Preliminary Geologic Map of the Bowen Mountain 

Quadrangle, Grand and Jackson Counties, 

Colorado

By James C. Cole, William A. Braddock, and Theodore R. Brandt 

View northward over Miocene boulder-gravel deposit toward Blue Ridge and Cascade Mountain 

(photo by J. Cole, 2007)

Pamphlet to accompany

Open-File Report 2011–1111

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director 

U.S. Geological Survey, Reston, Virginia 2011

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Suggested citation:

Cole, J.C., Braddock, W.A., and Brandt, T.R., 2011, Preliminary geologic map of the Bowen 

Mountain quadrangle, Grand and Jackson Counties, Colorado: U.S. Geological Survey OpenFile Report 2011–1111, 15 p., scale 1:24,000. 

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iii

Contents

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

Geologic Mapping ............................................................................................................................. 1

Stratigraphic Notes............................................................................................................................ 2

Structure ........................................................................................................................................... 3

Oil Exploration................................................................................................................................... 4

Acknowledgments............................................................................................................................. 6

References Cited .............................................................................................................................. 6

Description of Map Units ................................................................................................................... 9 

1

Preliminary Geologic Map of the Bowen Mountain 

Quadrangle, Grand and Jackson Counties, 

Colorado

By James C. Cole, William A. Braddock, and Theodore R. Brandt 

Abstract

The map shows the geology of an alpine region in the southern Never Summer 

Mountains, including parts of the Never Summer Wilderness Area, the Bowen Gulch 

Protection Area, and the Arapaho National Forest. The area includes Proterozoic 

crystalline rocks in fault contact with folded and overturned Paleozoic and Mesozoic 

sedimentary rocks and Upper Cretaceous(?) and Paleocene Middle Park Formation. The 

folding and faulting appears to reflect a singular contractional deformation (post-Middle 

Park, so probably younger than early Eocene) that produced en echelon structural uplift 

of the Proterozoic basement of the Front Range. The geologic map indicates there is no 

through-going “Never Summer thrust” fault in this area. The middle Tertiary structural 

complex was intruded in late Oligocene time by basalt, quartz latite, and rhyolite 

porphyry plugs that also produced minor volcanic deposits; these igneous rocks are 

collectively referred to informally as the Braddock Peak intrusive-volcanic complex 

whose type area is located in the Mount Richthofen quadrangle immediately north (Cole 

and others, 2008; Cole and Braddock, 2009). Miocene and upper Oligocene boulder 

gravel deposits are preserved along high-altitude ridges that probably represent former 

gravel channels that developed during uplift and erosion in middle Tertiary time.

Geologic Mapping

The geology was mapped primarily by W.A. Braddock (deceased, 2003) during 

the summers of 1985–1990. The eastern two-thirds of the quadrangle was compiled and 

published with the geologic map of Rocky Mountain National Park (Braddock and Cole, 

1990). Cole conducted additional field work, chiefly in the western part of the 

quadrangle, in 2006–2008, revised some of the previous mapping, and wrote the report 

and supporting materials. Brandt developed all of the digital database materials for the 

publication.

The Mount Richthofen quadrangle to the north was mapped in detail by O’Neill 

(1981), and the Trail Mountain quadrangle to the south was mapped by Izett (1974).

Reconnaissance mapping of the Rand 15-minute quadrangle to the west was completed 

by Kinney (1970). All of the Bowen Mountain quadrangle geology was compiled with 

the geologic map of the Estes Park 30’ x 60’ quadrangle (Cole and Braddock, 2009).

Metzger (1974) completed a detailed map of the Oligocene breccia-pipe in the Illinois 
2

River headwaters west of Bowen Mountain and most of his mapping is incorporated in 

this report.

Stratigraphic Notes 

The Middle Park Formation consists largely of sandstone and conglomerate 

deposited in response to mountain uplift in Late Cretaceous–early Tertiary time (the 

Laramide orogeny; Tweto, 1957, 1975). The most thorough description of the unit is 

presented by Izett (1968) for the area of the Hot Sulphur Springs quadrangle, 

immediately southwest of the Bowen Mountain quadrangle. In broad terms, the basal 

Middle Park is marked in this area by mafic volcaniclastic conglomerate and sandstone of 

the Windy Gap Volcanic Member. The Windy Gap is overlain by volcaniclastic 

sandstone and conglomerate that grades upward into increasingly arkosic sedimentary 

rocks. This sequence of compositions reflects erosion of a mafic volcanic center (location 

unknown) followed by erosion of Proterozoic basement rocks from the rising mountain 

blocks (Tweto, 1957; Izett, 1968). A similar lithic sequence is recorded in the Middle 

Park Formation in the Bowen Mountain quadrangle although the volcanic and arkosic 

subunits were not well characterized by Braddock and transitions between them were not 

studied in detail by Cole.

The Middle Park Formation appears to pass laterally northward into similar strata 

named Coalmont Formation in the North Park Basin (Hail, 1968; Izett, 1968; Tweto, 

1975). The arbitrary line of distinction between the two formations has traditionally been 

the Continental Divide across the Rabbit Ears Range, and it traverses the northwestern 

corner of the Bowen Mountain quadrangle between Willow Creek and the Illinois River.

For simplicity, all of these strata are referred to in this report as Middle Park Formation.

