Geologic Map of the Fort Collins 30'×60' Quadrangle, 

Larimer and Jackson Counties, Colorado, and 

Albany and Laramie Counties, Wyoming

Pamphlet to accompany

Scientific Investigations Map 3399

U.S. Department of the Interior

U.S. Geological Survey
Cover. Photograph looking east from Rockhole Lake in the Rawah Wilderness down the West Branch 

Laramie River as it descends steeply eastward to the north-flowing upper Laramie River. High upland areas 

in the foreground expose granite of the Rawah batholith (XgR) with glacial till of Pinedale age (Qtp) filling 

the valley below. The southern end of Green Ridge, capped by a gently sloping erosion surface below the 

skyline, is visible just beyond the mouth of the valley. This ridge, formed by the Laramie River fault zone, 

separates the Laramie River drainage to the west from the Cache la Poudre River drainage to the east. 

South Bald Mountain is visible on the skyline to the left with Crown Point visible on the skyline to the right. 

The Cache la Poudre River flows east, away from view through a deep, inset canyon between these two 

high points below the skyline. Photograph by C.R. Ruleman, May 24, 2005.
Geologic Map of the Fort Collins 30'×60' 

Quadrangle, Larimer and Jackson Counties, 

Colorado, and Albany and Laramie 

Counties, Wyoming

By Jeremiah B. Workman, James C. Cole, Ralph R. Shroba, Karl S. Kellogg, and 

Wayne R. Premo

Pamphlet to accompany

Scientific Investigations Map 3399

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

RYAN K. ZINKE, Secretary

U.S. Geological Survey

James F. Reilly II, Director

U.S. Geological Survey, Reston, Virginia: 2018

For more information on the USGS—the Federal source for science about the Earth, its natural and living 

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For an overview of USGS information products, including maps, imagery, and publications, 

visit https://store.usgs.gov.

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U.S. Government.

Although this information product, for the most part, is in the public domain, it also may contain copyrighted materials 

as noted in the text. Permission to reproduce copyrighted items must be secured from the copyright owner.

Suggested citation:

Workman, J.B., Cole, J.C., Shroba, R.R., Kellogg, K.S., and Premo, W.R., 2018, Geologic map of the Fort Collins 

30'×60' quadrangle, Larimer and Jackson Counties, Colorado, and Albany and Laramie Counties, Wyoming: 

U.S. Geological Survey Scientific Investigations Map 3399, pamphlet 83 p., scale 1:100,000, 

https://doi.org/10.3133/sim3399.

ScienceBase data release for this report available at https://doi.org/10.5066/F7G44PHV

ISSN 2329-132X (online)
iii

Acknowledgments

Funding for this study was provided by the National Cooperative Geologic Mapping Program 

through a joint project with the U.S. Geological Survey (USGS) Mineral Resources Program. Project coordination was provided by K.S. Kellogg and T.L. Klein (USGS). Constructive comments and 

suggestions were incorporated based on technical reviews by S.C. Lundstrom (USGS) and W.D. 

Nesse (University of Northern Colorado, emeritus). Preliminary reconnaissance geologic mapping 

was carried out in the Laramie River valley within the map area (parts of the Crazy Mountain, 

Glendevey, and Deadman quadrangles) by W.A. Braddock of the University of Colorado, Boulder 

(deceased) as part of a USGS funded project from 1981 to 1982. Geologic mapping, petrologic 

studies, and geochronologic investigations in the Comanche Peak quadrangle were completed 

over a number of years by T.G. Plymate and T.D. Moeglin of Southwestern Missouri University 

and by W.R. Van Schmus of Kansas State University; these results were graciously provided to 

the authors for this compilation prior to publication. W.A. Cobban (USGS; deceased) provided 

guidance in correlation of Cretaceous map units across the study area. The authors acknowledge 

and appreciate the thoroughness and thoughtfulness of these efforts. The final map and report 

are improved and refined as a result.

Contents

Acknowledgments........................................................................................................................................iii

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

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

Geography and Geomorphology ........................................................................................................2

Compilation Sources and Methods ...................................................................................................8

Geologic History...........................................................................................................................................10

Paleoproterozoic Layered Rocks .....................................................................................................14

Paleoproterozoic Intrusive Rocks....................................................................................................19

Paleoproterozoic Metamorphism ....................................................................................................20

Mesoproterozoic Intrusive Rocks....................................................................................................21

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

Pre-Cenozoic Sedimentary Rocks ...................................................................................................24

Laramide Synorogenic Sedimentary Rocks ...................................................................................27

Laramide Intrusive Rocks..................................................................................................................28

Post-Laramide Intrusive and Volcanic Rocks ................................................................................28

Post-Laramide Sedimentary Rocks .................................................................................................29

Quaternary Deposits...........................................................................................................................32

Main-Stream Fluvial and Pediment Deposits........................................................................32

Glacial and Glaciofluvial Deposits ..........................................................................................33

Mass-Movement Deposits.......................................................................................................34

Eolian Deposits...........................................................................................................................34

Structure........................................................................................................................................................34
iv

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

Mesoproterozoic Structures.............................................................................................................37

Sherman Batholith and Virginia Dale Ring-Dike Complex ..................................................37

Log Cabin Batholith ...................................................................................................................38

Longs Peak-St. Vrain Batholith................................................................................................38

Mylonite-Bearing Shear Zones ...............................................................................................38

Arapaho Uplift (Ancestral Rocky Mountains) ................................................................................39

Laramide Orogeny...............................................................................................................................39

Early Laramide Structures........................................................................................................39

Late Laramide Structures.........................................................................................................40

Post-Laramide Structures .................................................................................................................42

Braddock Peak Volcanic Complex..........................................................................................43

Laramie River Fault Zone..........................................................................................................43

Block-Glide Landslides .............................................................................................................43

Miocene-Pliocene Uplift and River Incision .........................................................................44

Economic Geology.......................................................................................................................................45

Manhattan District..............................................................................................................................45

Diamonds of the State-Line Kimberlite District .............................................................................45

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

Limestone and Gypsum......................................................................................................................45

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

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

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

Environmental Geology...............................................................................................................................46

Landslides and Rockfalls...................................................................................................................47

Expansive Soils....................................................................................................................................47

Floods....................................................................................................................................................47

Description of Map Units............................................................................................................................48

Surficial Deposits................................................................................................................................48

Artificial-Fill Deposit..................................................................................................................49

Alluvial Deposits.........................................................................................................................49

Alluvial and Mass-Movement Deposits.................................................................................51

Mass-Movement Deposits.......................................................................................................52

Glacial Deposits .........................................................................................................................53

Organic-Rich Deposit................................................................................................................55

Eolian Deposit.............................................................................................................................55

Post-Laramide Sedimentary Rocks .................................................................................................56

Post-Laramide Volcanic Rocks.........................................................................................................58

Post-Laramide Intrusive Rocks ........................................................................................................59

Laramide Intrusive Rocks..................................................................................................................59

Laramide Sedimentary Rock.............................................................................................................59

Pre-Cenozoic Sedimentary Rocks ...................................................................................................60

Paleozoic and Proterozoic Intrusive Rock......................................................................................63

Proterozoic Intrusive and Metamorphic Rocks.............................................................................64

Mesoproterozoic Intrusive Rocks...........................................................................................64
v

Mesoproterozoic and (or) Paleoproterozoic Intrusive and Metamorphic Rock ......................67

Paleoproterozoic Intrusive Rocks....................................................................................................67

Paleoproterozoic Metamorphic Rocks ...........................................................................................69

References Cited..........................................................................................................................................71

Figures

1. A. Geographic setting of the Fort Collins 30'×60' quadrangle. B. Simplified 

boundaries and areas of some of the Federal and State lands found in the 

Fort Collins, Colorado, map area ................................................................................................4

2. Physiography of the Fort Collins, Colorado, quadrangle. Dashed black lines are 

physiographic provinces. Black square indicates map area. Selected rivers, 

creeks, and roads shown ............................................................................................................6

3. Hydrography of the Fort Collins quadrangle and surrounding area, with the North 

and South Platte River Basins in the map area, and the Colorado River Basin to the 

southwest of the map area .........................................................................................................7

4. A, Index map showing sources of geologic map data used in compiling the Fort 

Collins 30’×60’ quadrangle. For U.S. Geological Survey publications, report number, 

authorship, and date of publication are shown. B, Index map of compilation 

responsibilities for new mapping in the Fort Collins 30'×60' quadrangle............................8

5. Regional geologic setting of the Fort Collins 30'×60' quadrangle ......................................11

6. Simplified geologic map of the Fort Collins 30'×60' quadrangle.........................................12

7. Proterozoic metamorphic rocks of the Fort Collins 30' x 60' quadrangle showing 

distribution of major metamorphic rock types within the map area..................................15

8. Photographs of thin sections and map showing mineralogy of Paleoproterozoic

metamorphic rocks in the northern Front Range. A, Sketch map showing

distribution of metamorphic-mineral isograds in pelitic rocks. B, Photomicrograph

showing matrix cordierite (Cd), quartz (colorless), and muscovite (Ms) with

porphyroblastic staurolite (St) in low-grade schist. 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 in partially melted biotite

gneiss............................................................................................................................................16

9. Proterozoic intrusive rocks of the Fort Collins 30'×60' quadrangle showing 

distribution of major plutons and batholiths of both the older Routt Plutonic 

Suite and younger Berthoud Plutonic Suite...........................................................................18

10. Laramide and older sedimentary rocks of the Fort Collins 30'×60' quadrangle. 

Paleozoic and Mesozoic sedimentary rocks are exposed on both the east and 

west flanks of the Front Range uplift in the Fort Collins quadrangle .................................25

11. Correlation of sedimentary rocks across Fort Collins 30'×60' quadrangle for the 

Denver Basin, Laramie Basin, Laramie River valley, and the North Park Basin..............26

12. Post-Laramide deposits and features of the Fort Collins 30'x60' quadrangle..................30

13. Map showing major structural elements of the Fort Collins 30'x60’ quadrangle.............35
vi

Conversion Factors

International System of Units to U.S. Customary units

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

Area

square meter (m2

) 0.0002471 acre

square kilometer (km2

) 247.1 acre

square kilometer (km2

) 0.3861 square mile (mi2)

Pressure

bar 14.5038 pounds per square inch (psi)

kilopascal (kPa) 0.01 bar

Volume

cubic meter (m3

) 6.290 barrel (petroleum, 1 barrel=42 gal)

Mass

metric ton (tonnes, t) 1.102 ton, short (2,000 lb)

metric ton (tonnes, t) 0.9842 ton, long (2,240 lb)

U.S. Customary units to International System of Units

Multiply By To obtain

Length

inch (in.) 2.54 centimeter (cm)

foot (ft) 0.3048 meter (m)

mile (mi) 1.609 kilometer (km)

Area

acre 4,047 square meter (m2

)

acre 0.004047 square kilometer (km2

)

square mile (mi2

) 2.590 square kilometer (km2) 

Pressure

kilopascal (kPa) 0.01 bar

Volume

barrel (bbl), (petroleum, 1 barrel=42 gal) 0.1590 cubic meter (m3

) 

gallon (gal) 3.785 liter (L) 

cubic foot (ft3

) 0.02832 cubic meter (m3) 

Temperature in degrees Celsius (°C) may be converted to degrees Fahrenheit (°F) as follows:

°F=(1.8×°C)+32

Datum

Horizontal coordinate information is referenced to the North American Datum of 1927 (NAD 27)

Altitude, as used in this report, refers to distance above Mean Sea Level.
vii

Divisions of Quaternary, Neogene, and Paleogene 

time used in this report1

Period or

subperiod Epoch Age

Quaternary

Holocene 0–11.7 ka

late 11.7–132 ka

Pleistocene middle 132–788 ka

early 788 ka–2.588 Ma

Neogene

Pliocene 2.588–5.332 Ma

Miocene 5.332–23.03 Ma

Oligocene 23.03–33.9 Ma

Paleogene Eocene 33.9–55.8 Ma

Paleocene 55.8–65.5 Ma

1

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2010) except those for the late-middle Pleistocene boundary and middle-early Pleistocene 

boundary, which are those of Richmond and Fullerton (1986). Ages are expressed in ka for 

kilo-annum (thousand years) and Ma for mega-annum (million years).

Geologic Map of the Fort Collins 30'×60' Quadrangle, 

Larimer and Jackson Counties, Colorado, and Albany and 

Laramie Counties, Wyoming

By Jeremiah B. Workman, James C. Cole, Ralph R. Shroba, Karl S. Kellogg, and Wayne R. Premo

Abstract

The rocks and landforms of the Fort Collins 30'×60'

quadrangle reveal a particularly 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. Regional

and textural evidence shows that the widespread metamorphism was essentially concurrent with intrusion of the Routt

Plutonic Suite, with the peak of deformation in the partially

melted high-grade rocks of the suite. 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 the Berthoud

Plutonic Suite into crust that was structurally neutral or moderately extending in an east-northeast direction. Mylonite

zones, including the Skin Gulch shear zone, formed during

Mesoproterozoic time perhaps during several events.

Evidence of the late Paleozoic Anasazi uplift (Ancestral

Rocky Mountains uplift) within the quadrangle is recorded

by removal of Permian and older sediments and deposition

of proximal Pennsylvanian and Permian strata unconformably onto the exhumed Proterozoic basement rocks. The

Phanerozoic sediments indicate a steady progression of

fluvial, eolian, and lacustrine environments throughout most 

of the Mesozoic Era which was a time of relatively slow

sediment accumulation. Early Cretaceous time was marked

by incursion of the Cretaceous 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 

significantly in the latter part of this period during downwarping related to distant crustal loading by thrusting along

the western continental margin.

With onset of the Laramide orogeny in latest 

Cretaceous time, mountain building resumed in this region. 

This deformation placed Proterozoic rock over Cretaceous 

and Paleocene strata along the western margin of the Front 

Range and Medicine Bow Mountains. The eastern margin of 

the range is marked by high-angle reverse faults and faultpropagation folds in the Phanerozoic strata, and by the deep 

syncline of the Denver-Cheyenne Basin. Dacitic to rhyodacitic porphyry dikes and plugs were emplaced during the 

Laramide orogeny in the Manhattan district, an historic mining area by the ghost town of Manhattan, Colo., that hosts 

sparse gold deposits. The Coalmont Formation on the western side of the quadrangle records erosion of the Laramide 

uplifts. Similar deposits transported east of the Front Range 

are not preserved in this quadrangle due to Neogene and 

younger erosion.

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 

White River Group record the early part of the Cenozoic 

geologic history across the entire area. Intrusive activity in 

the Never Summer Mountains south of the map area during 

middle Oligocene time led to eruption of basaltic, dacitic, 

and rhyolitic lavas and welded tuffs that blanketed the 

southwestern part of the map area. The upper Oligocene to 

Miocene North Park Formation contains clasts of this volcanic material and records erosion and fluvial deposition from 

the volcanic highland.

Erosion on the eastern slope of the Front Range in late 

Paleogene to early Neogene time 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 the Miocene and Pliocene(?) Ogallala Formation fluvial 

apron; this apron extends eastward from the Laramide mountain front. Low-gradient streams developed broad meandering paths across this aggraded apron. Renewed uplift of the 

central Rocky Mountains 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 led to 

glaciation during the Quaternary. In the Rocky Mountain 
2 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

region, the renewed uplift allowed erosion to accentuate the 

topographic relief across the high mountains of the map area 

and established the elevations necessary to trigger accumulation of persistent snow and ice. Mountain glaciers advanced 

and retreated during at least three glacial-interglacial cycles 

during the middle and late Pleistocene in this area.

Major landslide and debris flow deposits were active 

during the late Pleistocene to Holocene throughout the 

mountainous areas. Erosion continues to this day on the High 

Plains east of the mountain front, and progressive incision 

of the drainage is recorded by at least five major gravelclad terrace and pediment surfaces along the major fluvial 

channels that connect to the South Platte River system.

Introduction

This report describes the geology of the Fort Collins 

30'×60' quadrangle in north-central Colorado. This compilation and synthesis integrates geologic mapping and 

various topical studies carried out between 1960 and 

1990 under the U.S. Geological Survey (USGS) study of 

the northeastern Front Range (Braddock and Cole, 1979; 

Hutchinson and Braddock, 1987), geologic mapping of 

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

and reconnaissance geologic mapping in wilderness areas 

(Pearson and others, 1981, 1982). Academic studies contributed detailed geologic mapping of several areas north 

of the Cache la Poudre River (Walko, 1969; Burch, 1983), 

in the Medicine Bow Mountains and Laramie River valley 

(Beckwith, 1942; Kiver, 1968; Camp, 1979; Griswold, 

1980), and in the northern Mummy Range (Plymate and 

others, 2005). Authors of this report carried out detailed 

geologic mapping (Kellogg, Ruleman and others, 2008; 

Workman, 2008; Workman and Braddock, 2010) and 

reconnaissance mapping in previously unmapped areas of 

the quadrangle and reviewed field relations across the area 

to arrive at an integrated understanding of the geologic 

history. The ages of crystallization for numerous Proterozoic igneous and metamorphic units in the quadrangle were 

determined using single-crystal sensitive high-resolution 

ion microprobe (SHRIMP) techniques. This publication 

further 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 report is to summarize the geologic 

framework of a significant portion of the northern Colorado 

mountain terrain in support of updated mineral-resource 

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

Forest Service.

The geographic information system (GIS) data and related 

data files are available as a USGS data release in ScienceBase 

at https://doi.org/10.5066/F7G44PHV or https://www.sciencebase.gov/catalog/item/5a26d96ae4b03852baf7f16d (Workman 

and others, 2018).

Geography and Geomorphology

The Fort Collins 30'×60' quadrangle encompasses an 

area of 5,040 square kilometers (km2

), extends 93 kilometers 

(km) east-west and 57 km north-south, and includes parts of 

Jackson and Larimer Counties, Colorado, as well as a very 

thin sliver of the southernmost parts of Albany and Laramie 

Counties, Wyoming (fig. 1).

Most of the quadrangle is mountainous and much of the 

central terrain is rugged. The regional term Front Range is 

applied to the mountain uplift west of the Colorado Piedmont 

as far as the Laramie River valley in northern Colorado 

(fig. 2). The term Mummy Range applies to the highest 

parts of the Front Range south of the Cache la Poudre River, 

whereas the term Laramie Mountains refers to that part of the 

Front Range north of the river as shown on USGS topographic 

maps in the region (figs. 3 and 4A).

Elevations range between about 1,500 meters (m) in 

the Cache la Poudre valley in the southeastern part of the 

quadrangle to over 3,850 m in the northern Mummy Range 

and nearly 4,000 m in the Medicine Bow Mountains. Most 

elevations in the Laramie Mountains are between 2,300 m 

and 3,050 m. The Laramie River valley lies at about 2,500 m 

at its widest part and declines gently to the northwest. The 

southern margin of the Laramie Basin along the western part 

of the northern border of the Fort Collins quadrangle lies at 

about 2,300 m elevation. 

In the Fort Collins 30'×60' quadrangle, approximately 

one-half of the area lies within the Roosevelt National Forest, 

another five percent within the northern part of Rocky Mountain National Park, and another five percent in the southwest 

corner lies within the Colorado State Forest State Park. 

Wilderness areas are designated in some of the more remote, 

high alpine areas (both Forest Service and National Park lands), 

including the Neota and Comanche Peak Wilderness areas in 

the Front Range (Pearson and others, 1981) and the Rawah 

Wilderness Area in the Medicine Bow Mountains (Pearson 

and others, 1982). Most of the inner valley of the main Cache 

la Poudre River and the South Fork Cache la Poudre River 

are designated as Wild and Scenic Rivers, and about 26 km2

or about 10 square miles [mi2

])surrounding the confluence of 

these streams is designated the Cache la Poudre Wilderness.

Colorado State parks and wildlife areas encompass a few 

percent of the Fort Collins quadrangle. The Cherokee Park 

State Wildlife Area covers more than 52 km2

 (20 mi2) along 

the North Fork Cache la Poudre River and Trail Creek, and 

about 13 km2 (5 mi2

) surrounding the deep canyon of Lone 

Pine Creek are designated the Lone Pine State Wildlife Area. 

In the foothills area west of Fort Collins, Horsetooth Reservoir 

State Park includes the reservoir facility and several square 

miles of adjacent mountain land.

The eastern one-fifth of the Fort Collins quadrangle 

lies on the Colorado Piedmont below about 1,800 m elevation and includes the developed city center of Fort Collins, 

surrounding suburban areas, and farmland. The population of Fort Collins is just over 160,000, and the combined 
Introduction 3

WYOMING

ALBANY COUNTY

LARAMIE COUNTY

CARBON

COUNTY

ALBANY COUNTY

LARIMER COUNTY

LARAMIE COUNTY

LARIMER COUNTY

LARIMER COUNTY

WELD COUNTY

JACKSON

COUNTY

JACKSON COUNTY

LARIMER COUNTY

LARIMER

COUNTY

COLORADO

ALBANY

COUNTY

GRAND

COUNTY 

BOULDER

COUNTY

BOULDER

BOULDER

COUNTY

COUNTY

MAP

AREA

106°00' 105°45' 105°30' 105°15' 105°00' 104°45' 106°15'

41°30'

41°15'

41°00'

40°45'

40°30'

40°15'

40°00' Boulder

Granby

Estes

Park

Longmont

Loveland

Fort

Collins

Laramie

Cheyenne

Lyons

Livermore

Idylwilde

Pingree

Park

Masonville

Virginia Dale

Kinikinik

COLORADO

WYOMING

Base from U.S. Geological Survey National Elevation Dataset, 2010

Universal Transverse Mercator projection, zone 13N

North American Datum 1927

52

72

7

7

66

119

14

14

14

1

230

130

210

211

80

25

36

34

34

40 36

85

287

287

287

85

30

85

25

125

0 4 8 12 16 20 KILOMETERS

0 4 8 12 16 20 MILES

A

Figure 1 (continued on following page). A. Geographic setting of the Fort Collins 30'×60' quadrangle. B. Simplified 

boundaries and areas of some of the Federal and State lands found in the Fort Collins, Colorado, map area.
4 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Figure 1. A. Geographic setting of the Fort Collins 30'×60' quadrangle. B. Simplified boundaries and areas of some of 

the Federal and State lands found in the Fort Collins, Colorado, map area.—Continued

ROCKY

MOUNTAIN

NATIONAL

PARK

ROOSEVELT

NATIONAL FOREST

ROOSEVELT

NATIONAL

FOREST

COLORADO STATE

WILDLIFE AREAS

STATE FOREST

STATE PARK

ROOSEVELT

NATIONAL

FOREST

ROUTT

NATIONAL

FOREST

CACHE LA POUDRE

NATIONAL WILD AND SCENIC RIVER

CACHE LA POUDRE

WILDERNESS COMANCHE

PEAK

WILDERNESS

RAWAH WILDERNESS

NEVER

SUMMER

WILDERNESS

INDIAN

PEAKS

WILDERNESS

NEOTA

WILDERNESS

ARAPAHO

NATIONAL

FOREST

ARAPAHO

NATIONAL

FOREST

ARAPAHO

NATIONAL

FOREST

COLORADO STATE

WILDLIFE AREAS

COLORADO STATE

WILDLIFE AREA

ARAPAHO

NATIONAL

WILDLIFE

AREA

ROUTT NATIONAL FOREST

MEDICINE

BOW

NATIONAL

FOREST

PAWNEE

NATIONAL

GRASSLAND

BAMFORTH NATIONAL

WILDLIFE AREA

HUTTON LAKE

NATIONAL

WILDLIFE

AREA

MORTENSON LAKE

NATIONAL WILDLIFE

AREA

COLORADO STATE

WILDLIFE AREAS

MEDICINE

BOW

NATIONAL

FOREST

WYOMING

COLORADO

MAP

AREA

106°00' 105°45' 105°30' 105°15' 105°00' 104°45' 106°15'

41°30'

41°15'

41°00'

40°45'

40°30'

40°15'

40°00' Boulder

Granby

Estes Park

Longmont

Loveland

Fort

Collins

Laramie

Cheyenne

Lyons

Livermore

Idylwilde

Pingree

Park

Masonville

Virginia Dale

Kinikinik

Base from U.S. Geological Survey National Elevation Dataset, 2010

Universal Transverse Mercator projection, zone 13N

North American Datum 1927

Boundaries from a variety of Federal

and State sources, 2017

Boundaries are approximate. This map is not a legal document. 

There may be private inholdings within the boundaries of the 

National or State reservations shown on this map.

52

72

66

119

14

14 14

1

230

130

210

211

80

25

36

34

34

40

36

287 85

287

287 85

30

85

25

125 7

7

0 4 8 12 16 20 KILOMETERS

0 4 8 12 16 20 MILES

B

COLORADO

WYOMING
Introduction 5

population of Larimer County is estimated at being slightly 

over 333,000 (2015 estimates from 2010 U.S. Census Bureau, 

accessed Aug. 2, 2016, at http://www.census.gov/quickfacts/

table/PST045215/0827425,08) concentrated primarily in the 

Piedmont region which contrasts significantly with the sparsely 

populated areas in the mountainous central and western regions.

Highway access in the area is provided primarily by U.S. 

Route 287 and State Highway 14. Route 287 passes north-south 

through Fort Collins and then goes northwestward through the 

small historic town of Virginia Dale, Colo., toward Laramie, 

Wyoming. Colorado State Highway 14 is an east-west road that 

starts in the east off of Interstate 76, goes westward to Fort Collins 

where it runs concurrently with Route 287 for about 3 km (8 mi) 

north of the city. It then turns westward again and follows the 

canyon of the Cache la Poudre River to Cameron Pass, and then 

continues beyond the quadrangle to Walden, Colorado. Most other 

mountain areas are connected by a network of paved and unpaved 

roads, some of which are maintained year-round.

The Fort Collins quadrangle spans the north end of the 

Colorado Front Range uplift of the Southern Rocky Mountains (Eaton, 1986) and includes part of the northwesternmost Colorado Piedmont geographic province to the east 

(Fenneman, 1931). The Medicine Bow Mountains along the 

westernmost part of the quadrangle form a discrete structural 

uplift west of the Laramie River. The southwestern edge of 

the Laramie Basin extends into the northwest corner of the 

quadrangle. The eastern edge of the North Park Basin extends 

into the southwest corner of the quadrangle (fig. 2).

The general landforms of the quadrangle can be described 

in terms of several slightly overlapping geographic and elevation-bounded zones (fig. 2). The easternmost zone below an 

elevation of about 1,800 m consists of the gently inclined slopes 

and broad fluvial valleys in the Colorado Piedmont, including 

small remnants of the Miocene-Pliocene alluvial fan surface 

(the Gangplank surface or Cheyenne tablelands of McMillan 

and others, 2002) in the northeastern corner of the quadrangle. 

North of the town of Livermore, Colo., the Piedmont merges 

imperceptibly with the eroded top of the Proterozoic basement known as the Sherman surface (Eggler and others, 1969; 

Bradley, 1987) that marks the broad gentle crest of the Laramie 

Mountains north of Virginia Dale. The eastern margin of the 

Front Range is marked by a foothill zone consisting of hogback 

ridges formed by upturned sedimentary strata of contrasting erosional resistance. The foothill zone is segmented by a 

northwest-trending fault just north of Teds Place (junction of 

Route 287 and Highway 14) that offsets the hogback ridges. 

A similar but more dramatic fault offset of the hogback belt 

occurs just south of the map area (Cole and Braddock, 2009). At 

Livermore, the foothills belt steps westward into the Livermore 

embayment, a broad, fault-controlled trapezoidal graben marked 

by uplifted basement blocks flanking a gentle syncline in the 

upper Paleozoic strata. The northernmost part of the foothills 

belt has a greater width, lower relief, and is marked by subhorizontal Paleozoic-Mesozoic sedimentary strata forming mesas, 

separated by sections of inclined strata where the sediments are 

folded above faulted basement blocks (Matthews, 1978). 

The central part of the Front Range uplift within the 

map area (between about 2,300 and 2,750 m in elevation) is 

characterized by a gently rolling upland block of crystalline 

rocks sloping gradually to the east and cut by steep, narrow, 

and deeply incised canyons which drain to the east (Steven 

and others, 1997). This rolling upland is interpreted by many 

to be a relic of widespread erosion during the middle part of 

the Tertiary Period (for example, Epis and Chapin, 1975; Scott 

and Taylor, 1986; Steven and others, 1997). The highest part 

of the map area is the upland terrain of the Mummy Range 

and Medicine Bow Mountains that is above about 2,900 m in 

elevation and that is markedly steeper than the incised rolling 

upland zone to the east and north. The upper reaches of major 

valleys in the high upland were modified by glacial processes 

during the Quaternary Period and contain widespread glacial 

deposits (Madole and others, 1998). 

The physiography of the western part of the quadrangle is 

marked by the north-trending, fault-controlled Laramie River 

valley and the adjacent Medicine Bow Mountains to the west. 

The Laramie River drainage flows in a graben formed in a 

structural syncline composed of Paleozoic and Mesozoic rocks 

that are flanked by uplifted blocks of Proterozoic basement on 

the east and west. In the northwestern corner of the quadrangle, the Laramie River runs northwestward (fig. 3) along the 

foot of Bull Mountain, which consists of folded Phanerozoic 

strata along the southern margin of the Laramie Basin.

The Medicine Bow Mountains consist of a northwesttrending basement uplift block that is steepest and highest along 

its western side. The eastern flank of the mountain range is 

marked by a fairly smooth surface of beveled Proterozoic crystalline rocks that is incised by rather linear, narrow, northeasttrending drainages. Both sides of the range crest were sculpted 

by valley glaciers during the Pleistocene (Kiver, 1968, 1972).

Several major rivers have headwaters within and near 

the Fort Collins quadrangle (fig. 3). The Continental Divide 

intersects with the divide between the North and South Platte 

River Basins approximately 5 km southeast of Cameron Pass 

1.5 km south of the map boundary at the north end of the Never 

Summer Mountains. The headwaters of the Colorado River 

Basin sit just south of the Continental Divide in this location 

within Rocky Mountain National Park. River Basins are defined 

as that area drained by a river and its tributaries. The North 

Platte River Basin includes drainage areas within the western 

quarter of the map area. The Laramie River heads in the glaciated terrain near Cameron Pass and flows northward beyond the 

quadrangle to join the North Platte River northeast of Laramie, 

Wyo. The Michigan and Canadian Rivers similarly head on the 

west slope of the Medicine Bow Mountains and flow northwestward across the North Park Basin to join the North Platte River 

near Walden, Colo., to the west of the map area. The South 

Platte River Basin includes drainage areas across the central and 

eastern parts of the map area. The Cache la Poudre River drains 

most of this area and heads approximately 13 km southeast 

of Cameron Pass. The river flows north to Kinikinik and then 

eastward through a canyon across the Front Range towards 

Teds Place where it exits the Range and flows southeast to Fort 
6 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Figure 2. Physiography of the Fort Collins, Colorado, quadrangle. Dashed black lines are physiographic provinces. 

Black square indicates map area. Selected rivers, creeks, and roads shown.

Boulder Ridge

Green Ridge

106°15' 106°00' 105°45' 105°30' 105°15' 105°00' 104°45'

41°30'

41°15'

41°00'

40°45'

40°30'

40°15'

40°00'

Continental

Divide

MAP

AREA

Colorado Piedmont

MEDICINE BOW MOUNTAINS

Laramie

Basin

North

Park

Basin

Middle

Park

Basin

Cheyenne

tablelands

Foothill

MUMMY

RANGE

Cameron

Pass Mummy

Pass

NEVER SUMMER

MOUNTAINS

INDIAN

PEAKS

Denver

Basin

FRONT RANGE

Rolling upland

Sherman

surface

Foothill

hogbacks

L A R A

M I E

M O

U

N

TA I N

S

RABBIT EARS

RANGE

Rolling upland

Rolling upland

High

upland

High upland

hogbacks

Livermore

embayment

Laramie

River

valley

Bull

Mountain

Middle Bald

Mountain

Creek

Fletcher

Hill

Green

Mountain

14 14

80

25

25

287

287

287

Laramie River

Big Thompson River

Cache la Poudre River

Cache la Poudre River

South Platte

River

Boulder Creek

South Saint Vrain Creek

North Saint Vrain

Fraser River

Colorado River

Colorado River

Boulder

Granby

Estes Park

Longmont

Laramie

Cheyenne

Lyons

Livermore

Idylwilde

Virginia Dale

Kinikinik

Fort Collins

Loveland

Saint Vrain Creek

Masonville

Pingree

Park

WYOMING

COLORADO

Chimney

Rock

COLORADO

WYOMING

Base from U.S. Geological Survey National Elevation Dataset, 2010

Universal Transverse Mercator projection, zone 13N

North American Datum 1927 0 4 8 12 16 20 KILOMETERS

0 4 8 12 16 20 MILES
Introduction 7

Figure 3. Hydrography of the Fort Collins quadrangle and surrounding area, with the North and South Platte River 

Basins in the map area, and the Colorado River Basin to the southwest of the map area. River Basins are defined as 

that area drained by a river and its tributaries. Fort Collins quadrangle outlined in black.

Continental

Divide

WYOMING

COLORADO

MAP

AREA

106°00' 105°45' 105°30' 105°15' 105°00' 104°45' 106°15'

41°30'

41°15'

41°00'

40°45'

40°30'

40°15'

40°00'

Boulder

Granby

Fort

Collins

Laramie

Cheyenne

Lyons

Livermore

Idylwilde

Masonville

Virginia Dale

Kinikinik

Estes Park

Longmont

Pingree

Park

Bull

Mountain

Base from U.S. Geological Survey National Elevation Dataset, 2010

Universal Transverse Mercator projection, zone 13N

North American Datum 1927

52

72

66

119

14

14

14

1

230

130

210

211

80

25

36

34

34

40

36

85

287

287

287

85

30

85

25

125

7

7

COLORADO

WYOMING

0 4 8 12 16 20 KILOMETERS

0 4 8 12 16 20 MILES

NORTH

PLATTE

RIVER

BASIN

SOUTH

PLATTE

RIVER

BASIN

COLORADO

RIVER

BASIN

Colorado River

Willow

Creek

Williams Fork

Reservoir

Lake

Granby

Colorado

 River

Lake Hattie

Reservoir

James

Lake

Laramie River

Horse Creek

Lodgepole Creek

Crow Creek

Lone Tree Creek

Spring Creek

Cobb

Lake

Windsor

Reservoir

Black Hollow

Reservoir

Boxelder Creek

Terry

Lake

Douglas

Reservoir

Joe Wright

Reservoir

Park Creek

Reservoir

Halligan

Reservoir Panhandle

Creek

Cache la Poudre River

North Fork Cache la Poudre

Sheep Creek

Dale Creek

North Fork Cache la Poudre River

Sand Creek

Willow Creek

Carter Lake

Reservoir

Buckhorn Creek

Redstone Creek

Big

Thompson River

South Fork Cache la Poudre

Elkhorn Creek

McIntyre Creek

Canadian River

Michigan River

Illinois River

Chambers

Lake

Calkins

Lake Shadow Mountain

Lake

Grand Lake

Middle Saint Vrain

Left Hand Creek

Big Dry Creek

Laramie River

Timnath

Reservoir

Peterson

Lake

Fossil Creek

Reservoir

Horsetooth

Reservoir

Boyd

Lake

Cache la Poudre River

Lo n e Pine Creek

Fraser River

Loveland

Lake

Loveland

North Saint Vrain Creek

Middle S aint Vrain Creek

South Saint Vrain Creek

Little Thompson River

Boulder

Reservoir

Boulder Creek

South Platte River

Saint Vrai n Creek

Little Laramie River

North Platte River Basin

EXPLANATION

Colorado River Basin South Platte River Basin
8 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Collins; farther east, it joins the South Platte River near Greeley, 

Colo. The North Fork Cache la Poudre River runs southsoutheast along the eastern edge of the rolling upland block 

within the map area coalescing several smaller streams that 

flow east to east-southeast across the range and joins the main 

Cache la Poudre River near its mouth. Boxelder Creek flows 

south-southeast along the eastern edge of the foothill hogbacks 

collecting drainage from the low hills and upper piedmont along 

the eastern edge of the map area and joins the Cache la Poudre 

River just off the southeast corner of the map.

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

rolling upland. Local broad valleys and grass-covered flats are 

intermingled with the forested terrain, and are locally referred 

to as parks (Crosby, 1978). Most of the Front Range landscape 

is at elevations between about 2,300 m and 2,600 m, and 

merges gradually westward up into the steeper high upland terrain of the glaciated Mummy Range. On the western margin of 

the rolling upland adjacent to the higher elevation terrain, the 

through-flowing drainages coming out of the high upland have 

their lowest gradients which then increase as streams continue 

east across the range (Cole and Braddock, 2009, p. 3 and 4).

Compilation Sources and Methods

Much of the geology in the Fort Collins 30'×60' quadrangle was mapped at 1:24,000-scale under several USGS 

projects between the late 1960s and the 1990s (fig. 4). Proterozoic rocks in the core of the Front Range and the Phanerozoic 

strata of the foothills belt were mapped by W.A. Braddock and 

numerous graduate students from the University of ColoradoBoulder through the Northeastern Front Range Project and 

published at 1:24,000 scale (Abbott, 1976; Braddock, Abbott, 

Connor, and Swann, 1988; Braddock and Connor, 1988; 

Braddock, Connor, Swann and Wohlford, 1988; Braddock and 

LaFountain, 1988; Courtright and Braddock, 1988; Braddock, 

Wohlford, and Connor, 1988; Eggler and Braddock, 1988; 

Braddock, Calvert, O’Connor, and Swann, 1989; Braddock, 

Cole, and Eggler, 1989; Braddock, Eggler, and Courtright, 

1989; Braddock, O’Connor, and Curtin, 1989; Nesse and 

Braddock, 1989; Shaver and others, 1988). Braddock compiled the geology of Rocky Mountain National Park (RMNP) 

at 1:50,000 scale from his and graduate studentsʹ mapping at 

larger scales (Braddock and Cole, 1990).

Figure 4 (continued on following page). A, Index map showing sources of geologic map data used in compiling the Fort 

Collins 30'×60' quadrangle. For U.S. Geological Survey publications, report number, authorship, and date of publication 

are shown. B, Index map of compilation responsibilities for new mapping in the Fort Collins 30'×60' quadrangle. For U.S. 

Geological Survey publications, report number, authorship, and date of publication are shown.

OFR 81–578 

(Pearson and

others, 1981)

OFR 82–376

(Pearson and

others, 1982)

CRAZY

MOUNTAIN

SAND CREEK

PASS

EATON

RESERVOIR

DIAMOND

PEAK

CHEROKEE

PARK

VIRGINIA

DALE

TABLE

MOUNTAIN

ROUND

BUTTE

GLENDEVEY DEADMAN SOUTH BALD

MOUNTAIN

RED FEATHER

LAKES

HAYSTACK

GULCH

LIVERMORE

MOUNTAIN

LIVERMORE BUCKEYE

RAWAH

LAKES

BOSTON

PEAK

KINIKINIK RUSTIC BIG NARROWS POUDRE

PARK

LAPORTE WELLINGTON

CLARK PEAK CHAMBERS

LAKE

COMANCHE

PEAK

PINGREE PARK CRYSTAL MOUNTAIN BUCKHORN

MOUNTAIN

HORSETOOTH

RESERVOIR

FORT COLLINS

Burch, 1983

Camp, 1979

Walko, 1969

Griswold, 1980

T.G. Plymate and

others, 2005

SIM 3133

(Workman and

Braddock, 2010)

SIM 3029

(Workman, 2008)

GQ 1614

(Braddock and

others, 1989)

GQ 1615

(Eggler and

Braddock, 1988)

GQ 1617

(Braddock and

Connor, 1988)

GQ 1618

(Braddock, Wohlford,

and Connor, 1988)

GQ 1618

(Braddock, Connor,

Swann and

Wohlford, 1988)

GQ 1620

(Braddock, Abbott,

Connor,and

Swann, 1988)

GQ 1619

(Shaver and

others, 1988)

SIM 3010

(Kellogg, Ruleman

and others, 2008)

I 1973

(Braddock and Cole, 1990)

GQ 1622

(Nesse and

Braddock, 1989)

GQ 1623

(Braddock and

LaFountain, 1988) GQ 1624

(Braddock, O’Connor,

and Curtin, 1989)

I 855–G

(Colton, 1978)

GQ 1625

(Braddock, Calvert,

O’Connor, and

Swann, 1989)

GQ 1323

(Abbott, 1976)

GQ 1616

(Braddock, Eggler

and Courtright, 1989)

I 1805

(Courtright and

Braddock, 1988)

40°30'00"

106°00'00"

41°00'00"

41°52'30"

41°45'00"

41°37'30"

105°52'30" 105°45'00" 105°37'30" 105°30'00" 105°22'30" 105°15'00" 105°07'30" 105°00'00"

A
Introduction 9

Figure 4. A, Index map showing sources of geologic map data used in compiling the Fort Collins 30'×60' quadrangle. For U.S. 

