Geologic Map of the Fraser 7.5-minute Quadrangle, 

Grand County, Colorado

U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3130
COVER:

View to the southeast, from high gravel-capped hill near the town of Winter Park, looking toward snowclad peaks along the Continental Divide east of Berthoud Pass. High peak on the far left is James Peak, 

13,294 ft, (4,052 m). In foreground, the dense forest is composed of conifers and aspen, and is growing 

in soils formed in surficial deposits and weathered bedrock. Photograph by R.R. Shroba, September 26, 

2006.
Geologic Map of the Fraser 7.5-minute 

Quadrangle, Grand County, Colorado

By Ralph R. Shroba, Bruce Bryant, Karl S. Kellogg, Paul K. Theobald, 

and Theodore R. Brandt 

U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3130
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2010

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reproduce any copyrighted materials contained within this report.

Suggested citation: 

Shroba, R.R., Bryant, Bruce, Kellogg, K.S., Theobald, P.K., and Brandt, T.R., 2010, Geologic map of the Fraser 

7.5-minute quadrangle, Grand County, Colorado: U.S. Geological Survey Scientific Investigations Map 3130, 

scale 1:24,000, 1 sheet, 26 p. pamphlet. [Available at URL http://pubs.usgs.gov/sim/3130].

..
iii

Introduction.....................................................................................................................................................1

Description of Map Units..............................................................................................................................5

Surficial Deposits..................................................................................................................................5

Artificial Fill Deposits...................................................................................................................5

Alluvial Deposits...........................................................................................................................5

Alluvial and Mass-Movement Deposits...................................................................................8

Mass-Movement Deposits.........................................................................................................9

Glacial Deposits .........................................................................................................................10

Bedrock Units......................................................................................................................................11

Post-Laramide Sedimentary and Volcanic Rocks ................................................................11

Pre-Laramide Sedimentary Rocks ..........................................................................................12

Proterozoic Rocks......................................................................................................................12

Eolian Silt Contributions to the Troublesome Formation in the Fraser Basin.....................................14

Structural Setting of the Fraser 7.5-minute Quadrangle .......................................................................16

Geologic History of the Fraser 7.5-minute Quadrangle .........................................................................16

Proterozoic History.............................................................................................................................16

Paleozoic through Laramide History ...............................................................................................17

Post-Laramide Cenozoic History......................................................................................................17

Acknowledgments.......................................................................................................................................21

References Cited..........................................................................................................................................21

Contents

Figures

 1. Index of 7.5- and 15-minute quadrangles referred to in this report..............................3

2. Regional geologic setting of parts of the Front and Gore Ranges 

and adjacent basins..............................................................................................................4

3. Index of geologic mapping in figure 2................................................................................5

4. Plot of mean particle size and degree of sorting (standard deviation of 

the mean particle size) of silty sandstone, siltstone, and clayey siltstone

collected from the Troublesome Formation near Fraser, Colorado.............................15

Tables

1. Particle-size analyses of silty sandstone, siltstone, and clayey siltstone

collected from the Troublesome Formation near Fraser, Colorado......................................2

2. Approximate height, in meters, of top of gravelly stream alluvium 

(Qg and QNg) above the Fraser River and its major tributaries: 

Saint Louis Creek and Ranch Creek in the Fraser 7.5-minute quadrangle........................13

3. Mean annual precipitation near Kremmling, Granby, and Fraser, Colorado ....................15

4. Mean January and July temperatures near Fraser, Colorado, and estimated 

mean January and July temperatures near the Mary Jane site, about 9 km 

southeast of Fraser.....................................................................................................................20
iv

Conversion Factors

Multiply By To obtain

Length

centimeter (cm) 0.3937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

square kilometers (km2

) 0.386 square miles (mi2)

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

°F=(1.8×°C)+32

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

°C=(°F-32)/1.8

Altitude, as used in this report, refers to distance above the vertical datum.
v

Divisions of Quaternary, Neogene, and Paleogene 

time used in this report

Period or Subperiod Epoch Age

Holocene 0–11.5 ka 

 Quaternary

late 11.5–132 ka

Pleistocene middle 132–788 ka

early 788 ka–1.81 Ma

 Neogene Pliocene 1.81–5.33 Ma

 Miocene 5.33–23.0 Ma

 Paleogene Oligocene 23.0–33.9 Ma

Eocene 33.9–55.8 Ma

 Paleocene 55.8–65.5 Ma

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names 

Committee (2007) except those for the late-middle Pleistocene and middle-early Pleistocene 

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

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

North American land mammal ages used in this 

report

Land Mammal Age Age

Hemphillian 4.7–8.9 Ma

 Clarendonian 8.9–11.6 Ma

 Arikareean 19–29 Ma

Ages of time boundaries are those of Woodburne and Swisher (1995).