The Middle Park Formation lies disconformably on the sandy, middle unit of the 

Pierre Shale throughout much of this area. This relation implies that the upper several 

thousand feet of the Pierre Shale was eroded (without much tilting) prior to deposition of 

the lowermost Middle Park Formation.

Age of the lower part of the Middle Park Formation is not well known. Izett 

(1968) inferred it to be Late Cretaceous on the basis of one collection of fossil leaves 

from sandstone overlying the Windy Gap Volcanic Member east of Hot Sulphur Springs.

In addition, a single pollen sample “about 100 ft above the Windy Gap Member ... 

contains pollen and spores that ... indicate a Late Cretaceous age” (Izett, 1968, p. 25–26, 

quoting R.H. Tschudy); the possibility of reworked pollen was not addressed.

Several attempts to obtain a radiometric age on volcanic flows from the Windy 

Gap Volcanic Member have not been successful due to alteration and due to the lowpotassium content of the andesitic and basaltic rocks. One clast of hornblende-bearing 

andesite in the conglomerate at Windy Gap yielded a credible argon-plateau age of 65.2 +

0.3 Ma (G.L. Farmer, written commun., 2009). This result indicates the Windy Gap 

Volcanic Member is younger than about 65 Ma, and thus Paleocene, in this locality.

Until more definitive evidence is produced, the Windy Gap Volcanic Member is 

listed in this report as Upper Cretaceous(?).

The top of the Middle Park Formation is eroded in this area. Izett (1968, p. 25–26) 

summarized the paleobotanical data for the Middle Park Formation that are chiefly 

Paleocene. At the end of that discussion, he stated “beds included in the Coalmont 

Formation that may be partly equivalent to beds in the upper part of the Middle Park 
3

Formation contain pollen and spore assemblages that indicate an Eocene age (Hail and 

Leopold, 1960), perhaps early Eocene. The possibility exists that the upper part of the 

Middle Park Formation may also be of Eocene age, but no samples have yet yielded 

Eocene pollen and spores.” Hail (1968, p. 46–49) summarized abundant pollen data for 

the Coalmont Formation (exposed in the North Park area northwest of Bowen Mountain) 

that indicate the lower part is Paleocene but the middle and upper parts are Paleocene–

Eocene and Eocene, respectively. Distinction of the Middle Park and Coalmont

Formations is not clear along the Rabbit Ears Range divide, and it largely appears that 

they are continuous and coeval (see also Tweto, 1957, 1975).

Structure 

The major structural elements in the Bowen Mountain quadrangle are a series of 

sub-parallel folds and high-angle reverse faults that locally place Proterozoic basement 

rocks westward above the Upper Cretaceous Pierre Shale. These faults and folds are 

displayed in the Cretaceous and younger rocks in the Willow Creek drainage west of the 

Cascade Mountain–Blue Ridge divide. The easternmost fault that marks the structural 

contact between Proterozoic crystalline rocks and the Pierre Shale has been referred to as 

the “Stillwater overthrust” (Tweto, 1957) or, more commonly, the “Never Summer 

thrust” (O’Neill, 1981; Cole and Braddock, 2009). The position of the fault is well 

documented by the distribution of scattered outcrops of upper- and lower-plate rocks. The 

fault appears to dip 15o

–30o eastward (based on local three-point topographic relations

whose accuracies cannot be independently assessed) but is more likely 50o–65o eastward 

(based on the dip of overturned beds in the footwall block and on regional-scale 

topographic relations). This fault continues northward into the Mount Richthofen 

quadrangle where it dips 40o–60o eastward (O’Neill, 1981), but it cannot be traced south 

of the Porphyry Peaks area beyond the Bowen Mountain quadrangle, because it is 

intruded by Oligocene stocks and plugs.

The dip of this “Never Summer thrust” could also be inferred from the geology of 

the Neogene intrusive tuff-breccia pipe in the upper Illinois River drainage (Metzger, 

1974), but the geologic relations are not definitive. Metzger (1974) inferred that the pipe 

intruded only Proterozoic rocks (only upper-plate rocks) because he reported no breccia 

clasts of Phanerozoic rock; his cross-section shows the “Never Summer thrust”

steepening downward in a position completely west of the intrusive center of the pipe.

Braddock (in Braddock and Cole, 1990) reported that the tuff-breccia contained some 

clasts of probable Mesozoic sedimentary formations, suggesting that the fault has a 

gentler eastward dip and that the underlying footwall sedimentary units were penetrated 

by the breccia pipe. These observations were not confirmed by Cole and so the geometry 

of the fault remains undetermined.

A north-trending hangingwall anticline and reverse fault are inferred from the 

geologic relations between Lost Lake and the Pony Park area southward. The amount of 

throw on the fault appears to vary considerably over its length. North of Lost Lake, the 

hangingwall and footwall are both composed of Upper Cretaceous Pierre Shale. At the 

southern boundary of the Bowen Mountain quadrangle, the hangingwall carries Jurassic 

Morrison Formation over the Pierre. In the Trail Mountain quadrangle to the south of 

Pony Park (Izett, 1974), this fault places Triassic Chugwater Formation against the lower 

Pierre Shale, but farther southward the displacement declines over a mile strike-length to 
4

the juxtaposition of Cretaceous Benton Group against Pierre. Thus, this fault-fold pair is 

a relatively minor structure within the Never Summer fault zone.