Geological Survey publications, report number, authorship, and date of publication are shown. B, Index map of compilation 

responsibilities for new mapping in the Fort Collins 30'×60' quadrangle. For U.S. Geological Survey publications, report number, 

authorship, and date of publication are shown.—Continued

CRAZY

MOUNTAIN

SAND CREEK

PASS

EATON

RESERVOIR

DIAMOND

PEAK

CHEROKEE

PARK

VIRGINIA

DALE

TABLE

MOUNTAIN

ROUND

BUTTE

GLENDEVEY DEADMAN SOUTH BALD

MOUNTAIN

RED FEATHER

LAKES

HAYSTACK

GULCH

LIVERMORE

MOUNTAIN

LIVERMORE BUCKEYE

RAWAH

LAKES

BOSTON

PEAK

KINIKINIK RUSTIC BIG NARROWS POUDRE

PARK

LAPORTE WELLINGTON

CLARK PEAK CHAMBERS

LAKE

COMANCHE

PEAK

PINGREE PARK CRYSTAL MOUNTAIN BUCKHORN

MOUNTAIN

HORSETOOTH

RESERVOIR

FORT COLLINS

SIM 3133 SIM 3029 GQ 1614 GQ 1615

GQ 1617 GQ 1618

GQ 1619 GQ 1620 GQ 1618

SIM 3010 I 1973 GQ 1622 GQ 1623 GQ 1624 GQ 1625

GQ 1323

GQ 1616

40°30'00"

106°00'00"

41°00'00"

41°52'30"

41°45'00"

41°37'30"

105°52'30" 105°45'00" 105°37'30" 105°30’00” 105°22'30" 105°15'00" 105°07'30" 105°00'00"

B

Reconnaissance

mapping of

surficial deposits

by R.R. Shroba

(diagonal overprint

pattern)

Reconaissance

mapping

by J.B. Workman,

K.S. Kellogg,

W.A. Braddock,

and R.R. Shroba

Reconaissance mapping

by J.C. Cole

Reconaissance mapping

by K.S. Kellogg

Previous mapping

compiled by

K.S. Kellogg

Previous mapping

compiled by

J.B. Workman

Previous mapping

compiled by

J.B. Workman

Previous mapping

compiled by

J.B. Workman

Reconaissance mapping

by W.A. Braddock (USGS) Reconnaissance

mapping of

surficial deposits

by R.R. Shroba

(diagonal overprint

pattern)

The USGS did reconnaissance mapping in the southwestern part of the quadrangle in the 1970s to evaluate mineral 

resource potential in existing wilderness areas (Pearson and 

others, 1981, 1982). The Laramie River valley was mapped 

to explore regional structure by the State Geological Survey 

of Wyoming (Beckwith, 1942) and for graduate studies at 

Colorado State University (Camp, 1979). Adjoining areas 

of the southern Laramie Mountains west of Red Feather 

Lakes (Burch, 1983) and the central Medicine Bow Mountains (Griswold, 1980) were also mapped for graduate theses 

at Colorado State University. The glacial geology of the 

Medicine Bow Mountains was studied and mapped by Kiver 

(1968) from the University of Wyoming. A detailed study of 

the Proterozoic rocks north of the Cache la Poudre canyon in 

the vicinity of Idylwilde was completed by Walko (1969) from 

State University of New York at Buffalo.

New geologic mapping was carried out on the current 

project to cover areas not previously mapped, including the 

Eaton Reservoir quadrangle (Workman, 2008), western parts 

of the Clark Peak quadrangle (Kellogg, Ruleman, and others, 

2008), and parts of the South Bald Mountain, Boston Peak, 

Chambers Lake, Deadman, Haystack Gulch, and Red Feather 

Lakes quadrangles (Cole and Kellogg) (fig. 4B). Previous 

reconnaissance mapping by W.A. Braddock (USGS emeritus, 

deceased, 2003) in the Sand Creek Pass and Crazy Mountain 

quadrangles was updated and completed by Workman 

(Workman and Braddock, 2010) and Workman, Kellogg, 

and Shroba (fig. 4B), respectively. Geologic mapping, petrologic studies, and geochronologic investigations in the 

Comanche Peak quadrangle were completed over a number 

of years by T.G. Plymate and T.D. Moeglin of Southwestern 

Missouri University and by W.R. Van Schmus of Kansas State 

University (Plymate and others, 2005).

Surficial deposits in the Fort Collins 7.5' quadrangle 

and areas immediately to the west in the foothills belt were 

mapped by Colton (1978). Shroba mapped the surficial 

geology in the Colorado Piedmont and foothills belt north 

of Colton’s 1978 work based on photogeologic interpretation, limited field work, and comparisons with the geology 

of areas to the south (Kellogg, Shroba, and others, 2008). 

Shroba also reviewed and revised the portrayal of surficial 

geologic units in the Laramie River valley, the Medicine 

Bow Mountains, and the upper Cache la Poudre River valley, 

based largely on photogeologic interpretation using analytical 
10 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

stereophotogrammetric techniques. The Quaternary geologic 

history of these areas has not been studied in detail.

Comprehensive regional study of the Cretaceous marine 

deposits east of the Front Range was completed by Scott and 

Cobban (1965, 1986). Correlation of marine Cretaceous beds in 

the Laramie River valley to the definitive stratigraphy of Scott 

and Cobban (1986) was assisted by W.A. Cobban (USGS).

Geologic History

The area covered by the Fort Collins 30'×60'quadrangle 

displays a diverse assemblage of rocks and surficial units that 

range in age from Paleoproterozoic to Quaternary (figs. 5 

and 6). The geology records several major rock-forming 

events and deformations that are characteristic of the Rocky 

Mountains of Colorado and southern Wyoming.

The core of the Front Range consists of volcanic and 

sedimentary rocks that were deposited in the Paleoproterozoic between about 1,790 and 1,725 million years (Ma), 

and intruded by calc-alkaline magmas and metamorphosed 

between about 1,725 Ma and 1,695 Ma (Premo and others, 

2010b). They were subsequently intruded and locally 

deformed again during emplacement of widespread Mesoproterozoic granitic intrusions between about 1,440 Ma 

and 1,350 Ma (DeWitt and others, 2010). All of these older 

Proterozoic rocks were also deformed by Mesoproterozoic 

mylonite-bearing shear zones from roughly 1,400‒1,200 Ma. 

The east flank of the Front Range and the Laramie Basin are 

marked by ridges of upturned Phanerozoic sedimentary rock 

in the foothills flanking the uplifted Proterozoic rocks. Uplift 

and denudation of the Proterozoic basement in the late Paleozoic as early as the Pennsylvanian Period (Anasazi uplifts of 

Nesse, 2007) removed any earlier Paleozoic sediments. The 

sediments in these strata were initially deposited as early as 

the Pennsylvanian Period in response. The oldest Paleozoic 

sediments were deposited adjacent to these now-eroded 

mountains by braided fluvial systems draining to an adjacent 

marine environment, and were succeeded by eolian, deltaic, 

marine, lagoonal, and other fluvial deposits that accumulated 

slowly from Permian through Jurassic time. By Early Cretaceous time, the dominant depositional environment was a 

fluvial delta system and lagoons on the margin of the vast 

Western Interior Seaway; subsidence led to marine incursion 

and deposition of very thick shales and local limestones and 

sands. All of these Phanerozoic rocks underlie the foothill 

hogback belt and the Colorado Piedmont areas of the eastern 

part of the Fort Collins quadrangle, as well as the Laramie 

Basin and the lower Laramie River valley. The oldest Pennsylvanian and possibly Permian rocks are missing from 

sections exposed in the upper Laramie River valley and North 

Park recording the southwestward onlap of deposits onto the 

eroded Anasazi uplifts (Nesse, 2007).

A major regional tectonic event known as the Laramide 

orogeny affected this area beginning in latest Cretaceous and 

led to the initial uplift of the Proterozoic basement cores of 

the modern ranges (Tweto, 1975). The eastern margin of the 

Front Range within the quadrangle rose as a broad monoclinal 

flexure cut by steep basinward dipping faults that uplifted the 

Proterozoic basement as the overlying Phanerozoic sedimentary rocks were denuded. The western margin of the early 

Laramide Front Range in the Laramie River valley is marked 

by an east-dipping reverse fault that places Proterozoic crystalline rocks over steep and overturned beds of Phanerozoic 

sedimentary rocks (Beckwith, 1942; Camp, 1979). 

The early Laramide orogeny led to withdrawal of the 

inland sea from areas west of the Front Range and exposed the 

Upper Cretaceous marine and overlying nonmarine deposits 

to erosion in the Cameron Pass area (Gorton, 1953; Ward, 

Post-Laramide rocks

Ogallala Formation (lowermost Pliocene? to middle Miocene)

Troublesome Formation (Miocene to Oligocene)

North Park Formation (Miocene and upper Oligocene?)

Oligocene volcanic and intrusive rocks

White River Group (lower Oligocene and upper Eocene)

Other Cretaceous, Phanerozoic, Proterozoic, and Archean rocks

Cretaceous sedimentary rocks—Includes some older rocks North and

 Middle Parks

Jurassic to Pennsylvanian sedimentary rocks

Mesoproterozoic intrusive rocks

Paleoproterozoic intrusive rocks

Paleoproterozoic metasedimentary rocks

Paleoproterozoic metavolcanic and metasedimentary rocks

Paleoproterozoic and Archean rocks

Water

Faults—Undifferentiated

Contact

Laramide

Wind River Formation (Eocene)

Coalmont and Middle Park Formations (Eocene and Paleocene)

Hanna Formation (Paleocene)

Denver and Arapahoe Formations (Paleocene to Upper Cretaceous)

Eocene to Late Cretaceous intrusive rocks

No

Nt

Nn

vi

wr

Ks

Js

Yg

Xg

Xb

Xsv

Twdr XA

cm

h

Kda

Ki

EXPLANATION

Figure 5 (continued on following page). Regional geologic setting of the Fort Collins 30'×60' quadrangle. Modified from Tweto 

(1979), ver Ploeg and others (1998), Sutherland and Hausel (2004), ver Ploeg and Boyd (2007), and Cole and Braddock (2009).
Geologic History 11

Figure 5. Regional geologic setting of the Fort Collins 30'×60' quadrangle. Modified from Tweto (1979), ver Ploeg and 

others (1998), Sutherland and Hausel (2004), ver Ploeg and Boyd (2007), and Cole and Braddock (2009).—Continued

WYOMING

CARBON

COUNTY

LARIMER

COUNTY

JACKSON

COUNTY

GRAND

COUNTY

COLORADO

ALBANY

COUNTY LARAMIE

COUNTY

BOULDER

COUNTY

WELD

COUNTY

Continental

Divide

Ks

Ks

Ks

No

No

Ks

Yg

Xb

J*s

Xb

Yg

Yg Yg

Yg

Xg

Yg

J*s

Xb

Ks

Xg

Xsv

Xb

Ks

Yg

Yg

:vi

:vi

Xsv

Xb

Xb

Xg

Ks

J*s

Xsv

Xg

Xg

Xb

Yg

Xb

Xg

Xsv

XA

:vi

:vi

Yg

Xsv

Xb

Xsv

Yg

Yg

Xsv

Twdr

Xsv

Xg

Xb

Yg

Ks

Ks

Xsv

Ks

Ks

Xg

Xg

J*s

Xg

Xg

Xg

Xg

Xg

Xsv

Xsv

Xsv

Xg

Xsv

Yg

Yg

Yg Xsv

Xb

Xg

Xb

Xsv

J*s

J*s

Yg

Xsv

N:n?

N:n

N:t

:Ki :Ki

:Ki

N:t

:wr

:wr

:wr :wr

:Kda

:wr

:h

N:n

N:n

N:n?

N:n?

Braddock Peak

volcanic complex

MAP AREA

106°00' 105°45' 105°30' 105°15' 105°00' 104°45' 106°15'

41°30'

41°15'

41°00'

40°45'

40°30'

40°15'

40°00'

Boulder

Granby

Estes

Park

Longmont

Loveland

Fort

Collins

Cheyenne

Lyons

Base from U.S. Geological Survey National Elevation Dataset, 2010

Universal Transverse Mercator projection, zone 13N

North American Datum 1927

14

14

1

80

25

85

287

287

287

85

25

0 4 8 12 16 20 KILOMETERS

0 4 8 12 16 20 MILES

COLORADO

WYOMING
Figure 6 (continued on following page). Simplified geologic map of the Fort Collins 30'×60' quadrangle. Modified from the current study (this rhighlight general distribution of rock age, basic lithology, and structural geometry.

LARIMER COUNTY

JACKSON COUNTY

LARIMER COUNTY LARIMER C

ALBANY COUNTY LARAMIE C

ROCKY MOUNTAIN

NATIONAL PARK

ROCKY MOUNTAIN

NATIONAL PARK

14

287

Xg

Xg

Xg

Xg

Xg

Xg

Xgd

Xgd

Xgd

Xgd

Xgd

Xgd

Xgd

Xgd

Xgd

Yg

Yg

Yg

Yg

Yg

Yg

Yg

Yg

Yg

Yg

Yg

Xf Xf

Xf

Xf

Xf Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xf

Xh

Xh

Xh

Xh

Xh

Xh

Xh

Xh

Xh

Xh

Xh

Xh Xh

Xh

Xh Xh

Xh

Xb

Xb

Xb

Xb

Xb

Xb

Xb Xb

Xb

Xb

Xb

Xb

Xf

Xf

Xb

Xb

Xb

Js Js

Js

Xb

Xf

Xf

Xf

Xf

Xb

Xb

Xb

Xh

Xh

Xh

Xf

Xf

YgH

YgV

YgSH

YgSH

YgSH

YgSH

YgSH

Xb

Xb

Xh

Ks

Ks

Ks

Ks

Ks

Ks

Ks Ks

Ks

Ks

Js

Tb

b

wr?

wr?

wr?

c

c

c

r

r

Nn? Nn?

Nn

Nn

Nn?

Nn?

Nn?

Nn?

Nn?

Nn

Js Js

Js

Js

Js

Js

Js

Js

Js

Js

Js

b

v

YgSH

YgSH

40°30'

106°00'

41°00'

Teds Plac

Bellv

La

WYOMING

COLORADO

Kinikinik Rustic

Eggers

Idylwilde

Red Feather Livermore

Lakes

Log Cabin

Stove

Prairie

Landing

Virginia Dale

Cameron

Pass

Glendevey

Stove

Prairie

Dilky

Ranch

to Laramie

Base from U.S. Geological Survey digital data, 2010Universal Transverse Mercator projection, zone 13NNorth American Datum 1927 0 2 4 6 8 

0 2 4 

WY
Geologic History 13

Ogallala Formation (lowermost Pliocene? to middle Miocene)

North Park Formation (Miocene and Oligocene?)

Rhyolite flows (Oligocene)

Dacite and rhyolite flows (Oligocene)

Trachybasalt flows (Oligocene)

Virginia Dale ring-dike complex (Mesoproterozoic)

Jurassic to Pennsylvanian sedimentary rocks

Sherman Granite (Mesoproterozoic)

Granite of Hagues Peak (Mesoproterozoic)

Monzogranite (Paleoproterozoic)

Granodiorite and trondhjemite (Paleoproterozoic)

Biotite gneiss (Paleoproterozoic)

Hornblende gneiss and amphibolite (Paleoproterozoic)

Felsic gneiss (Paleoproterozoic)

Fault breccia

White River Group (lower Oligocene and upper Eocene)

Coalmont Formation (Eocene to Paleocene)

Cretaceous sedimentary rocks

Jurassic to Pennsylvanian sedimentary rocks

Silver Plume-type monzogranite (Mesoproterozoic) Fault—Undifferentiated. Dotted where approximate

Contact

No

Nn

r

v

b

YgV

Js

YgSH

YgH

Xg

Xgd

Xb

Xh

Xf

b

c

Ks

Js

Yg

EXPLANATION

wr

Figure 6. Simplified geologic map of the Fort Collins 30'×60' quadrangle. Modified from the current study (this report, 

map sheet) to highlight general distribution of rock age, basic lithology, and structural geometry.—continued

1957), and particularly in areas west of this quadrangle. Thick 

fluvial deposits of the upper Paleocene and Eocene Coalmont 

Formation (:c) accumulated in the North Park Basin west 

of the Laramide highlands. Subsequent folding, possibly in 

middle Eocene time (Hail, 1965; Tweto, 1975; Cole and others, 2010) deformed the Coalmont Formation west of the Front 

Range during a distinct period of late Laramide contraction. 

At this time, the Never Summer thrust fault south of the map 

area along the western edge of the Never Sumer Mountains 

uplifted Proterozoic crystalline rocks westward against folded 

Coalmont Formation sediments. The Medicine Bow Mountains may have risen at this time, as suggested by the absence 

of Coalmont beds in the Laramie River valley. Minor amounts 

of Paleocene-Eocene porphyritic intrusive rock in the Rustic 

7.5' quadrangle (a part of the historic Manhattan District) indicate some magmatic activity late in the Laramide orogeny.

Most of the Eocene appears to have been a time of 

erosion and deep weathering, accompanied by a hot, humid 

climate (Scott and Taylor, 1986). The Proterozoic crystalline rocks in the Laramide and Eocene uplifts were reduced 

to residual ridge crests and large areas of subdued rolling 

ground (Steven and others, 1997). This widespread erosion 

and planation is reflected today in broad areas with nearly 

concordant hilltop elevations across the northern Front Range, 

as well as by the Sherman erosion surface across the Laramie 

Mountains (Eggler and others, 1969; Bradley, 1987). Eocene 

and Oligocene deposits of the White River Group (:wr) are 

rare in the Fort Collins quadrangle, but are preserved in paleoriver channels in the mountain areas (Steven, 1957; Evanoff, 

1990) and on the eroded Cretaceous strata east of the Front 

Range (Courtright and Braddock, 1989; this report).

Volcanic activity during the latter part of the Oligocene 

Epoch is recorded by basalt flows and rhyolite ash-flow tuffs 

and flows near the southwestern corner of the quadrangle east 

and west of Cameron Pass, emanating from the Braddock Peak 

volcanic complex in the Never Summer Mountains (OʹNeill, 

1981; Cole and others, 2008; Cole and Braddock, 2009, p. 40). 

This younger magmatic activity was accompanied and followed by high-angle normal faulting, both of which might 

have been manifestations of broad regional uplift related 

to rise of aesthenospheric mantle (Eaton, 1986, 1987) and 

extensional faulting in the northern extents of the Rio Grande 

rift. Rapid erosion occurred during this uplift, as indicated by 

coarse gravel deposits preserved in mountain-area paleo-river 

channels and by volcaniclastic and arkosic sands and gravels 

of the North Park Formation (N:n). Extensional faulting was 

contemporaneous with North Park Formation deposition, particularly in the North Platte River drainage north of Walden, 

Colo. (Montaigne, 1957). Ongoing Miocene regional uplift 

led to broad erosion across the Front Range and deposition of 

upper Miocene and lower Pliocene gravels and sands of the 

Ogallala Formation (No) blanketing areas east of the Front 

Range (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, such as the 

Cache la Poudre River and Buckhorn Creek, have been deeply 

incised into the Proterozoic crystalline core of the Front 

Range. Drainage of the North Platte River west of the Front 

Range is not so deeply incised because base-level adjustments 

have been slowed by resistant knickpoints north of the North 

Park Basin (Steven, 1957; Blackstone, 1975).

Climate cooling that began during the Pliocene Epoch 

continued to the full glacial-interglacial conditions that characterize the Pleistocene (Bluemle and others, 2001). Two major 

phases of Pleistocene mountain-valley glaciation are well 
14 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

recorded by high-altitude deposits within the Fort Collins quadrangle (Madole and others, 1998), as well as several Holocene 

glacial advances (Kiver, 1968). The spectacular alpine scenery 

in the Mummy Range and along the crest of the Medicine 

Bow Mountains reflects both glacial sculpting in cirques and 

down-valley transport of rubble by streams of ice. The oldest 

glacial outwash deposits in the Laramie River valley may be 

affected by minor normal faulting, but more detailed study is 

needed. Pleistocene drainage patterns appear to be influenced 

by possible young movement of the Sheep Creek fault zone 

in the southern Laramie Basin (Workman, 2008). Quaternary 

mass-movement deposits are present throughout the mountainous areas of the study area. At lower elevations, the Quaternary 

is marked by several distinct levels of stream terrace deposits.

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 post-Archean, cratonic basement. 

The metamorphic schists and gneisses in the Fort Collins 

quadrangle were chiefly derived from mafic and felsic volcanic rocks and interlayered sediments (fig. 7) that were deposited south of the older (Archean) craton that has been documented just north of the Colorado-Wyoming State line (Reed 

and others, 1987; Reed, 1993). These sediments and volcanic 

rocks (erupted at about 1,782 Ma to 1,763 Ma) were deposited 

between 1,790 Ma to 1,725 Ma and were progressively 

metamorphosed from about 1,725 Ma to 1,695 Ma (Premo 

and others, 2010b) during the time they were being folded 

and intruded by calc-alkaline granodiorite, trondhjemite, and 

granite (Routt Plutonic Suite of Tweto, 1987).

The southeastern part of the Fort Collins quadrangle 

preserves a fairly complete record of this progressive regional 

metamorphism in metasedimentary rocks (see figs. 7 and 

8; from garnet-staurolite schist to sillimanite-biotite schist 

to partially melted biotite gneisses with residual high-grade 

cordierite and (or) garnet; Braddock and Cole, 1979; Nesse, 

1984; Cole and Braddock, 2009, p. 19‒22). Biotite gneisses 

farther north and west in the quadrangle universally show 

partial-melting textures, as do most metasedimentary rocks in 

the Front Range (Cole and Braddock, 2009, p. 19‒22).

Hornblende-bearing 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 

although some may have been shallow intrusive sills. These 

rocks typically occur as hornblende schist, hornblende gneiss, 

or amphibolite. Epidote-bearing calc-silicate gneiss (Xcg) is a 

rare but distinctive rock type in this part of the Front Range. It 

is commonly interlayered with the mafic metavolcanic rocks 

and may have originated from sediments eroded from those 

volcanic protoliths. A similar origin is inferred for faintly 

layered, dark-colored, fine-grained gneisses consisting of 

plagioclase, biotite, epidote, and minor quartz and iron oxide 

minerals (granoblastic biotite gneiss) that occur as interlayered 

masses in the canyon east of Bellvue, Colo., and in the northeastern Comanche Peak Wilderness north of the South Fork 

Cache la Poudre River (Braddock, Abbott, and others, 1988; 

Shaver and others, 1988; Nesse and Braddock, 1989).

Quartzofeldspathic leucocratic gneisses are conspicuous 

elements of the metamorphic terrane in parts of the Fort 

Collins quadrangle, especially north of the Skin Gulch shear 

zone. These gneisses typically display compositional banding marked by differing proportions of quartz, feldspar, and 

biotite, or by variations in grain size and their bulk compositions are similar to granite, granodiorite, or tonalite. These 

felsic gneisses (Xfl) are commonly interlayered with mafic 

hornblende gneisses and amphibolites (Xh) (Abbott, 1976; 

Braddock, Cole, and Eggler, 1989; Workman, 2008) and are 

interpreted to have formed from felsic volcanic rocks. Recent 

geochronologic study has demonstrated that these compositionally banded and dimensionally foliated felsic gneisses 

contain zircons that crystallized between about 1,782 Ma 

and 1,763 Ma (Premo and others, 2010b), consistent with 

the interpretation that they formed from primary volcanic 

rocks. Burch (1983) mapped these compositionally banded, 

medium- to coarse-grained rocks as a “porphyritic phase of 

the Rawah batholith,” but the zircon studies show that the 

typical igneous granite of the Rawah batholith (XgR) (our 

terminology) was not intruded until about 50 million years 

later. Plymate and others (2005) also demonstrated that these 

granitic gneisses are much older than the granite of the Rawah 

batholith in the Comanche Peak quadrangle, and they give an 

excellent summary of the physical characteristics they used to 

distinguish the two rock types in the field.

Figure 7 (following page). Proterozoic metamorphic rocks 

of the Fort Collins 30'x60' quadrangle showing distribution of 

major metamorphic rock types within the map area. Note the 

predominance of felsic gneiss mixed with hornblende gneiss 

and amphibolite with only isolated areas of biotite gneiss in the 

northern and western parts of the quadrangle as opposed to 

the southeastern part of the quadrangle where biotite gneiss 

dominates. The northern assemblage is indicative of interlayered 

volcanic and volcaniclastic source rocks (volcanic source 

dominated terrain) while the southern assemblage is indicative 

of pelitic source rocks (sedimentary source dominated terrain). 

The approximate boundary between these two terrains (thick 

gray dashed line) roughly follows the east-northeast trending Skin 

Gulch shear zone (wavy red lines). Metamorphic reaction isograd 

lines of Braddock and LaFountain (1988), Braddock, Calvert, 

O’Connor, and Swann (1989), Braddock, O’Connor, and Curtin 

(1989), and Nesse and Braddock (1989) are shown (dotted red line) 

indicating the increasing metamorphic grade to the north and 

west. See figure 8 for more complete explanation of reaction lines. 

Geology simplified from geologic map (this report, map sheet).
LARIMER COUNTY

JACKSON COUNTY

LARIMER COUNTY LARIMER COUNTY

ALBANY COUNTY LARAMIE COUNTY

S

-A

K M

K

M

M

M

K -A

S

-A

M

K

M

K

Skin Gulch

shear zone

VOLCANIC SOURCE DOMINATED TERRAIN

SEDIMENTARY SOURCE DOMINATED TERRAIN 

14

287

40°30'

106°00'

41°00'

Bellvue

Laporte

WYOMING

COLORADO

Kinikinik Rustic

Eggers

Idylwilde

Livermore Red Feather

Lakes

Log Cabin

Stove

Prairie

Landing

Virginia Dale

Cameron

Pass Pingree

Park

Glendevey

Stove

Prairie

Dilky

Ranch

to Walden

to Laramie

Teds Place

Hornblende gneiss and amphibolite

—

 Layered mafic dominated metavolcanic rocks

Felsic gneiss—Layered felsic dominated  metavolcanic rocks

Sillimanite, in

Proterozoic fold axes

Approximate boundary between  source raines

Shear zone

S

-A Andalusite, cordierite, and garnet, out

M Migmatite, in

K Potassium feldspar, in; lepidoblastic muscovite, out

Metamorphic reaction isograd lines

EXPLANATION

Biotite gneiss—Layered metasedimentary rocks

W

0 2 4 6 8 10 

Base from U.S. Geological Survey digital data, 2010 0 2 4 6 Universal Transverse Mercator projection, zone 13NNorth American Datum 1927
16 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Figure 8. Photographs of thin sections and map showing mineralogy of Paleoproterozoic metamorphic rocks in the northern Front Range 

(modified from Cole and Braddock, 2009). Definitions of features on the figures as taken from Cole and Braddock (2009). 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), 

Estes

Park

A

105°30' 105°22'30" 105°15'

+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

Big Thompson

Canyon

105°37'30"

40°

30'

40°

22'

30"

40°

15'

40°

37'

30"

Moose Mountain shear zone

Ms

1 mm

Cd St

Cd

St

St

Cd

B

Si

Si

Cd

Cd

Cd

Bi

1 mm

C

Si

Cd

Cd

Cd

Cd

G

Bi

Bi

1 mm

D

E

Biotite is the only (Fe, Mg) phase

 in this zone

Edge of the Proterozoic basement 

 exposure at the surface

Generalized metamorphic reaction isograd 

 lines as shown on the map sheet—Dashed 

 where inferred due to lack of data, exposure, 

 or the presence of younger rocks or structures

EXPLANATION

Garnet

Melt restites

Cordierite

Base from U.S. Geological Survey digital data

Universal Transverse Mercator projection, zone 13N

North American Datum 1927

COLORADO

WYOMING

0 2 4 6 8 10 KILOMETERS

0 2 4 6 8 10 MILES
Geologic History 17

quartz (colorless), and muscovite (Ms) with porphyroblastic staurolite (St) in low-grade schist (W.A. Braddock photomicrograph, 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 partialmelt 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 inches in diameter. (C, D, and E photomicrographs, J.C. Cole, 1972–1974).—Continued

Metamorphic rocks west of the Virginia Dale intrusive 

complex (fig. 9) in the Diamond Peak and Eaton Reservoir 7.5' 

quadrangles (Braddock, Cole, and Eggler, 1989; Workman, 

2008; figs. 6, 7, and 9) provide strong evidence of volcanic 

origin for these mafic and felsic gneisses. The three principal 

rock types are (1) hornblende gneiss and amphibolite (Xh), 

(2) leucocratic felsic gneiss (Xfl), and (3) biotite felsic gneiss 

with conspicuous microcline augen (Xfb). These three lithologic 

units are interlayered on the scale of several to hundreds of 

meters, and compositionally distinct layers have been mapped 

out for miles along their strike through complex fold structures. Local lenses and layers of biotite-rich sillimanite gneiss 

(Xbq) are conformable with compositional layering in the 

felsic and hornblende-rich gneisses. The three major units are 

compositionally similar to basalt, rhyolite, and dacite, whereas 

the biotite-sillimanite gneiss indicates a clastic, pelitic protolith. The outcrop evidence is most consistent with a folded, 

metamorphosed stack of diverse volcanic rocks and interbedded sediments (volcanic source dominated terrain of fig. 7). 

Rock types (2) and (3) above produced lead-uranium (U-Pb) 

sensitive high-resolution ion microprobe (SHRIMP) zircon 

crystallization ages within the map area of 1,766.6±8 Ma and 

1,775.5±4 Ma, respectively (Workman, 2008), consistent with 

an age of primary volcanic eruption (Premo and others, 2010b). 

Hornblendite within this interlayered metavolcanic sequence in 

the Eaton Reservoir 7.5' quadrangle produced one of the oldest 

U-Pb SHRIMP zircon ages determined to date in the map area 

(1,779±5 Ma) and may have originated as a komatiitic flow rock 

or ultramafic synvolcanic intrusive mass (Workman, 2008).

Reconnaissance mapping for this compilation in the 

forested mountains of the Comanche Peak Wilderness southsoutheast of Kinikinik revealed large areas of fine- to mediumgrained faintly layered gneiss containing hornblende and biotite in addition to quartz and feldspar. It is locally interlayered 

with hornblende gneiss and granitic gneiss, and so is probably derived from volcanic or volcaniclastic protoliths. It is 

intruded in complex fashion by granite of the Rawah batholith 

and forms large irregular inclusions in that body.

The area south of the Skin Gulch shear zone generally 

contains metamorphic rocks rich in biotite and sillimanite 

that are interpreted to be derived from sedimentary protoliths 

(sedimentary source dominated terrain of fig. 7). Contiguous 

areas of similar rock to the south of the map area show original sedimentary structures including graded bedding, crossbedding, scour-and-fill structures, thin conglomerate lenses, and 

current-bedding lineations (Braddock, 1970). These structures, 

and the generally fine-grained nature of the parent sediments, 

are consistent with submarine-fan, turbidity current depositional environments. Braddock (1970) estimated that more than 

12.2 km of marine sediment accumulated in this region. The 

two most common rock types are distinguished from each other 

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

(Xbk) was probably derived from shale; the quartz-feldspar-rich 

variety (Xbq) contains less than about 15 percent mica and was 

probably derived from sandstone and siltstone. Thin, persistent 

beds of very mica-rich rock distinguished by conspicuous porphyroblasts of biotite are mapped within the metasedimentary 

sequence in the southeastern part of the quadrangle (Braddock, 

OʹConnor, and Curtin, 1989). U-Pb SHRIMP zircon data indicate deposition of these sediments occurred between 1,770 and 

1,730 Ma (Premo and others, 2012).

The mineralogical and textural distinctions among these 

varieties of metasedimentary rock diminish in the northern and 

western parts of the quadrangle due to widespread recrystallization and partial melting during the peak of metamorphism. 

Most biotite-bearing metasedimentary rocks approximately 

north of the Skin Gulch shear zone and west of Pingree Park 

are compiled as biotite gneiss (Xb).

Figure 9 (following page). Proterozoic intrusive rocks of the Fort Collins 30’×60’ quadrangle showing distribution of major plutons and 

batholiths of both the older Routt Plutonic Suite and younger Berthoud Plutonic Suite. Various crosscutting dikes shown as colored lines. 

Note east-west trending, inferred Proterozoic shear zone which separates Rawah and Log Cabin Batholiths to the south from Sherman 

Granite and related rocks to the north. This structure is not exposed and is cut out by the younger East Jimmy Creek fault, Cornelius Creek 

shear zone, and Halligan Reservoir fault. Geology simplified from geologic map (this report map sheet). Ma, million years.
JACKSON COUNTY

LARIMER COUNTY

LARIMER COUNTY LARIMER COUNTY

ALBANY COUNTY

River

Redstone

Irondike LOG

CABIN

BATHOLITH

RAWAH-TYPE

PLUTON

SHERMAN

BATHOLITH

SHERMAN

BATHOLITH

VIRGINIA DALE

RING-DIKE

COMPLEX

BOULDER CREEK-TYPE INTRUSIONS

LONGS PEAK-ST. VRAIN

BATHOLITH

SILVER PLUME-TYPE INTRUSIONS

SHERMANBATHOLITH

RAWAH

BATHOLITH

RAWAH

BATHOLITH

SHERMANBATHOLITH

TRONDHJEMITE OF

THOMPSON CANYON

inferred

Proterozoic

lineament

inferred

concealed

Proterozoic

lineament

? ? ? ?

ROCKY MOUNTAIN

NATIONAL PARK

LARAMIE COUNTY

HAGUES PEAK

PLUTON

McIntyre

Creek

Sand Creek

Creek

Sheep

North

Fork

North

Fork

Cache la Poudre

Cache la Poudre

Lone

Pine

Creek

Creek

Elkhorn

Cache la Poudre

Buckhorn

Creek

Creek

South 

Fork

Laramie

River

Cache la Poudre

Kelsey

Lake

14

287

40°30'

106°00'

41°00'

Teds Place

Bellvue

Laporte

F

Colli

WYOMING

COLORADO

Kinikinik

Rustic Eggers Idylwilde

Livermore

Red Feather

Lakes

Log Cabin

Stove

Prairie

Landing

Virginia

Dale

Cameron

Pass

Glendevey

Stove

Prairie

to Walden

to Laramie STATE LINE MINING DISTRICT

Middle Bald

Mountain

Base from U.S. Geological Survey digital data, 2010Universal Transverse Mercator projection, zone 13NNorth American Datum 1927 0 2 4 6 8 1

0 2 4 6 

C

WYO

Silver Plume-type granite intrusions—Including granite of Log Cabin batholith (1,408–1,406 Ma) and granite of Longs Peak (1,420–1,390 Ma)

Berthoud Plutonic Suite

Boulder Creek-type granodiorite (1,714 Ma)—Granite of Rawah batholith (1,721–1,715 Ma) and trondhjemite of Thompson Canyon (1,726 Ma)

Routt Plutonic Suite

Virginia Dale ring-dike complex (1,412 Ma) Proterozoic rocks, undividedSherman Granite (1,412 Ma)Granite of Hagues Peak (about 1,480 Ma)

EXPLANATION

Kimberlite (Devonian to NeoprotGabbro of the Iron Dike (1,316 MaAplite dikes (various ages)Pegmatite (various ages)Mafic dikes (Mesoproterozoic)
Geologic History 19

Paleoproterozoic Intrusive Rocks

Paleoproterozoic intrusive rocks are common in the 

Fort Collins quadrangle (fig. 9) and consist of hornblende 

gabbro (Xgbh) and calc-alkaline granitoids ranging from 

monzogranite to tonalite in composition. These rocks belong 

to the regional Routt Plutonic Suite of Tweto (1987) intruded 

between 1,725 and 1,690 Ma (Premo and others, 2010a). Contacts with enclosing metamorphic rocks range from broadly 

conformable to crosscutting to extremely intricate, but are 

generally sharp at the outcrop scale.

Two small bodies of Paleoproterozoic hornblende gabbro 

(Xgbh) are present in the quadrangle and both are smaller than 

3 km2

 in extent. One body spans the Cache la Poudre River 

west of Kinikinik and consists of massive, dark greenishgray, hornblende metagabbro (Pearson and others, 1981). 

The second body is located north of Kinikinik near Middle 

Bald Mountain, is roughly circular in plan, and consists of 

very fresh-looking, gray, equigranular hornblende gabbro; 

U-Pb SHRIMP zircon studies indicate the emplacement age is 

1,715±7 Ma (Premo and others, 2007a).

Distinctive fine-grained biotite trondhjemite (XjT) intrudes 

the metasedimentary section in the southern part of the quadrangle. This rock typically forms continuous, concordant sheets that 

can be traced for miles along the strike of sedimentary bedding 

(Braddock and others, 1970; Braddock and LaFountain, 1988) 

or forms more irregular plug-like masses in the Comanche 

Peak and Pingree Park 7.5' quadrangles (Nesse and Braddock, 

1989; Plymate and others, 2005). Braddock and Cole (1979) 

inferred that the trondhjemite magmas south of this quadrangle 

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 

by U-Pb zircon methods that the trondhjemite was emplaced 

at 1,726±15 Ma. Plymate and others (2005) determined a 

U-Pb zircon age of 1,702±6 Ma for similar rock (leucotonalite 

mapped as XjT) in the south-central part of the quadrangle 

approximately 10 km south of Kinikinik.

Foliated biotite granodiorite and biotite-hornblende 

granodiorite (Xgd and Xgdp) are mapped in the area south 

of the Cache la Poudre River in the eastern part of the quadrangle that are similar to the Boulder Creek Granodiorite 

(Gable, 1980), named from its type area in the Boulder 

Creek batholith about 50 km south of this quadrangle. The 

Boulder Creek batholith is typically a foliated rock containing aligned mafic minerals that are concentrated in schlieren 

bands, aplitic-pegmatitic zones, and local feldspar porphyry 

textures. Internal dikes are not common, but generally consist 

of leucocratic granitic and aplitic rocks or pegmatite. Several 

large intrusive masses in the Stove Prairie region form 

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

folds that suggest emplacement during regional deformation 

(Braddock and Cole, 1979; Cole and Braddock, 2009, p. 18). 

The Boulder Creek batholith was emplaced at 1,713±4 Ma, 

based on the statistical weighted-mean of values determined 

from U-Pb SHRIMP on individual zircon grains (Premo and 

Fanning, 2000; Premo and others, 2010a).

The largest body of Paleoproterozoic intrusive rock is the 

Rawah batholith (XgR) and associated Rawah-type plutons 

(Pearson and others, 1981, 1982; Burch, 1983) that consist of 

leucocratic granitic rocks that underlie most of the western 

one-fourth of the Fort Collins quadrangle. The exposed part 

of the Rawah batholith forms most of the basement terrane 

of the Medicine Bow Mountains and extends eastward to 

the younger Mesoproterozoic Log Cabin batholith (YgLC), 

and southward to the latitude of Cameron Pass. East of the 

Laramie River valley, the northern boundary of the batholith 

is juxtaposed against older metamorphic rocks by the younger 

east-west trending Cornelius Creek shear zone and East Jimmy 

Creek fault. There is no exposure of granite of the Rawah 

batholith north of this structural feature and no external contact of the batholith with the country rock south of the feature 

(fig. 9; see discussion on Paleoproterozoic structures below).