Introduction 

The geology of the Fraser 7.5-minute quadrangle was 

compiled from previous and new mapping. Theobald mapped 

much of the southern one-third of the quadrangle during 

1959–1963. Many of the geologic units and geologic features 

observed by Theobald along old logging roads and some of the 

aqueduct cuts are no longer visible, due chiefly to the deposition of colluvium and regeneration of dense forest cover. 

Materials available from Theobald’s work in the Fraser quadrangle included a preliminary geologic map of the southern 

one-third of the quadrangle, field notes, and field sheets. None 

of his petrographic information, rock samples, or map-unit 

descriptions was available to the other co-authors.

Bryant made a reconnaissance geologic map of the 

northern two-thirds of the Fraser quadrangle during 1974 as 

part of his work on the Denver 1° × 2° quadrangle geologic 

map (Bryant and others, 1981). During 2005 and 2006, Bryant 

spent a few days each field season on the Fraser quadrangle 

making minor changes to Theobald’s mapping and reconnoitered some unmapped areas, chiefly in the northeast part of 

the quadrangle. Shroba mapped all of the Quaternary and late 

Neogene surficial deposits and Tertiary bedrock units in the 

Fraser quadrangle in eight weeks during 2003 and 2005–2008. 

Kellogg compiled Theobald’s unpublished mapping of the 

Fraser quadrangle and digitized the earlier version of the current geologic map. Brandt prepared the digital compilation of 

the geologic map, added shaded relief on one of the versions 

of the geologic map, and prepared the GIS database.

Surficial deposits within the Fraser quadrangle record 

alluvial, mass-movement, and glacial processes along the 

western boundary of the Front Range and in the eastern 

two-thirds of the Fraser basin during the Quaternary and late 

Neogene. Many of the surficial deposits are poorly exposed. 

Deposits that are limited in extent were not mapped, including 

(1) fill material along roads and at building sites, (2) storage 

piles composed of sand and gravel at an active gravel pit, and 

(3) thin colluvial deposits that commonly mantle the Troublesome Formation (N$t) and older bedrock units. 

The surficial units on this map are informal allostratigraphic units (disconformity-bound sequences) of the North 

American Stratigraphic Code (North American Commission 

on Stratigraphic Nomenclature, 1983), whereas the other map 

units (“bedrock units”) are informal or formal lithostratigraphic or informal lithodemic units. For these reasons, subdivisions of allostratigraphic or lithodemic units use time terms 

“late” and “early” as applied to surficial units or Proterozoic 

crystalline units, but we use position terms “upper” and 

“lower” where applied to lithostratigraphic units. 

Age assignments for surficial deposits within the map 

area are based chiefly on relative heights above present stream 

channels, the relative degree of modification of their original 

depositional morphology, and degree of soil development and 

clast weathering. 

The mapped distribution of surficial map units is based 

primarily on interpretation of 1:40,000-scale, color-infrared 

aerial photographs taken in 1988 (National Aerial Photography Program (NAPP), http://egsc.usgs.gov/isb/pubs/booklets/

aerial/aerial.html) and 1:18,000-scale, natural-color aerial 

photographs taken in 2000 (U.S. Department of Agriculture, 

Farm Service Agency, http://www.fsa.usda.gov/FSA/apfoap

p?area=apfohome&subject=landing&topic=landing). The 

approximate boundaries of one active and three inactive gravel 

pits (indicated by hachured lines) near the northwestern corner 

of the map area are those observed on year 2000 aerial photographs. Grain or particle sizes of surficial deposits in map-unit 

descriptions are field estimates. Size limits for sand (2–0.05 

mm), silt (0.05–0.002 mm), and clay (< 0.002 mm) are those 

of the Soil Survey Staff (1951). Size ranges for the particlessize analyses in table 1 are those of the Wentworth scale (Folk, 

1965). 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 descriptions of clast composition of 

surficial map units, the terms “granite” and “granitic” refer 

chiefly to granite, pegmatite, and monzogranite. The term 

“gneiss” refers chiefly to biotite gneiss and hornblende gneiss. 

