contrast to the less populated, but 

frequently visited mountainous terrain. 

This section of the report summarizes some of the 

geologic processes and conditions that impact human use 

of the land. It is not intended to be comprehensive, but 

rather to present geologic factors that have historically led 

to hazards or damage when they were ignored. Much of the 

information in this section regarding landslides and expansive 

soils is summarized from a 2004 field-trip guide covering 

environmental geology of the Denver metropolitan area 

(Abbott and Noe, 2004) and from several papers referenced in 

that guide. The Colorado Geological Survey (CGS) has also 

published several papers and booklets on geologic hazards that 

are especially pertinent (Soule and others, 1976; Noe, 1997; 

Noe and Dobson, 1997; Noe and others, 1997, 1999).

Landslides, Rockfalls, and Debris Flows

Landslides occur in several geologic settings in the 

Estes Park quadrangle. The large-scale block-glide landslide 

complexes of the Dakota Group (Kd) strata (Braddock, 

1978) were described previously in the section on “PostLaramide structures;” most are believed to be inactive, 

although excavation of the toe area could lead to further 

earth movement. Mountain landslides involving Proterozoic 

crystalline rocks are known from the Jackstraw Mountain area 

on the east side of Trail Ridge Road, but they are rare. 

Landslides and slumps occur in several areas of the 

quadrangle in association with rock units that contain 

abundant silt or expansive clay minerals. The Troublesome 

Formation (Nbt) in the Granby valley has numerous slumps 

and slides on moderate slopes that are due to this condition. 

A large landslide complex involves hydrothermally altered 

Oligocene intrusive rocks on the northwest slope of Porphyry 

Peaks, southwest of Grand Lake. The Laramie Formation (Kl) 

in the southeastern part of the quadrangle is also susceptible to 
48 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

slope failure due to high clay content. The Cretaceous Pierre 

Shale (Kp) contains several beds of weathered volcanic ash 

(referred to as bentonite) that are rich in expansive clays that 

have led to slope failure in the past.

The west side of Boulder is marked by a slow-moving 

landslide that leads to persistent road-maintenance issues. The 

westward extension of Baseline Road (southern quadrangle 

boundary) zig-zags up the face of Flagstaff Mountain and 

crosses a zone of slumping ground within the strongly tilted 

sedimentary beds deformed by the local Boulder thrust fault.

Rockfalls are fairly frequent in mountain canyons and 

occasionally disrupt highways and trails. They are most common during the spring and summer months when rainfall and 

snowmelt raise the local water table, increase soil pore-water 

pressure, and allow unstable rock masses to slide and fall.

Debris flows have occurred on several occasions within 

the quadrangle as a result of elevated pore-water pressure near 

water-conveyance systems. The historic Grand Ditch was 

constructed in the 1880’s to intercept runoff from the Never 

Summer Mountains and convey it to the Cache la Poudre 

drainage for agricultural use in the Greeley-Fort Collins 

area. This ditch traverses the east slope of the Never Summer 

Mountains at an elevation of about 10,200 ft and passes 

through La Poudre Pass and down to Long Draw Reservoir 

(north of the Estes Park quadrangle). Several debris flows 

appear to have originated just below the ditch, causing watercharged rock and mud to cascade down the lower slopes of the 

Never Summer Mountains to the Kawuneechee Valley below. 

One modern debris flow event occurred in 1978 east of Baker 

Mountain and impacted the Holzwarth Ranch in the valley 

below. The deposit of this flow was shown on the geologic 

map of Rocky Mountain National Park (Braddock and Cole, 

1990). A debris flow in the spring of 2005 caused damage to 

the historic mining camp of Lulu City near the headwaters of 

the Colorado River, but it originated above the Grand Ditch. 

A catastrophic debris flow occurred on the early morning 

of July 15, 1982, as a result of failure of the Lawn Lake dam 

at nearly 10,900 ft elevation in the Mummy Range section 

of Rocky Mountain National Park. The dam had been 

constructed prior to establishment of the Park in the early 

1900’s to serve as high-altitude water storage for the City of 

Loveland and other entities along the eastern Big Thompson 

River drainage. 

Over time, the 80-year-old earthen dam had developed 

a leak around the outlet pipe, leading to subsurface erosion 

and ultimately to collapse. When the dam gave way, more 

than 670 acre-feet of water were released into the Roaring 

River channel. The torrent cascaded down the 5-mile valley 

toward Horseshoe Park, eroding a deep 30-ft gash into the 

channel banks, and picking up boulders, soil, and trees in a 

fluid slurry. That debris flow reached the mouth of Roaring 

River in a matter of minutes and deposited a broad alluvial 

fan at the floor of Horseshoe Park. The maximum flood-stage 

flow is estimated to have been as high as 18,000 cubic feet per 

second. The flood transported many boulders more than 3 ft 

in diameter (as large as 14 × 17 × 21 ft), many of which were 

eroded from the north lateral glacial moraine of the Fall River 

drainage. 

Expansive Soils

Clay-rich soils in the Colorado Piedmont section of the 

Estes Park quadrangle have caused damage to foundations, 

structures, and roads due to expansion and contraction 

resulting from alternate wetting and drying. Flat-lying and 

gently inclined beds of the Pierre Shale (Kp) and the Laramie 

Formation (Kl) are most susceptible to this process due to 

the presence of zones rich in expansive clays. Experience 

during the last 40 years has shown that these conditions 

can be mitigated with engineered systems in most cases, 

and all county building departments require soil testing and 

appropriate mitigation for new construction in areas east of the 

foothills-hogback belt.