Granitic rocks of the Rawah batholith tend to be leucocratic (biotite and sparse hornblende generally make up about 

5 percent of the rock), medium- to fine-grained, and fairly massive. Mafic minerals locally are concentrated in schlieren, and 

the granite may display dimensional alignment of biotite grains, 

but much of the batholith consists of massive rock. Granite of 

the Rawah batholith commonly crops out as low knobs and tors 

with rounded forms due to thermal exfoliation of the massive 

rock. This characteristic locally gives the appearance that the 

exposed granite consists of rounded, transported blocks; however, subsurface exposures in widespread road cuts show that 

the granite of the Rawah batholith is jointed into equant blocky 

masses that acquire their rounded forms at the ground surface 

by exfoliation. Within the Medicine Bow Mountains, the Rawah 

batholith is cut by numerous faults as located by McCallum and 

others (1983). These faults are largely based upon lineations 

and only rarely by exposed breccias and are poorly located, 

but are interpreted to be Tertiary faulting related to uplift of the 

Medicine Bow Mountains and deposition of the Paleogene to 

Neogene North Park Formation.

The Rawah batholith intrudes the felsic and hornblenderich gneisses in a complicated manner. Large blocks of the 

older gneisses are surrounded by the massive granite and yet 

retain the regional alignment of compositional banding and 

mineral foliation (Burch, 1983). Smaller masses of the older 

gneisses (less than a few hundred meters in greatest dimension) are locally discordant to the regional metamorphic 

fabric, suggesting they rotated during emplacement of the 

younger Rawah batholith.

U-Pb data from zircon grains by Premo and Van Schmus 

(1989), Plymate and others (2005), and Premo and others 

(2010a) show that the Rawah batholith crystallized between 

about 1,724 Ma and 1,715 Ma with a statistical weightedmean of 1,718±4 Ma, although some of the U-Pb systematics 

are complicated. Several samples contained zircon cores that 

may have been derived from 1,850‒1,750 Ma material (Premo 

and others, 2010a).
20 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Paleoproterozoic Metamorphism

Metamorphic rocks of the Fort Collins quadrangle 

record evidence of a single, pervasive regional metamorphic 

event that culminated in widespread partial melting of the 

metasedimentary gneisses (Braddock and Cole, 1979; Cole 

and Braddock, 2009, p. 19‒22). Field relations indicate the 

peak of recrystallization largely coincided with intrusion 

of the Boulder Creek Granodiorite and equivalent magmas 

at about 1,715 Ma (Peterman and others, 1968; Braddock 

and Cole, 1979; Hutchinson and Braddock, 1987; Cole 

and Braddock, 2009, p. 19). Pelitic rocks (biotite and mica 

schists; Xbq and Xbk) in the southern part of the quadrangle 

preserve lower-grade mineral assemblages (middle amphibolite grade) that contain primary muscovite, staurolite, and 

andalusite. North and westward, these assemblages were 

metamorphosed to higher pressure-temperature stable assemblages, marked by the sequential (1) appearance of sillimanite 

(sillimanite-in), (2) disappearance of andalusite, cordierite, 

and garnet (andalusite-out), and disappearance of staurolite 

(south of the map area), (3) appearance of potassium feldspar 

(potassium feldspar-in) and disappearance of lepidoblastic 

muscovite (muscovite-out), and (4) the onset of partial melting 

(migmatite-in). Metamorphic reaction isograd lines define the 

areal occurrence of this progressive increase in metamorphic 

grade recorded by mineralogy changes across the map area 

(map sheet, figs. 7 and 8). All of the biotite gneisses in the 

northern and western parts of the Fort Collins quadrangle 

show textural evidence of partial melting. High-grade (magnesian) garnet and (or) cordierite are present in many of the 

partially melted biotite gneisses due to the incongruent melting 

of biotite (Cole, 1977; Cole and Braddock, 2009; fig. 8).

Partial melting (summarized from Cole, 1977; Cole and 

Braddock, 2009, p. 19‒20) occurred in the pelitic metasedimentary rocks 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 non-melted 

host rock is the characteristic of migmatite, and its first appearance has been systematically mapped in the southern part of the 

Fort Collins quadrangle (Braddock and Cole, 1979; Braddock 

and LaFountain, 1988; Braddock, Calvert, O’Connor, and 

Swann, 1989; Braddock, O’Connor, and Curtin, 1989; and 

Nesse and Braddock, 1989). The fact that the isograd delineating the first appearance of migmatite (migmatite-in isograd line 

on fig. 7; “+Melt” line on fig. 8) 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).

Regional metamorphism took place during the protracted 

orogenic event marked by the intrusion of the Boulder Creek 

Granodiorite and by the formation of kilometer-scale folds of 

two contrasting trends (Braddock and Cole, 1979; Hutchinson 

and Braddock, 1987; Cole and Braddock, 2009, p. 19‒20). The 

thermal maximum is interpreted to have coincided with the 

peak of deformation in the highest-grade rocks as supported 

by partial-melt leucosome masses that are phacolithic in fold 

hinges and elongate parallel to fold axes, and by coarse sillimanite and biotite aligned with fold axes as observed within 

the map area.

Interpreted conditions at the peak of metamorphism have 

been estimated by several studies. Cole (1977, 2004b) concluded that peak conditions were about 5.5 kilobars (kbar), 

675°‒725 °C, and undersaturated with water 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 (potassium feldspar-in 

on fig. 7; “+K” line on fig. 8) occurred at 3‒4 kbars and about 

650 °C and higher temperatures and pressures in the partially 

melted rocks, under the assumption of water-saturated conditions (higher pressures would have prevailed under waterundersaturated conditions; Cole, 1977). Both authors conclude 

that pressures greater than about 6 kbars 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), Munn and others (1993), and Munn (1997) reported 

thermobarometric calculations based on garnet, biotite, 

and hornblende chemistry that suggested pressures near 7 

kbars in the east-central part of the Fort Collins quadrangle. 

Selverstone and others (1995, 1997) asserted pressures of 

8 kbars to 10 kbars based on unpublished chemistry of garnet 

and plagioclase in staurolite-bearing schists near the Big 

Thompson River south of the map area, but did not explain 

how such extreme conditions could have been attained without 

formation of kyanite and in the absence of partial melting. 

We discount these conclusions and suggest that the chemical 

analyses were obtained from non-equilibrium assemblages 

(Cole, 2004b). Solway (2014) indicated that the mole fractions of grossular garnet and anorthite are too low within the 

samples analyzed by Munn (1997) to assume equilibrium and 

the high weight percent of oxides within garnets reported may 

indicate equipment calibration error.

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, 

1977; 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 
Geologic History 21

replace 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 metasedimentary rocks 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 argon-argon (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 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 chiefly 

emplaced between about 1,440‒1,350 Ma (Berthoud Plutonic 

Suite of Tweto, 1987; DeWitt and others, 2010) and are 

widespread in the Fort Collins quadrangle (fig. 9). These 

include the distinctly older granite of Hagues Peak (YgH) and 

the more widespread units of the Sherman Granite batholith 

(YgSH), the Virginia Dale ring-dike complex (YgVi, YgVo, 

and YdV), a regional swarm of northwest-trending mafic 

porphyry dikes (Yd), biotite granite of the Log Cabin batholith 

(YgLC, YgLCe, and YgLCp), the northern margin of the 

Longs Peak-St. Vrain batholith (YgLP), and smaller bodies 

of granodiorite (Yg, Yge, and Yghp), quartz diorite (Yqd), 

gabbro (Ygbh), aplite (Ygay and Ygao), and pegmatite 

(YXp). In addition, an unusually long, narrow persistent 

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

intruded at about 1,316 Ma and is exposed along the eastern 

flank of the Medicine Bow Mountains (rubidium-strontium 

[Rb-Sr] isochron age; 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 

(south-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 about1,442 Ma 

(U-Pb on zircon; Aleinikoff and others, 1993). Small masses 

of undated biotite-hornblende quartz diorite (Yqd) are present in the same area near Mummy Pass and are crosscut by 

the granite of Hagues Peak and by widespread granite of the 

Longs Peak batholith (Plymate and others, 2005). Small bodies of hornblende gabbro (Ygbh) were identified by Eggler 

(1968) peripheral to the Virginia Dale ring-dike complex 

(see below) so that he interpreted them to be related to the 

mafic rocks of the complex (although they may be much 

older). Similarly, irregular intrusions of hornblende granite 

porphyry (Yghp) have been mapped in the Paleoproterozoic 

metavolcanic rock terrane west of the Virginia Dale ringdike complex (Eggler and Braddock, 1988; Braddock, Cole, 

and Eggler, 1989; Workman, 2008) that are crosscut by the 

Sherman Granite; however, they are interpreted to be Mesoproterozoic because they crosscut the metamorphic rocks, 

they show no structural fabric, and their mineralogy and 

chemistry suggest affinities with the magmas of the Virginia 

Dale ring-dike complex.

The southern portion of the Sherman batholith is widely 

exposed in low hills and broad plains across the northern 

boundary of the quadrangle. The batholith continues northward in the Laramie Mountains of Wyoming for a distance of 

80 km north of the map area. In the Laramie Mountains, the 

batholith is exposed for 25 km east-to-west but to the south 

in the map area it spans the entire width of the quadrangle 

implying that it underlies a significant part of the Laramie 

Basin. The Sherman Granite is quite uniform throughout the 

batholith and consists of red-weathering, coarse-grained, equigranular syenogranite with biotite and sodium-rich amphibole 

aggregated in clots (Eggler, 1968). The batholith rocks are 

geochemically alkaline to peralkaline, consistent with the presence of accessory fluorite, common zircon, and the sodiumrich amphiboles (Anderson and Thomas, 1985; Anderson 

and Cullers, 1999). The age of intrusion is established at 

1,433±1.5 Ma, based on zircon studies (Frost and others, 

1999). The Sherman Granite typically weathers to a thick 

mantle of quartz-feldspar grus and is not well exposed (Eggler 

and others, 1969). Widespread chlorite alteration forms a 

high-standing, more resistant zone within the intrusion at the 

western end of Boulder Ridge (Workman, 2008; Workman and 

Braddock, 2010). Along Boulder Ridge at the southern edge 

of the Laramie Basin, a distinctive set of lineations within the 

Sherman batholith parallels the outer contact of the intrusion (see map sheet). Workman (2008) shows these features 

as faults that cut the younger, northwest-oriented mafic dike 

swarm, but these features are most likely related to late stage 

expansion of the batholith that influenced the propagation of 

later extensional fractures during intrusion of the dike swarm.
22 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

The southernmost part of the Sherman batholith is 

marked by the distinctive Virginia Dale ring-dike complex, 

which is genetically related to the batholith. It was studied 

in detail by Eggler (1968), and the following description is 

largely based on his work. The ring-dike complex consists 

of nested, nearly concentric, ring-shaped intrusions of 

distinctive cap rock and main phase granites and diorite that 

were emplaced in a cauldron-like structure. The outer ringdike zone is filled with Sherman Granite (YgSH) that merges 

northward with the main mass of the Sherman batholith. The 

next inner zone of the complex consists of a partial arcuate 

ring of granite-diorite hybrid rocks (YdV) and blocks of country rock; textures in the hybrid rocks indicate the granitic and 

dioritic magmas were both liquid at the time of emplacement. 

There are fragmental and magmatically mixed rocks, ranging 

from diorite to monzogranite, that form a partial annular zone 

between the distinctive cap rock monzogranites in the central 

part of the complex and the Sherman Granite in the outer zone 

of the Virginia Dale ring-dike complex. The distinctive central 

cap rocks of the complex are divided into two zones called 

the outer cap rock (YgVo) and the inner cap rock (YgVi) 

that both consist of porphyritic biotite monzogranite but are 

distinguished from each other by size of feldspar phenocrysts, 

by degree of flow alignment, and by the abundance of biotiterich schlieren bands. Eggler (1968) used gravity and structural 

data to infer the nested ring-dikes are steeply inclined outward 

and that the form of the complex arose through gravitational 

foundering of the central block of country rock.

Mafic porphyry dikes (Yd) form a persistent and widespread swarm that trends north-northwest through the metamorphic rocks of the Front Range (Peterman and others, 

1968). These distinctive dikes are typically 1 to 5 m thick and 

tens of meters to several kilometers long and are especially 

common in the southeastern and northern areas of the Fort 

Collins 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 the Virginia Dale ringdike complex) and are intruded and deformed by younger 

granites of the suite (Longs Peak and Log Cabin batholiths; 

Eggler, 1968; Peterman and others, 1968; Cole and Braddock, 

2009, p. 14 and 22). Their emplacement age is constrained by 

the ages of these batholith granites between about 1,430 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 

(about 80 km south of the Fort Collins quadrangle), where 

characteristic mineralogical and textural traits are displayed (Tweto, 1987). Granites of the Silver Plume type are 

typically biotite syenogranite or monzogranite, commonly 

porphyritic with flow-aligned feldspars and biotite grains, 

contain moderate amounts of biotite (5 to 12 percent), and 

are relatively homogeneous (Cole, 1977). Silver Plumetype granites are geochemically peraluminous and locally 

contain magmatic garnet or sillimanite (Cole, 1977), as well 

as post-crystallization secondary muscovite (Cole, 1977; 

Anderson and Thomas, 1985). Throughout the Front Range, 

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 used 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 and granite of the Log Cabin batholith (YgLC) 

denotes Silver Plume-type granitic rocks in that pluton.

The granite of Longs Peak is only mapped in the Fort 

Collins quadrangle along the south-central border where it 

forms the northernmost margin of the Longs Peak-St. Vrain 

batholith (Cole and Braddock, 2009, p. 22–23). The most 

typical rock is coarsely porphyritic, biotite monzogranite, 

which contains conspicuous tabular phenocrysts of floworiented microcline, typically 1 to 2.5 cm long, 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 batholith and defines internal structure 

formed by flow patterns of the magma as it intruded (Cole, 

1977; Cole and Braddock, 2009, p. 35). The granite of Longs 

Peak was intruded at 1,420±25 Ma (Rb-Sr isochron; Peterman 

and others, 1968). 

The Log Cabin batholith is exposed in the center of the 

Fort Collins quadrangle and forms a simple, ovoid pluton 

approximately 32 km (east-west) by 21 km (north-south) in 

dimension. The northern contact of the batholith with older 

metamorphic and intrusive rocks follows but is locally cut by 

a series of younger east-west brittle faults (Cornelius Creek 

shear zone and Halligan Reservoir fault). The batholith consists primarily of medium-grained, subporphyritic biotite syenogranite, with a coarsely porphyritic phase (YgLCp) along 

the western and southern margins intermingled with a distinctly more equigranular phase (YgLCe). The pluton sharply 

crosscuts the metamorphic country rock along steep contacts, 

and appears to have caused some plastic deformation in the 

wall rock due to inflation during emplacement (Abbott, 1976; 

Cole, 1977). Analysis of magmatic zircons (U-Pb SHRIMP) 

indicates the coarse, porphyritic phase crystallized at 

1,406±13 Ma and the medium-grained phase at 1,408±15 Ma 

(DeWitt and others, 2010; Premo and others, 2012); that is, 

the two phases are essentially contemporaneous although 
Geologic History 23

crosscutting field relations suggest the porphyritic phase was 

intruded first. The Log Cabin batholith is a prominent positive magnetic anomaly against the surrounding metamorphic 

rocks and a steep gradient coincides with its external contact 

(Zietz and Kirby, 1972); it also coincides with elevated values 

of uranium and thorium in airborne spectrometric data (Duval 

and Zietz, 2000).

Small bodies of Silver Plume-type granite (Yg) are 

present in scattered locations across the Fort Collins quadrangle and they are mapped as biotite syenogranite. The most 

prominent body is a small, sub-circular plug that was intruded 

near the center of the Virginia Dale ring-dike complex following emplacement of the peralkaline granites related to the 

Sherman batholith (Eggler, 1968). Numerous small bodies of 

biotite syenogranite are mapped along the Skin Gulch shear 

zone. Some of these bodies may be displaced blocks of the 

larger Log Cabin and Longs Peak-St. Vrain batholiths, but 

most are individual bodies that were deformed by the younger 

shear zone.

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±25 Ma along a broad 

belt between southern California and Labrador (Anderson and 

Thomas, 1985). 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, have few basalts 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; Cole, 1977) at 1,400 Ma. Anderson and Bender 

(1989) also argued that the widespread intrusion of anorogenic, 

potassic granite across North America at this time indicates a 

significant, non-tectonic event in which thermal upwelling of 

mantle caused major melting of undifferentiated lower crust.

The youngest Proterozoic rock in the Fort Collins 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-whole-rock Rb-Sr 

isochron. The Iron Dike consists of a swarm of northnorthwest trending sub-vertical dikes that can be traced 

from just west of Boulder, Colo., through Rocky Mountain 

National Park and into the Fort Collins quadrangle; the 

Iron Dike 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 150 km. 

Thickness ranges from 1 m in some of the thinner dikes to as 

much as 40 m. Contacts with the Proterozoic wall rock are 

typically chilled within a few centimeters of the margin (Cole, 

1977). The Iron Dike swarm was hot enough during intrusion 

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

granite (Cole, 1977; Braddock and Peterman, 1989).

Kimberlite

Numerous small bodies of kimberlite (DZk) intrusive 

rock (not all large enough to map) are present in the Fort 

Collins quadrangle and they have been actively prospected 

for diamonds since the 1970s. These kimberlites are collectively referred to as the State Line District because they have 

been identified a few kilometers north in Wyoming and as far 

as 20 km south of the Colorado State boundary in the North 

Fork Cache la Poudre River drainage. An excellent summary 

of these unusual rocks, their geologic setting, and the 

diamond production history was prepared by Hausel (1998), 

and the following remarks are summarized from that report. 

Numerous reports pertinent to this subject are referenced in 

Hausel (1998).

These kimberlites occur in pipes and dikes within the 

Sherman Granite, the granite of the Log Cabin batholith, 

and the surrounding metamorphic country rock. They 

typically contain abundant phlogopite, pyrope garnet, 

chromian diopside, and ilmenite with widespread carbonate 

and serpentine alteration products. Kimberlites also contain 

nodules of ultramafic mantle and lower crustal rocks, including peridotite, eclogite, pyroxenite, and granulite as well as 

megacrysts of garnet, illmenite, and diopside. 

As of the early 1990s, mining was underway on about 

a dozen kimberlite localities in the State Line District. More 

than 16,000 tonnes was processed from the 10 smaller sites, 

from which more than 130,000 stones were recovered. The 

largest mine at Kelsey Lake had installed a mill and operated 

between 1996 and 1999, recovering nearly 1,000 carats of 

industrial stones and several gem-quality diamonds larger than 

28 carats raw weight.

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 the kimberlites in the North Fork Cache la 

Poudre River drainage and some other Front Range localities were emplaced at about 700 Ma to 600 Ma during the 

Neoproterozoic. 
24 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

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 Fort Collins quadrangle (fig. 10). Comprehensive description and discussion of these units is largely beyond the scope 

of the regional summary that accompanies this map compilation. Excellent summaries of the Phanerozoic units are available for this area, including Scott and Cobban (1986), Weimer 

and LeRoy (1987), and Weimer (1996). Detailed measured 

sections and stratigraphic summaries are also compiled in Lee 

(1927), which despite its vintage retains much useful descriptive information. Beckwith (1942) and Camp (1979) contain 

observational descriptions of the Phanerozoic rocks in the 

Laramie River valley, and Workman and Braddock (2010) 

provide descriptions of the Phanerozoic rocks in the southern 

Laramie basin. These papers and many of the source geologic 

maps form the basis for the following short summations.

The oldest sediments deposited along the eroded margins 

of the uplifted Front Range Proterozoic crystalline basement are somewhat different along the eastern, northern, and 

western margins (fig. 11), due to variations in the uplift duration and depositional environments during the late Paleozoic 

deformation in Pennsylvanian and later time (Anasazi uplifts 

of Nesse, 2007). The Pennsylvanian and Permian Fountain 

Formation (P*f; arkosic conglomerate and sandstone; 198 

to 268 m thick) records the initial deposition of alluvial fans 

along the eastern and northern flanks of the uplift.

East of the Front Range in the Denver Basin, the Fountain Formation is overlain by Permian Ingleside Formation 

(Pi; planar-bedded sandstone deposited in shallow marine and 

tidal-flat; 46 to 67 m thick), Owl Canyon Formation (Po; red 

siltstone deposited in tidal-flat and desert-lake(?) settings; and 

pinches out south of the Fort Collins quadrangle; maximum 

thickness 84 m), and the unconformably overlying Lower 

Permian Lyons Sandstone (Pl; eolian cross-bedded sandstone 

and fluvial sand; 1 to 15 m thick). The Lyons is conformably 

overlain by Upper Permian and Lower Triassic Lykins Formation (^Pl; red siltstone, gypsum, fine sandstone, and thin 

limestone beds deposited in restricted marine or saline lake 

environments; includes the Upper Permian Forelle Limestone 

Member (Pgfsu); 152 to 183 m thick).

Along the northern margin of the Front Range (Laramie 

Basin and Laramie River valley), the Fountain Formation thins 

and eventually pinches out to the south. Within the Laramie 

Basin, it is overlain by the Middle Pennsylvanian to Lower 

Permian Casper Formation (P*c; facies contemporary of 

the Ingleside Formation; trough-cross-bedded eolian sandstones deposited in tidal-flat; maximum 29 m thick in map 

area), which pinches out just south of the Colorado-Wyoming 

border, but thickens north of the map area, interfingering 

with the Fountain Formation. The contact between the two is 

a time-transgressive facies boundary that becomes younger 

southwestward against the Anasazi uplift.

In the Laramie Basin, the Fountain and Casper 

Formations are overlain by the Goose Egg Formation (facies 

contemporary of the Owl Canyon Formation, the Lyons 

Sandstone, and part of the lower Lykins Formation), which 

is divided here into a lower (Pgl) and upper part (^Prgu). 

The lower part of the Goose Egg Formation includes the 

Lower and Upper Permian Satanka Shale Member (red shale, 

siltstone, and sandstone, gypsum, and dolomite deposited in 

restricted marine or saline lake environments; 88 m thick) and 

the Upper Permian Forelle Limestone Member (the Forelle 

Limestone Member is marine; about 7 m thick). The Satanka 

Shale Member is here divided into a lower part (Pgsl) and 

an upper part combined with the thin Forelle Limestone 

(Pgfsu). The lower part of the Satanka Shale Member (Pgsl)

encloses a poorly exposed unconformity (most of the lower 

Leonardian stage is missing) that is equivalent to the unconformity gap beneath the Lyons Sandstone (Pl). The top of a 

sandstone-rich horizon (top of the lower part, as mapped) is 

inferred to correlate with the top of the Lyons Sandstone in the 

Denver Basin. Red beds continue up section above the Forelle 

Limestone Member in the Laramie Basin which include the 

Upper Permian and Lower Triassic upper part of the Goose 

Egg Formation (Little Medicine, Freezeout Shale, Ervay, 

and Difficulty Shale Members) and Lower Triassic Red Peak 

Formation (^Prgu; facies contemporary of the upper Lykins 

Formation; red shale, siltstone, and sandstone and gypsum 

deposited in restricted marine or saline lake environments; 

combined thickness 177 m). The top of the Goose Egg Formation is not a mappable contact within the map area due to poor 

exposure. West of the Front Range in the Cameron Pass area, 

this equivalent interval above the Forelle Limestone Member 

(244 m thick) was previously called the Chugwater Formation 

by Kellogg, Ruleman and others (2008).

South and west of North Middle Mountain in the 

Laramie River valley, the Pennsylvanian and most of the 

Lower Permian rocks pinch out against the eroded Proterozoic crystalline rocks along the flank of the late Paleozoic 

Arapaho uplift (fig. 10). West of Cameron Pass along the eastern boundary of the North Park Basin, Proterozoic rocks are 

overlapped by only the uppermost few meters of the Lower 

and Upper Permian Satanka Shale and the Upper Permian 

Forelle Limestone Members of the Goose Egg Formation 

(included in unit ^Prgu at this location; Gorton, 1953; Ward, 

1957; Braddock and Cole, 1990; Kellogg, Ruleman and others, 2008), and this condition persists along the entire western 

side of the Medicine Bow Mountains (Kinney, 1970, 1971). 

This onlapping relationship reflects the fact that basement 

uplifts locally remained emergent until Late Permian time 

(Anasazi uplifts of Nesse, 2007).

In most areas of the Fort Collins quadrangle, the Upper 

Permian and Lower Triassic red bed sequence is truncated 

upward by a Middle Triassic disconformity, upon which the 

Upper Triassic Jelm Formation was deposited (orange-pink, 

cross-bedded, fine-grained, nonmarine sandstone; maximum 

79 m thick in northwest part of map). The Jelm Formation 

thins significantly southward along the Foothills Hogback and 

is absent in the Cameron Pass area (Gorton, 1953; Ward, 1957; 

Kellogg, Ruleman and others, 2008). The Upper and Middle 
Figure 10. Laramide and older sedimentary rocks of the Fort Collins 30’×60’ quadrangle. Paleozoic and Mesozoic sedimentary rocks are exposed 

west flanks of the Front Range uplift in the Fort Collins quadrangle. The thick dashed gray line depicts the northeast boundary of the Arapaho uplif

Laramie River

Creek

Sand Creek

Creek

Sheep

North

Fork

Lone

Pine

Creek

Creek

Elkhorn

Cache la Poudre

Buckhorn

Creek

Creek

Boxe

Creek

South 

Fork

North Fork Cache la Poudre

Cache la Poudre River

McIntyre

Redstone

Ca che la

Poudre

ROCKY MOUNTAIN

NATIONAL PARK

14

287

40°30'

106°00'

41°00'

WYOMING

COLORADO

Kinikinik

Idylwilde

Livermore

Virginia

Dale

Cameron

Pass

Sand Creek

Pass

to Walden

to Laramie

North

Middle

Mountain

LARIMER COUNTY

JACKSON COUNTY

LARIMER COUNTY LARIMER 

ALBANY COUNTY LARAMIE 

Base from U.S. Geological Survey digital data, 2010Universal Transverse Mercator projection, zone 13NNorth American Datum 1927

DENVER BASIN

LARAMIE

BASIN

NORTH

PARK

BASIN

Upper Permian rocks

deposited directly on

Proterozoic basement

Fountain Formation

thins dramatically

to south

Laramide

intrusive

plug

Livermore

embayment

MANHATTAN

MINING DISTRICT

Casper

Formation

pinches out

to south

Lower Permian

rocks deposited

directly on

Proterozoic

basement

Approximate northeast

boundary of Paleozoic

Arapaho uplift

0 2 4 6 8 10 KILOMETERS0 2 4 6 8 10 MILES

Laramide sedimentary basin-fill rocksCretaceous marine sedimentary rocksJurassic to Pennsylvanian sedimentary rocks

EXPLANATION

Laramide intrusive dikes and plugs (62–34 Ma) 

Fault

Laramide fold axis—AnticlineLaramide fold axis—Syncline
26 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Jurassic Sundance Formation (fine-grained, yellowish-tan, 

tabular sandstone deposited in shallow-water marine environments; about 10 to 40 m thick) disconformably overlies 

the Jelm Formation (Chugwater Formation at Cameron Pass 

where Jelm is absent) across the entire quadrangle. The 

Sundance represents the return of near-shore marine conditions in this area in Middle Jurassic time as the late Paleozoic 

uplift had been eroded to sea level. The Jelm and Sundance 

Formations (^js) are mapped as one unit in this study.

The Upper Jurassic section consists of about 85 to 

95 m of nonmarine Morrison Formation (Jm; variegated 

fluvial claystone, siltstone, and thin sandstone deposited 

in floodplains and freshwater lakes and swamps) that lies 

conformably above the Sundance Formation. The top of the 

Morrison Formation is a regional unconformity representing 

marine transgression.

The vast, shallow Western Interior Seaway flooded the 

center of North America beginning in Early Cretaceous time. 

The marginal-marine Lower Cretaceous Dakota Group (Kd;

sandstone, shale, and sparse limestone; 85 to 97 m thick) was 

deposited across the whole of the Fort Collins quadrangle. It 

generally crops out as three sandstone ledges with intervening 

shale beds in the Foothills Hogback. Here, the base is called 

the Lytle Formation (chert-pebble conglomerate and sandstone 

Proterozoic rocks

Fountain Formation

Ingelside Formation

Owl Canyon Formation

Lyons Sandstone

Lykins

Formation

Sundance Formation

Morrison Formation

Dakota Group

Benton and Mowry Shales

Niobrara Formation

Pierre Shale

Kph

Kpr

Fox Hills Sandstone

Laramie Formation

Denver and Arapahoe Formation

White River Group

Ogallala Formation

Jelm Formation

Casper

Formation

upper part

Red Peak Formation

Jelm Formation

Miss.?

Steele Shale

Mesaverde Formation

Proterozoic rocks

Sundance Formation

Morrison Formation

Dakota Group

Benton Group

Niobrara Formation

Goose Egg Formation Chugwater Group

Fountain

Formation

upper part

lower

part

North Park Formation

Proterozoic rocks

Fountain

Formation

Forelle Limestone Member Forelle Limestone Member

upper part

Red Peak Formation

Jelm Formation

Sundance Formation

Morrison Formation

Dakota Group

Benton Group

Niobrara Formation

Steele Shale

Mesaverde Formation

Goose Egg Formation Chugwater Group

upper part

lower

part

Satanka

Shale Member

Satanka

Shale Member

Chugwater Formation

Jelm Formation

Coalmont Formation

Proterozoic rocks

Satanka? Shale

Forelle? Limestone

Sundance Formation

Morrison Formation

Dakota Group

Benton Group

Niobrara Formation

Pierre

Shale

White River Group

North Park Formation

LARAMIE BASIN DENVER BASIN LARAMIE RIVER

VALLEY

NORTH PARK

BASIN

Tertiary Cretaceous

Jurassic Triassic Permian

Penn.

Miss.?

D–



Figure 11. Correlation of sedimentary rocks across Fort Collins 30'×60' quadrangle for the Denver Basin, Laramie Basin, Laramie River 

valley, and the North Park Basin. Kpr, Richard Sandstone Member, Larimer Sandstone Member, Rocky Ridge Sandstone Member and 

two intervening unnamed shale members; Kph, Hygiene Sandstone Member. <, Proterozoic rocks; D–_, Devonian through Cambrian; 

M, Mississippian; *, Pennsylvanian.
Geologic History 27

with overlying shaly beds deposited on a channeled floodplain; about 18 m thick) which is overlain by the transgressive 

Plainview Formation (9 m thick), deep-water Skull Creek 

Shale (about 27 m thick), and beach and tidal-flat sands of 

the Muddy Sandstone (12‒30 m thick). The Muddy Sandstone is locally divided into the lower Fort Collins and upper 

Horsetooth Members. The Plainview Formation, Skull Creek 

Shale, and Muddy Sandstone are equivalent to the South Platte 

Formation of the Denver Basin south of Boulder (MacKenzie, 

1965). Within the Laramie Basin, the lower Dakota-equivalent 

is called the Cloverly Formation (equivalent to both the Lytle 

Formation and the Plainview Formation; about 45 m thick) 

which is overlain by the Thermopolis Shale (equivalent to 

the Skull Creek Shale; 20 m thick) and the Muddy Sandstone 

(20 m thick) at the top.

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 Late Cretaceous time. 

Shale, thin-bedded limestone, and sparse sands accumulated 

in the Western Interior Seaway across this entire area and are 

known as the Lower and Upper Cretaceous Benton Group 

(Kbm) consisting in the Denver Basin (from bottom to top) of 

Mowry Shale (3 to 20 m thick but locally absent), Graneros 

Shale (49 m thick), Greenhorn Limestone (79 m thick), and 

Carlile Shale (24 m thick) and in the Laramie Basin (from 

bottom to top) of the Mowry Shale and Frontier Formation 

(168 m thick). The overlying Niobrara Formation (Kn) consists of the Fort Hays Limestone Member (5 m thick) and the 

Smoky Hill Shale Member (20 m thick).

Increased rates of subsidence are recorded by the Upper 

Cretaceous Pierre Shale across the Fort Collins quadrangle 

(Higley and Cox, 2005). Subsurface data show that the Pierre 

is approximately 1,675 m thick in North Park (partly eroded; 

west of this quadrangle) and 2,075 m thick in the deepest 

parts of the Denver Basin near Denver, Colo., and also near 

Cheyenne, Wyo. Scott and Cobban (1986) measured about 

2,300 m in the eastern part of the map area, all deposited 

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

Obradovich, 1993). The isopach patterns of Late Cretaceous 

deposition in the Denver Basin suggest that 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 is compiled on this map as 

the lower shale member (Kpl, about 792 m thick), Hygiene 

Sandstone Member (Kph, 74 m thick), middle shale member (Kpm, 259 m thick), an upper alternating sandstone 

and shale member (Kpr, 160 m thick), and the upper shale 

member (Kpu, about 1,012 m thick; Scott and Cobban, 

1986). Withdrawal of the Western Interior Seaway from the 

Denver Basin in latest Late Cretaceous is marked by strandline marine deposits (Kfh , Fox Hills Sandstone) and coeval, 

interfingering, and younger deposits (Laramie Formation) of 

coastal plain, coal-forming swamp, and delta-plain environments (Roberts, 2005). The Fox Hills is only preserved in the 

northeasternmost area of the Fort Collins quadrangle and the 

Laramie Formation lies entirely east of the quadrangle.

West of the Front Range in the Laramie Basin and 

Laramie River valley, the Upper Cretaceous beds above the 

Niobrara Formation are called the Steele Shale (Kss) and 

Mesaverde Formation (Kmv) according to practice in southcentral Wyoming (Lynds and Slattery, 2017). The Steele 

Shale is a thick, uniform, dark-gray, marine shale in this area 

and is at least 731 m thick (Beckwith, 1942; Camp, 1979). 

Sandstone beds are common in the upper exposed part of the 

Steele and they contain ammonites of the Baculites reesidei

Elias zone of Scott and Cobban (1986), equivalent to the 

upper sandy member (Richard Sandstone Member [Kpr]) 

of the Pierre Shale. The Steele Shale is overlain by marine 

shale and sandstone and nonmarine sandstone assigned to the 

Mesaverde Formation; Beckwith (1942) and Camp (1979) 

estimate more than 610 m of Mesaverde are preserved in 

the Laramie River valley syncline but the top is eroded. 

No equivalent to the regressive Fox Hills Sandstone or the 

Laramie Formation is preserved west of the Front Range due 

to Paleocene erosion.

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 due to worldwide sea-level 

decline combined with broad tectonic uplift driven by compression across the western half of North America. The 

outlines of most of the modern mountain ranges in this part 

of Colorado were established when compressional forces and 

low-angle subduction (Dickinson and Snyder, 1978) initiated 

uplift of mountain-scale crustal blocks. The deformation (tilting and uplift) began at somewhat variable times in different 

places (Tweto, 1975; Kluth and Nelson, 1988; Cole and others, 

2010). The end of marine deposition and the transition to 

coastal and fluvial environments are generally taken to mark 

the onset of the Laramide orogeny (Tweto, 1975; Kluth and 

Nelson, 1988; Raynolds, 2002), and are best recorded east of 

the Front Range.

As the basement blocks rose, denudation removed some 

or all of the Cretaceous and older sedimentary rocks from 

uplifted areas and shed synorogenic sediments into the adjoining Denver-Cheyenne foreland basin east of the Front Range 

(Tweto, 1975; Raynolds, 2004). In the eastern part of the Fort 

Collins quadrangle, the retreat of the Western Interior Seaway 

is marked by the recessional Fox Hills Sandstone (Kfh) and 

the overlying (but broadly contemporary) Laramie Formation 

consisting of coastal-plain, swamp, and delta-plain deposits of 

sandstone, claystone, carbonaceous shale, and coal (Roberts, 

2005). The Laramie Formation (Upper Cretaceous) has been 

eroded from the eastern part of the quadrangle, but is exposed 

southward in the Estes Park 30'×60' quadrangle (Cole and 

Braddock, 2009) and northward in the Cheyenne 30'×60' quadrangle (ver Ploeg and others, 1998). Southeast of the map area, 

the synorogenic deposits of the Denver Basin sit conformably 

above this regressive sequence (Raynolds, 2002; Dechesne 

and others, 2011)
28 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

The record of Laramide uplift is notably different west of 

the Front Range in this area. Late Cretaceous to early Paleocene 

erosion was much more protracted in the area of North Park 

than along the eastern range margin because neither the Fox 

Hills Sandstone nor the Laramie Formation-equivalent remains 

(Hail, 1965; Kinney, 1970; Tweto, 1975). Instead, the Pierre 

Shale was deeply (but variably) eroded prior to deposition of the 

synorogenic upper Paleocene and Eocene Coalmont Formation 

(:c). For example, all of the upper shaly part of the Pierre is 

preserved in the nearby Denver-Cheyenne and Laramie Basins 

(more than 610 m) but was removed from the North Park area 

west of the map area (Kinney, 1970; Tweto, 1975), whereas all 

1,675 m or more of the Pierre was removed from the Cameron 

Pass area prior to Coalmont deposition (Gorton, 1953; Ward, 

1957; Braddock and Cole, 1990; Cole and others, 2010).

In summary, the Late Cretaceous to early Paleocene 

uplift of the Front Range Proterozoic basement appears to 

have been strongly asymmetric. The Denver-Cheyenne Basin 

flexed downward during mountain uplift and accumulated 

deposits that recorded the recession of the inland seaway 

and the fluvial aggradation of synorogenic deposits. However, the western flank of the Front Range remained above 

sea level through early-middle Paleocene time while Upper 

Cretaceous rocks were eroded. The downwarp that created 

the North Park structural basin did not begin to subside until 

middle Paleocene time when the lower part of the Coalmont 

Formation was deposited (Roberts and Rossi, 1999).

The age of the Coalmont Formation is not known in 

detail and can only be loosely defined from paleobotanical 

evidence as middle Paleocene to early Eocene (Hail, 1965; 

Izett, 1968; Roberts and Rossi, 1999). The basal Coalmont 

was most likely time-transgressive over older, tilted and 

eroded formations as the Front Range rose and the North Park 

Basin subsided. Cole and others (2010) dated clasts within the 

basal Windy Gap Volcanic Member of the Middle Park Formation (Coalmont Formation equivalent) as 61 Ma, indicating a Paleocene age for basin formation. The duration of the 

erosional gap between the eroded Upper Cretaceous marine 

units and the basal, fluvial Coalmont Formation cannot be 

quantified on the basis of present data. The Coalmont Formation was broadly folded in the North Park Basin and beneath 

the west-vergent fault blocks of the southern Medicine Bow 

Mountains west of Cameron Pass sometime during or following deposition, most likely during the Eocene (Hail, 1965).

Laramide Intrusive Rocks

Late Cretaceous to Eocene intrusive rocks are common farther south in the Front Range along the trend of the 

Colorado Mineral Belt (Lovering and Goddard, 1950; Cole 

and Braddock, 2009). These plugs and dikes consist of maficalkalic and alkali-calcic varieties that were emplaced in two 

main pulses at about 78‒68 Ma and 62‒45 Ma.