In this report, the terms “alluvium” and “alluvial” refer to 

sediment transported by running water confined to channels 

(stream alluvium) as well as by running water not confined 

to channels (sheet flow or unconfined overland flow). The 

term “colluvium” refers to all rock and sediment transported 

downslope chiefly by gravity (Hilgard, 1892; Merrill, 1897). 

Material on slopes is transported chiefly by mass-movement 

Geologic Map of the Fraser 7.5-minute Quadrangle, 

Grand County, Colorado

By Ralph R. Shroba, Bruce Bryant, Karl S. Kellogg, Paul K. Theobald, and Theodore R. Brandt 
2 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

(gravity-driven) processes—such as creep, debris flow, and 

rock fall that are locally aided by running water not confined 

to channels. 

Surficial map units that include debris-flow deposits 

probably also include hyperconcentrated-flow deposits. 

These latter deposits have bedding and sorting characteristics 

intermediate between those of 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. Soil-horizon designations are those of the Soil Survey Staff (1999) and Birkeland 

(1999). Colors of soil horizons, surficial deposits, and siltstone 

of the Troublesome Formation (N$t) are for air-dried samples, and were determined with the aid of Munsell soil color 

charts (Munsell Color, 1973). 

Subsurface lithologic data referred to in this report were 

obtained from driller’s logs on file with the Colorado Division 

of Water Resources (Colorado Division of Water Resources, 

2007).

Most of the bedrock in the Fraser quadrangle is poorly 

exposed. A thin mantle of unmapped colluvium (Qcs) locally 

overlies large areas of Proterozoic rock in the southern and 

eastern parts of the quadrangle. In these and other parts of the 

map area, the Troublesome Formation (N$t) and rhyolite tuff 

($t) are concealed locally by deposits of unmapped colluvium 

(Qcc) derived chiefly from the Troublesome. 

Descriptions of Proterozoic units are in part modified 

from those of Kellogg and others (2008), and are based in part 

on petrographic studies in the Strawberry Lake quadrangle 

(fig. 1) north of the Fraser quadrangle (Schroeder, 1995a) and 

in Vasquez Peak Wilderness Area and nearby roadless areas 

south and southwest of the Fraser quadrangle (Eppinger and 

others, 1985). 

In this report, “migmatite” refers to biotite gneiss containing numerous layers and irregular pods of light-gray or 

white granitic rock (leucosomes), typically 1–5 cm thick but 

locally thicker, which in some places are strongly folded along 

with the enclosing biotite gneiss. Formation of leucosomes is 

generally attributed to partial melting of the biotite gneiss during metamorphism (Olsen, 1982; Johannes and Gupta, 1982; 

Cole and Braddock, 2009). The term “Laramide” (orogeny) 

refers to a protracted period of basement faulting, folding, 

magmatic activity, and regional uplift between Late Cretaceous and early Eocene across the southern Rocky Mountains 

(Tweto, 1975).

The community that was named Hideaway Park is now 

part of the town of Winter Park Colo., and the name has been 

removed from the base map used in this report. In keeping with 

current usage, the name Winter Park is used in this report. 

The ski area, about 3 km south of the old Hideaway Park area 

(now labeled Winter Park), is referred to as the Winter Park 

Ski Area.

Heading 1 1

Table 1. Particle-size analyses of clayey siltstone, siltstone, and silty sandstone collected from 

Troublesome Formation near Fraser, Colorado.

[Samples were disaggregated using a porcelain mortar and pestle, and were treated with hydrogen peroxide to remove 

organic matter and 15 percent hydrochloric acid (HCI) to remove carbonates. Sodium hexametaphosphate (NaPO ) 3 6

was then added as a dispersant to all samples. Following this procedure, the samples were shaken on a shaker table 

for about four hours to ensure deflocculation of the clay-size particles. Particle size was determined using a Malvern 

Mastersizer–S long-bed laser particle-size instrument. Each sample was introduced into an aqueous medium and 

pumped through the laser analyzer for grain-size measurements. Eric Fisher (USGS) analyzed the samples. Values are 

fraction of total volume (percent).]