The Middle Park Formation displays additional north-trending folds along the 

western margin of the Bowen Mountain quadrangle that are broadly conformable with the 

folds and faults described above. From the Windy Gap Volcanic Member westward, 

these comprise a broad syncline and a sharp-crested, asymmetric, west-vergent anticline.

The form of the anticline suggests it reflects a steep, east-dipping reverse fault at depth. A 

similar, sharp, west-vergent anticline was mapped in the Middle Park Formation to the 

west (Kinney, 1970) along La Fevre Ridge .

These north-trending folds and faults appear to be genetically related because they 

are geometrically similar, affect the same parts of the Middle Park Formation, and form 

an en echelon set of faults and folds in a belt of deformation along the western margin of

the Bowen Mountain quadrangle. Taken together, they suggest that the feature known as 

the “Never Summer thrust” is not a major crust-penetrating fault but rather a southwarddiminishing, high-angle reverse fault whose deformation is transferred to similar

structures progressively westward in the quadrangle. On the basis of all local evidence, 

the Never Summer structure is most accurately designated as a reverse fault, not a thrust,

because it is not laterally persistent; it (and parallel faults) probably steepens with depth, 

and there is no firm evidence of a major structural overhang of younger rocks. 

Folding and faulting probably occurred following deposition of the Middle Park 

Formation and laterally equivalent beds in the Coalmont Formation to the north. Local, 

minor unconformities were described in parts of the Coalmont Formation by Hail (1968),

but the strata above and below these minor angular discordances are similar fluvial 

sandstones and siltstones. No unconformities were described in the upper parts of the 

Middle Park Formation (Izett, 1968). With specific regard to the area near the Bowen 

Mountain quadrangle, Tweto (1975, p. 18) stated, “In the North Park-Middle Park basin, 

orogenic sediments of latest Cretaceous(?) to Eocene age lie with marked unconformity 

on older rocks ...” Regarding the time of deformation, he stated (p. 33), “In North Park, 

where the Coalmont Formation is involved in the mountain-front folds and faults, 

deformation must have been younger than early Eocene.”

Oil Exploration 

Three wildcat oil-exploration wells were drilled in the Bowen Mountain 

quadrangle between 1961 and 1981; one well was deepened in 1985. All are located near 

the Stillwater Pass road (Forest Road FR 123) along Willow Creek and appear to have 

been sited to explore the carbonaceous marine Cretaceous rocks that are folded and 

overturned in the footwall of the Never Summer reverse fault. No production was 

reported and all three wells are classified as “dry holes.”

Well name (year completed)

Well identification number (API = American Petroleum Institute system)

Public Land Survey System location by quarter-quarter-quarter section,

 Township, and Range; UTM Zone 13 coordinates (NAD 83)

Elevation of K.B. = Kelly bushing on the drilling platform

Depths to tops of formation boundaries

T.D. = total depth to bottom of well
5

All data reported to the Colorado Oil and Gas Commission.

Brooks EB Jr #1 (1973; deepened 1985)

API 05049 06005

NE. SE. sec. 13, T4N., R77W.; 418575 E., 4462400 N.

K.B. elev 9,675 ft

Top Pierre (eroded)........................................Ground surface

Top Niobrara..................................................2,259 ft

Top Dakota.....................................................3,722 ft

T.D. ................................................................4,380 ft (prior T.D. 2,505 ft)

Sunray DX #1-D (1961)

API 05049 05021

SE. NE. NE. sec.14, T4N., R77W.; 416860 E., 4462970 N.

K.B. elev. 9,350 ft

Top Coalmont/Middle Park (eroded).............Ground surface

Top volcanic (Windy Gap Member?) ...........3,600 ft

Top Niobrara..................................................5,942 ft

Top Dakota ....................................................6,923 ft

Top Morrison .................................................7,324 ft

Fault zone.......................................................7,506–7,592 ft

Niobrara beneath fault

T.D ................................................................7,882 ft

Voyager Fed-Gilsonite #1 (1981)

API 05049 06026

SW. SE. SE. sec. 10, T4N., R77W.; 415145 E., 4463325 N.

K.B. elev. 9,532 ft

Top Middle Park/Coalmont (eroded).............Ground surface

Top Niobrara (eroded) ...................................880 ft

Top Dakota.....................................................1,606 ft

Top Morrison .................................................1,799 ft

Top Proterozoic basement..............................2,178 ft

T.D. ................................................................6,161 ft

These data seem somewhat contradictory. For example, the Voyager FedGilsonite #1 well penetrates a very thin section of Middle Park/Coalmont and then enters 

the Niobrara. The “missing” Pierre Shale might indicate the Middle Park/Coalmont here 

was deposited on a pre-existing structural high, but no nearby evidence has been mapped.

The data for the Sunray DX #1-D well is consistent penetrating an overturned syncline, 

but Pierre Shale should have been noted beneath the “volcanic” unit. The 1985 log for the 

deepened rooks EB Jr #1 well indicates the Niobrara and Dakota (identified by M. 

Dechesne, written comm., 2011) are much higher than previously indicated, consistent 

with a stacked thrust beneath the Never Summer fault.
6

Acknowledgments 

This report was improved in response to technical review comments by J.M. O’Neill 

and K.S. Kellogg, both of USGS Denver. The lead author accepts responsibility for the 

content of this report and all interpretations presented within. 