In contrast, no Late Cretaceous intrusive rocks are present in the Fort Collins quadrangle, and Paleocene to Eocene 

intrusive rocks are rare and are chiefly limited to minor dikes 

and small plugs in the Manhattan mining district north of 

Rustic (Shaver and others, 1988) and in scattered localities 

farther south (Nesse and Braddock, 1989). A brief description 

of the Manhattan district intrusions (along with potassiumargon [K-Ar] and fission-track dating results) is presented by 

McCallum and Naeser (1977) and summarized here. Most of 

the dikes are dacite (:dp) and andesite (:ap) porphyries 

containing phenocrysts of feldspar with either hornblende or 

biotite. The dikes are a few meters to several tens of meters 

wide and as long as 1.2 km; trends are highly variable. Ages of 

intrusion based upon zircon and apatite fission track data range 

between about 62 and 55 Ma (Paleocene; adjusted to modern radiometric decay constants by J.C. Cole). Dacite dikes 

with tridymite phenocrysts altered to alpha quartz exposed 

within the mining district (mapped as unit :dp) produce 

fission track ages of 54 and 58 Ma (not adjusted; McCallum 

and Naeser, 1977). A small hourglass-shaped plug of flowbanded rhyolite (:rp) east of Manhattan, Colo., within the 

Proterozoic Log Cabin batholith was intruded in late Eocene 

time at about 37 to 34 Ma (fission track on zircon and apatite, 

adjusted; McCallum and Naeser, 1977). Mining activity in 

the Manhattan district was limited to a few decades in the late 

1800s and early 1900s and primarily exploited sparse gold in 

quartz-pyrite veins near the porphyry intrusions (Pearson and 

others, 1981).

Post-Laramide Intrusive and Volcanic Rocks

Post-Laramide 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). South of the Fort Collins quadrangle in the Never Summer Mountains and near 

Cameron Pass lay major centers of Oligocene activity in the 

northern Front Range. The following discussion is largely 

summarized from OʹNeill (1981) Braddock and Cole (1990), 

and Cole and others (2008).

The Braddock Peak volcanic complex consist of local 

flows of dark basalt, trachyandesite, and flow breccia (:b) 

that appear to lie in paleovalleys, and dacitic to rhyolitic ash 

and welded tuff (:v where undivided) that generally overlie 

the mafic rocks. The volcanic strata are only preserved in the 

southwestern part of the Fort Collins quadrangle east and 

southwest of Cameron Pass on the southern margin of the 

Medicine Bow Mountains, and farther north on Green Ridge, a 

long narrow hilltop in the west-central part of the Fort Collins 

quadrangle that is east of the Laramie River valley (fig. 12). 

Volcaniclastic sediment is interbedded in several locations 

(explicitly mapped as unit :bs North of Chambers Lake). 

The youngest preserved volcanics (:r) consist of a rhyolite 

ash-flow tuff with conspicuous smoky quartz phenocrysts, 

and an overlying crystal-poor, flow-banded rhyolite flow that 

displays contorted flow-folding above a local basal vitrophyre. 

The thickness of all units is highly variable because these 

volcanic rocks accumulated on a deeply eroded landscape and 

because of post-eruption erosion.
Geologic History 29

Figure 12 (following page). Post-Laramide deposits and features of the Fort Collins 30’x 60’ quadrangle. Thin fluvial deposits are 

preserved both east and west of the Front Range in the Fort Collins quadrangle that are correlated with the upper Eocene to lower 

Oligocene White River Group (:wr). Oligocene volcanic deposits of the northern edge of the Braddock Peak volcanic complex overlie 

these fluvial deposits in the southwestern corner of the quadrangle with distal flows extending north onto Green Ridge. Coarse fluvial 

gravel deposits overlie and contain clasts of the volcanic rocks. These gravels are also present in the Laramie River valley, eastern 

North Park Basin, and in high-level positions in the landscape on top of the Front Range. High-level gravel deposits are interpreted by 

Scott and Taylor (1986) to define the axis of several Oligocene to Miocene paleovalleys (Cache la Poudre, North Fork Cache la Poudre, 

and Buckhorn paleovalleys), which drained the volcanic terrain and uplifts in the west eastward onto the plains where sediments were 

deposited in the Miocene Arikaree Formation and Miocene to Pliocene(?) Ogallala Formation (No/Na?; combined here and on cross 

section E–E’). The upper Laramie River valley is interpreted as an Oligocene to Miocene structural graben formed by the inferred northsouth striking Laramie River fault zone. This graben was filled by gravels of the North Park Formation (N:n). Oligocene to Miocene 

block-glide landslides developed within the Dakota Group along the Foothill hogbacks belt on the eastern side of the Front Range which 

override White River Group sediments and are overlain by Ogallala Formation. p_, Precambrian; Ma, million years.

The basaltic and trachyandesitic rocks (29.6±0.2 Ma; 

Knox, 2005) are about the same age as granodiorite of the 

Mount Richthofen stock (29.7±3 Ma; Marvin and others, 

1974) exposed just south of the map area at the northern end 

of the Never Summer Mountains. The youngest rhyolitic 

rocks (28.6±0.03 to 27.7±0.1 Ma; Knox, 2005) are similar 

in age to the highly differentiated granite porphyry of the 

Mount Cumulus stock (28.2±0.7 Ma; Marvin and others, 

1974) exposed just south of the Mount Richthofen stock, 

consistent with these intrusive bodies being the source of the 

volcanic deposits.

Post-Laramide Sedimentary Rocks

Erosion characterized most of the early and middle parts 

of the Eocene Epoch in this region (Tweto, 1975; Epis and 

Chapin, 1975; Scott and Taylor, 1986). Thin fluvial deposits 

that are correlated with the upper Eocene to lower Oligocene 

White River Group (:wr; fig. 12) are exposed both east 

and west of the Front Range in the Fort Collins quadrangle. 

Courtright and Braddock (1989) mapped White River Group 

deposits in the northeastern part of the quadrangle (about 

250 ft thick), consisting of white, olive-gray, or pink, massive, clayey siltstone with thin beds of arkose, pebble gravel 

(Brule Formation) and overlying beds of red, purplish-red, or 

gray siltstone and minor sandstone and coarse conglomerate 

(Chadron Formation). Ash beds within the White River Group 

in this area range in age between about 35.5 Ma and 30 Ma 

(Larson and Evanoff, 1998). Paleocurrent analysis indicates 

drainage toward the southeast at this time in the Colorado 

Piedmont (Steven and others, 1997).

Sparse exposures in the southwestern part of the quadrangle of unnamed sediments (:s) around the shores of 

Chambers Lake (Izett, 1975), as well as near Peterson Lake 

display gray and pale orange, fine-grained tuffaceous mudstone, micritic limestone, and immature conglomerate that 

may correlate with the White River Group (Braddock and 

Cole, 1990) because they underlie middle Oligocene volcanic 

rocks (:r and :b) (about 30 Ma to 28 Ma). Older fluvial 

gravel deposits (:go) on Green Ridge east of the upper 

Laramie River valley also appear to underlie Oligocene volcanic rocks. These gravels are tentatively correlated with the 

White River Group due to stratigraphic position, but stratigraphic position is uncertain here due to poor exposure and 

possible unmapped faults in the area. Unmapped thin beds of 

fine-grained, ashy deposits poorly exposed at the base of the 

North Park Formation (N:n) in the Laramie River valley may 

also correlate to the Chadron Formation of the White River 

Group. Where these sediments are preserved in the mountainous areas of northern Colorado and southern Wyoming, they 

typically occupy drainage systems eroded into the Proterozoic 

basement (Steven, 1957; Evanoff, 1990).

Upper Oligocene and Miocene fluvial sediments were 

deposited in several locations in the Fort Collins quadrangle 

in somewhat contrasting geologic settings. In the northeastern 

area on the Colorado Piedmont, beds of light-brown, massive 

siltstone, very fine grained sandstone, and minor arkosic 

gravel are preserved disconformably on top of the White River 

Group sediments (Courtright and Braddock, 1989). These 

Arikaree Formation (Na?) sediments appear to have been 

transported by southeast-trending streams similar to those that 

deposited the White River Group (Steven and others, 1997).

Regional tilting is inferred to have taken place in 

early Miocene time based on the erosional truncation of the 

Oligocene and lower Miocene formations prior to deposition of middle Miocene sediments of the Ogallala Formation 

(Izett, 1975; Steven and others, 1997; Cole and Braddock, 

2009, p. 39‒41). The Ogallala Formation (No), exposed 

in the northeast corner of the map area, consists of brown, 

poorly sorted, medium- to coarse-grained arkosic sandstone 

and conglomerate rich in granite clasts (some as large as 1 m 

in diameter, derived from the Sherman Granite). Regional 

tilting is also indicated by a shift to northeast paleocurrent 

directions in the Ogallala and by abundant rhyolite clasts in 

southern Wyoming, northeast of the map area, which probably 

originated from the volcanic deposits of the Braddock Peak 

volcanic complex south of Cameron Pass (Blackstone, 1975).

Gravelly fluvial sediments of the North Park Formation 

(N:n) are preserved in this quadrangle in the northern part 
?

?

River

Redstone

ROCKY MOUNTAIN

NATIONAL PARK

14

287

McIntyre

Creek

Sand Creek

Creek

Sheep

North

Fork

Lone Pine

Creek

Creek

Elkhorn

Cache la Poudre

Buckhorn

Creek

Creek

South 

Fork

Laramie

River

Cache la Poudre

North Fork

Cache la Poudre

Cache la Poudre

NORTH

PALEOVALLEY

FORK

CACHE

PALEOVALLEY

POUDRE

LA

PALEOVALLEY

BUCKHORN

500 meters elevation

difference at base

of gravel deposits

Eastward tilting ofWhite River Group

LARAMIE RIVER

FAULT ZONE ? block

of Da

Block-glandsliddebris dGroup

Braddock Peak

volcanic complex

CACHE LA POUDRE

Prairie Divide

gravel deposits

Nunn Creek

basin gravel

deposits

Deadman Hill

North Middle gravel deposits

Mountain North

Park Formation

Laramie River valley

North Park Formation

G R E E N R I D G E

North Park

Coalmont

Formation

Eastward tilting ofWhite River Group

Gould Mtn gravel deposits andlarge block-glide deposits of 

= rocks

N:n

N:n

N:n

N:n

N:n

N:n

:wr

:wr

:wr

:c

N:n

N:n

N:n

:wr

N:n

40°30'

106°00'

106°00'

41°00'

Teds Place

Bellvue

Laporte

Fo

Colli

WYOMING

COLORADO

Kinikinik

Rustic

Eggers

Idylwilde

Livermore

Red Feather

Lakes

Log Cabin Owl

Canyon

Stove

Prairie

Landing

Virginia

Dale

Glendevey

Stove

Prairie

to Walden

to Laramie

Cameron

Pass

LARAMIE COUNTY

Green

Mountain

Fletcher

Hill

JACKSON COUNTY

LARIMER COUNTY

LARIMER COUNTY LARIMER COUNTY

ALBANY COUNTY

Base from U.S. Geological Survey digital data, 2010Universal Transverse Mercator projection, zone 13NNorth American Datum 1927

COLORADO

WYOMING

0 2 4 6 8 10 K0 2 4 6 

Ogallala Formation (Pliocene? to Miocene)North Park Formation (Miocene and Oligocene)

—

 And associated gravel depositsRhyolite flows (Oligocene; 27 Ma)Dacite and Rhyolite flows (Oligocene; 28.5 Ma)

Trachybasalt flows (Oligocene; 29.6 Ma)White River Group and pre-volcanic sediments (Oligocene to Eocene)Coalmont Formation (Eocene to Paleocene)Proterozoic rocks, undivided 

No/Na?

N:n

:c

EXPLANATION

:wr Fault zone—Dashed and queried whe

 is approximate. Ball and bar on dowPaleovalley—Dashed and queried wh is approximate. Arrow shows direc
Geologic History 31

of the Laramie River valley and are inferred to be upper 

Oligocene(?) and Miocene based on landscape position and lithologic similarity to strata in the type area west of the Medicine 

Bow Mountains (Montagne and Barnes, 1957; Hail, 1965; 

Camp, 1979; Cole and others, 2010). The North Park Formation 

in the Fort Collins quadrangle consists of tan to white, calcareous sandstone and conglomerate with lesser amounts of white, 

light-gray, and pink calcareous siltstone and shale. It contains 

abundant clasts of flow-banded rhyolite and quartz-bearing 

welded tuff that closely resemble the felsic rocks of the Braddock Peak volcanic complex near Cameron Pass. The North 

Park Formation has not been studied in detail here, but it 

appears to have been transported in a fluvial stream system that 

was probably fault-bounded between the Medicine Bow and the 

Laramie Mountains, respectively, and followed a course similar 

to the modern Laramie River (Blackstone, 1975).

Scattered relics of bouldery, very poorly sorted, coarsegrained gravel, conglomerate, and sandstone deposits (N:gy) 

are preserved in several localities in the quadrangle (fig. 12). 

Most of these were compiled in a regional map by Scott and 

Taylor (1986) and interpreted as remnant valley-fill of prior 

drainage systems that produced paleovalleys (Cache la Poudre, 

North Fork Cache la Poudre, and Buckhorn paleovalley on 

fig. 12) that incised the mature landscape that formed in late 

Oligocene time. These paleovalleys and the coarse bouldery 

fill deposits sit high in the modern landscape above deeply 

incised modern river valleys that suggest Miocene uplift of 

the Front Range (Steven and others, 1997) as well as primary 

Oligocene volcanic topography (Cole and others, 2010), but 

the ages of deposition are poorly known.

Volcanic rocks are inferred to be interbedded with the 

boulder gravel deposits in one location in the Fort Collins 

quadrangle, and this relationship provides an important 

constraint on the time of deposition. Gravel is preserved 

both below and above quartz-bearing rhyolite tuff (:r ) in 

the southern Laramie Mountains between Nunn Creek Basin 

and Deadman Hill on the Deadman 7.5' quadrangle on Green 

Ridge just east of the southern Laramie River valley. This tuff 

was dated at 27.7±0.1 Ma (Knox, 2005) and is correlated with 

the rhyolite erupted from the Braddock Peak volcanic complex 

and preserved southeast of Cameron Pass (OʹNeill, 1981). 

Gravel beneath the rhyolite ash-flow tuff contains clasts of 

Precambrian rock as well as mafic and intermediate volcanic 

porphyries eroded from the Braddock Peak volcanic complex. 

The overlying gravel contains common clasts of the 27.7-Ma 

rhyolite as well as some of the older intermediate and mafic 

volcanic rocks from the same complex. The paleovalley in the 

Nunn Creek Basin area that contains these gravel and volcanic deposits loses elevation as it goes northward towards the 

Deadman Hill gravel deposits (at about 3,050 m elevation) and 

is presumed to have connected northeastward approximately 

along the modern upper drainage of Panhandle Creek (fig. 3) 

to the North Fork Cache la Poudre paleovalley in the Prairie 

Divide area (at about 2,400 m elevation) where similar gravels 

are preserved (Prairie Divide gravel deposits of fig. 12). 

Farther east, these gravel deposits project downward to the 

Livermore valley where gravel rests on top of beveled Paleozoic strata at about 1,920 m elevation. These gravel deposits 

suggest a possible Miocene course of the North Fork Cache la 

Poudre River, but without proper age control or detailed clast 

provenance, correlation of these deposits is speculative.

Boulder gravel deposits are well preserved just north 

of the modern canyon of the Cache la Poudre River between 

about Eggers and the town of Livermore. The bottom of the 

paleovalley declines from about 2,180 m elevation on the west 

to about 1,920 m on the east of the Livermore embayment. 

At Green Mountain (about 3 km northwest of Stove Prairie 

Landing), modern erosion through these gravel deposits shows 

that they were at least 210 m thick or, stated differently, that the 

paleovalley was at least 210 m deep during deposition. Small 

patches of boulder gravel are preserved high on the landscape 

along the former drainage of the South Fork Cache la Poudre 

River as well between about 2,500 m and 2,300 m elevation.

The combined drainage of the North Fork and main 

fork of the Cache la Poudre paleovalleys passed across the 

Livermore embayment and crossed the upturned sedimentary 

strata of the foothills hogbacks and flowed southeastward 

down a course approximately through the modern Campbell 

Valley northeast of Owl Canyon. This interpretation is suggested by the elevation of the relict stream gravel deposits in 

relation to the modern heights of the upturned hogback ridges 

in this area. Scott and Taylor (1986) inferred the former Cache 

la Poudre River discharged northeastward across the eroded 

Upper Cretaceous strata, but that would not have been possible because the channel was already established at a lower 

elevation than the Paleozoic rock hogbacks (see Cole and 

Braddock, 2009, p. 41‒43, fig. 12).

The former drainage of the Buckhorn paleovalley is 

preserved by remnant gravel deposits between the modern Buckhorn Creek canyon and Stringtown Gulch in the 

southeastern part of the Fort Collins quadrangle (Braddock, 

OʹConnor, and Curtin, 1989). These Miocene to Pliocene(?) 

drainages were also established at lower elevation than the 

eastern edge of the Front Range (and tilted hogback ridges of 

Paleozoic strata), and were diverted near Fletcher Hill southeastward down a fault-controlled valley of modern Buckhorn 

Creek toward the town of Masonville, just southeast of the city 

of Fort Collins.

Boulder gravel is also preserved about 100 m higher than 

the modern drainage of the Michigan River near the southwestern corner of the Fort Collins quadrangle (Gould Mountain 

gravel deposits; fig. 12). These deposits contain common clasts 

of the Upper Oligocene quartz-bearing rhyolite tuff erupted 

from the Braddock Peak volcanic complex and are inferred to 

be Miocene because they are preserved at higher elevations 

than Pleistocene glacial gravels in the same drainage.

All of these fluvial gravel deposits share characteristics 

with the Oligocene(?) and Miocene North Park Formation 

(Hail, 1965; Camp, 1979) in that they locally overlie the 

27.7 Ma rhyolite tuff, they contain common clasts of the 

rhyolite and other Oligocene volcanic rocks, and they appear 

to indicate drainage that was approximately radial from the 
32 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Braddock Peak volcanic complex. The high-level gravel deposits preserved in canyons that cut into the Proterozoic basement 

only differ from the North Park Formation in that they lie on 

basement rock rather than Oligocene or Eocene strata.

Quaternary Deposits

Quaternary deposits in the Fort Collins quadrangle 

primarily reflect the climatic fluctuations during the 

last 1.8 million years. They are composed of stream deposits 

chiefly at lower elevations extending beyond the mouths of 

mountain canyons, glacial and glaciofluvial deposits at high 

elevations, and various other deposits related to mass wasting 

and eolian processes.

Main-Stream Fluvial and Pediment Deposits

The Quaternary geomorphic history of the quadrangle 

reflects the influences of persistent stream erosion and episodic 

deposition that has taken place since early Pliocene time. 

East of the pre-Quaternary mountain front (in the vicinity of the modern foothills hogback belt), the South Platte 

River drainage incised and removed much of the Tertiary 

sedimentary apron (Ogallala Formation and older units) and 

exposed the tilted resistant layers in the foothills hogback belt 

while upstream river valleys became deeply entrenched in 

the mountain block (for example, Steven and others, 1997). 

Continued erosion in the Colorado Piedmont has led to 

incomplete preservation of various surficial deposits near the 

eastern margin of the quadrangle. Similar processes have acted 

on the Laramie River drainage near the western margin of the 

quadrangle. The main Quaternary water-lain units east of the 

mountain front are modern and prehistoric stream alluvium on 

valley floors, flights of fluvial terrace deposits that record the 

positions of former floodplains, and pediment deposits (Qp) 

that probably were graded to the level of former floodplains.

Considerable study during the last five decades has been 

devoted to the fluvial terrace and pediment deposits along the 

western margin of the Colorado Piedmont east of the Front 

Range and to the record of progressive drainage incision and 

episodic deposition. Scott (1960, 1963) described five prominent 

alluvial units at different levels in the South Platte River drainage southwest of Denver. He used height above modern stream 

level, surface morphology, as well as soil and other weathering 

characteristics to establish a nomenclature scheme. Subsequent 

work has validated his observations, allowed correlation of 

terrace and pediment deposits elsewhere along the mountain 

front (Scott, 1975; Colton, 1978), and provided better control on 

the ages of these deposits (summarized by Madole, 1991).

Correlation of terrace and pediment deposits is uncertain 

from place to place, particularly among the older units because 

there are few numerical ages, dating methods have uncertainties, 

and local geology exerts considerable control on drainage evolution (Madole, 1991). For example, correlation based on terrace 

height above modern stream level can be unreliable locally, 

especially along the western margin of the Colorado Piedmont. 

In this area, stream capture is common owing to differences in 

flow regime, load characteristics, and gradients between stream 

channels that drain upland areas (flowing over cobble alluvium) 

versus those that head in the piedmont (flowing over erodible 

Pierre Shale, for example). Some terrace deposits may not have 

correlatives elsewhere along the western margin of the piedmont, 

because they are the products of local stream-channel avulsion 

and stream piracy (Madole [stops 2–6] in Dethier and others, 

2003). Figures stated below for deposit heights represent average 

values in locations near the mountain front. 

The oldest well-preserved deposit in the Colorado 

Piedmont is the Verdos Alluvium (Qv). It includes pediment 

deposits that cap southeast-sloping surfaces cut on Pierre 

Shale near the northeast corner of the quadrangle, as well as 

three valley-fill or terrace deposits on the east side of Dry 

Creek southwest of Wellington. These deposits are at one 

and locally two levels. Lower (younger) deposits are about 

34‒37 m above stream level. Two smaller, higher (older) 

deposits are about 43‒49 m above stream level. Higher 

(older) deposits of the Verdos Alluvium in the Denver area 

locally contain the Lava Creek B volcanic ash (640 ka; 

Lanphere and others, 2002) of early middle Pleistocene age 

(Madole, 1991). The top of the Verdos Alluvium is much 

lower than the top of the alluvium that underlies a highlevel surface, about 6 km east of Wellington, just east of the 

quadrangle. Deposits that underlie the high-level surface are 

about 73‒92 m above Boxelder Creek, and may be correlative with or predate the Rocky Flats Alluvium in the Denver 

area (Scott, 1960, 1963).

The next youngest deposit below the Verdos is designated 

the Slocum Alluvium (Qs), which is widespread for several 

miles east of the foothills hogback belt along the eastern margin 

of the Front Range. The Slocum consists of pediment and 

valley-fill or terrace deposits at two levels. Lower (younger) 

deposits are commonly pediment deposits about 9‒15 m above 

stream level. Higher (older) deposits are pediment and valleyfill or terrace deposits about 15‒18 m above stream level. These 

latter deposits typically cap small hills. Slocum Alluvium south 

of the Denver area is considered to be about 240 ka, but its age 

is not well constrained (Madole, 1991).

Although the Louviers Alluvium underlies terraces along 

some of the major streams immediately east of the foothills hogback belt south of Loveland (Colton, 1978), it was not identified 

within the Fort Collins quadrangle. Deposits of Louviers Alluvium may locally underlie eolian deposits along the Cache la 

Poudre River and Boxelder Creek near Fort Collins or they may 

be buried by younger alluvium. Louviers Alluvium in the Denver 

area was deposited during the Bull Lake glaciation (Scott, 1975; 

Madole, 1991), about 170–120 thousand years (ka) (Schildgen 

and others, 2002; Sharp and others, 2003; Pierce, 2004).

The Broadway Alluvium (Qb) underlies prominent low 

terraces and includes valley-fill deposits along the Cache la 

Poudre River and its major tributary streams east of the foothills hogback belt. Deposits underlie valley-floor alluvium 

(Qa) in floodplains along major streams as well as terraces 
Geologic History 33

about 7.5 m above the Cache la Poudre River and about 3 

to 7.5 m above major tributary streams. The Broadway also 

forms gravelly alluvial aprons along Sand Creek and the 

upper reaches of Rawhide Creek and Spottlewood Creek, 

near the northeast corner of the quadrangle. Radiocarbon 

ages indicate that deposition of the Broadway was broadly 

coeval with Pinedale glaciation, about 30–12 ka (Madole and 

Shroba, 1979; Nelson and others, 1979; Madole, 1986, 1991). 

Deposition ceased by about 10–11 ka (Holliday, 1987).

Alluvium on valley floors (Qa) in the Colorado Piedmont 

postdates the Pinedale glaciation and is probably of Holocene 

age based on its topographic position relative to Broadway 

Alluvium. Post-Pinedale alluvium (Qa) fills stream channels, 

forms floodplain deposits and underlies adjacent low terraces 

less than 4.5 m above major streams and their major tributaries 

east of the foothills hogback belt and in the Laramie River. 

However, this youngest alluvial unit (Qva) in the mountains 

and the Laramie River valley locally contains narrow deposits 

of Pinedale outwash (Qgp).

Glacial and Glaciofluvial Deposits

Landforms and surficial deposits in the high mountain 

country of the Fort Collins quadrangle record the history of 

Pleistocene and Holocene glaciation in considerable detail. 

The prominent glacial cirques along range crests, classic 

U-shaped valley cross-sections formed by glacial action, and 

characteristic forms of lateral and terminal moraines all attest 

to the extent of glaciation and are major scenic attractions.

Early work by Lee (1917, 1923) and Richmond (1960) 

in the Front Range identified the main elements of the glacial 

landscape and established evidence for multiple glacial 

advances and retreats. Subsequent detailed work by Madole 

(1969), Madole and Shroba (1979), Shroba and Birkeland 

(1983), and Birkeland and others (2003) in the Front Range 

and by Kiver (1968, 1972) in the southern Medicine Bow 

Mountains has added further information about the glacial 

stratigraphy, soil characteristics, and ages of glacial events. A 

comprehensive regional investigation by Madole and others 

(1998) provides a summary of much of the relevant information on glaciation within the quadrangle. The following 

paragraphs are summarized in part from the above sources as 

well as from other sources cited elsewhere in this report.

Deposits of two major Pleistocene glacial episodes 

are recognized in high mountain valleys within the quadrangle, representing the regional Bull Lake and Pinedale 

glaciations (Richmond, 1960). Although scattered remnants 

of deeply weathered till of pre-Bull Lake age may be locally 

preserved near the lower limit of glaciation in the Laramie 

River valley (Kiver, 1968), none were mapped owing to poor 

exposure and lack of strongly weathered and disintegrated 

surface boulders. The oldest mapped deposits accumulated 

during the Bull Lake glaciation. These deposits consist of 

till and glaciofluvial sediment that formed during one or 

more glacial advances during late and middle Pleistocene 

(roughly 190–120 ka; possibly as old as 300 ka, Madole and 

others, 1998). The Pinedale glaciation occurred during the 

late Pleistocene (about 30–12 ka) and produced the most 

extensive and best preserved moraines and glaciofluvial terraces within the quadrangle. The area of ice accumulation 

during the last two major glacial episodes seems to have been 

similar. Glaciers were largely confined to mountain valleys 

above an elevation of about 2,325 m. Perennial snowfields 

probably locally mantled parts of the intervening upland areas 

during glacial episodes, and high ridgelines appear to have 

remained exposed during most of the Pleistocene Epoch.

Till and outwash deposits of the Bull Lake glaciation 

(Qtb) are preserved beyond the Pinedale terminal moraines 

in the Laramie River valley. The form of the moraine crests 

is typically subdued but still discernible, some of the moraine 

boulders are partly disintegrated, and soils have clay-enriched 

B horizons. Multiple moraine-crest ridges suggest minor 

advances and retreats during the Bull Lake glaciation.

Till and outwash deposits of the Pinedale glaciation (Qtp) 

are widespread in the upper part of major mountain valleys in 

the western part of the quadrangle. Sharp moraine-crests, fresh 

glacial boulders (some retain glacial polish and striations), and 

thin weakly developed soils all indicate that these deposits 

accumulated during the last major glaciation. Pinedale terminal moraines are only slightly incised by stream erosion and 

locally impound outwash sediments that cover flat-floored 

valleys that underlie low terraces along the upper reaches of 

the Laramie and Cache la Poudre Rivers. Kettles and kettle 

lakes are locally common in areas of Pinedale drift. Wellpreserved Pinedale moraines show that two or more advances 

and retreats occurred during the Pinedale glaciation; parallel 

lateral moraines and nested terminal moraines are conspicuous 

in many of the major glacial valleys.

Two additional glacial units and one minor post-glacial 

unit are shown on the map sheet. Till of latest Pleistocene 

age forms two or more moraines as much as 0.5 to 2.2 km 

beyond cirque headwalls at elevations as low as 3,220 to 

3,415 m near present timberline on the east flank of the 

Medicine Bow Mountains. Rock-glacier deposits (Qr) are 

present in the cirque-basin in the Mummy Range, Never 

Summer, and Medicine Bow Mountains at elevations greater 

than about 3,200 m. Most of these tongue-shaped, ice-cored 

and lobate, ice-cemented masses of talus boulders and 

locally ablation drift near cirque headwalls formed during 

Holocene time, but some of the larger deposits well beyond 

the cirque headwall may date to late Pleistocene (Meierding 

and Birkeland, 1980; Braddock and Cole, 1990). Organicrich sedimentary deposits (peat in fens and bogs and carbonaceous silt; Qo) are shown in several areas on till of Pinedale age (Qtp). These deposits are preserved chiefly north of 

Chambers Lake and along tributaries to the upper Cache la 

Poudre canyon in the southwestern part of the quadrangle. 

Detailed records of late Pleistocene and Holocene glacial 

environments have been constructed from study of these 

lake and bog sediments in the Front Range south of the Fort 

Collins quadrangle (Madole, 1976, 1980; Davis, 1988).
34 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Mass-Movement Deposits

Mass-movement deposits are widespread in parts of the 

Fort Collins quadrangle, but they have been mapped chiefly in 

areas where they are especially thick, extensive, or important 

as indicators of geologic and past climatic conditions.

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 frost action causes bedrock to fracture and move 

slowly downslope under the influence 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 near the base of steep 

bedrock slopes that were scoured by glacial ice.

Landslides (Qls) are present in the Fort Collins quadrangle in a number of contrasting geologic settings; some are 

old and seem to be relatively stable at present, while others 

remain active today. Several high-altitude landslide deposits 

involve masses of Proterozoic crystalline rock, particularly in 

the Laramie River valley near Chambers Lake where the rock 

has been weakened by faulting. Landslides are also common within the foothills hogback belt. These masses involve 

unconsolidated clay, silt, and sand mixed with blocks of rock 

several meters in diameter that have slid down the dip-slope of 

the Dakota Group (Kd), Fountain Formation (P*f), and Lyons 

Sandstone (Pl), in order of decreasing frequency.

Some landslide masses within the Dakota Group (fig. 12) 

involve bedding-plane slip beneath the first sandstone member 

of the Plainview Formation that allowed large blocks to glide 

down-dip and override younger strata to the east. Some of 

these block-glide landslides, near the northeast corner of the 

quadrangle, are known to be Oligocene in age because they 

are overlain by Miocene and Pliocene(?) Ogallala Formation 

(Braddock, 1978). This distinctive class of landslides is 

described in greater detail in the following Structure section.

Eolian Deposits

Loess (Qlo) consists chiefly of sandy silt, and is common 

east of the foothills hogback belt where it mantles gently 

sloping surfaces. Loess deposits are commonly 3‒11 m thick. 

Loess-derived soils are well suited for agricultural use; they are 

among the most productive soils in the quadrangle. Although 

inactive today, loess commonly accumulated downwind of 

active floodplains and decomposed Cretaceous shale and minor 

sandstone as a result of high-wind conditions that accompanied 

glaciations during the late and late middle Pleistocene.

Structure

The geology of the Fort Collins quadrangle records a 

complex structural history spanning from the Paleoproterozoic to the late Cenozoic (fig. 13). The earliest deformation 

is recorded within the metamorphic rocks of the Front Range 

(fig. 7), followed by structures that formed during intrusion of 

the Mesoproterozoic granitic rocks of the Berthoud Plutonic 

Suite (fig. 9) and subsequent faulting under deep-crustal conditions. Phanerozoic deformation related to the Ancestral Rocky 

Mountains orogeny (Pennsylvanian age) is not explicitly 

recorded in the quadrangle, but is inferred from depositional 

patterns of sediments shed from the uplifted mountain blocks 

(fig. 10). The Laramide orogeny, beginning in Late Cretaceous 

time, exerted a major influence on the Front Range region by 

raising Proterozoic crystalline rocks to similar elevations as 

Upper Cretaceous marine sediments. Synorogenic sediments 

were folded and faulted during a second phase of Laramide 

contraction and uplift in Eocene time. Localized volcanic and 

intrusive activity was accompanied by renewed uplift and 

extensional block faulting in late Oligocene and early Miocene 

time (fig. 12). The most recent uplift of the northern Front 

Range and the Medicine Bow Mountains, in conjunction with 

climate change since the Pliocene, has produced most of the 

modern landscape and removed much of the lower Tertiary 

cover from the foothills and plains regions.

Paleoproterozoic Structures

Within areas of low metamorphic grade along the southeastern edge of the map, Proterozoic rocks of the sedimentary 

source dominated terrain (fig. 7) preserve graded bedding and 

cross-bedding which record the facing direction of beds that 

define original major east-west folds (Braddock and Cole, 

1979; Nesse and Braddock, 1989; Cole and Braddock, 2009). 

These first-generation, regional-scale folds are interpreted 

to have formed while the original sediments were saturated 

with water (Braddock, 1970) prior to metamorphism at 

1,713±30 Ma (Peterman and others, 1968). All folds of this 

period are overturned toward the north and the spacing of 

adjacent anticlinal culminations implies a fold wavelength 

greater than 15 km (Nesse and Braddock, 1989).

Folds attributable to this first stage of deformation can 

only be reliably identified in the southern part of the Fort 

Collins quadrangle where metamorphic recrystallization was 

less pervasive and primary sedimentary structures are preserved. All of the central, northern, and western parts of the 

quadrangle were strongly metamorphosed to the extent that 

pelitic metasedimentary rocks characteristically show evidence 

of partial melting (Cole and Braddock, 2009; p. 19‒20). Folds 

formed in the dominantly volcanic terrane west of Virginia 

Dale, for example, share a dominant east-west orientation with 

Figure 13 (following page). Map showing major structural 

elements of the Fort Collins 30'x60' quadrangle. Dark gray areas 

are Proterozoic crystalline basement rocks and light gray are 

Phanerozoic deposits. Major faults and shear zones are shown 

with names (FLT, fault) and inferred sense of offset (U, upthrown 

block; D, downthrown block; teeth on upper plate of thrust fault) 

where known. Proterozoic fold axes (red lines) are shown within 

crystalline basement rocks and Laramide folds (magenta lines; 

arrows indicate fold type) are shown within Phanerozoic rocks.
ROCKY MOUNTAIN

NATIONAL PARK

14

287

40°30'

106°00'

41°00'

Te

B

WYOMING

COLORADO

Kinikinik Rustic

Eggers

Idylwilde

Red Feather Lakes Livermore

Log Cabin

Stove

Prairie

Landing

Virginia

Dale

Glendevey

Stove

Prairie

to Walden

to Laramie

Cameron

Pass

JACKSON COUNTY

LARIMER COUNTY

LARIMER COUNTY LARIMER 

ALBANY COUNTY LARAMIE 

Base from U.S. Geological Survey digital data, 2010Universal Transverse Mercator projection, zone 13NNorth American Datum 1927

?

D U

D U D

U

D U

D

U

U D

DU

D U

D

U

D

U

U D

D

U

D

U

D

U

D

U

D U

D U D U

D U

D U

D U

D

U

D

U

D

U D

D U

U

D U

U D

U D

U D

D

U

D U

D U D DU D

U

D U

D

U

D

U D U

D

U

D U

D U

D

U

D

U

D

U

U D

D

U

D

U

D U

D U

D

U

D

U D U

D

U

D U

D

U

D U D U

D

U

DU

D U

D

U

D U

CORNELIUS CREEK SHEAR ZONE

SHEEP CREEK FAULT ZONE

BULL MOUNTAIN FAULT

MCINTYRE CREEK FAULT

GREEN RIDGE

FAULT

PANHANDLE CREEK

RED FEATHER

ELKHORN CREEK

POUDRE RIVER SHEAR ZONE

SKIN GULCH SHEAR ZONE 

CAMERON PASS 

SHIPMAN PARK FAULT

BLUE LAKE

KELLY LAKE

FAULT

GRASSY PASS

FAULT

EAST JIMMY CREEK FAULT

SKIN GULCH SHEAR ZONE

ELEPHANT

MOUNTAIN

BOSTON PEAK

FAULT

FAULT ZONE

HALLIGAN RESERVOIR

FLETCHER HILL

STOVE PRAIRIE FAULT

BUCKHORN CREEK

THOMPSON

CANYON FAULT

LIVERMORE

NORTH LIVERMORE

WEST

LIVERMORE

DILKY RANCH FAULT

NORTH FORK RIST CANYON

REDSTONE CREEK

REDSTONE CREEK

CAMPBELL

MOUNTAIN

MILL CREEK

SHEAR ZONE

SHEAR ZONE

LAKES SHEAR ZONE

MOUNTAIN THRUST

NORTH MIDDLE

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

FAULT

BELLVUE

HEWLETT

FAULT

STINK CREEK

FAULT

BUCKHORN CREEK

FAULT

SHELL CREEKFAULT

 FAULT

FAULT 

0 2 4 6 KILOM0 2 4 

EXPLANATION

Fault—U indicates upthrown block; D downthrown; teeth on upper plate of thrust faults

Proterozoic rocks, undivided Proterozoic fold axis

Laramide fold axis—AnticlineLaramide fold axis—Syncline U

D
36 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

the first-generation soft-sediment folds in the sedimentary 

terrane, but they do not show northward overturning and may 

have formed at a somewhat different time.

Second- and third-generation Paleoproterozoic folds are 

widely developed across the Fort Collins quadrangle consisting 

of antiforms and synforms with wavelengths of (at most) a 

few kilometers (summarized from Braddock and Cole, 1979; 

Hutchinson and Braddock, 1987). Second-generation folds (and 

the crenulation cleavages related to them) generally trend east or 

northeast; third-generation folds and cleavages trend northwest 

or north. Both fold sets produce structures of similar scale and 

geometry with steep fold axes, and display axial-plane cleavage.

Kilometer-scale folds related to the second event 

are present in the southeastern and eastern parts of the 

quadrangle. Compositional layering and mineral foliation 

are predominantly oriented east-northeast in the central and 

western parts of the quadrangle, probably reflecting secondgeneration folding. Third-generation folds are well developed 

in the southeastern area between Pingree Park and Stove Prairie 

(Braddock, OʹConnor, and Curtin, 1989; Nesse and Braddock, 

1989), and related crenulation cleavages are widespread in the 

metasedimentary rocks. This northwesterly fold trend is known 

to be the younger of the two because it consistently overprints 

second-generation structures, although the two fold trends 

probably represent essentially conjugate shear directions that 

formed during a protracted contractional event. 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 first-phase folding indicates contraction oriented 

roughly north-south, and the same orientation is suggested by 

the younger northeast and northwest (conjugate) folds.

All of this folding took place at lower metamorphic grades 

(sillimanite-grade or lower) before the thermal peak of metamorphic recrystallization passed through these rocks (summarized 

from Cole, 1977; Cole and Braddock, 2009; p. 19‒20). The 

index porphyroblastic minerals biotite, garnet, cordierite, staurolite, and andalusite consistently overprint the crenulated fabric 

displayed by the matrix micas (muscovite and biotite). The porphyroblasts 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), including most of the Fort Collins 

quadrangle (see fig. 7) show evidence of mineral growth and 

recrystallization more nearly coincident with peak metamorphism (Cole, 1977; Cole and Braddock, 2009; p. 19‒20). 

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 during deformation.

The difference in timing between peak deformation and 

peak recrystallization in low- vs. high-grade metamorphic 

rocks is expectable. As a consequence of the low thermal 

conductivity of rocks, the variation in peak recrystallization 

timing reflects the delay in transmitting heat through the 

deforming orogen. The deformation was probably broadly 

synchronous across the region and at depth, but the thermal 

maximum arrived later in the lower-grade terranes as the temperature front migrated upward and outward from the greater 

pressure and temperature conditions in the core of the orogen 

after folding had ceased.

The broadly coeval second- and third-generation folding 

coincided with the intrusion of Paleoproterozoic magmas at 

about 1,715±10 Ma (Braddock and Cole, 1979; Hutchinson and 

Braddock, 1987; Cole and Braddock, 2009). This interpretation 

is supported by the observation that granodiorite bodies near 

Stove Prairie, for example, intruded and expanded within the 

hinges of these folds. The trondhjemite of Thompson Canyon 

locally contains a biotite foliation that is parallel to third-phase 

crenulation cleavage in adjacent rocks but discordant to the margins of the intrusive bodies (Braddock and Cole, 1979; Barovich, 1986). The Rawah batholith (fig. 9) does not show any 

consistent internal foliation beyond localized flow alignment 

of biotite, and inclusions of metamorphic wall rocks frequently 

indicate rotation of foliation planes from the regional patterns.