Wentworth

size class1

size range 

 in microns

size range 

 in phi units

silty

sandstone2 siltstone

clayey

siltstone

Very coarse sand 2000–1000 -1–0 0.0

Percent

0.0 0.0

Coarse sand 1000–500 0–1 3.3 0.0 0.0

Med. sand 500–250 1–2 18.4 4.2 0.2

Fine sand 250–125 2–3 19.3 16.7 4.9

Very fine sand 125–62.5 3–4 19.5 25.4 12.6

Coarse silt 62.5–31 4–5 15.7 22.5 17.2

Medium silt 31–16 5–6 8.7 12.4 14.1

Fine silt 16–8 6–7 5.3 6.8 11.8

Very fine silt 8–4 7–8 3.5 4.0 11.0

“coarse” clay 4–2 8–9 2.4 2.5 9.1

“medium” clay 2–1 9–10 1.5 1.6 6.8

“fine” clay 1–0.5 10–11 0.9 1.2 5.2

“very fine” clay <0.5 >11 1.6 2.7 7.2

sum 100.0 100.0 100.0

1

Size ranges are those of the Wentworth scale (Folk, 1965). Terms in quotation marks are informal terminology. 

2

Lithologic designations are based on field descriptions. 

Table 1. Particle-size analyses of silty sandstone, siltstone, and clayey siltstone 

collected from the Troublesome Formation near Fraser, Colorado.
00 5 10 MILES

5 10 KILOMETERS

HOT SULPHUR SPRINGS

KREMMLING

TRAIL

MOUNTAIN

 SHADOW

MOUNTA

STRAWBERRY

LAKE

GRANBY

BOTTLE PASS

FRASER

BYERS PEAK

BERTHOUD

PASS

Kremmling

Gore Canyon

Parshall

Granby

 canyon

River

Fraser Tabernash

Fraser

Winter

Park

Green MountainReservoir

Williams ForkReservoir

Williams

Fork

106°30’

40°15’

106°15’ 106°00’ 

40°00’

39°45’

Hot SulphurSprings

Mary Jane site

Blue

Red Mountain

Lake

Granby

COLORADO

RIVER

River

Bertho

Pass

GrandLake

Shadow MountLake

)(

Figure 1. Index of 7.5- and 15-minute quadrangles referred to in this report. Fraser 7.5-minute quadrangle in yellow. Canyons are depicted with black line is the Continental Divide.
4 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

SHEAR ZONE

5 10 20 30 KILOMETERS

5 10 15 MILES

0

0

Ng

Granby

Fraser basin

Tabernash

Fraser

Winter Park

Yp

Yp

Yp

Yp

Yp

106°00'

Granby basin

Lake 

Granby

Fraser 

Xm

Xm

Xm

Xm

Xm

Xm

Xm

}

}

}

}

}

}

}

}

COLORADO

Troublesome

Williams Fork Reservoir

basin

Ng

Kremmling

Range

Green Mountain

Reservoir

Yp

Yp

Silver Plume

batholith

Hot Sulpher

Xp Springs

Williams Fork

|

Yp Yp VASQUEZ PASS bi

106°30' 106°15' 105°45'

40°15'

40°00'

39°45'

Gore

1 2

3

4 5

6 7

11 8

7

River

Vasquez Mountains

Q

Xp

Xp

Xp

Front

Range

Ng

Xp

$Km

N$r $i

$i

$i $i

$km

$km

$km

$ki

$ki

$i

N$r

N$r

N$t

N$t

N$t

N$t

N$t

$a

N$t

RABBIT EARS RANGE

Ng

Xm

Xm

Xm

Xm

Xm

Xm

Xm Xm

Yp

Xp

Xp

Grand

Lake

Shadow Mountain

Lake

Q

Q

Yp

Yp

Yp

Yp

MIDDLE

PARK

RIVER

Blue

River

SAINT LOUIS LAKE 

ST LOUIS CREEK FAULT

BERTHOUD

PASS

FAULT

ZONE

FAULT

CONTINENTAL

DIVIDE

Never Summer North Park Mtns.