References Cited 

Braddock, W.A., and Cole, J.C., 1979, Precambrian structural relations, metamorphic 

grade, and intrusive rocks along the northeast flank of the Front Range in the 

Thompson Canyon, Poudre Canyon, and Virginia Dale areas, in Ethridge, F.G., ed., 

Northern Front Range and northwest Denver Basin, Colorado: Geological Society of 

America Field Guide, Rocky Mountain Section, field trip 2, p. 106-121.

Braddock, W.A., and Cole, J.C., 1990, Geologic map of Rocky Mountain National Park 

and vicinity: U.S. Geological Survey Miscellaneous Investigations Series Map I-1973, 

scale 1:50,000.

Cole, J.C., 1977, Geology of east-central Rocky Mountain National Park and vicinity, 

with emphasis on the emplacement of the Precambrian Silver Plume Granite in the 

Longs Peak-St Vrain batholith: Boulder, University of Colorado, Ph.D. dissertation, 

343 p.

Cole, J.C., 2004, Elegant simplicity in prograde-retrograde metamorphism of Early 

Proterozoic metasediments, Front Range, Colorado: Geological Society of America 

Abstracts with Programs, v. 36, no. 5, p. 454.

Cole, J.C., and Braddock, W.A., 2009, Geologic map of the Estes Park 30’ x 60’ 

quadrangle, north-central Colorado: U.S. Geological Survey Scientific Investigations 

Map 2009-3039, scale 1:100,000, 56 p.

Cole, J.C., Larson, Ed, Farmer, Lang, and Kellogg, K.S., 2008, The search for 

Braddock’s caldera–Guidebook for Colorado Scientific Society fall 2008 field trip, 

Never Summer Mountains, Colorado: U.S. Geological Survey Open-File Report 2008-

1360, 30 p.

DePaolo, D.J., 1981, Neodymium isotopes in the Colorado Front Range and crust-mantle 

evolution in the Proterozoic: Nature, v. 291, p. 193-196.

Gable, D.J. and Madole, R.F., 1976, Geologic map of the Ward quadrangle, Boulder 

County, Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ-1277, scale 

1:24,000.

Gable, D.J., and Sims, P.K., 1969, Geology and regional metamorphism of some highgrade cordierite gneisses, Front Range, Colorado: Geological Society of America 

Special Paper 128, 87 p.

Hail, W.J., Jr.,1968, Geology of southwestern North Park and vicinity, Colorado: U.S. 

Geological Survey Bulletin 1257, 119 p.

Hail, W.J., Jr., and Leopold, E.B., 1960, Paleocene and Eocene age of the Coalmont 

Formation, North Park, Colorado, in Short papers in the geological sciences: U.S. 

Geological Survey Professional Paper 400-B, p. B260-B261. 

Higley, D.K., and Cox, D.O., 2005, Oil and gas exploration and development along the 

Front Range in the Denver Basin of Colorado, Nebraska, and Wyoming, Chapter B, in 

Fishman, N.S., ed., Energy resource studies, northern Front Range, Colorado: U.S. 

Geological Survey Professional Paper 1698, p. 9-53.
7

Izett, G.A., 1968, Geology of the Hot Sulphur Springs quadrangle, Grand County, 

Colorado: U.S. Geological Survey Professional Paper 586, 79 p., scale 1:48,000.

Izett, G.A., 1974, Geologic map of the Trail Mountain quadrangle, Grand County, 

Colorado: U.S. Geological Survey Geologic Quadrangle Map GQ-1156, scale 

1:24,000.

Kinney, D.M., 1970, Preliminary geologic map of the Rand quadrangle, North Park, 

Jackson County, Colorado: U.S. Geological Survey Open-File Report 70-183, scale 

1:48,000.

Madole, R.F., 1982, Possible origins of till-like deposits near the summit of the Front 

Range in north-central Colorado: U.S. Geological Survey Professional Paper 1243, 31 

p.

Madole, R.F., VanSistine, D.P., and Michael, J.A., 1998, Pleistocene glaciation in the 

upper Platte River drainage basin, Colorado: U.S. Geological Survey Geologic 

Investigations Map I-2644, scale 1:250,000.

Marvin, R.F., Young, E.J., Mehnert, H.H., and Naeser, C.W., 1974, Summary of 

radiometric age determinations on Mesozoic and Cenozoic igneous rocks and uranium 

and base metal deposits in Colorado: Isochron/West, no. 2, 41 p.

Meierding, T.C., 1977, Glacial geology on the western slope around Lake Granby:

Boulder, University of Colorado, Ph.D. dissertation, 353 p.

Metzger, C.W., 1974, Geology, mineralization, and geochemistry of the upper Illinois 

River drainage basin, Grand and Jackson Counties, Colorado: Golden, Colorado School 

of Mines M.S. thesis, 92 p.

Naeser, C.W., Izett, G.A., and White, W.H., 1973, Zircon fission-track ages from some 

middle Tertiary igneous rocks in northwestern Colorado (abs.): Geol. Soc. America 

Abs. with Programs, v. 5, no. 6, p. 498.

O'Neill, J.M., 1981, Geologic map of the Mount Richthofen quadrangle and the western 

part of the Fall River Pass quadrangle, Grand and Jackson Counties, Colorado: U.S. 

Geological Survey Miscellaneous Investigations Series Map I-1291, scale 1:24,000.

Peterman, Z.E., Hedge, C.E., and Braddock, W.A., 1968, Age of Precambrian events in 

the northeastern Front Range, Colorado: Journal of Geophysical Research, v. 73, no. 6, 

p. 2277-2296.