The northernmost exposure of the granite of the Rawah 

batholith (XgR) in the southern Laramie Mountains is defined 

by the east-west oriented Cornelius Creek shear zone (fig. 13). 

This structure is a brittle breccia zone of mixed lithologies and 

inferred Laramide age (Workman, 2008). No mylonitic rocks 

have been found in this area, but no systematic structural study 

has been conducted to date of the Cornelius Creek shear zone. 

Rocks of the Rawah batholith are completely absent north of 

this shear zone within the older Paleoproterozoic metavolcanic 

section (volcanic source dominated terrain of figure 7). 

Although inclusions of wall rock are present south of the shear 

zone, there is no northern outer contact of the Rawah batholith 

present in the area. Based upon external contact relationships 

of the Rawah batholith with older metamorphic rocks in other 

areas, the juxtaposition of rock types across the shear zone 

implies a large offset. However, the potential Laramide offset 

along this structural trend is moderate at best as observed to the 

east where Mesoproterozoic and Phanerozoic contacts are offset along the Halligan Reservoir fault and to the west along the 

East Jimmy Creek and Bull Mountain faults where Phanerozoic 

rocks in the Laramie River valley are offset. Therefore, the 

Laramide motion along the Cornelius Creek shear zone is 

inferred to cut out or reactivate a post-Rawah-age structure of 

uncertain orientation and nature (inferred Proterozoic lineament 

on fig. 9). The northern boundary of the Mesoproterozoic 

Log Cabin batholith to the east of this inferred structure is 

surprisingly linear east to west along the strike of the younger 

Laramide faults. This contact shows normal intrusive relationships along the trend with only moderate (hundreds of meters) 

to minor (tens of meters) offsets on the younger faults within 

the trend. The largest offsets (approximately 1.5 km) occur 

east of Halligan Reservoir. Therefore, the inferred structure to 

the west must pre-date intrusion of the Log Cabin batholith. 
Structure 37

The southern contact of the Mesoproterozoic Sherman batholith (YgSH) is sharp in the vicinity of Boulder Ridge where it 

appears to closely follow the structural trend within the older, 

intruded metamorphic terrain. The western end of the Cornelius 

Creek shear zone separates Sherman Granite to the north from 

the Rawah batholith to the south. It is unclear if the Sherman 

batholith was cut by the inferred Proterozoic structure in this 

location or if the Mesoproterozoic intrusive contact was following an older structural trend. We prefer the later interpretation, but the relationships are ambiguous. The northern contact 

of the Log Cabin batholith, the younger Laramide faults, and 

the older inferred Proterozoic structure all define an east-west 

striking lineament which crosses the entire width of the Front 

Range (fig. 9).

Mesoproterozoic Structures

The oldest intrusive rocks of the Mesoproterozoic 

Berthoud Plutonic Suite of Tweto (1987) are only exposed in 

the southern part of the map area. The granite of Hagues Peak 

(1,480 Ma; Braddock and Cole, 1990) 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. Small masses of older biotite-hornblende quartz 

diorite are cut by the granite of Hagues Peak (fig. 9).

The most voluminous intrusive rocks of the Berthoud 

Plutonic Suite are the younger biotite monzogranite to syenogranite plutons and batholiths emplaced at about 1,400 Ma 

(Silver Plume Granite and equivalents; 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). Based on 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; Anderson and Cullers, 1999). 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 Bender, 1989; Anderson and Cullers, 1999).

The Mesoproterozoic granites have been described as 

“anorogenic” because many of these 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-northwesttrending mafic dikes (fig. 9) that were intruded during the 

Silver Plume event (Cole, 1977; Anderson and Cullers, 1999; 

Cole and Braddock, 2009; p. 22–23).

In the Fort Collins quadrangle, the four major Mesoproterozoic intrusive bodies are the Sherman batholith, the 

Virginia Dale ring-dike complex, the Log Cabin batholith, and 

the northern margin of the Longs Peak-St. Vrain batholith. The 

Sherman batholith is exposed across the very northern boundary of the quadrangle and extends several tens of kilometers 

northward in the Laramie Mountains of Wyoming. The Virginia 

Dale ring-dike complex is genetically and magmatically related 

to the Sherman batholith but represents a discrete intrusive 

center where both mafic and felsic magmas were emplaced 

together. The Log Cabin batholith is an oval intrusion near the 

center of the Fort Collins quadrangle. Small bodies of leucocratic biotite granite are present in various locations within 

the quadrangle and they are similar to the granites of the Log 

Cabin body in terms of mineralogy, texture, and crosscutting 

relations. The Longs Peak-St. Vrain batholith lies mostly to 

the south of this area in the Estes Park quadrangle (Cole and 

Braddock, 2009; p. 35), but its northernmost lobe is exposed in 

the Comanche Peak area (Plymate and others, 2005).

Sherman Batholith and Virginia Dale Ring-Dike 

Complex

The Sherman batholith is a compositionally uniform 

body of red-weathering, coarse-grained granite that contains 

sodic amphiboles, common zircon, and fluorite (metaluminous 

to slightly peralkaline chemistry; Eggler, 1968). Most of the 

batholith is exposed in the Laramie Mountains of southern 

Wyoming but it continues southward to surround the Virginia 

Dale ring-dike complex and is exposed in isolated locations 

west of the Laramie Basin. The total exposed extent of the 

main Sherman batholith is approximately 80 km × 25 km, but 

regional aeromagnetic data and outcrop patterns suggest it is a 

larger, continuous body beneath the cover of Phanerozoic sedimentary rocks in the Cheyenne and Laramie Basins. Similar 

granite correlated with the Sherman is also exposed in tilted 

basement blocks south and southwest of Livermore, Colorado. 

Geophysical modeling (Eggler, 1968) shows the Sherman 

batholith as a sheeted intrusion dipping to the southeast, which 

is supported by apparent offset along faults at the intrusion 

margins. Northeast-trending structures on Boulder Ridge 

within the Sherman Granite are parallel to the inferred sheeting orientation and may be fractures or joints formed during 

late-stage cooling and expansion in the batholith. These structures appear to offset younger mafic dikes (Workman, 2008) 

although kinematic relationships are not well constrained.

The well-exposed ring-dike complex near and west of Virginia Dale, Colo., is a distinctive composite intrusive body that 

was studied in detail by Eggler (1968); the following summary 

is based largely on his work. The Virginia Dale complex consists of concentric rings of texturally distinct intrusive granites, 

diorite, and hybrid magmatic mixtures of granitic and dioritic 

magmas, as well as a crescent-shaped relic of the metamorphic 

country rock. Most intrusive contacts dip steeply outward away 

from the ring center. This geometry was interpreted by Eggler 

(1968) to indicate that successive phases of the intrusive complex were emplaced following subsidence of conical, central 

blocks into the underlying Sherman batholith. The hybrid rocks 
38 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

are significant because they show chilled contacts of dioritic 

(basaltic) magma against fluid granite magma, as well as evidence of commingling of mixed magmas. The contacts of the 

intrusive rocks with metamorphic country rock are sharp, and 

the only sign of contact alteration in the surrounding terrane is 

the addition of fluorite. Metamorphic mineral assemblages and 

textures are not modified, indicating the Virginia Dale complex 

was emplaced in the crust at pressure-temperature conditions in 

equilibrium with the high-grade metamorphism that had taken 

place 300 million years earlier (Cole, 2004b).

Log Cabin Batholith

The Log Cabin batholith is a steep-sided, oval, pluton 

consisting of porphyritic and equigranular varieties of biotite 

monzogranite. It measures about 23 km by 32 km with its 

long dimension aligned east-west, parallel to the prevailing 

metamorphic foliation in the country rock. The external 

contacts are sharp and no contact metamorphic effects were 

noted. Some preexisting folds in the metamorphic country 

rock appear to have been compressed into tighter configurations as a result of outward expansion during emplacement of 

the Log Cabin batholith (Abbott, 1970, 1976), but otherwise 

the regional stress conditions are inferred to have been neutral 

to mildly extensional (Cole, 2004b). The northern boundary of 

the batholith is roughly parallel to an inferred older east-west 

trending structure as described above defining the northern 

boundary of the Rawah batholith (fig. 9).

Longs Peak-St. Vrain Batholith

The Longs Peak-St. Vrain batholith is a large, irregular 

body of porphyritic biotite monzogranite that is exposed in 

the core of the Front Range from the southern margin of the 

Fort Collins quadrangle southward for more than 65 km. Cole 

(1977) and Cole and Braddock (2009; p. 35‒36) summarize 

the main compositional and structural features of this complex 

body. Its form is distinctive because the Silver Plume-type 

magmas that intruded it were anomalously dry and viscous 

and the wall rocks responded with intense plastic deformation 

and rotation during emplacement.

Mylonite-Bearing Shear Zones

Several mylonite-bearing fault zones are present in the 

Fort Collins quadrangle and they are interpreted to have 

formed during Mesoproterozoic time. The most prominent 

such zone, and the most thoroughly studied, is the Skin Gulch 

shear zone (fig. 13). The following summary is based on 

Abbott (1970, 1972) and Nesse and Braddock (1989). The 

Skin Gulch shear zone consists of a broad band of steeply 

dipping, plastically deformed rock, ranging from mylonite 

gneiss to phyllonite to mylonite that traverse lithologic-unit 

contacts. Sense of movement on the zone is inferred to be 

largely vertical with southeast side up based on S-C fabric 

within the mylonites and a pervasive mineral alignment that 

plunges steeply in the foliation plane. The 300‒600 m-wide 

zone trends east-northeast through the south-central part of the 

quadrangle. East of Stove Prairie Landing, where it crosses 

the Cache la Poudre canyon, the shear zone turns eastward and 

disperses into numerous thinner shear zones. West of Pingree 

Park where it diverges from the South Fork Cache la Poudre 

River, the Skin Gulch shear zone similarly turns westward 

and thins and disperses. A parallel mylonite-bearing fault zone 

is present farther west (near Peterson Lake), but north of the 

western end of the continuous Skin Gulch shear zone. The 

total amount of offset across the shear zone is hard to quantify 

because no major rock-unit contacts are obviously displaced. 

Rock units north and south of the zone are largely similar, 

although felsic gneisses are far more prominent to the north 

and Paleoproterozoic granodiorite intrusions are far more 

common to the south. The Skin Gulch shear zone also generally coincides with the northern margin of strongly magnetic 

Paleoproterozoic basement rocks, which contrast markedly 

with subdued magnetic susceptibility in similar rocks north of 

the zone; this contrast has not been adequately investigated or 

explained. Solway (2014) estimated offset of approximately 

1 to 6.5 km based upon estimated pressure and temperature 

values from pseudosection analysis. The fault zone appears to 

slightly offset 1,400 Ma mafic dikes, and Abbott (1972) determined homogenization of rubidium and strontium within the 

shear zone at about 1,200 Ma but with large reported errors. 

Allen (2005) dated similar mylonites in the Front Range using 

Ar-Ar techniques and determined a 1,400 Ma age arguing for a 

post-Berthoud suite episode of mylonitization.

Abbott (1976) describes the Elkhorn fault zone as similar 

in character to the Skin Gulch shear zone and it traverses the 

southern margin of the 1,400 Ma Log Cabin batholith. It has 

a generally east-west orientation, dips moderately to steeply 

southward, and ranges in width from a few tens of meters to 

600 m. The Elkhorn fault may connect with the Livermore 

fault to the east where mylonites also dip moderately southward. Brittle deformation is pervasive within the Elkhorn fault 

zone which may indicate younger faulting that cuts the older 

mylonite zone.

Cavosie and Selverstone (2003) report physical, isotopic, 

and structural characteristics of deformed amphibolites and calcareous rocks in the south-central part of the Fort Collins quadrangle that they designate the Buckhorn Creek shear zone (unit 

Xh mapped just south of and cut by the Buckhorn Creek fault). 

They interpret this narrow zone as a relic of original ocean-floor 

basalt and gabbro that was deformed in an oceanic transform 

fracture zone. This interpretation is unlikely for several reasons. 

The rocks in this zone are surrounded by many thousands of 

feet of metamorphosed shales and sandstones, and are interlayered with calc-silicate gneisses and probable metavolcanic 

rocks, not only here but in numerous locations throughout the 

Front Range. That is, the rock assemblages north and south 

of the proposed “suture” are largely indistinguishable. Other 

differences cited by Cavosie and Selverstone (2003) north and 

south of the feature are gradational, not abrupt, and subsequent 
Structure 39

analysis of detrital zircons in metasedimentary rocks (Premo 

and others, 2007) shows no distinction in age populations across 

the feature. The entire metamorphic terrane was intensely folded 

and metamorphosed as a unit. The more straightforward interpretation is that these amphibolites and related rocks are simply 

interlayered with the surrounding metasedimentary rocks.

Arapaho Uplift (Ancestral Rocky Mountains)

Compression along the southern edge of North America 

during the late Paleozoic generated by the collision and suturing 

of North America with South America and Africa and (or) intervening arc terranes generated intracratonic block uplifts known 

as the Ancestral Rocky Mountains (Kluth, 1986). Nesse (2007) 

proposed renaming these tectonic highlands the Anasazi uplifts 

and used the term Arapaho uplift for the highlands of northern 

Colorado in the map area to clarify the lack of coincidence of 

these uplifts with the later Rocky Mountain and Front Range 

uplifts of the Late Cretaceous to Tertiary. There is no direct evidence to indicate any mapped faults within the map area were 

active during this late Paleozoic event, but the presence of the 

Arapaho uplift is indicated by the sedimentary record of alluvial 

fans and the onlapping sequence recorded above the eroded 

Proterozoic basement (fig. 10). A comprehensive analysis of the 

Paleozoic Arapaho uplift is beyond the scope of this study, but 

some observations are worth noting here.

Within the eastern (Denver Basin) and northern (Laramie 

Basin) parts of the map area, coarse to medium arkosic 

conglomerates of the Pennsylvanian and Permian Fountain 

Formation (P*f) record the erosion and transport of clasts of 

the Proterozoic basement towards the east and northeast from 

the inferred uplift over a slope of coalescing alluvial fans. 

To the north, in the Laramie Basin, the Pennsylvanian and 

Permian Casper Formation (P*c), with abundant carbonate 

beds and carbonate-cemented sandstones, interfingers with and 

overlies the Fountain Formation recording the presence of a 

marine slope and intermittent marine incursions into the area. 

The Casper Formation thins rapidly at the southern end of the 

Laramie Basin and pinches out just north of Sand Creek Pass 

(Workman and Braddock, 2010) indicating the edge of marine 

incursion and increasing paleo-elevations to the southwest. To 

the south of this location, the Fountain Formation also thins 

dramatically to the south. It is not clear if onlapping relationships or localized unconformities cause the thinning, but at 

North Middle Mountain and to the south in the Laramie River 

valley, the Lower and Upper Permian Satanka Shale Member 

of the Goose Egg Formation (Pgfsu) is deposited unconformably on the Proterozoic basement. Along the western side of 

the Medicine Bow Mountains, most of the Satanka is absent 

and only the Upper Permian part of the unit is present above 

the Proterozoic basement. These relationships weakly constrain the location of the northeast boundary of the Arapaho 

uplift (thick gray dashed line on fig. 10).

Depositional patterns in the Fountain Formation along the 

eastern side of the Front Range indicate a steep mountain front 

existed (Howard, 1966); however, Kluth and McCreary (2006) 

indicate that no major structure defined the northeast boundary 

of the northwest-oriented uplift. The sparse preservation of upper 

Paleozoic rocks due to uplift and erosion of the Front Range 

during the Cretaceous to Tertiary Laramide orogeny make more 

detailed study difficult. Without the sedimentary record, it is not 

possible to determine if any of the faults cutting the Proterozoic 

basement in the Front Range were active during the late Paleozoic.

Laramide Orogeny

Tweto (1975) described the Laramide orogeny as a prolonged regional structural event that began in Late Cretaceous 

time (about 69 Ma), marked by withdrawal of the Cretaceous 

Western Interior Seaway and by deposition of synorogenic, nonmarine fluvial sands at about 68 Ma. As deformation continued, 

basement blocks rose and shed debris into flanking subsiding 

basins. The Denver Basin east of the Front Range contains 

a relict alluvial fan complex that records rather continuous 

sedimentation over at least 6 million years, producing more 

than 600 m of fluvial sediment (Raynolds, 2003; Dechesne 

and others, 2011). Tweto considered that the Laramide uplift 

and sedimentation continued constantly on a regional basis 

through the Paleocene Epoch and into the Eocene. Raynolds 

(2002, 2004) has shown that the Denver Basin deposits comprise two fluvial sequences that are separated by a significant 

gap of approximately 8 million years (roughly 64 Ma‒56 Ma), 

indicating a more episodic history. The evidence in the Fort 

Collins quadrangle also indicates uplift and deposition were 

episodic, and that the absolute timing, orientation, and style of 

deformation may have been quite variable.

Structures related to the Laramide orogeny are common 

in the Fort Collins quadrangle. They include reverse faults and 

fault-propagation folds along the eastern mountain front, as 

well as low-angle thrusts, reverse faults, and footwall folds on 

the western margin of the range. Syn-tectonic deposits of the 

Paleocene and Eocene Coalmont Formation in the Laramide 

North Park Basin are exposed in the southwest corner of the 

map area (fig. 10). Major brittle fault zones within the Proterozoic basement rocks of the mountain blocks are inferred 

to have moved during Laramide deformation, or later, and are 

marked by broad zones of brecciation, iron-oxide alteration, 

and some silicification.

Early Laramide Structures

The upturned sedimentary units in the foothills hogback 

belt on the east flank of the Front Range record the tilting 

and folding that accompanied mountain uplift in the early 

Laramide deformation. In contrast to areas farther south, no 

master west-dipping fault is exposed here and the range front 

is best described as a broad monoclinal flexure. From the 

area of Horsetooth Reservoir north to about Livermore, the 

range margin consists of fault-bounded, gently tilted blocks 

of Proterozoic basement and inclined Phanerozoic strata that 

are progressively folded over the rigid blocks (Matthews and 

Work, 1978; Sterns, 1978; Matthews, 1987; Erslev, 1991). 
40 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Faults and related fault-propagation folds similar in timing 

and style to those exposed at the range front are present west 

of the range front along Redstone Creek and in the vicinity 

of Stove Prairie and Dilky Ranch (fig. 6) as well as east of 

the range front where folds in the Wellington and Whittier oil 

fields (fig. 10) reflect blind basement faults below. The main 

contractional faults are moderate-displacement (as much as 

300 m), high-angle cylindrical reverse faults that dip basinward to the east (see cross sections E–Eʹ, F–Fʹ, and G–Gʹ). 

Erslev (1991) proposes a trishear fault-propagation fold 

model for this style of structure where a zone of complex 

deformation propagates shear upwards through the upper 

plate of a blind thrust. This blind thrust beneath the eastern 

edge of the Front Range is proposed as an east-vergent backthrust accommodating the large west-vergent overthrusting 

of the Never Summer thrust fault on the western edge of the 

Front Range (Erslev, 1993; Erslev and Rogers, 1993; Erslev 

and Selvig, 1997; Erslev and Holdaway, 1999; Erslev and 

others, 2004; Erslev and Larson, 2006; Erslev and Koenig, 

2009). Within the range front on the eastern side of the map 

area, this study provides no new information about any crustal 

scale fault model; however, new mapping along the western 

edge of the Front Range in the map area does not support the 

degree of overthrusting described by OʹNeill (1981) in the 

Never Summer Mountains to the south of the map area. The 

Livermore embayment (fig. 10) is a trapezoidal structural 

depression bounded by east-northeast-trending high-angle 

basement faults. The absence of tilting in this area is consistent with primarily vertical motion on steep faults and indicates that, at least locally, considerable uplift was achieved in 

the absence of significant lateral contraction. However, this 

does not contradict the possibility of a low-angle structure at 

depth accommodating large amounts of lateral contraction. 

Tetreault and others (2008) describe the role of oblique slip in 

forming some of the asymmetrical folding in the Livermore 

area. From Livermore north to the Wyoming State line, the 

range front is marked by uplifted, non-tilted basement blocks 

flanked by fault-propagation folds in the overlying sedimentary strata that die out up section, but no exposure of faults at 

the surface (Matthews and Work, 1978).

Within the map area, the Denver-Cheyenne basin is a 

broad, symmetrical structure with about 3 km of structural relief 

on the Proterozoic basement between its deepest part and the 

foothills hogback belt (Haun, 1968). The basin axis is located 

about 40 km east of the range front. None of the syn-tectonic 

basin fill deposits are exposed within the map area. In contrast, 

the basin is much deeper and more asymmetric near Golden, 

Colo., to the south (Dechesne and others, 2011), where the axis 

is only 3 km east of the range front and where the basement is 

faulted over the Phanerozoic strata on a west-dipping master 

reverse fault (Weimer, 1996; Higley and Cox, 2005).

Other north-trending, east-vergent structures may 

also have formed during early Laramide deformation. The 

reverse fault on the west side of Red Mountain (north segment of “Bull Mountain fault” of Beckwith [1942] and Camp 

[1979]), the Stink Creek fault between Crazy Mountain and 

Bull Mountain, the Shell Creek fault between east and west 

Bull Mountain, and the West Jimmy Creek fault south of east 

Bull Mountain dip steeply at depth because the surface of the 

uplifted basement block and Phanerozoic strata are nearly 

horizontal (that is, neither folded nor tilted). The folding of 

the sedimentary strata is extremely localized near the uplifted 

edge of the basement block. The low-angle inclinations of 

these faults shown in cross-section by both Beckwith (1942) 

and Camp (1979) are unlikely, based on the rather straight 

trajectory of the fault trace over steep topography. Farther west 

(outside the Fort Collins quadrangle), the western margin of 

the Laramie River valley (in southern Wyoming) is formed by 

a north-trending monocline of tilted Paleozoic strata adjacent 

to uplifted Proterozoic basement (Beckwith, 1942).

The north-trending, east-vergent North Middle Mountain 

thrust fault is exposed along the eastern edge of North Middle 

Mountain in the Laramie River. This moderate- to low-angle 

fault is significantly deformed by younger high-angle structures 

which cross it at several locations. The North Middle Mountain 

thrust fault is defined by a tight fault-propagation fold cut 

locally by the thrust fault with offset of up to hundreds of 

meters but generally minimal offset accompanied by significant 

attenuation of the near-vertical to overturned Phanerozoic section within the middle limb of the fold. At the north end of the 

structure along the Laramie River near Bull Mountain, the entire 

section from the lower Satanka Shale Member of the Goose Egg 

Formation to the Niobrara Formation is approximately 15‒20 m 

thick across a vertically dipping exposure (see cross section 

A–Aʹ). The dip of the fault and axial fold planes steepens to the 

south as inferred vertical structural relief increases. The depth of 

the narrow basin south of North Middle Mountain is constrained 

by drill hole data (Continental Oil Co., Pollock 1–B, Colorado 

Oil and Gas Conservation Commission, 2008) which indicate 

1,642 m depth to the top of the Dakota Group and therefore 

approximately 2 km of vertical relief (see cross section C–Cʹ).

The north-trending structures are interpreted to have 

formed in latest Cretaceous to Paleocene time because they 

can be reasonably related to patterns of withdrawal of the 

Cretaceous Interior Seaway, deposition of the regressional 

Upper Cretaceous Fox Hills Sandstone and Laramie Formation, 

and deposition of syndeformational strata, particularly east of 

the Front Range (Tweto, 1975; Raynolds, 2002, 2004). The 

Laramie Basin (north of this quadrangle; fig. 2) also preserves 

a sedimentary record of sea-level decline, shoreline regression, 

and syn-uplift deposits that spans the Late Cretaceous to early 

Tertiary interval (Blackstone, 1975).

In contrast, the western side of the Front Range and the 

Medicine Bow Mountains record a different chronology as 

well as contrasting styles and orientations of deformation. 

Taken together, these features suggest a distinct, younger 

period of deformation that is described in the next section.

Late Laramide Structures

The Upper Cretaceous Fox Hills Sandstone (Kfh) marks 

the withdrawal of the Cretaceous Interior Seaway and it was 
Structure 41

deposited at about 68‒67 Ma across much of eastern Colorado 

and Wyoming (Scott and Cobban, 1986; Dechesne and others, 

2011). However, the Fox Hills is not present in the Laramie 

River valley (Beckwith, 1942), the North Park Basin west of the 

Medicine Bow Mountains (Hail, 1965; Kinney, 1970), or in the 

Kremmling area (Izett and Barclay (1973). Rather than accumulating sediment, these areas were eroded during Late Cretaceousearly Paleocene time and at least 600 m of the upper Pierre 

Shale (Kp) and equivalent Steele Shale (Kss) and overlying 

Mesaverde Formation (Kmv) were removed in most areas (Hail, 

1965; Tweto, 1975) prior to deposition of the syn-uplift Coalmont Formation (:c). The age of the lower Coalmont is not 

well known and so the onset of sedimentation is also uncertain. 

Fossil pollen from the lower part of the formation in North Park 

(Sudduth coal beds and others) is middle Paleocene (Roberts and 

Rossi, 1999), whereas Paleocene and Eocene pollen are present 

in the middle and upper parts of the formation (Hail, 1968).

The Coalmont Formation rests on the uplifted and eroded 

middle part of the Pierre Shale within the North Park Basin and 

onlaps successively older formations around the margins of the 

basin (Cole and others, 2010). Hail (1965, 1968) estimated the 

Coalmont to be more than 2,750 m thick at its greatest preserved extent, and so the North Park Basin represents a substantial structural reversal from Late Cretaceous to Paleocene time. 

The area where marine Pierre Shale was removed subsequently 

subsided and accumulated thick fluvial deposits of the Coalmont 

Formation. Sedimentology of the Coalmont is not well known, 

but the interplay of mountain uplift, tilting, and basin subsidence is indicated in its gross architecture. Hail (1968) reported 

that the lower Coalmont consists of arkosic sandstone with 

conglomerate beds marked by pebbles and cobbles of porphyritic volcanic rocks inferred to be derived from southern source 

areas. The middle part of the Coalmont (as thick as 1,830 m) 

consists of arkosic sandstone, conglomeratic sandstone (Proterozoic, Dakota, and volcanic porphyry clasts), and mudstone, 

probably derived from adjacent uplifts or reworked in the basin. 

The incomplete upper part of the Coalmont (more than 400 m 

thick) overlies the middle part with slight angular unconformity; 

it consists of carbonaceous shale, sandstone, mudstone, and 

economic coal beds (Roberts and Rossi, 1999).

The nature of the Coalmont Formation indicates uplift 

was occurring east and west of the North Park Basin during 

sedimentation and that volcanic material was removed early 

from the uplifts along with Proterozoic debris from the basement blocks. This is similar to the “unroofing stratigraphy” 

recorded in the Denver Basin by the andesite-clast-bearing 

Denver Formation (Upper Cretaceous to lower Paleocene) 

and the overlying Proterozoic-clast-bearing “D1 sequence” 

(Raynolds, 2004; Dechesne and others, 2011). However, the 

Coalmont sequence is distinctly younger (middle Paleocene to 

lower Eocene) and documents a separate period of mountain 

uplift in the North Park area (Cole and others, 2010).

The late Laramide structures within North Park all 

deform the younger parts of the Coalmont Formation and 

are thus post-late Eocene in age, or possibly younger. In the 

part of the North Park Basin that is immediately west of the 

Fort Collins quadrangle, the most conspicuous structures are 

regularly spaced, upright, asymmetric anticlines and synclines 

that trend northwest (Kinney, 1970, 1971). These folds are 

essentially parallel to the southwestern margin of the uplifted, 

Proterozoic-cored Medicine Bow Mountains. The mountain 

front is marked by a prominent topographic step where the 

less resistant Phanerozoic strata meet the resistant Proterozoic 

rocks, but the contact is unfaulted at least as far south as the 

Canadian River drainage. The dip of the nonconformity at the 

base of the Permian red-beds (Satanka Shale) increases from 

about 35° to 65° from north to south as the crest of the range 

rises from about 3,050 m to nearly 3,960 m elevation. South 

of the Canadian River drainage, the nonconformity contact is 

offset by an east-dipping reverse fault and locally the footwall beds are overturned, but the amount of offset appears to 

be small (Braddock and Cole, 1990; Kellogg, Ruleman, and 

others, 2008). South of the map area in the Mount Richthofen 

7.5' quadrangle, 10–15 km of horizontal overhang is indicated 

along the Never Summer thrust fault (OʹNeill, 1981), but there 

is no indication of this fault exposed within the map area. 

Kellogg, Rulman, and others (2008) project this thrust faulting 

above the surface of the map area north of the Michigan River 

and indicated isolated outcrops of Proterozoic rocks west of 

the range front as klippe, but we reinterpreted these blocks as 

simple slide blocks off of the range front. There is no other 

evidence of a major overthrust north of the Michigan River. 

The structural incongruity north and south of the Michigan 

River remains unresolved by the current mapping.

The western flank of the Front Range uplift (southern 

Laramie Mountains) is marked by a conspicuously linear, 

north-south escarpment of elevated Proterozoic basement rocks 

(Green Ridge area east of the southern Laramie River) against 

folded Phanerozoic rocks in the Laramie River valley. From 

Middle Mountain northward to Bull Mountain, the Paleozoic 

strata are steep to moderately overturned westward along the 

east side of the valley, consistent with the east-dipping Green 

Ridge fault. The dip of the lower plate sediments implies a 

low- to moderate-angle fault, but the trend as mapped across 

significant topographic changes does not support a low angle. 

The Paleozoic through Lower Cretaceous strata are strongly 

attenuated and locally cut out along the fault trace (Beckwith, 

1942; Camp, 1979). The Green Ridge fault appears to lose 

throw suddenly to the north dying out in the Phanerozoic section north of the Deadman Hill Road, where the entire section 

is intact with the Paleozoic strata dipping gently northwestward 

into the Laramie Basin. To the south, this fault swings around 

to a west-northwest orientation, cutting across the southern 

end of Green Ridge, where it juxtaposes older felsic gneisses 

against the granite of the Rawah batholith (Boston Peak fault). 

The structure is well exposed as a narrow breccia zone where it 

crosses the Cache la Poudre canyon just north of Boston Peak. 

The overall form of this structure is an arcuate, moderate-angle, 

southwest-vergent fault. At the valley’s southeastern end, the 

fault merges with a fault zone that trends northwest through the 

Proterozoic rocks north of the Cache la Poudre canyon. Near 

the mouth of Deadman Creek, the nearly vertical Green Ridge 
42 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

fault cuts abruptly down section in the lower plate across the 

strongly tilted, north-south striking Lower Cretaceous into the 

Proterozoic basement. This geometry suggests that the fault is 

cutting across an older fold. The orientation and cross cutting 

relationships of these structures indicate that they are part of the 

late Laramide deformation described above in North Park.

The Bull Mountain fault at the south end of Bull Mountain, 

the East Jimmy Creek fault, and the Panhandle Creek shear zone, 

which extends across most of the width of the Front Range, 

form a left-stepping, en echelon set of west-northwest oriented 

faults which cut and deform the older north-south oriented faults 

and folds along the Laramie River valley. The Bull Mountain 

anticline, just north of the Bull Mountain fault, is a tight, vertical 

fold which closely parallels the fault indicating significant compression across the fault. The fault geometry suggests possible 

left-lateral oblique-slip motion, but no definitive slip indicators 

were observed. Several smaller faults with similar orientations to 

the north also cut the older north-south structural grain. Within 

the map area, the similarly west-northwest trending Sheep Creek 

and Mill Creek fault zones and the Red Feather Lakes shear zone 

may be contemporaneous with the Bull Mountain fault zone, but 

the lack of sufficient crosscutting relationships precludes more 

specific age control on these structures. The north end of North 

Park is defined by a similar set of west-northwest trending faults 

which cut and deform older north-south oriented faults and folds 

(Steven, 1957; Cole and others, 2010).

These northwest and west-northwest structures appear to 

form an identifiable group of elements that lie in subparallel 

proximity to each other forming a pervasive structural grain 

throughout the northwestern Front Range and North Park 

Basin, extending north into the southern Laramie Basin and 

Saratoga Valley of Wyoming. They deform the Coalmont Formation (where it is present) and therefore appear to be post-late 

Eocene and younger than the north-trending faults and folds of 

the Front Range that were mostly active in Late Cretaceous and 

Paleocene time, about 67 Ma to 63 Ma (Tweto, 1975).

It is significant that no Coalmont-age strata are present in 

the Laramie River valley, either in the Fort Collins quadrangle 

or northward in the southern Laramie Basin (Blackstone, 

1975). This observation suggests these areas were part of the 

elevated terrain that was being stripped of the upper Pierre 

Shale prior to Coalmont deposition, and that it remained high 

during Coalmont time. It seems contrived that the Coalmont 

Formation might have been deposited in the Laramie River 

valley, only to be completely removed prior to deposition of 

the North Park Formation during the Miocene.

If the post-late Eocene folding of the Coalmont 

Formation is genetically related to the parallel form of the 

Medicine Bow Mountains, then the Medicine Bow uplift must 

be considerably younger than the Front Range uplift. Kelley 

and Chapin (2004) used partial-annealing of fission tracks in 

apatite crystals to show that the northern Front Range was 

uplifted during the Late Cretaceous to early Paleogene (about 

76 Ma to 44 Ma). Prior reconnaissance data from the southern 

Medicine Bow Mountains were inconclusive (Cervany, 1990, 

quoted in Kelley and Chapin, 2004).

An enigmatic transverse east-west trending structure is 

inferred in the Cameron Pass-Michigan River area that is, most 

likely, contemporaneous with the Never Summer thrust fault 

exposed to the south of the map area (Gorton, 1953; Ward, 1957; 

OʹNeill, 1981; Kellogg, Ruleman and others, 2008). This fault 

is largely covered by Quaternary deposits in the floor of the 

valley and by Oligocene volcanic rocks to the east. The fault is 

reasonably inferred because structures to the north and south do 

not correlate properly along the trend of the Michigan River valley. Conspicuous folds that involve the whole section from the 

Satanka Shale to the Coalmont Formation north of the valley are 

absent to the south. The Coalmont rests unconformably on Benton Group in the footwall north of the valley (that is, all Pierre 

Shale eroded), but at least 1,220 m of Pierre Shale is preserved 

beneath the Never Summer thrust fault to the south (OʹNeill, 

1981). Most high-angle normal faults that post-date the Eocene 

also truncate or terminate at the valley, suggesting the underlying 

structure is fundamentally different in the two areas.

Post-Laramide Structures

The Eocene Epoch was marked by declining Coalmont 

sedimentation (North Park area), accelerated weathering due 

to an intensely warm, moist climate, and widespread erosion 

throughout the Southern Rocky Mountain region (Epis and 

Chapin, 1975; Tweto, 1975). These factors formed a topographically subdued landscape over large areas characterized by rolling hills and broad valleys (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 central part of the Front Range block 

in the Fort Collins quadrangle, lying between about 2,285 m and 

2,750 m elevation, represents relics of the Eocene landscape, 

but it is equally or more likely that this area of broadly concordant hilltop summits continued to develop during Oligoceneearly Miocene time (Evanoff, 1990; Steven and others, 1997). 

Beveled, rolling upland areas include the Green Ridge area and 

all of the mature, lower elevation Laramie Mountains landscape 

north of the Cache la Poudre River from Red Feather Lakes 

eastward across the eroded Virginia Dale ring-dike structure and 

across the Paleozoic and Mesozoic sediments to the east.

Oligocene sediments and volcanic strata are present in a 

few areas of the Fort Collins quadrangle (fig. 12), but most have 

been removed by subsequent erosion. Fluvial and lacustrine volcaniclastic sediments of the White River Group crop out above 

tilted and eroded pre-Cenozoic strata in the northeastern corner 

of the quadrangle where they record a prolonged period of basin 

aggradation and storage of far-traveled volcanic ash (Blackstone, 

1975; Larson and Evanoff, 1998). Similar tuffaceous mudstones 

are preserved in scattered localities surrounding Chambers Lake 

and Peterson Lake (Izett, 1975; Braddock and Cole, 1990), 

which might represent an old valley system buried by volcaniclastic sediment (see Evanoff, 1990, for similar conditions in 

the Laramie Mountains of Wyoming). No obvious structures are 

associated with this time period in the region.
Structure 43

Braddock Peak Volcanic Complex

Late Oligocene time is marked by volcanism and 

intrusion in the southwestern corner of the Fort Collins quadrangle. This magmatic activity and related faulting reflects the 

Never Summer igneous complex, most of which is exposed 

immediately south of this area (OʹNeill, 1981; Braddock 

and Cole, 1990; Cole and Braddock, 2009). Cole and others 

(2008) introduced the term Braddock Peak complex to replace 

Never Summer igneous complex. Only minor exposures of 

Oligocene intrusive rocks occur in the map area so the term 

Braddock Peak volcanic complex is used here. Oligocene 

volcanic rocks include alkali basalts and trachyandesite flows 

in the vicinity of Joe Wright Reservoir and slightly younger 

rhyolite ash-flow tuffs and extrusive flows farther east in the 

Bald Mountain-Flat Top Mountain-Iron Mountain area. Thin 

ash-flow tuff is also present north of Gould Mountain in the 

extreme southwestern corner of the quadrangle, and dacitic 

flows are mapped nearby to the north along the North Fork 

Michigan River (Kellogg, Ruleman and others, 2008). Recent 

geochronologic study by Knox (2005) and previous work (see 

Cole and others, 2008) indicates all volcanic and intrusive 

rocks of the Braddock Peak volcanic complex were emplaced 

between about 29.6 Ma and 27.1 Ma. The mafic lavas occupy 

lower positions in the landscape and may have flowed down 

contemporaneous canyons, whereas the rhyolitic rocks primarily blanket the higher parts of the Oligocene landscape.

Local extensional faulting and uplift are inferred to have 

accompanied magmatic activity in the area of the Braddock 

Peak volcanic complex. Interlayered mafic and felsic volcanic rocks are tilted and anomalously thick in the Little 

Yellowstone area (Braddock and Cole, 1990), which is a synvolcanic, fault-bounded half-graben south of this quadrangle 

in Rocky Mountain National Park. In the Fort Collins quadrangle, boulder gravel deposits are present in paleochannels 

both above and below the rhyolite ash-flow tuff in the Nunn 

Creek basin areas (fig. 12). Contact relations in these localities indicate a valley system existed prior to volcanic eruption, 

and that the gradients were sufficient to allow transport of 

15- to-30-cm-diameter boulders many kilometers north from 

their source areas. Once the late Oligocene volcanic rocks 

were erupted across the landscape, the valley system remained 

intact and the streams were still capable of transporting 30 cm 

boulder gravel containing many clasts of the rhyolite ash-flow 

tuff. The paleovalley system that was developed across the 

mature rolling landscape of the Green Ridge area appears to 

have been integrated with paleodrainage below and to the east 

between Prairie Divide and Livermore (North Fork Cache la 

Poudre paleovalley on fig. 12).

Very similar sand and gravel deposits correlated with the 

Oligocene(?) and Miocene North Park Formation (N:n) are 

present in the Laramie River valley north of Middle Mountain 

(Camp, 1979). These fluvial sediments have not been studied 

in detail, but they contain clasts of the upper Oligocene rhyolite 

ash-flow tuffs exposed within the Braddock Peak volcanic complex along with clasts of durable Proterozoic basement units. The 

nature and clast-size attributes of all these channel-bound fluvial 

units strongly suggest that stream gradients were steep (renewed 

uplift) and they form a regional network that seems to have radiated outward from the Braddock Peak volcanic complex.

Laramie River Fault Zone

For several reasons, we infer substantial extensional 

faulting in the Laramie River valley at or following the time of 

deposition of these upper Oligocene to Miocene fluvial units. 