Williams

Fork

Mountains

Q

Ng

$a

N$t

N$r

$i

$km

$ki

}

|

Yp

Xp

Xm

Surficial deposits (Quaternary)—Mostly till and outwash

Gravel (Pliocene? and Miocene)

Trachyandesite flows (late Oligocene)

Troublesome Formation (Miocene and upper Oligocene)

Rabbit Ears Volcanics (early Miocene? and Oligocene)

Latite and rhyolite plutons (Oligocene)

Middle Park Formation (Paleocene and Upper Cretaceous?)

Laramide plutons (Paleocene and Late Cretaceous)

Mesozoic sedimentary rocks

Paleozoic sedimentary rocks

Contact

Normal Fault with Neogene movement—Bar and

ball on downthrown side. Dashed where approximate, 

dotted where concealed

Thrust or reverse fault with inferred Laramide

movement—Teeth on upthrown side. Dashed where 

approximate

Fault, unspecified—Mostly Laramide. Dashed where 

approximate, dotted where concealed

Continental Divide

Shear zone 

Mesoproterozoic plutonic rocks

Paleoproterozoic plutonic rocks

Paleoproterozoic metamorphic rocks and small

plutons

EXPLANATION

Figure 2. Regional geologic setting of parts of the Front and Gore Ranges and adjacent basins. Quadrangles and 

corresponding geologic maps are as follows: (1), Kremmling, 15 minute (Izett and Barclay, 1973); (2) Hot Sulphur 

Springs 15 minute (Izett, 1968); (3) Trail Mountain (Izett, 1974); (4) Granby (Schroeder, 1995b); (5) Strawberry Lake 

(Schroeder, 1995a); (6) Bottle Pass (Taylor, 1975); (7) Fraser (this report); and (8) Berthoud Pass (Theobald, 1965). 

Oligocene age of trachyandesite flows south of Green Mountain Reservoir is based on 40Ar/39Ar sanidine ages of 

two of these flows dated 23.62±0.6 Ma and 23.63±0.06 Ma (Naeser and others, 2002).
Description of Map Units 5

Tweto (1979a)

Cole and 

Braddock 

(2009)

Kellogg and 

others 

(2008)

Shroba,

unpublished

mapping

(2008)

Winter

Park

106°30' 106°15' 106°00' 105°45'

40°15'

40°00'

39°45'

KREMMLING HOT SULPHUR

SPRINGS

TRAIL

MOUNTAIN

GRANBY STRAWBERRY

LAKE

BOTTLE

PASS

FRASER

BERTHOUD

PASS

SHADOW

MOUNTAIN

Figure 3. Index to sources of geologic mapping. Mapping by Tweto in yellow, Shroba in 

purple, Cole and Braddock in green, and Kellogg and others in blue. Gray lines indicate U.S. 

Geological Survey quadrangles. Kremmling and Hot Sulpher Springs are 15-minute 

quadrangles, all other quadrangles depicted are 7.5 minute.

Figure 3. Index of geologic mapping in figure 2.

Description of Map Units

Surficial Deposits

Artificial Fill Deposits

af Artificial fill (latest Holocene)—Compacted fill material composed mainly of silt, sand, and rock fragments. 

Mapped beneath railroad tracks near the Winter Park Ski Area and the town of Tabernash, Colo., the highway 

US 40 overpass east of Tabernash, and parking lots and man-made structures at the Winter Park Ski Area. 

Distribution based on interpretation of color aerial photographs taken in 2000. Estimated thickness is 1–10 m 

Alluvial Deposits

Qva Valley-floor alluvium (Holocene and late Pleistocene?)—Mostly poorly sorted coarse sand and pebbly to cobbly gravel in stream channels and flood plains along the Fraser River and streams in glaciated tributary drainages where it probably overlies, and locally may include, gravelly stream-terrace alluvium (Qg1

). Unit Qva

probably consists mostly of poorly sorted sand and gravel along non-glaciated tributary streams and locally 

contains a significant amount of silty alluvium in areas underlain by the Troublesome Formation (N$t). 