Premo, W.R., and Fanning, C.M., 2000, SHRIMP U-Pb zircon ages for Big Creek gneiss, 

Wyoming, and Boulder Creek batholith, Colorado: Implications for timing of 

Paleoproterozoic accretion of the northern Colorado province: Rocky Mountain 

Geology, v. 35, p. 31-50.

Premo, W.R., Kellogg, K.S., and Bryant, B., 2007, SHRIMP U-Pb zircon ages for 

Paleoproterozoic basement rocks from the northern and central Colorado Front Range: 

A refinement of the timing of crustal growth in the Colorado Province (abs.): 

Geological Society of America Abstracts with Programs, v. 39, no. 6, p. 221.

Scott, G.R., and Cobban, W.A., 1986, Geologic, biostratigraphic, and structure map of 

the Pierre Shale between Loveland and Round Butte, Colorado: U.S. Geological 

Survey Miscellaneous Investigations Map I-1700, scale 1:50,000.

Tweto, Ogden, 1957, Geologic sketch of southern Middle Park, Colorado, in Guidebook 

to the geology of North and Middle Parks Basin, Colorado: Rocky Mountain 

Association of Geologists , p. 18-31.
8

Tweto, Ogden, 1975, Laramide (Late Cretaceous-early Tertiary) orogeny in the Southern 

Rocky Mountains, in Curtis, B.F., ed., Cenozoic history of the southern Rocky 

Mountains: Geological Society of America Memoir 144, p. 1-44.
9

DESCRIPTION OF MAP UNITS

[Color terms are descriptive of fresh rock/sediment material]

ALLUVIAL UNITS

Qva Mountain valley alluvium (Holocene and upper Pleistocene)—Gravel, sand, 

and silt along courses of major and intermittent streams in bedrock-cut 

valleys, in alluvial fans along montane valley margins, and in outwash 

deposits downstream from terminal moraines of Pinedale and Bull Lake 

age. Unit underlies narrow, high-altitude valleys. Thickness variable, 

generally less than 20 ft

Qgy Stream-terrace gravel deposit (Holocene and upper Pleistocene)—Gravel, 

sand, and silt deposit preserved beneath low terrace (about 20–40 ft above 

modern drainage) along Willow Creek at western quadrangle boundary.

Deposit probably reflects redeposition of till (glacial outwash) during 

deglaciation of the drainage basin. Deposit appears to be about 20 ft thick

GLACIAL UNITS

Qr Rock glacier deposit (Holocene and upper Pleistocene)—Lobate mass of 

rock rubble, silty sand, and ice that is veneered by angular blocks and 

boulders in alpine cirques and valleys above 10,000-ft elevation. Surface 

typically marked by conspicuous arcuate ridges and a steep lobate front 

(Gable and Madole, 1976). Thickness variable

Qo Organic-rich sediment (Holocene and upper Pleistocene)—Bog and marsh 

deposits in formerly glaciated terrain, locally overlain by glacial-outwash 

sediments. Bog deposits also form near the basal contact of Neogene gravel 

deposits (Ng), for example east of Blue Ridge, probably due to 

groundwater discharge from the gravels. Thickness indeterminate, but 

probably less than 20 ft

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

boulders, cobbles, and pebbles in a silty sand matrix. Prominent, sharpcrested moraines are numerous, most clasts are slightly weathered, and 

soils are thin. Unit contains small bodies of Holocene till in cirque basins.

Some areas mapped as till in the southwest corner of the quadrangle may 

include slightly transported cobbly material that was weathered from the 

underlying gravels and sandstones of the Middle Park Formation. Pinedale 

glaciation occurred between about 30 ka and 12 ka in this region (Madole

and others, 1998). Thickness variable, but probably several tens of feet in 

most areas and thickest beneath moraine ridges, based on more detailed 

work in adjacent areas (Madole and others, 1998)

Qtb Till of Bull Lake age (upper and middle Pleistocene)—Subangular to 

subrounded boulders, cobbles, and pebbles in a silty-sand matrix. Morainal 

form subdued, clasts variably weathered, and soils well developed. Bull 

Lake glaciation occurred between about 300 ka and 120 ka in this region 

(Madole and others, 1998). Thickness indeterminate, but probably less than 

a few tens of feet in most areas
10

MASS-MOVEMENT UNITS

Qc Colluvium (Holocene and upper Pleistocene)—Deposits of rock debris 

ranging in size from silt to boulders, formed by a variety of mass-wasting 

processes. Unit chiefly mapped on nonglaciated, moderate slopes west of 

the trace of the Never Summer reverse fault, in local colluvial aprons 

surrounding Oligocene intrusive plugs, and in extensive areas near and 

above timberline where freeze-thaw action has aided mass wasting and 

downslope movement. Thickness variable

Qta Talus deposits (Holocene and upper Pleistocene)—Angular blocks of rock 

on steep slopes below cliffs. Unit is chiefly shown in alpine and subalpine 

areas in the northern part of the quadrangle at the bases of steep slopes that 

expose Proterozoic gneiss. Thickness variable

Qls Landslide deposits (Holocene and upper Pleistocene)—Includes slumps, 

earth flows, and minor debris flows. Landslides common in glacial deposits 

north of Bowen Gulch and on the southeast flank of Gravel Mountain.