First, we note that the North Park Formation (N:n) deposits 

on North Middle Mountain are approximately 425 m lower 

in elevation than the probable correlative channel deposits 

preserved on Green Ridge to the east (Deadman Hill and Nunn 

Creek basin gravel deposits on fig. 12). This disparity suggests 

a north-trending normal fault between Middle Mountain and 

Green Ridge that might be responsible for tilting the Medicine 

Bow Mountains block eastward and producing the moderate 

slope of its eastern flank.

Second, Izett (1975), Camp (1979), and McCallum and 

others (1983) all inferred a significant north-northwest-trending 

normal fault along the west side of the Laramie River valley 

between North Middle Mountain and the main part of the 

Medicine Bow Mountains. We concur that the McIntyre Creek 

fault exists and reinterpret faulting along the east side of North 

Middle Mountain originally mapped as part of the North Middle 

Mountain thrust fault as younger normal faulting. A fault is 

required to explain the juxtaposition of Cretaceous rocks against 

the Proterozoic rocks along the west side of Middle Mountain 

which is inferred to be a normal fault. This normal faulting is 

responsible for the alignment of the upper Laramie River valley 

southward to Chambers Lake, where Eocene and Oligocene 

White River Group (:wr) is tilted eastward along the lakeshore. 

The Shipman Park fault is also interpreted here to be a younger 

normal fault. All these normal fault displacements must have 

taken place following eruption of the 28 Ma rhyolites of the 

Braddock Peak volcanic complex, but are poorly understood.

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. Detailed 

mapping by Braddock (Braddock and Eicher, 1962; Braddock, 1978) showed that these landslides are characterized by 

gravity-driven detachment of parts of the tilted Dakota section 

along stratigraphic zones of weakness, causing detached blocks 

to slide down the dip slope over the contemporaneous land 

surface overriding younger strata at the toe of the dip slope. 

The detached blocks were folded in the process of sliding 

over the underlying, intact strata. These landslide complexes 

have been mapped between Laporte and the Wyoming State 

line (Braddock and Eicher, 1962; Courtright and Braddock, 

1989). Evidence 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 and Pliocene(?) Ogallala Formation 
44 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

(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. 

These displaced blocks are represented on the geologic map 

as a “broken block” pattern overprinted on the Dakota Group 

(Kd) rocks. Landslide debris deposits (N:ls) composed of 

fragmented Dakota Group rocks overlie these intact block-glide 

masses in the northernmost exposure (fig. 12) and are considered part of the block-glide complex. These landslide deposits 

probably derived from incompetent rocks exposed upslope near 

the head of the landslide complex after initial detachment of the 

intact blocks below.

Miocene-Pliocene Uplift and River Incision

Middle Miocene time was marked by onset of significant renewed uplift of the Front Range in this area (Bolyard, 

1997; Steven and others, 1997). Eaton (1986, 1987) assembled 

regional evidence to argue that this younger uplift was subcontinental in scale and most likely due to 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 along the extended crest of the 

regional uplift (Eaton, 1986; Chapin and Cather, 1994).

In the area of the Fort Collins quadrangle, this Miocene 

uplift is suggested by the persistent boulder gravel deposits 

(N:gy) in east- and northeast-trending paleocanyons of the 

early Cache la Poudre River drainage and Buckhorn Creek as 

discussed above (Scott and Taylor, 1986; fig. 12). These gravel 

deposits that mark the positions of paleocanyons define fairly 

straight drainages. Stream gradients may have been moderate to 

steep across the Oligocene-Miocene surface beveled over Proterozoic crystalline rocks or drainage may have followed planar 

faults and fractures within the basement rocks. Steven and 

others (1997) noted that these Miocene drainages cross the preexisting Oligocene drainages of the White River Group at a high 

angle and indicate regional post-Oligocene tilting of the Front 

Range block toward the east and northeast (that is, due to uplift 

south and west of this locale). Middle to late Miocene uplift and 

erosion of the modern Front Range and Laramie Mountains are 

recorded by the Miocene sand, gravel, and conglomerate of the 

Ogallala Formation (about 17 Ma to 6 Ma; Izett, 1975).

These high-level (high in the landscape) gravel deposits 

have been interpreted as paleovalley backfill along Oligocene 

to Miocene rivers that carried sediment to the alluvial fan 

complex of the Ogallala Formation (No) on the eastern plains of 

Colorado and Wyoming (Blackstone, 1975 ; Tweto, 1975; Scott 

and Taylor, 1986; Steven and others, 1997). The paleovalleys 

(North Fork Cache la Poudre paleovalley, Cache la Poudre 

paleovalley, and Buckhorn paleovalley on fig. 12) within the 

mountain blocks appear to grade close to the upslope limit of 

the preserved Ogallala, and the clast compositions are similar in 

the two deposits, but gravels as young as Pliocene not differentiated within the poorly exposed deposits may obscure any 

direct correlations. Absolute timing of most of these deposits 

remains highly uncertain. The gravel deposits on Green Ridge 

are interbedded with 28 Ma rhyolite ash-flow tuff, whereas the 

Ogallala is dated by faunal remains as about 17 Ma to about 

5 Ma (Izett, 1975). The base of the gravelly deposits is likely 

to be time-transgressive from higher (older) to lower (younger) 

landscape positions and the drainage pattern may have evolved 

over a considerable span of time.

In any event, the modern stream courses of the Cache 

la Poudre drainage and (possibly) Buckhorn Creek probably 

developed on an aggraded alluvial surface linked to the Ogallala Formation on the Colorado Piedmont. The modern drainage displays significant, kilometer-scale meandering paths, 

even where the river flows through deep canyons cut into the 

Proterozoic basement (for example, the Big Narrows of the 

Cache la Poudre River along the northern edge of the Cache la 

Poudre Wilderness). Such large meanders suggest the drainage 

path was established on a low- to moderate-gradient surface 

prior to incision into the basement rock (Steven and others, 

1997). The late Miocene to Pliocene alluvial surface of the 

Ogallala Formation probably merged with the Sherman surface in the Laramie Mountains straddling the state line where 

the Sherman Granite was deeply weathered to a uniform feldspathic grus (see summary in Eggler, 1973; Blackstone, 1975).

Erosion has dominated Front Range geomorphology since 

the end of Ogallala Formation deposition in middle Miocene 

(Blackstone, 1975; Steven and others, 1997; Leonard, 2002). The 

main drainages tied to the South Platte River removed almost all 

of the Oligocene and Miocene deposits east of the Continental 

Divide on the Colorado Piedmont (Steven and others, 1997). 

This erosion was certainly triggered in part by renewed uplift of 

the area. 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 (post 5 Ma) did not allow time 

for streams to adjust to contrasts in canyon-wall fabric or rock 

types. Analysis of depositional gradients for the Ogallala Formation in the area immediately north of the Fort Collins quadrangle 

indicates that this part of the Colorado Piedmont has been tilted 

eastward since Ogallala deposition, due primarily to tectonic 

uplift of the Front Range and to lesser flexural rebound related to 

incision (McMillan and others, 2002).

The accelerated erosion during Pliocene time may also 

reflect the marked change to a 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. Leonard 

(2002) examined the Pliocene and Quaternary incision of the 

North and South Platte River drainages and concluded that 

both tectonic uplift and flexural adjustment to erosion were 

required to account for the gradients of modern streams and 

post-depositional warping of the Ogallala.
Economic Geology 45

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 involved diffuse adjustments in irregular faultbounded blocks within the main ranges. North-trending fault zones 

in the upper Laramie River valley (Laramie River fault zone, 

fig. 12) may have also undergone Pliocene-Quaternary displacement, but more detailed study is necessary to test this possibility.

Economic Geology

The geologic resources of the Fort Collins quadrangle 

have supported commercial development and economic 

growth over the years. The mountain regions contain deposits of precious metals that were explored in the late 1800s, 

and diamonds have been recovered from kimberlite bodies 

along the Colorado-Wyoming State line in the last several 

decades. The foothills hogback zone has been extensively 

quarried for various rock and stone products over the years. 

The Denver-Cheyenne 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.

Manhattan District

Early Tertiary intrusive rocks (:dp, :ap, and :io) 

emplaced during the latter part of the Laramide orogeny 

provided heat sources and hosts for several minor mineral 

deposits in the Manhattan District north of Rustic (summarized 

from Pearson and others, 1981). Similar small-vein systems 

were explored southwest of the main district identified as the 

Home and Mayesville Districts located just west of Rustic 

along the Cache la Poudre River. Most of the weak mineralization consists of quartz veins with pyrite, chalcopyrite, and 

galena that show modest enrichment in gold. Very little ore was 

produced and most activity had ceased by 1900.

Diamonds of the State-Line Kimberlite District

Neoproterozoic and Devonian kimberlite pipes (DZk) have 

been found at several localities within the Proterozoic basement 

rocks west of Virginia Dale. Diamonds were first discovered in 

the kimberlite breccia in the 1970s and commercial-scale processing began in the 1990s. This area is known as the State Line 

District and it includes kimberlite pipes in northern Colorado 

and southern Wyoming, and is comprehensively summarized by 

Hausel (1998). More than 130,000 industrial-grade stones were 

recovered, as were several gem-quality diamonds larger than 

28 carats raw weight. The largest mine at Kelsey Lake, about 

14 km northwest of Virginia Dale, operated a mill between 1996 

and 1999, and recovered nearly 1,000 carats of industrial and 

gem-quality diamonds. Operations had ceased by 2006.

Coal, Oil, and Gas

The marine deposits of the Cretaceous Western Interior 

Seaway contain abundant carbonaceous debris that serve as 

source rocks for significant hydrocarbon deposits of the region. 

Oil and gas resources in the Denver Basin are described and 

summarized by Higley and Cox (2005), and coal resources 

in the same area were summarized by Roberts (2005). Both 

reports form the primary sources for the following summary; 

additional information and extensive bibliographies are 

contained in these summary reports.

More than 245 million barrels of oil (MMBO) and more 

than 2.2 trillion cubic feet (TCFG) of natural gas have been 

produced by about 2,000 wells in the Denver Basin since early 

development began in the late 1800s. The principal source 

rocks are in the middle part of the South Platte Formation 

(upper Dakota Group, Kd) as well as the shaly formations of 

the Benton Group (Kbm) between the Dakota Group and the 

Niobrara Formation. 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. The high rates of subsidence and sedimentation within 

the Denver Basin during Late Cretaceous time led to burial, 

heating, and the subsequent 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- and gas-producing wells in the Fort 

Collins quadrangle are limited to the area west of Wellington 

where anticlinal folds provide the structural traps.

Minor coal was mined in this area between 1897 and 

1942 from thin beds in the Laramie Formation.

Limestone and Gypsum

Limestone is extensively quarried for cement manufacture in the foothills hogback belt between Laporte and 

Livermore. These operations primarily exploit the Fort 

Hays Limestone Member of the Upper Cretaceous Niobrara 

Formation (Kn) and limestones within the Lower Permian 

Ingleside Formation (Pi).

Gypsum is interbedded at several stratigraphic levels 

within the Lower Triassic and Upper Permian Lykins 

Formation (^Pl) in the northeastern part of the Fort Collins 

quadrangle. Small-scale quarry operations have extracted 

gypsum stone over the years in the area near Table Mountain 

west of Wellington.
46 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Gypsum, in the variety known as alabaster, is present 

in thick blocks and well-crystallized veins within the Lykins 

Formation. Alabaster has been quarried discontinuously for 

several decades by artisans in the Fort Collins area and fashioned into decorative and utilitarian objects on a small scale.

Sand and Gravel

Quaternary floodplain and fluvial terrace deposits of sand 

and gravel have been widely developed in the eastern part of 

the quadrangle for construction materials (Schwochow and 

others, 1974). The youngest terraces (Holocene valley-floor 

alluvium [Qa] and Broadway Alluvium [Qb]) have been most 

thoroughly developed because the clasts are freshest and postdepositional soils are relatively thin. Sand and gravel quarries 

are common along the floodplain of the Cache la Poudre 

River. Extensive sand and gravel deposits are present in the 

Laramie River valley, but currently they are too distant from 

markets to be economic.

Quarry Stone

Several quarries have been opened in the foothills 

hogback belt east of the Front Range to exploit resources 

of dimension stone and flagstone for building materials and 

landscaping rock. Sandstone of the Dakota Group (Kd) is 

quarried in a few locations along the range front where it 

hosts abundant lichen that produce an aesthetically pleasing 

appearance referred to as “moss rock.” Sandstone from the 

Ingleside Formation is crushed and sold for road ballast and 

landscaping purposes.

Proterozoic crystalline rock is quarried at a few sites in 

the area where it is crushed and sized, for example, along the 

Red Feather Lakes road west of Livermore. This material is 

chiefly for road base, rip-rap, and landscape dimension stone.

Water

The earliest western settlers in this region recognized that 

water was a critical commodity to develop and control. Some 

of the earliest development projects in this region were ditch 

systems and reservoirs 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.

Further water development began in the high mountain 

valleys during the late 1800s and early 1900s. 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; Cole and Braddock, 2009, p. 46-47). 

Local cities own most of these systems to provide 

fresh-water resources for their municipalities. The City of 

Fort Collins obtains some of its water from the North Platte 

River system by diversion of flow in the Michigan River 

headwaters just south of Cameron Pass. The Michigan 

Ditch conveys spring runoff to Cameron Pass and then to 

Joe Wright Creek that is operated to control summer-season 

flow for the city. The city also operates Eaton Reservoir and 

Halligan Reservoir on the North Fork Cache la Poudre River 

for seasonal water storage.

The City of Greeley also obtains North Platte River water 

by trans-mountain diversion. The Skyline Ditch system in 

the upper Laramie River valley conveys spring runoff to the 

Laramie-Poudre Tunnel (constructed 1909–1911) and then to 

the main branch of the Cache la Poudre River approximately 

7 km southwest of Kinikinik. The City of Greeley also owns 

and operates six mountain reservoirs: Milton Seaman, Barnes 

Meadow, Peterson Lake, Comanche, Hourglass, and Twin 

Lakes Reservoirs.

Both municipalities obtain substantial water from the 

Colorado-Big Thompson (C-BT) Project, administered by 

the U.S. Bureau of Reclamation. The project was constructed in stages between 1938 and 1957 to meet numerous needs in northern Colorado (see Cole and Braddock, 

2009, p. 46‒47). It collects and diverts water from the 

Colorado River headwaters west of the Continental Divide 

and transmits it through the 21-km (13 mi)-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 241 km 

(150 mi) of waterways and includes 12 reservoirs, 56 km 

(35 mi) 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 2,490 km2

 (615,000 square 

acres) along the Big Thompson, Cache la Poudre, and 

South Platte River drainages.

Environmental Geology

Geology is a fundamental consideration when any 

part of the landscape is evaluated for a particular use. The 

geology of the Fort Collins quadrangle is both diverse and 

complex. Different aspects of geology and topography 

are important in both the relatively populated areas of the 

Colorado Piedmont and the less populated, but frequently 

visited mountainous terrain.

This section of the report summarizes some of the geologic 

processes and conditions that can pose hazards in this area. 

This section of the report 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 has also published 
Environmental Geology 47

several papers and booklets on geologic hazards that are 

especially pertinent (Soule and others, 1976; Noe, 1997; Noe 

and Dodson, 1997; Noe and others, 1997, 1999).

Landslides and Rockfalls

Landslides occur in several geologic settings in the Fort 

Collins quadrangle. The large-scale block-glide landslide 

complexes of the Dakota Group strata (Braddock, 1978) were 

described previously in the section on “Post-Laramide Structures,” most are believed to be inactive, although excavation of 

the toe areas could lead to further earth movement. Mountain 

landslides involving Proterozoic crystalline rocks are known 

from the upper Laramie River valley and the southernmost 

Medicine Bow Mountains (Kellogg, Ruleman and others, 2008).

Landslides and slumps occur on the slopes below the 

Oligocene rhyolitic volcanic rocks in the southwestern corner 

of the quadrangle, perhaps due to underlying clay-rich, Tertiary 

sediment of the White River Group.

Rockfalls are fairly frequent in mountain canyons and 

disrupt and damage highways and trails. They are most common during the spring and summer months when rainfall 

and snowmelt raise the local water table. This increases soil 

pore-water pressure and can cause unstable rock masses to 

slide and fall.

Expansive Soils

Clay-rich expansive or swelling soils in the Colorado 

Piedmont section of the Fort Collins quadrangle have caused 

damage to foundations, structures, and roads due to expansion 

and contraction resulting from wetting and drying. Flat-lying 

and gently inclined beds of the Pierre Shale are most susceptible 

to this process due to the presence of zones rich in expansive 

clays. More steeply inclined beds are susceptible to bedrock 

heaving due to vertical variations in expansive clays within 

the section. Of particular concern are the lower (Kpl), middle 

(Kpm), and upper (Kpu) shale members which contain discrete 

bentonite layers as well as significant bentonitic claystones. The 

intervening sandstone-rich layers of the Pierre Shale (Kph and 

Kpr) pose a much lower risk. The Morrison Formation (Jm), 

Benton Group (Kbm), and Smokey Hill Shale Member of the 

Niobrara Formation (Kn) also pose moderate risk but development on these units is very sparse within the map area. 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 from about 1,676 m to 2,743 m (5,500 to 9,000 ft) 

elevation. Major summer thunderstorms concentrate their 

most intense rainfall between elevations of about 2,134 m 

to 2,743 m (7,000 to 9,000 ft) due to orographic conditions, 

and so the potential for mountain-valley flash flooding is both 

high and persistent (Cole, 2004a). Buckhorn Creek and the 

South Fork and main stem of the Cache la Poudre River are 

uncontrolled below this potential high-rainfall zone down to 

the mouth of the canyons where they empty onto the Colorado 

Piedmont. The North Fork Cache la Poudre River is controlled 

by Halligan Reservoir and Seaman Reservoir, but the terrain 

along the North Fork is neither as high nor as steep as the 

terrain farther south in the quadrangle (thus, less susceptible to 

orographically enhanced rainfall).

The City of Fort Collins has undergone occasional minor 

to severe flooding through the years (following information 

summarized from City of Fort Collins website, accessed May 

2008 at http://www.fcgov.com). Although the Cache la Poudre 

River flows along the northeast side of the old historic downtown area, most of this area has escaped significant damage 

because it sits on Slocum Alluvium, 10 to 30 ft above the 

floodway. The original Camp Collins was located on the floodway and was largely destroyed in 1864 when a severe spring 

rainstorm in the mountains caused a major flood surge that 

swept through the mountain canyon and discharged through 

Laporte and the settlersʹ colony. A similar spring flood (caused 

by heavy mountain rain combined with rapid snowmelt) in 

1904 also swept away numerous structures that had been built 

in the floodway.

The flood of June 9, 1891, initiated by the failure of 

one of the dams that impounded Chambers Lake, may have 

affected developed areas of Fort Collins. This flood destroyed 

bridges across the Cache la Poudre River between the junction 

with Joe Wright Creek and Fort Collins. Very fresh, boulder 

alluvium deposited during this flood now underlies the area 

that is now Aspen Glen Campground along Colorado State 

Highway 14 in the Roosevelt National Forest.

Subsequent flooding events affected developed areas of 

Fort Collins in 1938, 1951, 1977, and 1992, and were caused 

by the heavy rains of persistent and slow-moving summer 

storms in the piedmont area, not in the mountains. The 1997 

Spring Creek flood was one of the most damaging of these 

piedmont-storm events. Heavy rain on the night of July 27 

(about 100 mm to 150 mm or 4 to 6 inches [in]) was followed 

by nearly 254 mm (10 in) of rain during a 5-hour span on 

the night of July 28, and the resulting discharge in Spring 

Creek exceeded the estimated 500-year event by more than 

30 percent.

Every major stream on the east slope of the Front Range 

has undergone moderate to significant flooding at one time or 

another in recorded history and the likelihood of future flooding is high. As modern development encroaches further on 

floodplains and floodways, the potential for damage continues 

to rise.
48 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

DESCRIPTION OF MAP UNITS

SURFICIAL DEPOSITS

[Surficial deposits in the Fort Collins quadrangle record the alluvial, mass-movement, glacial, and eolian 

processes during the Quaternary and late Neogene. Many of the surficial deposits in these areas are poorly 

exposed. Deposits that are of limited extent (less than about 200 m wide), including (1) small earth-fill 

dams, (2) waste rock, commercial rock products, and fill material at quarries such as near La Porte and 

Livermore, and (3) small organic-rich deposits (Qo) in glaciated areas, were not mapped. Also, thin 

deposits (less than about 1.5 m thick), including (1) some mass-movement deposits above treeline (such as 

block-stream and some block-slope deposits) and (2) some sheetwash deposits (Qsw) that locally mantle 

gently sloping bedrock and surficial map units, were not mapped.

Most of the surficial units on this map are informal allostratigraphic units (discontinuity-bound 

sequences) of the North American Stratigraphic Code (North American Commission on Stratigraphic 

Nomenclature, 2005), whereas the other Phanerozoic bedrock units are informal or formal lithostratigraphic 

units. For this reason, subdivisions of stratigraphic units use time terms “late” and “early” where applied 

to surficial units, but use position terms “upper” and “lower” where applied to lithostratigraphic units. 

Formal names used for fluvial and pediment deposits east of the mountain front are those established by 

Scott (1960, 1963). Tentative correlation of surficial deposits east of the mountain front with those formally 

established by Scott in the Denver area is based on the mapping of Colton (1978) near Fort Collins and on 

relative height above present stream level, physical properties, and genesis in the piedmont north of Fort 

Collins determined by Shroba.

A fractional map symbol (No/Na) is used on the cross sections where individual units are too thin to 

show separately. The units occur from top to bottom as listed. This fractional unit is not described here; 

instead refer to descriptions of individual units. Fractional map units are not shown on the Correlation of 

Map Units diagram on the map sheet.

Informal names (such as terrace alluvium of Pinedale age, Qgp, and pediment deposits) are applied to 

fluvial and pediment deposits in the Laramie River valley and in other areas west of the mountain front.

The mapped distribution of surficial units on the Round Butte, Buckeye, Wellington, Crazy Mountain, 

Glendevey, and Rawah Lakes 7.5' quadrangles and parts of the Deadman, Kinikinik, Boston Peak, 

Chambers Lake, and Fort Collins 7.5' quadrangles is based primarily on interpretation by Shroba of 

1:40,000-scale, black-and-white aerial photographs (NAPP, National Aerial Photography Program images 

accessed in 1999 at https://lta.cr.usgs.gov/NAPP) . Surficial deposits in other areas are compiled from existing mapping (fig. 4). Age assignments for surficial deposits within the quadrangle are based chiefly on the 

relative degree of modification of inferred original surface morphology and relative heights above present 

stream channels. Degree of soil development and clast weathering were used to distinguish till of Pinedale 

age (Qtp) from till of Bull Lake age (Qtb). Soil-horizon designations are based on those of the Soil Survey 

Staff (1999) and Birkeland (1999). 

Grain or particle sizes of surficial deposits are field estimates. Size limits for sand (0.05‒2 mm), 

silt (0.002‒0.05 mm), and clay (<0.002 mm) are those of the Soil Survey Staff (1951). In descriptions of 

surficial map units, the term “clasts” refers to granules and larger particles (>2 mm in diameter), whereas 

the term “matrix” refers to sand and finer particles (≤2 mm in diameter).

In this report, the terms “alluvium” and “alluvial” refer to material transported by running water 

confined to channels (stream alluvium) as well as by running water not confined to channels (sheetwash). 

The term “colluvium” refers to all rock and sediment transported downslope chiefly by gravity (Hilgard, 

1892; Merrill, 1897). Colluvial material on slopes is transported chiefly by mass-movement (gravitydriven) processes—such as creep, sliding, debris flow, and rock fall—locally aided by sheetwash.

Surficial map units that include debris-flow deposits probably also include hyperconcentrated-flow 

deposits. These latter deposits are intermediate in character between stream-flow and debris-flow deposits 

(Pierson and Costa, 1987; Meyer and Wells, 1997).

In this report the terms “soil” and “soils” refer to pedogenic soils formed in surficial deposits (for 

example, Birkeland, 1999)]
Description of Map Units 49

ARTIFICIAL-FILL DEPOSIT

af Artificial-fill (latest Holocene)—Engineered, compacted fill material composed of rock 

fragments and finer material in four large earth-fill dams at Horsetooth Reservoir near 

Fort Collins, and in three smaller earth-fill dams at Eaton, Panhandle, and Long Draw 

Reservoirs in the western part of the map area. Estimated thickness is 3–60 m

ALLUVIAL DEPOSITS

Qa Valley-floor alluvium (Holocene)—Sand and gravel in stream channels as well as sand, 

sandy and clayey silt, and gravel underlying floodplains and adjacent low terraces 

less than 4.5 m above major streams and their major tributaries east of the foothills 

hogback belt of the Front Range. Unit includes post-Piney Creek alluvium and Piney 

Creek Alluvium (Colton, 1978). The younger alluvium commonly lies within stream 

channels cut in the older alluvium. Unit Qa locally includes small fan deposits (Qf) and 

sheetwash aprons (Qsw) along the margins of major valleys, and overlies Broadway 

Alluvium (Qb) in major valleys. The abundance of silty and clayey sediment, particularly along Boxelder Creek and its major tributaries, suggests that unit Qa was derived 

in part from sediments eroded from silty and clayey bedrock units such as the Pierre 

Shale and probably from loess deposits (Qlo). Low-lying deposits are prone to periodic 

stream flooding. Estimated thickness is 1–5 m

Qva Mountain valley alluvium (Holocene and late Pleistocene)—Pebbly to bouldery gravel, 

sand, and minor deposits of silty alluvium in stream channels, floodplains, and low 

terraces along courses of major and tributary streams in the mountains west of the 

foothills hogback belt and along the Laramie River near the northwest corner of the 

quadrangle. Unit Qva locally includes outwash deposits (Qgp) downstream from till 

of Pinedale age (Qtp), small fans deposits (Qf) and minor deposits of colluvium (Qc) 

along valley margins, as well as organic-rich sediments (Qo) in bogs, fens, marshes, 

and meadows. Deposits of unit Qva, near cirques in the Medicine Bow Mountains, 

locally may include outwash deposits of post-Pinedale age. Unit is chiefly present along 

the Cache la Poudre River, Laramie River, and their tributaries. Outwash deposits are a 

potential source of coarse aggregate. Low-lying deposits are prone to periodic flooding. 

Estimated thickness is 3–9 m

Qsw Sheetwash alluvium (Holocene and late Pleistocene)—Mostly slightly pebbly to pebbly, 

slightly silty to silty sand that is commonly deposited on gentle slopes. Low-lying areas 

of unit Qsw are susceptible to sheet flooding due to unconfined overland flow, and 

locally to stream flooding and gullying. Recently disturbed areas of unit Qsw may be 

susceptible to minor wind erosion. Estimated thickness is 1–4.5 m

Qb Broadway Alluvium (late Pleistocene)—Mostly cobbly pebble gravel and pebble gravel in 

terrace and valley-fill deposits along the Cache la Poudre River and its major tributary 

streams east of the foothills hogback belt. Deposits of unit Qb underlie unit Qa in 

floodplains along major streams as well as terraces about 7.5 m above the Cache la 

Poudre River and about 3–7.5 m above major tributary streams. Unit Qb also forms 

gravelly alluvial aprons along Sand Creek and the upper reaches of Rawhide Creek 

and Spottlewood Creek, near the northeast corner of the quadrangle. Deposition is 

considered to be broadly coeval with Pinedale glaciation, about 30–12 ka (Madole and 

Shroba, 1979; Nelson and others, 1979; Madole, 1986, 1991). The upper part of the 

Broadway probably was deposited after Pinedale-age glaciers reached their maximum 

extent about 21 ka (Madole, 1986), and ceased deposition by about 10–11 ka (Holliday, 

1987). Unit Qb may be locally overlain by thin, unmapped loess deposits (Qlo). Unit 

Qb in the floodplain of the Cache la Poudre River near Fort Collins is a major source 

of coarse aggregate. Thickness of valley-fill deposits beneath unit Qa in the Cache la 

Poudre River valley is typically 3–6 m (Schwochow and others, 1974); estimated thickness of terrace deposits along the Cache la Poudre River and Boxelder Creek near Fort 

Collins is 7.5–15 m
50 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Qgp Terrace alluvium of Pinedale age (late Pleistocene)—Fluvial deposits composed chiefly of 

cobbly pebble gravel rich in granitic clasts. Deposits underlie one and locally two terraces 

and terrace remnants about 3–6 and 12–15 m above the Laramie River downstream of 

Four Corners; about 9 m above McIntyre Creek downstream of Glendevey; about 3–6 m 

above Joe Wright Creek downstream of Chambers Lake; about 3–12 m above the Cache 

la Poudre River downstream of the confluence with Joe Wright Creek; and about 12 m 

above the Michigan River and North Fork Canadian River near the southwest corner 

of the map area. The proximity of moraines composed of till of the Pinedale glaciation (Qtp) to terraces underlain by unit Qgp suggests that much of unit Qgp is outwash 

deposited by glacial meltwater during the Pinedale glaciation, about 12–30 ka (Nelson 

and others, 1979; Madole, 1986; Schildgen and others, 2002; Benson and others, 2004, 

2005, 2007). Deposits along Cache la Poudre River downstream of Kinikinik, and probably those along the Laramie River and McIntyre Creek, locally include alluvium derived 

from tributary drainages that were not glaciated, but probably were subject to periglacial 

processes. Unit Qgp locally includes small bodies of valley-floor alluvium (Qva) and till 

of Pinedale age (Qtp) as well as sheetwash alluvium (Qsw) and fan deposits (Qf) along 

valley margins. Unit also includes bouldery flood gravel along Joe Wright Creek that is 

too narrow to map separately. The gravel contains boulders as large as 120×140×210 cm3

, 

and may have been deposited during the flood of June 9, 1891, that flowed from the failed 

dam at Chambers Lake. Deposits of unit Qtp are a potential source of coarse aggregate. 

Estimated thickness is 3–15 m; possibly as much as 90 m on the up-valley sides of Pinedale terminal moraines (Madole and others, 1998)

Qgb Terrace alluvium of Bull Lake age (late and middle Pleistocene)—Fluvial deposits 

composed chiefly of bouldery to slightly bouldery, cobbly pebble gravel rich in granitic 

rocks of the Rawah batholith (XgR) that underlies terrace remnants along the Laramie 

River and locally along McIntyre Creek near the northwest corner of the quadrangle. 

Terrace remnants are about 37–43 m above the Laramie River downstream of Four 

Corners. Terrace remnants along McIntyre Creek are about 30 m above stream level 

downstream of Glendevey. A few terrace remnants along the Cache la Poudre near 

Stove Prairie Landing are about 24–40 m above stream level. Deposits of unit Qgb are 

about 30 m above the North Fork Canadian River near the southwest corner of the map 

area. Deposits along the Laramie River near the down-valley limit of glaciation contain 

boulders about 30–70 cm in diameter. Large pebbles, cobbles, and boulders commonly 

are subangular to subrounded. Deposits 5.5 km farther downstream near Four Corners 

are composed of cobbly pebble gravel that contains lenses of slightly bouldery gravel. 

The proximity of moraines composed of till of the Bull Lake glaciation (Qtb) to terrace 

remnants underlain by unit Qgb suggests that much of unit Qgb is outwash deposited 

by glacial meltwater during the Bull Lake glaciation (about 120–170 ka; Sharp and 

others, 2003; Pierce, 2004). Unit Qgb is a potential source of coarse aggregate. 

Estimated thickness is 3–15 m

Qg Gravelly stream alluvium (late and middle Pleistocene)—Small deposits of poorly 

sorted and poorly to well stratified stream gravel and probably finer-grained sediment 

in valleys, west of the Dakota hogback, in the eastern part of the map area. Clasts 

are commonly of Precambrian rock, but locally are derived from nearby sedimentary 

rocks; many of the clasts are weathered. Calcium carbonate cement is locally abundant 

(Braddock, Wohlford, and Connor, 1988; Courtwright and Braddock, 1989). Deposits 

near Table Mountain are about 18 m and locally 24 m above Boxelder Creek and about 

12 m above some of its tributary streams. Deposits west of Livermore, Colo., are about 

24 m and 37 m above the North Fork Cache la Poudre River and about 24, 37, 60, and 

85 m above some of its tributary streams. Unit locally includes fan deposits (Qf) and 

pediment deposits (Qp). Deposits of unit Qg are a potential source of fill and road 

subbase material. Estimated thickness is 3–15 m

Qs Slocum Alluvium (middle Pleistocene)—Unit was deposited by streams and probably 

locally by debris flows in the piedmont east of the foothills hogback belt where it commonly consists of widespread deposits of slightly cobbly, pebble gravel interbedded 

with pebbly sand and slightly pebbly silty sand. The Slocum consists of pediment and 
Description of Map Units 51

valley-fill or terrace deposits at two levels. Lower (younger) deposits are commonly 

pediment deposits about 9–15 m above stream level. Higher (older) deposits are pediment and valley-fill or terrace deposits about 15–18 m above stream level. The latter 

deposits typically cap small hills. Slocum Alluvium in the Denver area is considered 

to be about 240 ka (Madole, 1991), based on a uranium-series age of 190±50 ka near 

Canon City, Colo. (Szabo, 1980). Terrace or valley-fill deposits of unit Qs are a potential source of fill and road subbase material. Estimated thickness is 2–9 m

Qv Verdos Alluvium (middle Pleistocene)—Unit was deposited by streams and probably 

locally by debris flows in the piedmont east of the foothills hogback belt where it 

consists of non-cobbly to very slightly cobbly, pebble gravel. Locally, the gravel is 

interbedded with pebbly sand and slightly pebbly silty sand. Unit Qv consists of pediment and, locally, valley-fill or terrace deposits at one and locally two levels. Lower 

(younger) deposits are about 34–37 m above stream level and a small, higher (older) 

deposit east of Rawhide Creek is about 45 m above stream level. Beds and lenses of 

water-laid Lava Creek B tephra (about 640 ka; Lanphere and others, 2002) are present 

at several localities in the Denver area. The tephra locally occurs at the base of higher 

(older) pediment deposits of Verdos Alluvium (Scott, 1963), and is present within or at 

the top of higher (older) main-stream fluvial deposits of the Verdos in and near Denver 

(Hunt, 1954; Scott, 1972; Baker, 1973; Van Horn, 1976; Kirkham, 1977). Terrace or 

valley-fill deposits of unit Qv are a potential source of fill and road subbase material. 

Estimated thickness is 2–9 m

ALLUVIAL AND MASS-MOVEMENT DEPOSITS

Qac Alluvium and colluvium, undivided (Holocene to middle? Pleistocene)—Chiefly 

undifferentiated valley-floor alluvium (Qa), mountain-valley alluvium (Qva), sheetwash 

alluvium (Qsw), fan deposits (Qf), colluvium (Qc), and locally other mass-movement 

deposits along minor streams and on adjacent lower (toe) slopes. Low-lying areas of unit 

adjacent to stream channels may be subject to periodic stream flooding and debris-flow 

deposition. The adjacent toe slopes may be susceptible to local sheet flooding due to 

unconfined overland flow, and locally to gullying. Estimated thickness is 3–15 m

Qsc Sheetwash alluvium and colluvium, undivided (Holocene to middle? Pleistocene)—

Thin mantle composed chiefly of slightly pebbly to pebbly, silty sand and sandy silt that 

overlies the North Park Formation (N:n) in the Laramie River valley near Glendevey. 

Unit Qsc locally includes small outcrops of the North Park Formation (N:n), and 

locally may include pebbly lag deposits. Deposited chiefly by unconfined overland flow 

on gentle slopes and by mass-movement processes on moderate slopes. Low-lying areas 

of unit are susceptible locally to sheet flooding due to unconfined overland flow, and 

locally to stream flooding and gullying. Estimated thickness is 1–1.5 m

Qf Fan deposits (Holocene and late Pleistocene)—Mostly poorly sorted, slightly bouldery 

pebble and cobble gravel and locally pebbly and cobbly silty sand. Deposited chiefly 

by streams and debris flows in fan- and tongue-shaped accumulations near base of 

moderate to steep slopes along valley sides west of the foothills hogback belt where 

unit commonly overlies unit Qgp. Lower limits of deposits are about 3–12 m above 

stream level. Unit locally may include sheetwash alluvium (Qsw), colluvium (Qc), and 

probably hyperconcentrated-flow deposits. Low-lying areas on the unit may be subject 

to stream flooding and debris-flow deposition. Estimated thickness is 3–9 m

Qp Pediment deposits (late and middle Pleistocene)—Gravelly and sandy alluvium that 

overlies gently sloping surfaces cut on sedimentary rocks in the foothills hogback belt 

and locally on sedimentary rocks and granite in the mountains. Unit Qp also includes 

gravelly alluvium and debris-flow deposits that overlies gently sloping surfaces cut on 

sediments and sedimentary rocks in the Laramie River valley. Deposits consist chiefly 

of crudely stratified and non-sorted to poorly sorted, clast- and locally matrix-supported 

sediment. Deposits southeast of Virginia Dale are composed chiefly of slightly cobbly, pebble gravel that contains abundant granules and resembles grus weathered from 

the Sherman Granite (Eggler and others, 1969; Braddock, Eggler, and Courtright, 
52 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

1989). Deposits north of Eaton Reservoir that overlie the Sherman Granite (YgSH) 

are composed of granite-rich pebble gravel and pebbly sand derived from the Sherman 

Granite. Those that overlie the Fountain Formation (P*f) are composed of arkosic, 

conglomerate- and sandstone-rich gravel and pebbly sand derived from the Fountain 

Formation. Deposits north of Bull Mountain are composed of sandstone-rich gravel and 

non-pebbly to pebbly silty sand. Deposits in the Laramie River valley are composed of 

non-bouldery to slightly bouldery, cobbly, pebble gravel as well as matrix-supported 

small boulders to granules. Bouldery debris-flow deposits locally contain sandstone 

casts eroded from the Dakota Group (Kd) that are as large as 30×45×45 cm3

. Unit Qp

locally may include small unmapped bodies of sheetwash alluvium (Qsw), colluvium 

(Qc), and probably hyperconcentrated-flow deposits. The approximate heights above 

stream level of the lower (downslope) limits of pediment deposits are 37 m near 

Virginia Dale, 18 m near Eaton Reservoir and Bull Mountain, and 12–18, 37–55, and 

64–73 m above stream level in the Laramie River valley as it flows northward near 

the northwest corner of the quadrangle. Deposits near their lower limits in the Laramie 

River valley may be graded to, and locally may include, fluvial gravel deposited by 

the Laramie River. Low-lying areas on unit Qp may be subject to stream flooding and 

locally to debris-flow deposition. Estimated thickness is 1–4.5 m

MASS-MOVEMENT DEPOSITS

Qc Colluvium (Holocene to middle? Pleistocene)—Non-sorted deposits that consist of clay, silt, 

sand, and clasts that range in size from granules to large boulders, formed by a variety 

of mass-movement processes. Unit Qc chiefly includes small deposits in and near the 

foothills hogback belt and on slopes in montane areas, as well as extensive areas of blockslope deposits near and above present day treeline at about 3,475 m where freeze-thaw 

action has promoted mass-movement and downslope transport. The composition of the 

deposits reflect that of the bedrock or sediment from which the colluvium was derived. 