Locally may include organic-rich sediments in marshes and other wet areas. The depositional top of the unit 

is commonly less than 5 m above the Fraser River and its major tributary streams near the towns of Fraser 

and Tabernash, Colo. Gravel deposits along Fraser River and glaciated tributary streams are potential sources 

of coarse aggregate. Low-lying deposits are prone to periodic stream flooding. Estimated thickness is 1–5 m
6 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

Qsa Sheetwash deposits and valley-floor alluvium, undivided (Holocene and late Pleistocene?)—Chiefly silty 

and sandy sheetwash deposits (Qsw) on gentle slopes and a minor amount of undifferentiated sandy and 

pebbly valley-floor alluvium (Qva) along and near ephemeral streams along the south side of Ranch Creek, 

in the northeast part of the map area. Low-lying areas of unit Qsa in and adjacent to stream channels may 

be subject to periodic stream flooding, gullying, and possibly debris-flow deposition; adjacent slopes may be 

subject to periodic sheet flooding due to sheet flow. Estimated thickness is 1–5 m

Qsw Sheetwash deposits (Holocene and late Pleistocene)—Pebbly, slightly silty sand to sandy, clayey silt derived 

chiefly from Troublesome Formation (N$t) and Proterozoic bedrock along Ranch Creek; mostly sandy, 

clayey silt derived chiefly from the Troublesome Formation along Elk Creek in the southwest part of the map 

area. Low-lying areas of unit Qsw may be subject to sheet flooding due to sheet flow, and locally to stream 

flooding and gullying. Estimated thickness is 1–5 m

 Gravelly stream-terrace alluvium (late to early? Pleistocene)—Glacial outwash gravel deposited chiefly by 

the Fraser River and its major tributaries Saint Louis Creek and Ranch Creek during two or more glaciations 

as well as fluvial gravel of non-glacial origin along streams tributary to the Fraser River that head in nonglaciated drainage basins. Some of the fluvial gravel of non-glacial origin may be coeval with glacial episodes. 

Units Qg1 through Qg8 are subdivided based on height above present stream level (table 2).

Qg1 Unit 1 (late Pleistocene)—Glacial outwash composed of poorly sorted, clast-supported, cobbly pebble gravel 

deposited by meltwater streams during the Pinedale glaciation and fluvial gravel of non-glacial origin on 

the north side of Pole Creek near Tabernash. The unit locally contains lenses of pebble gravel, pebbly sand, 

and fine to medium sand about 15–40 cm thick. Deposits near outer limit of glaciation locally contain a few 

small boulders about 25–50 cm in diameter. Large pebbles and cobbles commonly are subangular and subrounded granite and gneiss; some are rounded. Matrix is mostly coarse to very coarse sand that is unoxidized 

(10YR 7/2) at a depth of >1 m. Thin (20–35 cm) overbank sediments composed of slightly pebbly silty sand 

and locally sandy silt and sandy clayey silt locally overlie the gravel. Fluvial deposits of non-glacial origin 

on the north side of Pole Creek, about 1 km southwest of Tabernash, consist of slightly cobbly pebble gravel 

overlain by about 35 cm of slightly pebbly, sandy, clayey silt overbank sediment. Unit underlies one and 

locally two terrace treads, and probably locally underlies valley-floor alluvium (Qva). Terrace treads are 

about 6 and 12 m above the Fraser River near the towns of Winter Park and Fraser; about 6 m above the 

Fraser River near Tabernash and Saint Louis Creek near Fraser; about 6–9 m above Ranch Creek; and about 

6 m above Crooked Creek and Pole Creek (table 2). Unit locally may include a minor amount of valley-floor 

alluvium (Qva). Unit Qg1

 is a potential major source of coarse aggregate. Deposits of unit Qg1 in glaciated 

valleys are composed of glacial outwash deposited during the Pinedale glaciation, about 12–30 ka (Nelson 

and others, 1979; Madole, 1986; Benson and others, 2004, 2005). Cosmogenic dating of Pinedale outwash, 

4–14 m above stream level in Boulder Canyon east of Nederland, Colorado, yielded surface-exposure ages of 

10–32 ka (Schildgen and others, 2002). Unit thickness decreases in a downstream direction; estimated thickness along the Fraser River about 10 m near Winter Park and Fraser, and about 1–2 m near Tabernash