Prominent landslide on northwest slope of Porphyry Peaks involves 

hydrothermally altered Oligocene intrusive rocks that probably overlie 

Pierre Shale at shallow depth; jumbled blocks and irregular landform 

suggests late Pleistocene or younger age. Most landslide masses are 

shallow and less than 30 ft thick; the large slide northwest of Porphyry 

Peaks appears to be more than 100 ft thick, based on topographic relations

Qdf Debris-flow deposit (Holocene)—Prominent and distinctive deposit of coarse

angular blocks of Oligocene quartz latite of Apiatan Mountain (bqla) that 

underlies a sparsely forested, arcuate, elongate mass on the northwest slope 

of Porphyry Peaks. This debris-flow deposit lies on top of an older, 

complex landslide jumble of similar materials that probably rests on Pierre 

Shale at shallow depth. Debris-flow deposit contains many clasts larger 

than 5 ft and preserves sharp lateral levees adjacent to the flow centerline.

Several small topographic depressions within the debris flow enclose 

shallow, ephemeral lakes. Thickness indeterminate, but probably less than 

50 ft

EOLIAN DEPOSIT

Qlo Loess (Holocene and upper Pleistocene)—Pale-brown sandy to clayey silt 

deposited by wind. Unit is limited to a singular deposit east of Gravel 

Mountain where prevailing westerly winds appear to have transported silt 

and sand from the disaggregated Middle Park Formation to the lee slope of 

a nearby 11,240+ ft peak. Thickness variable but appears to be less than 

20 ft

POST-LARAMIDE SEDIMENTARY ROCKS

Nbg Fluvial gravel (Miocene and upper Oligocene)—Unsorted, unstratified 

deposits of boulders (as large as 10-ft diameter), cobbles, and pebbles in a 

dominantly coarse sandy matrix with some silt. High-level fluvial-gravel 

deposits at Gravel Mountain, Blue Ridge, and flanking Apiatan Mountain

probably mark paleocanyons of the early Colorado River drainage. Most 
11

clasts are Silver Plume-type granite, pegmatite, and fine-grained, biotiterich, equigranular granite, minor biotite gneiss, and rare quartz sandstone 

(well-sorted, well-rounded, frosted quartz grains cemented by silica; 

resembles Dakota Sandstone). Volcanic rocks of the late Oligocene 

Braddock Peak complex (source in northern Never Summer Mountains; 

Cole and others, 2008; Cole and Braddock, 2009) were not found at Blue 

Ridge, but “porphyritic andesite pebbles of Laramide age” were noted in 

the adjoining Hot Sulphur Springs quadrangle by Izett (1968). These gravel 

deposits may locally be as old as late Oligocene because some clasts 

closely resemble the quartz latite of Apiatan Mountain (bqla) that is about 

28 Ma. Madole (1982) concluded these kinds of deposits might be formed 

in ancient valley systems that were subsequently disrupted by younger 

Neogene uplift of the Front Range. Thickness unknown, but appears to 

locally exceed 200 ft

Nbt Troublesome Formation (lower Miocene and upper Oligocene)—Pale 

brown and grayish brown, tuffaceous mudstone and sandstone, volcanic 

ash beds, and granule conglomerate; locally interbedded with basalt and 

trachyandesite (unit bba). Unit is sparsely preserved along the eastern 

margin of the quadrangle; base is a significant unconformity with as much 

as 650 ft of local relief on older rocks (Braddock and Cole, 1990; Cole and 

Braddock, 2009). Lower part of Troublesome Formation is locally 

interlayered with Oligocene basalt (26.4 + 0.9 Ma) and latite breccia (28.4 

+ 2.3 Ma) in the Granby Basin to the south (Izett, 1974). Thickness 

variable due to pre- and post-Troublesome erosion, but locally exceeds 

2,500 ft in the Granby Basin (Izett, 1974); only a few tens of feet of 

Troublesome remain in the Bowen Mountain area

POST-LARAMIDE INTRUSIVE ROCKS

Nbi Intrusive rocks, undivided (Miocene(?) to Oligocene)—Dikes, sills, and 

small plugs; typically very fine grained, porphyritic, and altered; 

compositions range from basalt and andesite to rhyolite. Intrusions are 

chiefly located in Never Summer Mountains and westward in Illinois River 

drainage (Metzger, 1974; O’Neill, 1981)

brp Rhyolite porphyry (late Oligocene)—Variegated pale-pink, cream, or lightreddish-brown, flow-banded, porphyritic rock containing 20–40 percent 

phenocrysts of quartz, sanidine, oligoclase, biotite, and brown hornblende 

in an aphanitic matrix; forms small plugs. Intrusive body south of Porphyry 

Peaks was dated at 25.6 + 2.0 Ma and 26.2 + 2.3 Ma (zircon fission-track; 

ages corrected in 1979 for revised decay constants) by Naeser and others 

(1973)

blx Intrusive tuff-breccia (late Oligocene)—Yellowish-gray to grayish-yellowgreen, poorly sorted, weakly bedded tuff-breccia consisting of fragments of 

cognate volcanic rock (lapilli- and ash-sized vesicular clasts of the erupted 

magma), comminuted Proterozoic rocks, and Oligocene igneous rocks 

(andesite porphyry and quartz latite). Forms a singular pipe-like body 

northwest of Bowen Mountain that covers an area about 6,000 ft by 4,000 
12

ft (Metzger, 1974) within Proterozoic rocks; planar fabric (pyroclastic 

layering) defined by clast sorting dips moderately inward. Tuff-breccia is 

intruded by quartz latite and latite porphyry (Metzger, 1974), presumably 

of late Oligocene age. Metzger (1974) reported no Phanerozoic 

sedimentary clasts in the tuff-breccia, but Braddock (in Braddock and Cole, 

1990) described “sparse siltstone and sandstone of possible Mesozoic age” 

within the pipe deposits.