Unit Qc includes material transported by frost creep, solifluction, nivation and other 

periglacial processes, sheetwash, landslide, debris flow, hyperconcentrated flow, and rock 

fall. Unit Qc locally includes small bodies of undifferentiated valley-floor alluvium (Qa), 

mountain-valley alluvium (Qva), pediment deposits (Qp), sheetwash alluvium (Qsw), 

landslide deposits (Qls), and talus deposits (Qta). Estimated thickness is 3–15 m

Qls Landslide deposits (Holocene to middle? Pleistocene)—Deposits of unsorted and 

unstratified debris, on slopes at or near the base of slopes. These deposits commonly 

have hummocky topography. Many of the landslides and landslide deposits form on 

unstable slopes that are underlain by shale, siltstone, and claystone near the eastern 

and western boundary of the quadrangle. An extensive hummocky mass west of 

Chambers Lake consists mainly of blocks of Proterozoic granite of Rawah batholith 

and is interpreted to be a landslide complex that likely blocked the Joe Wright Creekupper Laramie River drainage (Kiver, 1968) and formed a large lake (at the present 

site of Chambers Lake) after the retreat of Pinedale ice upvalley of the landslide 

complex. The original dam formed by the landslide complex lead to diversion of 

Joe Wright Creek into the Cache la Poudre River system. Large landslide complexes 

flank the hogbacks west of Fort Collins and the margins of Bull Mountain near the 

northwestern corner of the quadrangle where the Dakota Group (Kd) sandstone 

caprock has slid over weaker, less competent sedimentary rock (Braddock and Eicher, 

1962; Braddock, 1978; Courtright and Braddock, 1989). Younger landslide deposits 

are commonly bounded upslope by crescent-shaped headwall scarps (hachured line 

symbol) and downslope by lobate toes. The unit locally includes material displaced 

chiefly by rotational rockslides, rotational earth slides, debris slides, debris flows, 

earth flows, and earth slide-earth flows as defined by Varnes and Cruden (1996). 

Some deposits probably are formed by translational slides and rock or earth creep. 

The sizes and lithologies of the clasts and the grain-size distributions of the matrices 

of these deposits reflect those of the displaced bedrock units and surficial deposits. 

Landslide deposits are prone to continued movement or reactivation due to both 
Description of Map Units 53

natural and human-induced processes. Deposits on gentle slopes along and east of the 

foothills hogback belt locally include minor bodies of sheetwash (Qsw) and creepdeformed deposits. Deposits in the mountains locally include small rock-fall deposits. 

Some of the landslide deposits composed of material derived from the Dakota Group 

(Kd) on the east flank of the foothills hogback belt and the margins of Bull Mountain 

may be of middle Pleistocene age. Estimated thickness is 5–45 m

Qta Talus deposits (Holocene to middle? Pleistocene)—Large angular boulders and smaller 

rock fragments deposited chiefly by rock and snow avalanche, rock fall, rock slide, and 

debris flow at the base of cliffs and steep slopes where debris forms aprons, cones, and 

fan-shaped deposits. Unit locally includes rubbly scree deposits and probably tills of 

Holocene age near cirque headwalls (for example, Benedict, 1973a; Benson and others, 2007). Much of the talus in glaciated valleys and cirques postdates the retreat of 

Pinedale ice. Unit Qta is mapped chiefly in alpine and subalpine areas along the crest of 

the Medicine Bow Mountains and Never Summer Mountains, and in the upper part of 

the Cache la Poudre River valley upstream of Kinikinik. Small talus deposits adjacent or 

surrounded by colluvium (Qc) are mapped as colluvium. Estimated thickness is 3–15 m

GLACIAL DEPOSITS

Qr Rock-glacier deposits (Holocene and latest Pleistocene)—Lobate and tongue-shaped 

masses of boulder- to silt-sized rock debris that commonly have steep fronts, and flanks 

and form in cirques, along valley walls, and on valley floors above an elevation of 

2,900 m commonly in areas of high talus production. Deposits consist of a veneer of 

angular boulders that overlies a thick mass of smaller debris. Rock material accumulated chiefly as talus that is mobilized downslope by flowage that is probably due to 

the deformation of interstitial ice or an ice core (Braddock and Cole, 1990). The lower 

part of the unit is ice cemented or has an ice core. Lobate rock glaciers form along 

valley walls, and are ice cemented. Tongue-shaped rock glaciers resemble glaciers, 

form on valley floors, and commonly have debris-covered ice cores (Benedict, 1973b; 

White, 1976). Ice-cemented rock glaciers likely formed under periglacial conditions 

(Barsch, 1987), whereas ice-cored rock glaciers probably are debris-covered glaciers 

(Janke, 2007). Inactive ice-cored rock glaciers commonly have depressions adjacent 

to headwall cliffs (where glacial ice melted), longitudinal marginal-and-central meandering furrows, and collapse pits. Ice-cemented rock glaciers commonly lack these 

surface features (White, 1976). Rock fragments on and within rock-glacier deposits 

are derived from steep slopes chiefly by rockfall and locally by sliding and avalanche. 

Unit Qr locally may include minor talus deposits (Qta) displaced by post-depositional 

creep or flowage, colluvium (Qc), other mass-movement deposits, and probably tills of 

Holocene age near cirque headwalls (for example, Benedict, 1973a; Benson and others, 

2007). Many of the rock-glacier deposits in Colorado are of latest Pleistocene or early 

Holocene age (Meierding and Birkeland, 1980). Estimated maximum thickness is 45 m

Qtpp Till of post-Pinedale age (latest Pleistocene)—Non-sorted and non-stratified, ice-deposited 

angular boulders to granules in a sandy matrix that commonly forms two or more small 

moraines as much as 0.5–2.2 km beyond cirque headwalls (Kiver, 1968) at elevations 

as low as 3,220–3,415 m near present timberline on the east flank of the Medicine Bow 

Mountains. The unit probably is correlative with deposits that form the paired moraines 

of the Satanta Peak advance of Benedict (1973a, 1985) in the Arapahoe Pass area of the 

Front Range, about 50 km south of the quadrangle. Deposits of the Satanta Peak advance 

have minimum-limiting radiocarbon (14C) ages of 9,915±380 years before present (before 

A.D. 1950 [B.P.]) and 9,700±215 yr B.P. (Benedict, 1973a), and probably date from 

12‒10 ka B.P. (Benedict, 1985). These deposits probably are coeval with glacial deposits 

formed during the Younger Dryas climatic event (11,000–10,000 14C yr B.P.) in northwestern North America and northern Europe (Menounos and Reasoner, 1997). Surface 

soils formed in deposits of the Satanta Peak advance have either thin (1525 cm) cambic 

(Bw) or argillic (Bt) horizons that contain 5–9 percent clay in the <2 mm-sized fraction 

(Birkeland and others, 1987). Unit locally may include till of Pinedale age (Qtp), talus 
54 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

deposits (Qta), and younger tills of late Holocene age. Deposits of unit Qtpp that are too 

small to map separately, such as those near Iceberg Lake, are combined with and mapped 

as till of Pinedale age (Qtp). Estimated thickness is 1.5–15 m

Qtp Till of Pinedale age (late Pleistocene)—Mostly non-sorted and non-stratified, subangular 

to subrounded boulders to granules in a silty sand to sandy silt matrix. Material <2 mm 

in diameter is estimated to be 20–50 percent of the unit. This matrix consists chiefly 

of poorly sorted sand and a minor to moderate amount of silt and clay. The unit locally 

may contain a significant amount of silt down valley of the Chambers Lake area where 

glaciers have eroded small, unmapped deposits that consist in part of weakly consolidated siltstone and sandstone that probably are of Oligocene age (Izett, 1975; Braddock 

and Cole, 1990). End moraines near the down-valley limit of glaciation are probably 

composed, in part, of outwash (Madole and others, 1998). Unit Qtp commonly forms 

large prominent, sharp-crested lateral and end moraines that are very bouldery and have 

distinct constructional morphology. Deposits locally have unfilled and undrained kettles, 

moraine-dammed lakes, and swamps (Braddock and Cole, 1990; Madole and others, 

1998). Surface soils formed in till of Pinedale age (Qtp) have either thin (10–40 cm) 

cambic (Bw) or weakly developed argillic (Btj) horizons or they lack B horizons and 

have A/Cox profiles. Soil B horizons formed in till of Pinedale age in the Front Range, 

south of the map area, commonly contain 7–8 percent clay in the <2 mm-sized fraction 

(Shroba, 1977). Most of the biotite-rich granitic and gneissic clasts within the soil are 

unweathered and disintegrated clasts are rare. Unit locally includes deposits of stratified 

drift deposited by glacial meltwater, till of Bull Lake age, colluvium (Qc), other massmovement deposits, and valley-floor alluvium (Qva); unit locally may include till of 

pre-Bull Lake age. As mapped in the Laramie River valley, unit Qtp includes deposits 

of younger and older till. The younger till forms terminal and lateral moraines south of 

Stubb Creek, 3 km southeast of Glendevey. The top of the outwash gravel graded to the 

younger till is about 3–6 m above the Laramie River. Deposits of older till form remnants of lateral moraines as much as 2.4 km down valley of the maximum down-valley 

extent of the younger till near Glendevey. The top of the outwash gravel graded to the 

older till is about 40 m above the Laramie River just downstream of the older till, but 

decreases in height to about 12–15 m above the Laramie River 8–19 km downstream of 

the older till. Radiocarbon and cosmogenic-exposure ages indicate that till of unit Qtp is 

about 12–30 ka (Nelson and others, 1979; Madole, 1986; Schildgen and Dethier, 2000; 

Benson and others, 2004, 2005, 2007). Much of the till of unit Qtp within the map area 

probably is similar in age. The older till of unit Qtp near Glendevey may be much older, 

because the higher outwash gravel graded to the older till (1) is substantially higher than 

the lower outwash gravel graded to the younger till and (2) the higher outwash lacks well 

preserved bar-and-channel topography characteristic of the lower outwash graded to the 

younger till. The older till near Glendevey may be correlative with an early advance of 

Pinedale ice about 34–47 ka (Sturchio and others, 1994), or it may be early Wisconsin 

in age (about 55–70 ka; Lisiecki and Raymo, 2005). During the Pinedale glaciation, 

the largest ice mass in the quadrangle formed in the headwaters of the Cache la Poudre 

River, Joe Wright Creek, and the Laramie River (Madole and others, 1998). The ice 

mass was as much as 600 m thick and flowed 45 km down the valley of the Cache la 

Poudre River to an altitude of about 2,325 m (Madole and others, 1998) and 23 km down 

the Laramie River valley to an altitude of about 2,540 m. Glaciers on the west flank of 

the Medicine Bow Mountains flowed about 2–7 km down-valley to altitudes of about 

2,720–2,915 m. Estimated thickness is 1.5–30 m

Qtb Till of Bull Lake age (late and middle Pleistocene)—Mostly non-sorted and non-stratified, 

subangular to subrounded boulders to granules in a silty sand matrix. Material <2 mm in 

diameter estimated to be 20–40 percent of the unit. Unit Qtb commonly forms prominent 

lateral moraines that have rounded crests beyond the down-valley limit of till of Pinedale 

age (Qtp) in the Laramie River valley near Glendevey and south of Kelley Creek on the 

west flank of Medicine Bow Mountains. Undrained depressions are uncommon (Madole 

and others, 1998). Surface boulders typically are less abundant on moraines of the Bull 

Lake glaciation than on those of the Pinedale glaciation. Surface soils formed in till of 
Description of Map Units 55

Bull Lake age (Qtb) have clay-enriched argillic (Bt) horizons that are thicker and contain 

more clay than those formed in till of Pinedale age (Qtp). Soil B horizons formed in till 

of Bull Lake age in the Front Range, south of the map area, commonly are 50 cm thick 

and contain 11–19 percent clay in the <2 mm-sized fraction (Shroba, 1977). Many of the 

biotite-rich granitic and gneissic pebbles and cobbles within the soil are weathered and 

are partly to completely disintegrated. 230Th/U analyses that constrain the ages of glaciofluvial deposits near the type area for the Bull Lake glaciation along the north flank of 

the Wind River Range, Wyo., and ages of glacial deposits near West Yellowstone indicate 

that the Bull Lake glaciation probably began prior to 167±6.4 ka (possibly 190 ka) and 

may have continued until about 122±10 ka (Sharp and others, 2003; Pierce, 2004). 10Be 

and 26Al analyses of surface boulders on moraines composed of till of Bull Lake age 

(Qtb) near Nederland, Colo., yielded minimum age estimates of 101±21 and 122±26 ka 

(Schildgen and others, 2002). These age estimates are in accord with a uranium-trend 

age estimate of 130±40 ka for till of Bull Lake age (Shroba and others, 1983) near Allens 

Park, Colo. Unit locally includes deposits of stratified drift deposited by glacial meltwater, till of Pinedale age (Qtp), colluvium (Qc), and other mass-movement deposits; near 

the down-valley limit of glaciation, unit locally may include till of pre-Bull Lake age that 

lacks depositional morphology. Ice during the Bull Lake glaciation flowed 24 km down 

the Laramie River valley to an altitude of about 2,530 m and 5 km down Kelley Creek 

valley to an altitude of about 2,780 m. The extent of ice during the Bull Lake glaciation 

in the valley of the Cache la Poudre River is unknown, but was less than that during the 

Pinedale glaciation. Estimated thickness is 1.5–15 m

ORGANIC-RICH DEPOSIT

Qo Organic-rich sediment (Holocene and latest Pleistocene)—Bog, fen, and marsh deposits 

associated with high or fluctuating water tables and frequent or continuous saturation 

in formerly glaciated terrain underlain by till of Pinedale age (Qtp) near the southwest 

corner of the quadrangle. Plant remains in these deposits are partly to completely decomposed. Unit Qo locally includes muck, peat, and organic-rich mud as well as layers of 

mineral-rich sediment. Peat and organic-rich mud locally overlie mineral-rich lacustrine 

or pond deposits. Organic-rich deposits are prone to significant compression under load 

owing to the very low density of organic material and high water content. Fen deposits 

in the Laramie River valley southwest of Boston Peak were studied during the 1980s as 

an example of sediment-hosted uranium deposits related to biological, geochemical, and 

hydrologic controls on the deposition of uranium from uranium-enriched ground waters 

(Schumann, 1990; Zielinski, 1990). Estimated thickness is 1–4.5 m

EOLIAN DEPOSIT

Qlo Loess (late and late middle? Pleistocene)—Non-stratified, well-sorted, wind-deposited, 

plastic to very plastic, sandy silt and locally sandy, clayey silt derived by wind erosion 

from floodplains and possibly Cretaceous bedrock sources by northwesterly or westerly paleowinds. Loess overlies deposits as young as Broadway Alluvium (Qb) near 

the northeast corner of the quadrangle. Deposits of sandy silt, too small to show at map 

scale, locally mantles Slocum Alluvium (Qs) on the west side of Boxelder Creek near 

Buckeye. Thin (≤50 cm) layers of pebbly, clayey, sandy silt that locally mantle other 

deposits of Slocum Alluvium (Qs) and Verdos Alluvium (Qv) in the piedmont east 

of the foothills hogback belt may consist in part of loess that that was mixed with the 

underlying gravelly alluvium by biotic processes. Deposits of unit Qlo locally reworked 

by unconfined overland flow contain a minor amount of coarse sand and granules, and 

locally a few pebbles. Unit Qlo locally may contain small deposits of silty eolian sand. 

Loess in northeastern Colorado records two episodes of deposition at about 20–14 ka and 

13–10 ka (Muhs and others, 1999). Holocene loess is locally extensive in the eastern part 

of Colorado (Madole, 1995), but none has been recognized within or near the quadrangle. Unit Qlo locally overlies and locally may include deposits of older loess. These 
56 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

deposits of older loess are correlative with the older loess mapped by Scott (1960, 1963) 

in the southern part of the Denver area. The deposits of older loess within the quadrangle 

and those mapped by Scott may be as old as 120–170 ka. They may be correlative with 

eolian silt and sand about 65 km southeast of the Fort Collins quadrangle that yielded 

thermoluminescence age estimates of about 150 ka (Forman and others, 1995). Some of 

the closed depressions on till of Pinedale age (Qtp) may locally contain thin (≤50 cm) 

deposits of loess or silty sheetwash alluvium (Qsw) derived chiefly from loess. Thin silty 

sheetwash deposits (Qsw) may locally overlie unit Qlo. West of Boxelder Creek near 

Fort Collins and Wellington, unit Qlo has a few linear dune-like features. They are as 

much as 10 m high, commonly trend N.52°W., and are composed of sandy silt. Estimated 

thickness is 3–11 m

POST-LARAMIDE SEDIMENTARY ROCKS

No Ogallala Formation (lowermost Pliocene? to middle Miocene)—Reddish-brown to 

brown, poorly sorted, medium- to coarse-grained arkosic sandstone and conglomerate 

rich in granite clasts, minor siltstone, and rare calcrete or limestone beds preserved 

east of the Front Range near the northeastern corner of the quadrangle. Clasts include 

rounded pebbles to boulders, as large as 1 m in diameter, of flow-banded, slightly porphyritic rhyolite (Moore, 1959), which was probably eroded from rhyolite flows in the 

headwaters of the Cache la Poudre River and transported by large streams northeastward into extreme northern Colorado and across southeastern Wyoming into Nebraska 

(Swinehart and others, 1985). Northeast of the quadrangle in eastern Wyoming, 

the conglomerate-rich facies of the Ogallala grades eastward into beds of siltstone, 

sandstone, volcanic ash, and limestone (Flanagan and Montagne, 1993). The Ogallala 

disconformably overlies the Arikaree(?) Formation (Na?) and Brule Formation of 

the White River Group (:wr) and its depositional top is not exposed (Moore, 1959; 

Courtwright and Braddock, 1989). Possible Pliocene age is based on fission-track 

age from zircon (4.6±1.0 Ma) for ash bed near the preserved top of the Ogallala in 

Nebraska; oldest included ash beds are about 17 Ma (Izett, 1975). Unit is combined 

with Arikaree Formation (Na?) on cross sections and designated as No/Na?. Thickness of Ogallala Formation is greater than 90 m and is locally as much as 140 m

Na? Arikaree(?) Formation (Miocene)—Light-brown or very pale brown massive siltstone; 

silty, very fine grained sandstone; and minor arkosic gravel. Slightly cobbly, granite-rich 

pebble gravel as much as 12 m thick is locally present at the base. Preserved underneath Ogallala Formation (No) near the northeast corner of the quadrangle. Correlation 

uncertain due to lack of outcrop continuity with Miocene rocks farther east (Courtwright 

and Braddock, 1989); however, a fragment of an oredont jaw bone considered to be 

of Miocene age was collected just east of the quadrangle (sec. 26, T. 12 N., R. 68 W.; 

Moore, 1959). Northeast of the Fort Collins quadrangle in eastern Wyoming, the 

Arikaree Formation consists of siltstone, fine-grained sandstone, limestone, and volcanic 

ash (Flanagan and Montagne, 1993). Thickness ranges from 0 to 79 m

N:ls Landslide debris deposits (lower Miocene or Oligocene)—Irregular, slope-mantling deposits consisting of massive blocks of sandstone and fragmented shale of the Dakota Group 

(Kd) that were formed by slope collapse on the margins of extensive, tabular, largely 

intact slide blocks of the Dakota Group that overlie the Oligocene and Eocene White 

River Group (:wr) (Braddock and Eicher, 1962; Braddock, 1978); landslide debris 

deposits are unconformably overlain by Pliocene(?) and Miocene Ogallala Formation 

(No) (Courtwright and Braddock, 1989). Maximum preserved thickness is about 36.5 m

N:n North Park Formation (Miocene and upper Oligocene?)—Light-tan to white, calcareous 

sandstone and conglomerate, with lesser amounts of white, light-gray, and pink calcareous siltstone. Camp (1979) described five distinct lithologies. They are, in approximate 

descending order: (1) pink, fine-grained calcareous sandstone and siltstone composed 

of poorly sorted quartz and feldspar cemented by calcite; (2) interbedded buff to white, 

coarse-grained, locally trough cross-bedded, calcareous sandstone and conglomerate 

containing pebbles to small boulders composed of pink to lavender, flow-banded rhyolite 
Description of Map Units 57

and porphyritic volcanic rocks (intermediate composition) and, locally, Proterozoic 

granite and gneiss in a medium- to coarse-grained sandstone matrix cemented by coarsegrained calcite; (3) white, very fine-grained, calcareous, tuffaceous sandstone containing fine-grained quartz, feldspar, and as much as 15 percent glass shards; (4) white 

calcareous sandy mudstone containing poorly sorted fine- to medium-grained quartz, 

chert, and biotite; and (5) a basal cherty limestone, which we reassign to the basal unit 

of the Permian and Pennsylvanian Fountain Formation (P*f) based on correlation with 

exposures east of Bull Mountain. Exposures are generally small and discontinuous, so 

exact relationship of these lithologies is unclear, although the lower units appear to be 

composed of finer-grained sediments that interfinger up-section with the coarser-grained 

units (Camp, 1979, p. 64). No fossils have been recovered from the Laramie River valley. 

The unit is correlated by Camp (1979) with the North Park Formation (N:n) to the 

west (Hail and Lewis, 1960; Izett, 1975) of middle and late Miocene age, with which 

we agree, although Beckwith (1942) and Knight (1953) correlated deposits surrounding 

North Middle Mountain with Miocene and Oligocene rocks in Wyoming and Colorado. 

Without better age control, we cannot rule out an upper Oligocene age. Unit may contain 

some undifferentiated Pliocene gravel deposits. Contains clasts of rhyolite tuff (:r) 

dated at 27.7±0.05 Ma (40Ar/39Ar age for sanidine; Knox, 2005). Total thickness probably 

as much as 200 m

N:gy Younger high-level fluvial gravel (Miocene? and Oligocene)—Unsorted unstratified 

deposits of boulders (as large as 1.25 m), cobbles, and pebbles in a dominantly coarse 

sandy matrix with some silt; numerous clasts derived from underlying rhyolite tuff 

(:r) dated at 27.7±0.05 Ma (40Ar/39Ar age for sanidine; Knox, 2005). Madole (1982) 

concluded similar deposits to the south of the map area formed in ancient valley 

systems that were subsequently disrupted by Neogene uplift of the Front Range. These 

high-level gravel deposits mark the trace of paleo-valleys of the early North Fork Cache 

la Poudre River from the Green Ridge area north and east through Prairie Divide to 

Livermore. Similar gravel deposits mark the courses of the early Michigan River west 

of Cameron Pass (Braddock and Cole, 1990; Kellogg, Ruleman and others, 2008), 

the main Cache la Poudre River east of the Big Narrows area, and the early Buckhorn 

Creek drainage in the south-central area of the quadrangle. Generally poorly exposed 

with lag gravel and younger colluvial cover that obscures deposit and basal contact. 

Western exposures may correlate with parts of the North Park Formation (N:n) in 

Laramie River valley or lateral equivalent. Deposits west of Livermore on eastern edge 

of Front Range may contain significant amounts of younger undifferentiated gravel 

as young as Pliocene where deposits grade slightly above Quaternary stream terrace 

deposits. Unit thickness is highly variable and difficult to estimate due to inferred relief 

on basal contact (Cole and others, 2008). Thickest exposure is above Cache la Poudre 

River on Green Mountain where at least 100 m thick (Abbott, 1976) but may exceed 

200 m above axis of paleovalley

:wr White River Group, undivided (lower Oligocene and upper Eocene)—Mapped in the 

northeastern corner of the map area where unit unconformably overlies deformed and 

eroded Upper Cretaceous Fox Hills Sandstone (Kfh) and Pierre Shale. Total minimum 

thickness approximately 76 m. Includes:

Lower Oligocene Brule Formation of upper part of White River Group—Consists 

of white, olive-gray, or pink, massive, clayey siltstone with thin beds of arkose, pebble 

gravel, silicic volcanic ash, and minor expansive claystone. Siltstone in the Brule 

Formation locally contains abundant calcareous concretions.

Upper Eocene Chadron Formation of lower part of White River Group—Underlies the 

Brule, and is red, purplish-red, or gray siltstone and minor sandstone interbedded with 

silicic volcanic ash and very poorly sorted fluvial gravel and conglomerate containing 

cobbles of Proterozoic crystalline rock and boulders of Paleozoic sandstone (as large as 

60×65×120 cm3

), and minor very coarse arkose.). Gravel and conglomerate are more 

abundant in the lower part of the Chadron Formation than in the upper part. Volcanic ash 

beds in the White River Group range in age from about 35.5 Ma to 30 Ma (Prothero and 

Swisher, 1992; Larson and Evanoff, 1998).
58 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

:s Sedimentary rocks, undivided (Oligocene and Eocene?)—Small exposures of sedimentary 

rocks that interfinger with or underlie Oligocene volcanic deposits in Pearson Park and 

just northwest of Long Draw Reservoir in southern part of quadrangle. Also includes 

small exposures of sediments surrounding Chambers Lake. Include blocky-weathering, 

orange-pink, tan, or light-gray tuffaceous mudstone, conglomeratic sandstone containing pebbles and cobbles of Oligocene volcanic rocks from the Braddock Peak volcanic 

complex, and fine-grained vitric tuff. Tentatively correlated with The White River Group 

(:wr) exposed east of the Front Range. Similar deposits exposed below the North 

Park Formation (N:n) west of the map area in the Laramie River valley produced an 40Ar/39Ar age from biotite of 36.0±0.3 Ma (Shroba, 2016). Thickness not known

:bs Tuffaceous sandstone and basalt (Oligocene)—Massive, very poorly indurated, finegrained, well-sorted, pale tan tuffaceous sandstone containing quartz, feldspar, glassy 

shards, and a few percent lithic grains. Sandstone overlies dark gray-brown, breccia 

of scoriaceous basalt that contains about 8 percent plagioclase and 15 percent olivine 

phenocrysts (both 1 to 2 mm); scoria vesicles partially filled with pale yellow zeolite(?). 

Deposit only exposed on the east slope of Laramie River valley about 1 km north 

of Chambers Lake; sandstone is probably equivalent to upper part of Oligocene and 

Eocene? Sedimentary rocks (:s) and basalt is probably equivalent to Oligocene basalt 

(:b) mapped farther south in the headwaters of the Cache la Poudre River (Braddock 

and Cole, 1990; Knox, 2005). Sandstone is about 30.5 m thick (top not exposed)

:go Older high-level fluvial gravel (Oligocene)—Unsorted, unstratified deposits of cobbles (as 

large as 38 cm) and pebbles in sandy matrix that appear to underlie rhyolite tuff (:r) 

dated at 27.73±0.05 Ma (40Ar/39Ar age for sanidine; Knox, 2005). Contains porphyritic 

clasts of dacitic and andesitic(?) volcanic material of uncertain age, as well as more 

common clasts of durable Proterozoic igneous and metamorphic rocks. Deposit only 

mapped in the Green Ridge area near Nunn Creek Basin where relationship with 

rhyolite tuff is inferred. Thickness not known

POST-LARAMIDE VOLCANIC ROCKS

:r Rhyolite tuff (upper Oligocene)—Combined unit consisting of two ash-flow units that 

were erupted coeval with emplacement of the Mount Cumulus stock in the Never 

Summer Mountains. Well exposed to the north and east of Iron Mountain. Isolated 

exposures located on Green Ridge near Nunn Creek suggest that unit underlies 

younger paleochannel gravel deposits (N:gy) and overlies older gravel deposits 

(:go). Lower ash flow is light-gray and brownish-gray with conspicuous phenocrysts 

of smoky-brown quartz, sanidine, and minor biotite in a devitrified matrix. Knox 

(2005) determined an 40Ar/39Ar age for sanidine of 27.73±0.05 Ma from exposures on 

Green Ridge. The younger tuff is gray, stony, massive, and contains phenocrysts of 

sanidine (4‒5 percent), plagioclase (4‒5 percent), biotite (3‒5 percent), and about one 

percent of hornblende and pyroxene, but no quartz. Combined thickness of both units 

at least 152 m at Bald Mountain (northeast of Cameron Pass)

:rw 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 (OʹNeill, 1981; 

Cole and Braddock, 2009). Unit represents welded zone of rhyolite ash-flow tuff (:r; 

27.73±0.05 Ma from 40Ar/39Ar on sanidine; Knox, 2005) in vicinity of Iron Mountain

:v Volcanic rocks, undivided (Oligocene)—Undifferentiated rhyolite, dacite, trachyte, 

and andesite flows west of Cameron Pass near Gould Mountain. Porphyritic rhyolite 

resembles upper flow unit of Iron Mountain rhyolite tuff (:r); lacks quartz phenocrysts. 

Knox (2005) determined 40Ar/39Ar ages for sanidine of 28.46±0.08 Ma from the rhyolite 

and 28.52±0.02 Ma from the thick dacite porphyry flow that caps Gould Mountain. 

Several undifferentiated flows of dacite, trachyte, and andesite underlie the main dacite 

flow north of Gould Mountain which are probably of similar age to the trachybasalt 

flows (:b) east of Cameron Pass. Thickness of the younger rhyolite unit just south of 

the map area is about 366 m, possibly thickened by rheomorphic folding (O’Neill, 1981; 
Description of Map Units 59

Kellogg, Ruleman and others, 2008). Thickness of main dacite flow is unknown. At 

least 100 m thick section of flows below main dacite north of Gould Mountain

:b Basalt (Oligocene)—Black, aphanitic, massive, dense trachybasalt flows that weather dark 

brown. Rock consists of “small interlocking labradorite laths, slightly larger sanidine 

crystals, considerable interstitial augite, and a small amount of glass” (Gorton, 1953, 

p. 91) with small phenocrysts of feldspar, hornblende, and augite (Corbett, 1968). 

Exposed along Highway 14 near Joe Wright Reservoir and on the flanks of Iron 

Mountain as high as about 3,400 m. Described as trachyandesite (Pearson and others, 

1982; Corbett, 1968), but chemical analysis indicates basaltic composition (Kellogg, 

Ruleman and others, 2008). 40Ar/39Ar whole-rock age is 29.58±0.22 Ma from exposure 

near Joe Wright Reservoir and 28.52±0.15 Ma from exposure on west side of Iron 

Mountain (Knox, 2005). Top and bottom not exposed, and orientation uncertain due 

to lack of internal flow layering; thickness unknown. Includes small area of andesite 

welded tuff just northeast of Flat Top Mountain which occupies similar stratigraphic 

position below Iron Mountain rhyolite flows

POST-LARAMIDE INTRUSIVE ROCKS

Located primarily in the Braddock Peak volcanic complex near Cameron Pass (southwestern corner of 

the quadrangle) with minor occurrences in the Manhattan mining district north of Rustic (Cache la Poudre 

River canyon)

:rp Rhyolite porphyry and felsite (Oligocene)—Dikes and small plugs of white, tan, or lightreddish-brown porphyritic rock containing variable amounts of phenocrysts (quartz, 

sanidine, oligoclase, biotite and brown hornblende) in an aphanitic matrix. Small stock 

in the Manhattan district is leucocratic, sparsely porphyritic, flow-banded, and internally 

brecciated; zircon fission-track ages are 33‒37 Ma (McCallum and Naeser, 1977; 

Shaver and others, 1988)

:iy Younger intrusive rocks, undivided (Oligocene)—Small dikes or plugs of various colors 

and compositions, typically altered to the extent that phenocryst minerals not identifiable. 

Includes felsic rocks of late Oligocene age in the Cameron Pass area

LARAMIDE INTRUSIVE ROCKS

Located primarily in the Manhattan mining district north of Rustic (Cache la Poudre River canyon)

:dp Dacite porphyry (Eocene)—Dikes and small plugs of gray, dense, porphyritic rock containing common phenocrysts of hornblende and plagioclase, plagioclase and biotite, or biotite 

alone, with sparse resorbed quartz (Shaver and others, 1988) in the Manhattan district. 

Zircon fission-track ages range between about 53 and 60 Ma (McCallum and Naeser, 1977)

:ap Andesite porphyry (Eocene)—Dikes of gray, porphyritic rock with abundant hornblende 

phenocrysts in a very fine grained matrix of feldspar, hornblende, and minor quartz 

(Shaver and others, 1988) in the Manhattan district. Zircon fission-track ages range 

between about 53 and 55 Ma (McCallum and Naeser, 1977)

:io Older intrusive rocks, undivided (Eocene)—Small dikes or plugs of various colors and 

compositions, typically altered to the point that phenocryst minerals not identifiable. 

Includes felsic and intermediate rocks of Eocene age in the Manhattan district

LARAMIDE SEDIMENTARY ROCK

:c Coalmont Formation (Eocene and Paleocene)—Fluvial sedimentary unit preserved on 

the margin of the North Park Basin (west of Cameron Pass) where it lies on Upper and 

Lower Cretaceous Benton Group (Kb). Gray, brownish-gray, and brown, fine- to coarsegrained arkose, dark-gray carbonaceous mudstone, and lenses of coarse conglomerate 

with clasts of Proterozoic rocks as large as 1.5 m and clasts of intermediate volcanic 

rocks; thickness exceeds 518 m here (OʹNeill, 1981; Braddock and Cole, 1990; Kellogg, 

Ruleman and others, 2008), but exceeds 2,740 m in North Park west of this quadrangle 

(Hail, 1968); top eroded everywhere
60 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

PRE-CENOZOIC SEDIMENTARY ROCKS

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 strand-line and delta-front environments during 

withdrawal of the Late Cretaceous Western Interior Seaway (Roberts, 2005); consists 

of several overlapping tabular sand bodies. Preserved beneath Oligocene and Eocene 

White River Group (:wr ) deposits near the northeast corner of the quadrangle. 

Thickness variable, but locally as much as 24 m

Kp Pierre Shale, undivided (Upper Cretaceous)—Thick, monotonous black shale in the 

Denver Basin, east of the Front Range. Subunits are well described by Scott and 

Cobban (1965; 1986); total thickness about 2,297 m. Unit Kp in cross section D–D′

only. Subdivided for this map into five units

Kpu Upper shale member—Friable sandstone and soft shaly sandstone (upper transition 

member) and gray concretionary silty shale (unnamed upper shale member); thickness 

about 1,012 m

Kpr Richard Sandstone Member, Larimer Sandstone Member, Rocky Ridge Sandstone 

Member and two intervening unnamed shale members—Light-brown or brownishyellow, micaceous or glauconitic sandstone interlayered with gray shale; thickness 

about 160 m

Kpm Middle shale member—Claystone and sandy siltstone; light-gray or yellowish-gray 

calcareous sandstone (Terry Sandstone Member) poorly exposed near center of unit; 

thickness about 259 m

Kph Hygiene Sandstone Member—An upper hard, glauconitic, ridge-forming sandstone 

separated from a lower friable sandstone by a shale; thickness about 74.5 m

Kpl Lower shale member—Mostly dark olive-gray bentonitic shale (Mitten Member and local 

Sharon Springs Member and unnamed lower shale member); thickness about 792.5 m

Kmv Mesaverde Formation (Upper Cretaceous)—Transitional marine and nonmarine beds 

in the Laramie Basin west of the Front Range; dark-gray, calcareous marine shale and 

siltstone interbedded with ledge-forming marine sandstone with ironstone concretions 

(Beckwith, 1942; Camp, 1979). Contains local carbonaceous beds, but no commercially 

exploited coal deposits. Conformably overlies Steele Shale (Kss); base is defined by the 

lowest prominent marine sandstone ledge above the Steele Shale (Kss). Total thickness 

about 610 m (top eroded) in this area

Kss Steele Shale (Upper Cretaceous)—Thick, monotonous, black to dark-gray, marine shale 

and mudstone in the southern Laramie Basin west of the Front Range; conformably 

overlies the Niobrara Formation (Kn); correlates with the lower and middle parts of the 

Pierre Shale. Steele contains zones of thin bedded sandstone about 305 m above the base 

(unnamed) and 518–580 m above the base (Shannon Sandstone Member; Beckwith, 

1942). A white sandstone near the top of the preserved section contains Baculites reesidei 

Elias ammonites, indicating equivalence to the upper middle sandy unit of the Pierre 

Shale (Scott and Cobban, 1986). Total thickness about 731.5 m

Kn Niobrara Formation (Upper Cretaceous)—Gray, calcareous marine shale and biomicrite 

beds; thinly laminated; typically weathers to light-gray or buff near top; commonly contains oyster fossils (Inoceramus sp.); conformably overlies Benton Group (Kbm). East of 

Front Range, Niobrara consists of Smoky Hill Shale Member in upper part and the basal, 

thin Fort Hays Limestone Member (extensively quarried for cement manufacture; Scott 

and Cobban, 1986). West of Front Range, Niobrara consists of the upper chalky member, 

upper shale member, lower chalky member, and the basal Sage Breaks Shale Member 

(Cobban and Reeside, 1952; Camp, 1979). Thickness of combined units varies from about 

106.7 m (southeast) to 88.4 m (northeast) to 140.2 m (northwest) across the quadrangle

Kbm Benton Group (Upper and Lower Cretaceous)—Includes several marine shale, calcareous 

shale, bentonite, and limestone units that lie conformably above the Dakota Group (Kd) 

and conformably below the Niobrara Formation (Kn); exposure is typically poor. The 

Benton Group is locally subdivided east of the Front Range (from top to bottom) into 

the olive-gray, silty-sandy Carlile Shale, dark-gray to olive-gray Greenhorn Limestone, 
Description of Map Units 61

dark-gray Graneros Shale, and underlying Mowry Shale (Scott and Cobban, 1986). West 

of the Front Range, the Benton Group has been subdivided into (from top to bottom) 

the Wall Creek Sandstone Member and the shaly member of the Frontier Formation and 

the underlying Mowry Shale (Shaw, 1957; Camp, 1979). Mowry Shale is wavy-bedded, 

rusty, siliceous shale that contains fish scales, weathers light gray, and is locally absent 

east of the Front Range. Thickness of combined units varies from about 147.8 m (southeast) to 173.7 m (northeast) to 115.8 m (northwest) across the quadrangle

Kd Dakota Group (Lower Cretaceous)—Light-gray to brown-gray, fine-grained, ripple-marked 

sandstone and interbedded gray carbonaceous shale in upper part, and chert-pebble 

conglomerate and sandstone in lower part; base is major regional unconformity. In 

eastern foothill belt, the Dakota consists of South Platte Formation in upper part and Lytle 

Formation in lower part. Sandstones are strongly cemented with silica and form prominent 

ridges; blocks of weathered Dakota sandstone litter slopes surrounding outcrops. The 

Dakota was deposited in fluvial and swamp environments during a major marine 

transgression. Thickness ranges from about 88.4 m (southeast) to 74.7 m (northeast) to 

76.2 m (northwest) to 75 m (southwest) across the quadrangle

Jm Morrison Formation (Upper Jurassic)—Green, red, yellow, and white, blocky-weathering 

claystone and siltstone, gray micrite, and gray, fine- to medium-grained, cross-bedded 

sandstone. Morrison Formation deposited by low gradient rivers in floodplains and in 

freshwater lakes and swamps. Thickness varies from about 97.5 m along the eastern 

foothills belt to 85.3 m (northwest) to 90 m (southwest) across the quadrangle

Jms Morrison Formation (Upper Jurassic) and Sundance Formation (Upper and Middle 

Jurassic), undivided—Mapped in the southwest corner of the map area where unit 

is predominantly Morrison Formation (Jm), but poor exposure along the flanks of the 

Medicine Bow Mountains makes it difficult to differentiate contact between Morrison 

and underlying Sundance Formations. Base of map unit locally includes about 3 m 

of light-orange, medium-grained, cross-bedded sandstone that may be equivalent to 

southern trailing edge of Upper Triassic Jelm Formation (Kellogg, Ruleman and others, 

2008). Thickness of combined units about 134 m

J^msj Morrison (Upper Jurassic), Sundance (Upper and Middle Jurassic), and Jelm (Upper 

Triassic) Formations, undivided—Shown on cross sections only. Includes all of 

mapped units Jm and J^sj

J^sj Sundance (Upper and Middle Jurassic) and Jelm (Upper Triassic) Formations, 

undivided—The Sundance Formation, in upper part of unit, dominantly consists of 

fine- to very fine-grained, tabular-bedded, variably colored sandstone that weathers 

yellowish tan; base is a regional unconformity. Sundance Formation deposited in 

shallow, near-shore marine environments (Pipiringos and OʹSullivan, 1976). The 

underlying Jelm Formation (only the Red Draw Member is present) consists of 

orange-pink or reddish-brown, fine-grained, cross-bedded, calcareous sandstone and 

local red-brown shale and limestone-pebble conglomerate; pinches out southwestward 

near Cameron Pass (Kellogg, Ruleman and others, 2008), and southeastward in the 

foothills hogback belt about 48 km south of this quadrangle. Thickness of combined 

units varies from 42.7 m (southeast) to 56.4 (northeast) to 118.9 m (northwest) across 

the quadrangle

^Pl Lykins Formation (Lower Triassic and Upper Permian)—Thick sequence of dominantly 

red and red-brown siltstone and fine-grained sandstone that forms a valley or subdued 

slope between the Lyons Sandstone and the Jelm Formation on the east side of the 

Front Range. Persistent limestone sequences present at several stratigraphic levels, 

including the Forelle Limestone Member (Upper Permian) in the lower third of the 

Lykins; gypsum beds are conspicuous in the lowermost Lykins and are locally quarried in the Table Mountain quadrangle (Broin, 1957; Courtright and Braddock, 1989). 