Qg2 Unit 2 (late and middle Pleistocene)—Glacial outwash composed of poorly to moderately well sorted, clastsupported, cobbly pebble gravel deposited by meltwater streams during the Bull Lake glaciation and fluvial 

deposits of non-glacial origin along Crooked Creek near Tabernash. The unit locally contains lenses of 

pebble gravel, pebbly granule-rich sand, and sand. Deposits near outer limit of glaciation, locally contain 

a few small boulders about 25–35 cm in diameter and are rarely as large as 70–120 cm in diameter. Large 

pebbles and cobbles commonly are subangular and subrounded granite and gneiss; some are rounded and 

well rounded. Matrix is mostly coarse to very coarse sand that is slightly oxidized (10YR 7/4). Pebbly sand 

or sandy silt overbank sediments, about 1–3 m thick, locally overlie the gravel. Unit underlies till of the Bull 

Lake glaciation (Qtb) about 0.5 km north of Vasquez Creek in Winter Park. Fluvial deposits of non-glacial 

origin along Crooked Creek, about 3 km southwest of Tabernash, consist of slightly cobbly pebble gravel 

overlain by about 2.5–3 m of slightly pebbly and cobbly, sandy, clayey silt overbank sediment that contains 

lenses of cobbly pebble gravel. Unit underlies one and locally two terrace treads about 24 m above the Fraser 

River near Winter Park and Fraser; about 12 and 18 m above the Fraser River east of Tabernash; about 12 m 

above Saint Louis Creek near Fraser; about 18 m above Crooked Creek; and about 18–24 m above Ranch 

Creek (table 2). About 15 km downstream of Tabernash at Granby, Colo. (fig. 1), glacial outwash of Bull 

Lake age underlies a terrace tread about 12 m above the Fraser River (Meierding, 1977). The soil that formed 

in pebbly alluvium beneath the 12-m high, fan-shaped terrace along Saint Louis Creek west of Fraser has 

a brown (7.5YR 5/4) argillic B (Bt) horizon 35 cm thick. Deposits of unit Qg2

, about 3 km northwest of 

Fraser, are a major source of sand and gravel for construction and other uses. The proximity of till of the Bull 

Lake glaciation (Qtb) to outwash terraces underlain by unit Qg2

 suggests that unit Qg2 was deposited by 
Description of Map Units 7

glacial meltwater during the Bull Lake glaciation (about 120–170 ka; Sharp and others, 2003; Pierce, 2004). 

Recent cosmogenic dating of Bull Lake outwash, 16 m above stream level in Boulder Canyon east of Nederland, Colo., yielded a surface-exposure age of 130 ka (Schildgen and others, 2002). Estimated thickness is 10 m 

along the Fraser River near Winter Park, and 5 m downstream near Tabernash; 18–20 m along Saint Louis 

Creek near Fraser; and 3–6 m along Ranch Creek and Crooked Creek

Qg3 Unit 3 (middle Pleistocene)—Probably glacial outwash composed of poorly sorted, clast-supported, cobbly 

pebble gravel deposited by meltwater streams during a pre-Bull Lake glaciation. The unit locally contains 

lenses of pebble gravel, pebbly sand, and sand. Large pebbles and cobbles commonly are subangular and 

subrounded granite and gneiss; some are rounded. Cobbles are commonly as large as 15–25 cm in diameter. 

Matrix is mostly coarse to very coarse sand that is slightly (10YR 7/4) to strongly oxidized (10YR 6/6). 

Overbank sediments composed of pebbly sand or clayey silt, about 1 m thick, locally overlies the gravel. 

Probably much of this clayey silt is derived from siltstone of the Troublesome Formation (N$t). A small 

deposit of pink (7.5YR 6/4) sandy (mostly very fine to fine sand) silt, too small to show on the map, mantles 

a low east-facing fluvial scarp at the eastern boundary of unit Qg3, about 1 km south of Tabernash. This 

sandy silt probably is eolian sediment (loess) deflated from nearby flood plains, possibly during the Pinedale 

glaciation. Unit consists of six terrace and stream-channel deposits about 30 m above the Fraser River south 

and southeast of Fraser; about 35 m above the Fraser River near Tabernash; and about 30-35 m above Ranch 

Creek east of Fraser (table 2). Estimated thickness is 10 m near Fraser and 8–10 m south of Tabernash 