bbi Intrusive basalt (late Oligocene)—Dark gray to black, dense rock intruded 

into Pierre Shale in a single body located about 1.4 mi east of Gravel 

Mountain. Age is inferred from correlation to similar mafic igneous rocks 

in adjoining areas (Izett, 1974; Cole and Braddock, 2009)

bqla Quartz latite of Apiatan Mountain (late Oligocene)—Gray to brownish-gray 

porphyry containing about 25–40 percent phenocrysts of pale-yellow 

oligoclase-andesine (as long as 3 cm), glassy twinned sanidine, minor 

hexagonal biotite, and minor clinopyroxene in a microcrystalline 

groundmass rich in potassium feldspar; intruded at 28.4 + 2.3 Ma (zircon 

fission-track; Marvin and others, 1974; Izett, 1974; corrected in 1979 for 

revised decay constants)

POST-LARAMIDE VOLCANIC ROCKS

bv Volcanic rocks, undivided (upper Oligocene)—A singular, small deposit of 

felsic volcanic ash and breccia is located in the south side of the 

topographic saddle between the Porphyry Peaks; thickness less than 40 ft

bba Basalt and trachyandesite (upper Oligocene)—Dark-gray and dark greenishgray, very fine grained rock that weathers to grayish brown or grayish 

purple. Locally contains 10–15 percent small phenocrysts of pyroxene and 

altered olivine(?); vesicles locally filled with fibrous zeolite minerals. Unit 

only observed near intrusive quartz latite (bqla) and rhyolite porphyry 

bodies (brp) in the southwestern part of the quadrangle. Interlayered with 

the Troublesome Formation (Nbt) in the adjoining Trail Mountain 

quadrangle (Izett, 1974); erupted at 26.7 + 0.9 Ma (whole-rock K-Ar; Izett, 

1974; Marvin and others, 1974; corrected in 1979 for revised decay 

constants)

LARAMIDE SEDIMENTARY AND VOLCANIC ROCKS

Middle Park Formation (Paleocene and Upper Cretaceous?)—Thick 

fluvial and alluvial deposits transported into the broad, synclinal Middle 

Park–North Park structural basin during late Laramide uplift of the adjacent 

Front Range and Vasquez Mountains Proterozoic basement blocks. Only 

partially preserved in this area, but thickness exceeds 7,000 ft in region 

west of this quadrangle in Middle Park Basin (Izett, 1968). The volcanic 

and arkosic units were mapped in adjacent areas to the west by Kinney 

(1970).

bma Arkosic unit (Paleocene)—Interbedded light- to medium-brown and gray, 

arkosic sandstone and siltstone, and lenses of pebble to boulder 

conglomerate characterized by clasts of Proterozoic Silver Plume-type 
13

granite, pegmatite, felsic gneiss, biotite gneiss, and rare hornblende gneiss, 

in addition to mafic-to-intermediate volcanic porphyries. Contact with 

underlying volcanic unit is gradational, and felsic, Proterozoic clasts 

increase in abundance upward. Thickness incomplete due to erosion but 

exceeds 2,000 ft on east slope of Gravel Mountain

bmv Volcanic unit (Paleocene)—Interbedded medium-brown, greenish-gray, 

and gray sandstone, siltstone, and pebble-cobble conglomerate marked by 

conspicuous clasts of brown, purple, and greenish-gray, mafic volcanic 

porphyry; locally contains some felsic Proterozoic clasts. Generally 

overlies the Windy Gap Volcanic Member; thickness approximately 1,500 

ft on east slope of Gravel Mountain

Kmw Windy Gap Volcanic Member (Upper Cretaceous?)—Brown or 

greenish-brown, gray, or purple, massive volcanic conglomerate and 

discontinuous volcanic sandstone; interbedded with rare, probable flows of 

trachyte porphyry or basalt. Thickness about 200 ft along western 

quadrangle boundary

PRE-LARAMIDE SEDIMENTARY ROCKS

Pierre Shale (Upper Cretaceous)—Thick, dark-gray to black, marine shale; 

unit poorly exposed in the quadrangle in the footwall of the Never Summer 

reverse fault. Sandy beds in the middle of the Pierre are inferred to 

correlate with sandstone members (for example, the Hygiene, Richard, 

Larimer, or Rocky Ridge Sandstone Members) recognized in the region

(Izett, 1968, 1974; Scott and Cobban, 1986); total thickness is as great as 

6,200 ft in Denver Basin (Higley and Cox, 2005)