Floods

The mountainous terrain of the Front Range is drained by 

major rivers that flow through deeply incised, narrow canyons 

between about 9,000 ft and 5,500 ft elevation. Major summer 

thunderstorms concentrate their most intense rainfall between 

elevations of about 7,000 ft and 9,000 ft due to orographic 

conditions, and so the potential for mountain-valley flash 

flooding is both high and persistent (Cole, 2004a). Most of 

the incised canyons in this region contain no flood-control 

dam below the high-rainfall zone, largely because most of the 

deep canyons are major transportation routes and have been 

since settlement began in the 1800’s (Button Rock Reservoir 

on North St Vrain Creek is the sole exception). Every major 

stream on the east slope of the Front Range has experienced 

moderate to significant flooding at one time or another in 

recorded history, and the likelihood of future flooding is high. 

Modern development has encroached further on floodplains 

and floodways, and so the potential for damage continues to 

rise.

The Big Thompson flood of 1976 is instructive in this 

regard (see hydrologic and geologic summaries; McCain and 

others, 1979; Shroba and others, 1979). The summer afternoon of July 31 began with typical thunderstorm activity that 

centered around the head of the deep-valley section of the Big 

Thompson River below the dam at Lake Estes. The thunderstorm grew in size and remained stationary in this position, 

resulting in more than eight inches of rainfall over large portions of the drainage system within a period of little more than 

one hour (McCain and others, 1979; Cole, 2004a). The resulting flash flood swept rapidly through the Big Thompson River 

canyon, destroying numerous buildings, bridges, roadways, 

and other structures, and drowning more than 143 people. 

Most of the land downstream from the mouth of the canyon 

was agricultural and sparsely settled and so damage and loss 

of life were rare east of the mountain front.
References Cited 49

The conditions are quite different for Boulder Creek and 

St Vrain Creek. The communities of Boulder and Lyons, 

respectively, have grown up around the canyon mouths 

of these major streams and many structures lie within the 

geologically defined flood plains. The last major flood in 

Boulder occurred in the summer of 1894, during which large 

portions of low-lying sections of the downtown were flooded 

and several creek-side structures were lost. Lyons has also 

been spared major flooding for decades, but the potential for 

a 1976-Big Thompson flood event is significant over decadal 

intervals.

Failure of the Lawn Lake dam in 1982 (described above 

in the discussion of debris flows) produced flooding downstream from the alluvial fan deposition site and through the 

town of Estes Park. As the flood discharge spread out across 

the low-gradient Horseshoe Park, it deposited significant sand 

and silt more than 0.5 mile downstream (Blair, 1987). At the 

eastern end of Horseshoe Park, the flow returned to a narrow 

channel cut in bedrock and cascaded down toward the town of 

Estes Park, picking up more debris in the process. Thanks to 

advance warning, the downtown area was evacuated before the 

flood surge arrived. No lives were lost in town, but 18 bridges 

were destroyed and most of Estes Park was buried in 2 ft of 

mud and debris. The flood discharge was contained in the 

Lake Estes reservoir.

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Anderson, J.L., and Thomas, W.M., 1985, Proterozoic 

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177–180.

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deformation in the northern Front Range, Colorado: 

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chronosequences in the western United States: U.S. 

Geological Survey Bulletin 1590, p. E1–E21.

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sequences, and geomorphology of the Roaring River 

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and magnitude of past global climate changes, chapter 

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Colorado Front Range, in Bolyard, D.W., and Sonnenberg, 

S.A., eds., Geologic history of the Colorado Front Range: 

Rocky Mountain Association of Geologists field-trip guidebook: Denver, Rocky Mountain Association of Geologists, 

p. 125–133.

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Range: Longs Peak-St Vrain batholith: Geological Society 

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Braddock, W.A., 1970, The origin of slaty cleavage: Evidence 

from Precambrian rocks in Colorado: Geological Society of 

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Slides and Avalanches: New York, Elsevier Scientific 

Publishing Company, Inc., p. 439–480.

Braddock, W.A., Calvert, R.H., Gawarecki, S.J., and Nutalaya, 

Prinya, 1970, Geologic map of the Masonville quadrangle, 

Larimer County, Colorado: U.S. Geological Survey 

Geologic Quadrangle Map GQ-832, scale 1:24,000.

Braddock, W.A., and Cole, J.C., 1979, Precambrian structural 

relations, metamorphic grade, and intrusive rocks along 

the northeast flank of the Front Range in the Thompson 

Canyon, Poudre Canyon, and Virginia Dale areas, in

Ethridge, F.G., ed., Northern Front Range and northwest 
50 Geologic Map of the Estes Park 30’ x 60’ Quadrangle, North-Central Colorado

Denver Basin, Colorado: Geological Society of America 

Field Guide, Rocky Mountain Section, field trip 2, p. 

106–121.

Braddock, W.A., and Cole, J.C., 1990, Geologic map of Rocky 

Mountain National Park and vicinity: U.S. Geological 

Survey Miscellaneous Investigations Series Map I-1973, 

scale 1:50,000.

Braddock, W.A., and Eicher, D.L., 1962, Block-glide landslides in the Dakota Group of the Front Range foothills, 

Colorado: Geological Society of America Bulletin, v. 73, 

p. 317–324.

Braddock, W.A., Houston, R.G., Colton, R.B., and Cole, J.C., 

1988, Geologic map of the Lyons quadrangle, Boulder 

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