Unit generally correlates with, in descending order, the Red Peak Formation and upper 

part of Goose Egg Formation (^Prgu) and Forelle Limestone Member and uppermost 

shale (Glendo Shale of Burk and Thomas, 1956) of the Satanka Shale Member of the 

Goose Egg Formation (Pgfsu) west of the Front Range. Thickness varies from 183 m 

(southeast) to 152 m (northeast)
62 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Pl Lyons Sandstone (Lower Permian)—Unit exposed east of the Front Range. Moderate 

orange, pink, or pinkish-gray, fine- to medium-grained, well-sorted, cross-stratified, 

eolian quartz sandstone (firmly cemented with silica and local calcite), and tan, gray, 

and red laminated siltstone. Lyons Sandstone deposited in widespread eolian dune complexes that graded northward to coastal sabkhas and mudflats (Walker and Harms, 1976; 

Berman, 1979); unconformably overlies the Owl Canyon Formation (Po). Unit not 

mapped west of the Front Range in this quadrangle, but Maughan and Wilson (1963) 

suggest that the unnamed sandstone at the top of the lower part of the Satanka Shale 

Member of Goose Egg Formation (Pgsl) is equivalent to the Lyons Sandstone. Unit 

thins from 15 m in the southeast corner of the map to 1–9 m north of Bellvue, Colo., 

where the unit is generally mapped as a linear ‘key bed’ between the overlying Lykins 

Formation (^Pl) and underlying Owl Canyon Formation (Po)

Pu Lyons Sandstone and Owl Canyon and Ingleside Formations, undivided (Lower 

Permian)—Mapped only to the south of the Fletcher Hill fault along Buckhorn Creek 

on the southern edge of the quadrangle, where the Permian section is folded near vertical and attenuated against the fault

Po Owl Canyon Formation (Lower Permian)—Red siltstone and fine-grained, red, ripplelaminated sandstone; interbedded locally with the Ingleside Formation below and with 

the Lyons Sandstone above; not mapped west of the Front Range. Owl Canyon was 

deposited in tidal flats and estuaries and possibly in large desert lake environments 

(Pearson, 1976; Berman, 1979). Unit generally correlates with the base of the Satanka 

Shale Member of Goose Egg Formation (lower part of unit Pgsl) west of the Front 

Range; thickness varies from 70 m (southeast) to 84 m (northeast)

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 

cross-bedded; limestones present at several stratigraphic levels; exposed only east of the 

Front Range. Ingleside Formation was deposited in coastal sabkhas and shallow marine 

lagoons (Berman, 1979). Limestones are unusually thick east of Livermore where they 

have been extensively quarried for cement production. Thickness varies from 46 m 

(southeast) to 67 m (northeast); mapped as a linear ‘key bed’ between overlying Owl 

Canyon Formation (Po) and underlying Fountain Formation (P*f) south of Fletcher 

Hill and Redstone Creek faults where unit is folded near vertical and attenuated against 

the fault

^Prgu Red Peak Formation (Lower Triassic) and upper part of Goose Egg Formation 

(Lower Triassic and Upper Permian), undivided—Unit exposed west of the Front 

Range. Red to red-brown, planar laminated, ripple-marked, cross-bedded siltstone and 

micaceous shale interbedded with gypsum near base. Thin carbonate bed defines top 

of Goose Egg Formation, but entire unit is poorly exposed and generally underlies 

valleys, so formational contact is not mapped. Unit was deposited in shallow-water, 

hypersaline marine environments. Mapped in two small areas near the southwestern 

corner of quadrangle west of Cameron Pass includes 1–2 m of light gray, laminated 

algal limestone near the base (Forelle? Limestone Member of Goose Egg Formation) 

as well as a few meters of underlying red sandy shale (Satanka? Shale Member of 

Goose Egg Formation) which may correlate to the top of the lower Goose Egg Formation (Pgfsu) (Ward, 1957; Braddock and Cole, 1990). Map unit rests unconformably 

on Proterozoic basement in southwestern part of quadrangle (Berman, 1979) and 

conformably above lower Goose Egg Formation in northwestern part of quadrangle 

(Pgfsu). Unit ^Prgu generally correlates with the middle and upper parts of the 

Lykins Formation (^Pl) east of the Front Range. Total thickness ranges from about 

243.8 m (southwest) to 176.8 m (northwest)

Pgl Lower part of Goose Egg Formation (Upper and Lower Permian)—Unit exposed west 

of the Front Range. Consists of the Forelle Limestone and Satanka Shale Members. 

Shown undivided on cross sections only. On map, divided into two subunits as follows:

Pgfsu Forelle Limestone Member (Upper Permian) and upper part of Satanka Shale 

Member (Upper and Lower Permian), undivided—The overlying Forelle Limestone 

Member consists of two beds of gray, microcrystalline, wavy laminated, algal limestone 
Description of Map Units 63

(each about 2.1 m thick) separated by about 3 m of purple and tan laminated shaly dolomite. The underlying upper part of the Satanka Shale Member consists of red sandy shale 

and siltstone interlayered with lesser amounts of fine-grained sandstone, blocky gypsum, 

and wavy-laminated dolomitic limestone near base of unit. Base of unit defined by thin 

gray dolomitic limestone with white to gray chert nodules. Workman and Braddock 

(2010) correlate this basal limestone to the base of the Minnekahta Limestone Member of 

the Goose Egg Formation of Burk and Thomas (1956). Unit Pgfsu is the lateral equivalent of the lower part of the Lykins Formation (^Pl) east of the Front Range (Broin, 

1957; Rascoe and Baars, 1972). Total thickness of the two units is about 60 m

Pgsl Lower part of Satanka Shale Member (Lower Permian)—Reddish-orange to dark 

maroon, thin laminated, sandstone and siltstone with increasing interbedded dolomitic 

sandstone and gypsum near top of section. Percentage of sandstone increases to south 

across map area. Workman and Braddock (2010) correlate unit with Opeche Shale 

Member of Goose Egg Formation of Burk and Thomas (1956); Pearson (1972) correlates the upper part of the unit with the Lyons Sandstone (Pl) and Maughan and Wilson 

(1963) correlate the lower part of unit Pgsl with the Owl Canyon Formation (Po) both 

mapped on the east side of the Front Range; the regional unconformity described by Cole 

and Braddock (2009) below the Lyons Sandstone is inferred to exist within unit Pgsl

based upon these correlations. Camp (1979) included this unit as part of the underlying 

Casper Formation (P*c) which we restrict here to exposures of “festoon” cross-bedded 

sandstones and interbedded limestones which pinch out near the northern edge of the 

quadrangle; within Laramie River valley unit may include thin exposures of unmapped 

Casper Formation at the base. Interfingers with underlying Fountain Formation (P*f) to 

south (Pederson, 1953). Thickness about 35 m at Sand Creek Pass (northwest)

P*c Casper Formation (Lower Permian to Middle Pennsylvanian)—White to buff, yellowweathering, fine- to medium-grained, well-sorted, carbonate cemented quartzose 

sandstone and interbedded thin lenses of freshwater limestone; sandstones display 

prominent ripple marks and large-scale trough (festoon) cross-bedding accompanied 

by significant soft-sediment deformation; mapped only in the vicinity of Sand Creek 

in the northwestern part of the quadrangle. Formation thins markedly southward and 

pinches out just north of Sand Creek Pass (Workman and Braddock, 2010). The Casper 

Formation was deposited in a coastal dune complex (Steidtmann, 1976) and the upper 

part is the approximate lateral equivalent of the marine shelf and sabkha deposits of the 

Ingleside Formation (Pi) east of the Front Range. Thickness is about 29 m at Chimney 

Rock; mapped as a linear ‘key bed’ between overlying Goose Egg Formation (Pgsl) and 

underlying Fountain Formation (P*f) where exposures are too thin to map, but outcrops 

form prominent, readily identified low cliffs and mesas east of Bull Mountain

P*f Fountain Formation (Lower Permian to Lower Pennsylvanian)—Reddish-brown, 

purplish-gray, and light greenish-gray, arkosic conglomerate; medium- to coarse-grained, 

feldspathic sandstone; dark-reddish-brown and pale green micaceous siltstone and shale; 

and minor thin limestone; unconformably overlies Proterozoic crystalline rocks. The 

Fountain Formation contains material eroded from the uplifted Proterozoic basement of 

the Ancestral Rocky Mountains; sediments were deposited in rapidly subsiding basins 

by braided stream systems and (locally) by eolian transport (Berman, 1979). Thickness 

varies from about 198 m (northeast) to 210 m (northwest) to 268 m (southeast) across the 

quadrangle; thins south along Laramie River valley and absent in southwestern part of 

quadrangle where inferred Ancestral Rocky Mountain uplift occurred; includes Casper 

Formation (P*c) where too thin to show in cross section A–Aʹ

PALEOZOIC AND PROTEROZOIC INTRUSIVE ROCK

DZk Kimberlite (Devonian? and Neoproterozoic)—Dark-gray or dark-olive-green, altered 

kimberlite breccia, carbonatitic kimberlite, and porphyritic kimberlite; typically contains olivine, chromian pyroxene, pyrope garnet, ilmenite, chromite, and phlogopite, 

variably altered to serpentine and carbonate minerals (McCallum and Eggler, 1979; 

Hausel, 1998). Kimberlite forms irregular dikes and deeply eroded diatremes that 
64 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

contain nodules of lower crustal granulite, peridotite, and pyroxenite. Outcrop is poor, 

but kimberlite soil shows characteristic blue color and correlates with grassy vegetation anomalies in otherwise forested land. Kimberlites are clustered in several small 

groups across the Fort Collins quadrangle and in adjoining areas north in Wyoming; 

thousands of carats of industrial-grade stones and some gem-quality diamonds have 

been recovered by commercial extraction (Carlson and Marsh, 1986; Hausel, 1998). 

Radiometric studies confirm both Devonian and Neoproterozoic emplacement ages in 

this quadrangle and across the Front Range (Naeser and McCallum, 1977; Lester and 

others, 2001)

PROTEROZOIC INTRUSIVE AND METAMORPHIC ROCKS

Proterozoic intrusive rocks in the Fort Collins 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 Mesoproterozoic Berthoud Plutonic Suite 

(approximately 1,350 to 1,440 Ma; DeWitt and others, 2010) and the Paleoproterozoic Routt Plutonic Suite 

(approximately 1,690 to 1,725 Ma; Premo and others, 2010a). 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) and rocks of similar 

composition, texture, and ages have been correlated with the named lithodemic units (see Tweto, 1987). 

Later isotopic studies have shown that some of these correlations were not valid (Premo and Van Schmus, 

1989). 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: Y = Mesoproterozoic (1,600 to 1,000 Ma); X = Paleoproterozoic (2,500 to 1,600 Ma)

Composition: d, diorite, diabasic mafic rock; g, granite; gb, gabbro; gd, granodiorite; j, trondhjemite; qd, 

quartz diorite

Intrusive body (where named): H, granite of Hagues Peak; LC, Log Cabin batholith; LP, Longs Peak 

batholith; R, Rawah batholith; SH, Sherman batholith; T, Thompson Canyon sills; V, Virginia Dale 

ring-dike complex

Textural and (or) mineralogical descriptors (where appropriate): h, hornblende-bearing; p, porphyritic; 

e, equigranular; a, aplitic

Temporal or spatial descriptors (where appropriate): y, younger; o, older or outer; i, inner

MESOPROTEROZOIC INTRUSIVE ROCKS

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 15 m). 

Intruded 1,316±50 Ma (Rb-Sr isochron age; Braddock and Peterman, 1989). Iron 

Dike forms a narrow north-northwest trending swarm along the eastern front of the 

Medicine Bow Mountains (Pearson and others, 1982) and in the Mummy Range just 

north of Long Draw Reservoir; this distinctive dike swarm has been traced as far north 

as the Wyoming State line (Tweto, 1987) and as far south as Boulder, Colo. (Cole and 

Braddock, 2009), over a total distance of about 120 km

Ygay Younger aplite dikes and plugs—Exposed in the central and southern parts of the quadrangle; unit includes light-gray to buff, very fine- to medium-grained, xenomorphic 

granular alkali-feldspar syenogranite to monzogranite with variable accessory biotite 

and muscovite that grades into pegmatite; large bodies just north of the Cache la Poudre 

River contain sparse garnet and intrude biotite granite (Yg and Yge); north-northwest 

striking dikes exposed south of Greyrock Mountain are porphyritic, biotite-rich, and 

intrude mafic dikes (Yd). Whole-rock Rb-Sr age from correlative rocks to south of 

quadrangle is 1,361±30 Ma (Peterman and others, 1968); probably related to intrusion 

of Silver Plume-type granites (Braddock, Abbott, and others, 1988)
Description of Map Units 65

Yg Biotite granite—Gray to buff, fine- to medium-grained, equigranular to seriate porphyritic 

biotite syenogranite to monzogranite, correlated with granite of Log Cabin batholith and 

other Silver Plume-type granites throughout the Front Range (Eggler and Braddock, 

1988; Shaver and others, 1988); associated with smaller unnamed intrusive bodies and 

dikes. Color index generally 2 to 5. To the north of the Log Cabin batholith, unit forms 

an oval, irregular plug intruded into the Sherman Granite phases of the Virginia Dale 

ring-dike complex; intrusion age established as 1,390±30 Ma (Rb-Sr isochron; recalculated from Peterman and others, 1968). To the south and east of the Log Cabin batholith, 

unit includes numerous irregular shaped intrusive bodies and dikes along the 1.2 Ga 

(Abbott, 1972) Skin Gulch shear zone which might be displaced or satellitic bodies of 

the main Log Cabin batholith or may be separate plugs and dikes intruded between the 

Log Cabin and Longs Peak-St. Vrain batholiths. Unit is primarily porphyritic to south 

where equigranular phase (Yge) is mapped separately; includes mafic quartz monzonite 

of Abbott (1976)

Yge Equigranular biotite granite—Gray, equigranular to slightly porphyritic, fine- to mediumgrained biotite syenogranite to monzogranite mapped to south and east of the Log Cabin 

batholith; weathers to pink or tan; correlated with granite of Log Cabin batholith and 

other Silver Plume-type granites throughout the Front Range (Shaver and others, 1988)

Granite of Log Cabin batholith—Light-gray, grayish-orange, pale pink, or tan 

monzogranite to syenogranite that contains 1‒8 percent biotite; texture ranges from 

equigranular to seriate porphyritic to distinctly porphyritic, and grainsize ranges from 

fine to coarse. Coarse-grained, porphyritic phase is prominent along the western and 

southern batholith margins and in the Red Feather Lakes area and displays a flow 

alignment of microcline phenocrysts that dips steeply outward. Mylonitic foliation 

is locally present in the vicinity of the Skin Gulch shear zone. Granite of Log Cabin 

batholith typically weathers tan and light orange-brown and forms a landscape of 

rounded tors surrounded by grus aprons. Age of intrusion is similar to granite of Longs 

Peak (YgLP) and other Silver Plume-type granites of the Front Range; U-Pb SHRIMP 

age on zircon grains from the coarse, porphyritic phase is 1,407±13 Ma, and from the 

medium-grained, equigranular phase is 1,408±15 Ma (DeWitt and others, 2010; Premo 

and others, 2012). Along southern margin of batholith, porphyritic and equigranular 

phases have been mapped separately

YgLC Granite of Log Cabin batholith, undivided—Mapped north of the Elkhorn Creek shear 

zone through most of the Log Cabin batholith where coarse-grained, porphyritic phase 

is predominant, but lack of detailed mapping does not allow differentiation of phases

YgLCe Equigranular granite of Log Cabin batholith—Mapped separately south of the Elkhorn 

Creek shear zone and along the western edge of the batholith where more detailed 

mapping exists (Abbott, 1976; Shaver and others, 1988); gray, equigranular to slightly 

porphyritic, fine- to medium-grained biotite syenogranite to monzogranite; flow foliation defined by alignment of biotite is common; weathers to pink or tan

YgLCp Porphyritic granite of Log Cabin batholith—Mapped south of the Elkhorn Creek shear 

zone where detailed mapping exists (Abbott, 1976; Shaver and others, 1988); gray to buff, 

coarse-grained, porphyritic to seriate porphyritic biotite syenogranite to monzogranite; 

2–4 cm-long subhedral phenocrysts of perthitic microcline and sparse oligoclase are commonly aligned to form flow foliation; weathers to tan and light orange-brown

YgLP Granite of Longs Peak—Light- to medium-gray, grayish-orange, orange-pink, or redpurple syenogranite to monzogranite that contains characteristic tabular microcline 

phenocrysts. Biotite is the principal dark mineral, 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. Color 

index generally 5 to 8. Granite intruded the Longs Peak-St. Vrain batholith as a viscous 

magma that extensively deformed metamorphic wall-rocks (Cole, 1977; Braddock 

and Cole, 1979, 1990; Cole, 2004b; Cole and Braddock, 2009); only the northernmost 

part of the batholith is exposed in this quadrangle. Age of intrusion is established as 

1,420±25 Ma (Rb-Sr isochron; Peterman and others, 1968) and 1,393±25 Ma (U-Pb 

zircon discordia; Plymate and others, 2005). Secondary muscovite (non-oriented, por-
66 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

phyroblastic) is locally conspicuous, particularly in small satellite intrusions; formed by 

subsolidus hydration of sillimanite+potassium feldspar or breakdown of primary biotite 

to muscovite+magnetite

Yd Mafic dikes and plugs—Black, greenish-black, or dark-gray, fine- to medium-grained 

diabasic rocks of basaltic or andesitic composition that form a north- to northeasttrending dike swarm throughout much of northern Front Range. Plagioclase forms 

phenocrysts in most dikes and matrix consists of finer-grained feldspar, hornblende, 

pyroxene, and magnetite, or secondary minerals related to contact metamorphism or 

hydration alteration. The dike swarm cuts 1,433 Ma-old Sherman Granite (YgSH) 

(Frost and others, 1999) and is cut and deformed by 1,395 Ma-old granite of Longs 

Peak (YgLP) (Cole, 1977; Plymate and others, 2005) and 1,406 Ma-old granite of the 

Log Cabin batholith (YgLC) (Shaver and others, 1988)

Ygao Older aplite dikes and plugs—Exposed in the northern part of the quadrangle; unit 

includes pinkish-gray to red, fine- to medium-grained, xenomorphic to hypidiomorphic 

granular biotite monzogranite that forms dikes and small slightly porphyritic plugs that 

parallel the margins of the Virginia Dale ring dike structure; some dikes contain hornblende; intrudes rocks of the Virginia Dale ring dike complex (YgVi and YgVo) and is 

intruded by mafic dikes (Yd)

Virginia Dale ring-dike complex—A roughly circular composite intrusion, about 9 mi in 

diameter, is exposed in the northern part of the Fort Collins quadrangle and extends 

northward into Wyoming. As summarized from Eggler (1968), the ring-dike complex 

is a distinct structural and magmatic element of the Sherman Granite batholith, based 

on geochemical similarities, intrusive relationships, and the merging of the outer ringdike with the main Sherman Granite batholith. The Virginia Dale ring-dike complex is 

intruded by biotite monzogranite (Ygao) and by mafic dikes (Yd). The age of intrusion 

is established by the emplacement of the Sherman Granite (YgSH) at 1,433±1.5 Ma 

(U-Pb zircon;Frost and others, 1999) and 1,430±20 Ma (Rb-Sr isochron; Zielinski and 

others, 1981)

YgVi Inner cap rock monzogranite of the Virginia Dale ring-dike complex—Pinkish-gray, 

medium-grained, porphyritic biotite monzogranite that displays flow foliation defined by 

aligned microcline phenocrysts, biotite-rich streaks, and aligned dioritic inclusions; color 

index generally 6 to 8. Contact with outer cap rock monzogranite (YgVo) is gradational

YgVo Outer cap rock monzogranite of the Virginia Dale ring-dike complex—Pinkish-gray, 

medium-grained, porphyritic biotite monzogranite, similar in all respects to inner cap 

rock monzogranite (YgVi) except that phenocrysts are smaller and better aligned, and 

biotite-rich streaks are more common; color index generally 7 to 9. Contact with inner 

cap rock monzogranite is gradational

YdV Dioritic and hybrid rocks of the Virginia Dale ring-dike complex—Fragmental and 

magmatically mixed rocks (Vasek and Kolker, 1999), ranging from diorite to monzogranite, that form a partial annular zone between the cap rock monzogranites (YgVi and 

YgVo) and the Sherman Granite (YgSH) outer ring-dike of the Virginia Dale ring-dike 

complex. Eggler (1968) interprets the hybrid zone as evidence of simultaneous intrusion 

of dioritic and monzogranitic magmas. Irregular blocks of metamorphic country rock 

are also contained within the hybrid zone

YgSH Sherman Granite—Red-weathering, coarse-grained, equigranular, subalkalic syenogranite 

with hornblende and biotite forming clotted aggregates (color index generally 6 to 11 in 

the Virginia Dale ring-dike complex; somewhat higher in the main Sherman batholith). 

Small amounts of accessory fluorite are typical. Sherman Granite characteristically 

weathers to thick quartz-feldspar grus and is poorly exposed (Eggler and others, 1969). 

Age of crystallization is determined at 1,433±1.5 Ma (U-Pb from zircon; Frost and 

others, 1999) from a sample in the Laramie Mountains north of the map area

Yghp Hornblende granite porphyry—Orange-weathering, yellowish-gray, fine-grained, massive, xenomorphic granular granite porphyry with conspicuous rounded phenocrysts of 

gray quartz, equant feldspar, and minute prismatic hornblende (color index about 5); 

typically weathers to smooth rounded forms. Present west of the Virginia Dale ring-dike 

complex; locally includes abundant inclusions of metamorphic wall rock; crosscut by 
Description of Map Units 67

mafic dikes and plugs (Yd) and by Sherman Granite (YgSH) (Eggler and Braddock, 

1988; Braddock, Cole and Eggler, 1989)

Ygbh Hornblende gabbro—Black, medium-grained, massive rock consisting of hornblende, 

calcic plagioclase, and a little biotite and pyrite. Mapped only in the country rock 

northeast of the Virginia Dale ring-dike complex where it intrudes felsic gneiss (Xfl) and 

amphibolite (Xh). Gabbro is intruded by Sherman Granite (YgSH), but is interpreted to 

belong to the Berthoud Plutonic Suite because it lacks dimensional fabric and because 

it is geochemically related to the mafic intrusive rocks of the Virginia Dale complex 

(Eggler, 1968; Braddock, Eggler, and Courtright, 1989)

YgH Granite of Hagues Peak—Tan, coarse-grained to very coarse grained, porphyritic biotiterich monzogranite and granodiorite. Contains conspicuous flow-aligned phenocrysts 

of microcline 2 to 6 cm long. Color index generally 10 to 15. The granite of Hagues 

Peak is intruded by the granite of Longs Peak (YgLP), and yields a whole-rock Rb-Sr 

isochron age of approximately 1,480 Ma (Braddock and Cole, 1990). Plymate and 

others (2005) indicate bulk geochemistry of this unit strongly resembles the granite of 

the Mt. Evans batholith (exposed 100 km south of the map area) intruded at 1,442±2 Ma 

(U-Pb on zircon; Aleinikoff and others, 1993)

Yqd Quartz diorite—Dark-gray, medium-grained, massive, biotite-hornblende quartz diorite and 

diorite; forms several small bodies along the southern quadrangle boundary contained 

within the granite of Hagues Peak (YgH) (Nesse and Braddock, 1989; Braddock and 

Cole, 1990; Plymate and others, 2005); may represent a mafic precursor to the granite 

of Hagues Peak

MESOPROTEROZOIC AND (OR)

PALEOPROTEROZOIC INTRUSIVE AND METAMORPHIC ROCK

YXp Pegmatite—White, medium-grained to very coarse grained pegmatite with accessory 

biotite, muscovite, garnet, and (or) tourmaline. Pegmatites are widespread in the metamorphic rocks and likely formed during emplacement of both the Routt (approximately 

1,690 to 1,725 Ma; Premo and others, 2010a) and Berthoud (approximately 1,350 to 

1,440 Ma; DeWitt and others, 2010) Plutonic Suites; reliable field criteria for distinguishing pegmatite ages have not been established

YXu Undivided intrusive and metamorphic rocks—Undivided crystalline basement rocks 

shown beneath Phanerozoic sedimentary deposits in cross sections only. May include 

any Proterozoic intrusive and (or) metamorphic rocks included on the map as well as 

other unidentified crystalline rocks not exposed at the surface. No internal relationships 

are shown within unit

PALEOPROTEROZOIC INTRUSIVE ROCKS

Xgd Granodiorite—Light- to medium-gray, medium- to coarse-grained, weakly to strongly 

foliated granodiorite and monzogranite containing biotite and variable amounts of 

hornblende and local feldspar phenocrysts. These rocks are physically and mineralogically similar to the Boulder Creek Granodiorite of the Boulder Creek batholith 45 km 

south (Gable, 1980), but were intruded in discrete plutons. Emplacement age inferred to 

be similar to Boulder Creek batholith (1,713±4 Ma mean age from U-Pb SHRIMP on 

zircon from multiple samples in and south of the map area; Premo and others, 2010a) 

because these granodiorites intruded during main stage of regional folding, which is 

typical of the Boulder Creek Granodiorite (Braddock and Cole, 1979)

Xgdp Porphyritic granodiorite—Gray to dark brown, medium- to coarse-grained, porphyritic 

hornblende-biotite granodiorite with conspicuous lensoid microcline phenocrysts as large 

as a few centimeters; color index varies between 10 and 25. Forms bladed outcrops with 

steeply plunging mineral-segregation lineation. Unit occurs in and near the Cache la 

Poudre canyon between Rustic and Kinikinik. Age unknown, but assumed to be related 

to Boulder Creek-type granodiorite (Xgd; 1,713±4 from U-Pb SHRIMP zircon data; 

Premo and others, 2010a) based upon similar composition and structural relationships
68 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

Xgbh Hornblende gabbro—Dark-gray to greenish-black, medium-grained, equigranular, massive hornblende gabbro, consisting largely of plagioclase, hornblende, and minor 

pyroxene and iron-titanium (Fe-Ti) oxides. Forms a roughly circular pluton (1 km 

wide) near North Bald Mountain and a smaller body in the Cache la Poudre canyon 

west of Kinikinik. Considered youngest phase of Rawah batholith which intrudes main 

granite phase (XgR) based upon 1,715 Ma U-Pb SHRIMP zircon age from North Bald 

Mountain exposure (Premo and others, 2010a)

XgR Granite of the Rawah batholith—Pink to gray, fine- to medium-grained, xenomorphic 

equigranular biotite granite; mineral foliation and compositional banding uncommon. 

Contains sparse accessory biotite and (or) hornblende and minor sphene, apatite, zircon, 

allanite, and rare sillimanite, rutile, and garnet. Biotite percentage varies significantly 

(gradationally and in schlieren) and biotite-poor varieties are generally not foliated. 

Unit contains abundant inclusions of older metamorphic rocks. Hornblende gneiss (Xh) 

inclusions are commonly rotated relative to regional metamorphic foliation (Burch, 

1983). Biotite gneiss (Xb) inclusions are far less common and generally smaller in size. 

Inclusions of leucocratic felsic gneiss (Xfl) may be common, but are hard to distinguish 

from Rawah due to similar composition and texture; exposures in the southern Medicine Bow Mountains are suspected to be largely older felsic gneisses based upon an 

older age reported by Kellogg, Ruleman and others (2008) and a change in geophysical 

signature to the south of Kelly Lake (Zietz and Kirby, 1972), but no mappable contact 

has been found. Burch (1983) describes coarser-grained, biotite-rich, foliated porphyritic granitic rocks as a separate phase of the Rawah; we reinterpret these rocks as 

inclusions of older felsic gneiss (Xfl) based on parallel foliation and interlayering with 

adjacent hornblende gneiss (Xh) inclusions. The Rawah batholith, proper, spans across 

the Laramie Mountains west of the Log Cabin batholith (YgLC) and the Medicine Bow 

Mountains; the northern boundary in the Laramie Mountains is marked by the Cornelius 

Creek shear zone of cataclasis and breccia (probably post-Proterozoic deformation). The 

smaller, isolated, northeast-southwest elongate intrusive body in the northern Mummy 

Range is correlated with the Rawah batholith based upon geochemistry and mineralogy (Plymate and others, 2005), but the exact relationship of these rocks to the Rawah 

batholith, proper, are still unclear. McCallum and Hedge (1976) report a whole-rock 

Rb-Sr age of 1,710 Ma; preferred age of batholith intrusion determined from U-Pb 

SHRIMP on zircon grains from several locations is 1,724‒1,715 Ma with a mean age of 

1718±4 Ma (Premo and others, 2010a). Plymate and others (2005) obtained an approximate crystallization age of 1,695±20 Ma by composite regression of discordant U-Pb 

isotope dilution-thermal ionization mass spectrometry (ID-TIMS) zircon-isochron data 

from three samples in the southern intrusive body

XjT Trondhjemite of Thompson Canyon—Light-gray, very fine-grained porphyritic to 

medium-grained equigranular, biotite-bearing 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). U-Pb discordia 

age is 1,726±15 Ma (Barovich, 1986) in the Big Thompson Canyon south of this quadrangle; Plymate and others (2005) obtained a zircon-isochron age from U-Pb TIMS-ID 

analysis of 1,702±6 Ma from a single body in the northern Mummy Range and correlate 

these rocks to the rocks in Big Thompson Canyon

Xp Gneissic pegmatite—White to pink, fine- to very coarse-grained, foliated granitic rocks 

composed of quartz, plagioclase, and microcline; exposed in the southern wall rocks 

of the Virginia Dale ring-dike complex where they are interlayered, folded and foliated 

parallel to the older metavolcanic and metasedimentary rocks (Xh, Xfb and Xbq) which 

they intrude; unit is intruded by granite of Log Cabin batholith (YgLC) and by younger 

pegmatite dikes (YXp)
Description of Map Units 69

PALEOPROTEROZOIC METAMORPHIC ROCKS

Biotite-bearing schist and gneiss units (Xb, Xbq, Xbk, Xbp, Xbg) and calc-silicate gneiss units (Xcg) are 

probably metasedimentary rocks; hornblende gneiss units (Xh) and felsic gneiss units (Xf, Xfl, Xfb) are 

probably metavolcanic rocks; mixed lithology units (Xbh, Xbqh, Xbkh) are probably interbedded metavolcanic and metasedimentary rocks. The biotite-bearing schists vary in metamorphic grade and minerology 

as shown by the metamorphic zone boundaries. Metasedimentary lithologies are notably sparse north of 

(approximately) the Skin Gulch shear zone, just as metavolcanic rocks are relatively sparse to the south 

(fig. 7). The age of regional metamorphism is 1,713±30 Ma (whole-rock Rb-Sr age of Peterman and others, 

1968, recalculated by Braddock, Cole, and Eggler, 1989), similar to the emplacement age for widespread 

granodiorite (Xgd) equivalent to the Boulder Creek Granodiorite (1714.4±4.6 Ma [weighted mean of multiple SHRIMP and ID-TIMS U-Pb zircon ages]; Premo and Fanning, 2000), although some recrystallization 

may have continued later to about 1,693±5 Ma (SHRIMP U-Pb zircon age from migmatitic melt phase; 

Premo and others, 2007b). The age of deposition is inferred to be as old as 1,780 Ma based on comparison 

with primary volcanic ages, consistent with samarium-neodymium (Sm-Nd) model age of 1,800 Ma for the 

formation of continental crust in 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. This unit is typical of the 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 in 

the rock during partial melting at the peak of metamorphism. Unit is subdivided as follows in areas of more detailed mapping and (or) better exposure (Cole, 1977; Braddock 

and Cole, 1990)

Xbq Quartzofeldspathic mica schist—Mica-poor schist interbedded with quartzofeldspathic 

metasandstone; typically shows identifiable sillimanite, and garnet is locally prominent. 

Unit contains interbedded knotted mica schist (Xbk) and sparse, thin beds of granulesized metaconglomerate

Xbk Knotted mica schist—Biotite-rich schist that displays 3‒10 mm porphyroblasts or clots 

of metamorphic minerals that exhibit a lumpy or knotted outcrop appearance. Contains 

interbedded quartzofeldspathic mica schist (Xbq) and sparse, thin beds of granule-sized 

metaconglomerate

Xbp Porphyroblastic biotite schist—Biotite- or sillimanite-rich variety of knotted mica schist 

that is characterized by large biotite porphyroblasts, typically about 0.635 cm (0.25 in) 

diameter. Unit forms an important stratigraphic marker symmetrically disposed about 

the axial surface of a major first-generation syncline in the Estes Park quadrangle to the 

south (Cole and Braddock, 2009)

Xbg Granoblastic quartzofeldspathic biotite gneiss—Medium- to dark-gray, fine-grained, 

thickly bedded to finely laminated, granoblastic gneiss consisting mainly of quartz, 

plagioclase, and biotite with smaller amounts of epidote, hornblende, or potassium 

feldspar; locally contains small calc-silicate clots that are flattened in the plane of compositional layering; typically interlayered with quartzofeldspathic mica schist (Xbq) and 

calc-silicate gneiss (Xcg)

Xbh Biotite gneiss and hornblende gneiss, undivided—Light- to dark-gray, medium-grained, 

layered to massive, moderately foliated gneiss consisting of interlayered bands of 

hornblende gneiss and amphibolite (Xh), biotite schist and gneiss (Xb), and lesser 

amounts of felsic gneiss (Xf) and calc-silicate gneiss (Xcg); felsic layers locally 

contain as much as 10 percent microcline. Unit mapped where exposure is poor and 

heterogeneity of layering makes mapping individual units difficult. Interpreted as 

interlayered metavolcanic and metasedimentary rocks. Forms an extensive part of 

the metavolcanic terrane intruded by the Rawah batholith (XgR) south of the Cache 

la Poudre River between about Rustic and Cameron Pass, and occurs as widespread 

blocky inclusions in the batholith within the Medicine Bow Mountains and Laramie 
70 Geologic Map of the Fort Collins 30'×60'Quadrangle, Colorado, and Wyoming

River valley. Primary age of deposition determined as 1,735±14 Ma (U-Pb SHRIMP 

from zircon; Kellogg, Ruleman, and others, 2008) from a sample near Kinikinik along 

the Cache la Poudre canyon

Xbqh Quartzofeldspathic mica schist and hornblende gneiss, undivided—Mapped north of 

Poudre Park where quartzofeldspathic mica schist is the predominant rock type, but 

multiple bands of hornblende gneiss and amphibolite (Xh), too thin to show at map 

scale, are interlayered parallel to foliation composing a significant percentage of the 

overall map unit

Xbkh Knotted mica schist and hornblende gneiss, undivided—Mapped north of Poudre Park 

where knotted mica schist (Xbk) is the predominant rock type, but multiple bands of 

hornblende gneiss and amphibolite (Xh), too thin to show at map scale, are interlayered 

parallel to foliation comprising a significant percentage of the overall map unit

Xh Hornblende gneiss and amphibolite—Dark-gray, greenish-gray, fine- to medium-grained, 

weakly to strongly layered hornblende-plagioclase gneiss and hornblende-biotite schist, 

locally interlayered with massive amphibolite. Chiefly consists of hornblende with variable amounts of plagioclase and magnetite, with minor amounts of biotite and (or) quartz. 

Contains thin layers and pods of white to light-green calc-silicate gneiss (Xcg); typically 

interlayered with granitic metavolcanic units and locally with metasedimentary biotite 

gneiss units. Age determined as 1,779±5 Ma (U-Pb SHRIMP from zircon; Workman, 

2008) from a sample south of Eaton Reservoir in the northwestern part of the map

Xcg Calc-silicate gneiss—Greenish gray, gray, and white, moderately to strongly layered rock 

with granoblastic texture; consists mainly of plagioclase, epidote, diopside, calcite, and 

quartz with minor hornblende, garnet, or biotite; typically interlayered with hornblende 

gneiss (Xh) or with biotite gneiss (Xb)

Xf Felsic gneiss—Gray to pinkish-gray, medium- to coarse-grained, banded and moderately 

to strongly foliated felsic gneiss of monzogranitic to tonalitic composition. Contains 

common layers and lenses rich in biotite or biotite and hornblende that are elongate 

parallel to regional mineral foliation; typically interlayered with hornblende gneiss (Xh) 

and leucocratic granitic gneiss (Xfl). May be metamorphosed volcanic or intrusive felsic 

rocks. Primary ages of crystallization determined as 1,763±6 Ma and 1,755±23 Ma 

(U-Pb SHRIMP from zircon; Kellogg, Ruleman, and others, 2008) and 1,764±12 Ma 

(U-Pb TIMS-ID from zircon; Plymate and others, 2005). Where color index is low, this 

unit is difficult to distinguish from the younger granite of the Rawah batholith (XgR) 

which intruded about 50 million years later

Xfl Leucocratic felsic gneiss—Light- to medium-gray, pink- and orange-weathering, fine- to 

coarse-grained, weakly to moderately foliated rock composed primarily of quartz, oligoclase, microcline, and biotite; hornblende, garnet, and rare sillimanite locally present. Rocks 

have been described as gneissic granite, gneissic alaskite, gneissic quartz monzonite, or 

gneissic granodiorite. Compositional banding defined by changes in grain size or mineral 

proportions is typical in outcrop on the scale of inches to several feet. Unit typically interlayered with hornblende gneiss and other metavolcanic units. Primary age of volcanism is 

indicated by U-Pb SHRIMP age on zircon grains of 1766.6±8.3 Ma (Workman, 2008) 

Xfb Biotite felsic gneiss—Dark-gray to pinkish-gray, medium- to coarse-grained, foliated and 

strongly lineated granodioritic rock with conspicuous microcline augen and aggregates 

that measure 1 to 2 in diameter; garnet and biotite are typical, and hornblende is locally 

present. Unit is intimately interlayered with hornblende gneiss (Xh) and with leucocratic felsic gneiss (Xfl), consistent with all three units comprising a compositionally 

diverse volcanic terrane. Primary age of volcanism is indicated by U-Pb SHRIMP age 

from zircon grains of 1,775.5±4.0 Ma (Workman, 2008)
References Cited 71

References Cited

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of living with geology—A model field trip for the general 

public (2d ed.), in Nelson, E.P., and Erslev, E.A., eds., Field 

trips in the Southern Rocky Mountains, USA: Geological 

Society of America Field Guide 5, p. 185‒206.

Abbott, J.T., 1970, Geology of Precambrian rocks and isotope 

geochemistry of shear zones in the Big Narrows area, northern Front Range, Colorado: Boulder, Colo., University of 

Colorado Ph.D. dissertation, 239 p.

Abbott, J.T., 1972, Rb-Sr study of isotope redistribution in 

a mylonite-bearing shear zone, northern Front Range, 

Colorado: Geological Society of America Bulletin, v. 83, 

p. 487‒493.

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Geologic Quadrangle Map GQ‒1323, scale 1:24,000. 

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For more information concerning the research in this report, 

contact the 

Center Director, 

USGS Geosciences and Environmental Change Science Center

Box 25046, Mail Stop 980

Denver, CO 80225

(303) 236-5344

Or visit the Geosciences and Environmental Change Science 

Center website at

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Workman and others—Geologic Map of the Fort Collins 30’×60’Quadrangle, Colorado, and Wyoming—Scientific Investigations Map 3399

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3399