Kpu Upper shale unit—Primarily dark-gray to black silty shale above the 

Middle sandy unit; erosionally truncated northward by the Windy Gap 

Volcanic Member of the Middle Park Formation. Thickness unknown

Kps Middle sandy unit—Light-brown, fine- to medium-grained marine 

sandstone beds and interlayered black shale; thickness estimated to be 

about 300–500 ft

Kpl Lower shale unit—Primarily dark-gray to black silty shale beneath the 

Middle sandy unit; thickness indeterminate due to folding and poor 

exposure

Kn Niobrara Formation (Upper Cretaceous)—Light-gray-weathering, platy, 

calcareous shale; weathers buff near top; thin, light-gray biomicrite beds 

near middle; thin, light-gray micritic limestone at base. Thickness 

indeterminate due to folding and poor exposure

Kd Dakota Sandstone (Lower Cretaceous)—Light-gray, fine-grained sandstone 

and interbedded shale in upper part, and chert-pebble conglomerate and 

sandstone in lower part; total thickness about 175–230 ft in nearby areas 

(Izett, 1986), but no complete sections exposed here

Jm Morrison Formation (Upper Jurassic)—Green, red, yellow, and white 

blocky-weathering claystone, siltstone, and gray micrite, and gray, fine- to 
14

medium-grained sandstone. Singular occurrence is located along the 

southern quadrangle boundary just west of Apiatan Mountain; thickness 

indeterminate due to poor exposure

dPc Chugwater Formation and older units (Lower Triassic and Upper 

Permian)—Reddish-brown to orange-red shale, siltstone, and fine-grained 

sandstone; laminated to thin bedded; poorly exposed. Locally contains 

light-gray to white finely laminated limestone (possible Forelle Limestone 

Member of Late Permian age) and underlying red shale (possible Satanka 

Shale). Occurrence is limited to two small localities immediately 

underneath the Never Summer reverse fault. Thickness indeterminate due 

to poor exposure and structural disruption

MESOPROTEROZOIC INTRUSIVE ROCKS 

YgLP Granite of Longs Peak batholith—Light- to medium-gray, grayish-orange, 

orange-pink, or red-purple monzogranite to syenogranite that contains 

characteristic tabular microcline phenocrysts. Biotite is the principal dark 

mineral (5–12 percent), locally accompanied by magmatic sillimanite and 

(or) garnet plus minor accessory minerals. Equigranular matrix is locally 

fine, medium or coarse grained and weakly foliated by aligned biotite 

grains. Granite intruded the Longs Peak–St. Vrain batholith as a viscous 

magma that extensively deformed metamorphic wall-rocks (Cole, 1977; 

Braddock and Cole, 1979, 1990). Intruded at 1,420 +

MESOPROTEROZOIC AND (OR) PALEOPROTEROZOIC INTRUSIVE ROCKS 

25 Ma (Rb-Sr 

isochron; Silver Plume Granite of Peterman and others, 1968). Secondary 

muscovite (non-oriented, porphyroblastic) is locally present and formed by 

subsolidus hydration of sillimanite + potassium feldspar or breakdown of 

primary biotite to muscovite + magnetite

YXp Pegmatite—White, medium-grained to very coarse grained pegmatite with 

accessory biotite, muscovite, garnet, and (or) tourmaline. Most pegmatites 

probably intruded along with granite of Longs Peak batholith (YgLP) but 

some are folded with enclosing metamorphic rocks and may be related to 

older intrusive events; reliable field criteria for distinguishing pegmatite 

ages have not been established

PALEOPROTEROZOIC METAMORPHIC ROCKS 

Biotite-bearing schist and gneiss unit (Xbq) is metasedimentary rock; hornblende gneiss 

units (Xh) and granitic-gneiss units (Xf, Xfb) were probably volcanic rocks prior to 

metamorphism (Cole and Braddock, 2009). Protolith of biotite granofels (Xfb) is 

uncertain. The age of metamorphism is 1,713 + 30 Ma (Rb-Sr isochron; Peterman and 

others, 1968) and 1,715–1,690 Ma (U-Pb zircon; Premo and others, 2007), similar to the 

emplacement age for widespread intermediate-composition, calc-alkaline igneous rocks 

of the Routt Plutonic Suite (Premo and Fanning, 2000). The primary age of interbedded 

volcanic rocks ranges between about 1,790 Ma and 1,760 Ma (U-Pb zircon; Premo and 

others, 2007), consistent with model Sm-Nd ages of about 1,800 Ma for formation of 

earliest continental crust in northern Colorado (DePaolo, 1981)
15

Xbq Quartzofeldspathic 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. Dark-gray to black, medium- to coarsegrained layers (uncommon) are rich in biotite, sillimanite, and magnetite, 

and locally contain high-grade cordierite and (or) garnet (Gable and Sims, 

1969; Cole, 1977, 2004b). Irregular quartzofeldspathic layers, bordered by 

selvages rich in biotite, sillimanite, and oxide minerals, formed in place 

during partial melting at peak of metamorphism (Cole and Braddock, 2009)

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

granoblastic to weakly foliated rock, commonly contains garnet and 

sillimanite; locally interlayered with minor quartzofeldspathic biotite gneiss 

and schist (Xb) and amphibolite (Xh). Unit only occurs in northern part of 

this quadrangle and adjoining parts of the southern Never Summer 

Mountains (O’Neill, 1981)

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. Contains thin layers and pods of white to light-green calcsilicate gneiss

Xf Granitic gneiss—Light- to medium-gray, fine- to coarse-grained, moderately

to strongly foliated rock composed primarily of quartz, oligoclase, and 

biotite. Microcline is generally present but amount varies from sparse to 

abundant; hornblende, garnet, and rare sillimanite locally present. Rocks 

have been described as gneissic granite, gneissic quartz monzonite, or 

gneissic granodiorite in adjoining areas, depending on mineralogy or bulk 

chemistry