Qg4 Unit 4 (middle Pleistocene)—Probably glacial outwash composed of poorly sorted, clast-supported, cobbly 

pebble gravel deposited by meltwater streams during a pre-Bull Lake glaciation, as well as fluvial gravel 

of non-glacial origin along Spring Branch (tributary of Crooked Creek). The unit locally contains lenses of 

pebble gravel, pebbly, granule-rich sand, and sand. Deposits near Fraser locally contain a few small boulders 

about 25–35 cm in diameter. Large pebbles and cobbles commonly are subangular and subrounded granite 

and gneiss. Cobbles are commonly as large as 20 cm in diameter. Matrix is mostly coarse to very coarse 

sand. Overbank sediment, about 1–1.5 m thick, composed of (1) slightly pebbly, silty, mostly very fine to 

medium sand as well as (2) slightly pebbly, sandy, clayey silt locally overlies the gravel. Unit consists of 

small terrace and probably stream-channel deposits about 50–55 m above the Fraser River east and southeast 

of Fraser, along Ranch Creek northeast of Fraser, and along Spring Branch southwest of Fraser (table 2). 

Estimated thickness is 16 m east of Fraser and 3 m or more along Ranch Creek 

Qg5 Unit 5 (middle Pleistocene)—Fluvial gravel of non-glacial origin and possibly glacial outwash composed 

of slightly cobbly pebble gravel deposited during a pre-Bull Lake glaciation. Large pebbles and cobbles 

commonly are subangular and subrounded granite and gneiss. Cobbles are commonly as large as 10 cm in 

diameter. Matrix is mostly coarse to very coarse sand. Unit consists of three small terrace or steam-channel 

deposits about 65 m above the Fraser River southeast of Tabernash; about 65 m above Ranch Creek northeast 

of Fraser; and about 60 m above Pole Creek west of Tabernash (table 2). The deposits southeast of Tabernash 

and northeast of Fraser may have accumulated along a former course of Ranch Creek. Estimated thickness is 5–10 m 

Qg6 Unit 6 (middle Pleistocene)—Fluvial gravel of non-glacial origin and possibly glacial outwash composed of 

slightly bouldery, cobbly, pebble gravel deposited during a pre-Bull Lake glaciation. Large pebbles and 

cobbles commonly are subangular and subrounded granite and gneiss. Cobbles are commonly as large as 

20–25 cm in diameter; boulders are as large as 50 cm in diameter. Matrix is mostly coarse to very coarse 

sand. Unit consists of two terrace or stream-channel deposits about 85–90 m above the Fraser River, southeast and northeast of Fraser (table 2). A stream tributary to the Fraser River, possibly a former course of 

Ranch Creek, deposited the alluvium northeast of Fraser. Estimated thickness is 5–10 m 

Qg7 Unit 7 (middle or early Pleistocene)—Fluvial gravel of non-glacial origin composed of cobbly pebble gravel 

that was deposited by streams tributary to the Fraser River, possibly during a pre-Bull Lake glaciation. The 

upper part of the unit locally consists of matrix-supported, slightly pebbly and cobbly, slightly sandy, clayey 

silt. The clayey silt probably was derived chiefly from siltstone of the Troublesome Formation (N$t). Unit 

locally contains a few small boulders about 35–45 cm in diameter that are composed of granite, gneiss, 

and sandstone probably derived from the Dakota Group (Kd) as well as a few rounded red (10R) siltstone 

pebbles that may have been eroded from the Morrison Formation (Jm). Cobbles are commonly as large as 

10–25 cm in diameter. Large pebbles and cobbles commonly are subangular granite and gneiss along with 

a minor amount of sandstone probably derived from the Dakota Group. Matrix of gravel deposits is mostly 

coarse to very coarse sand. Unit consists of four small stream-channel(?) deposits about 105–115 m above 

the Fraser River northeast and southeast of Fraser (table 2). Estimated thickness is 3–10 m 

Qg8 Unit 8 (middle or early Pleistocene)—Fluvial gravel of non-glacial origin or possibly glacial outwash composed 

of poorly sorted, clast supported, slightly bouldery, cobbly pebble gravel deposited by the Fraser River, 
8 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

during a pre-Bull Lake glaciation. The unit locally contains (1) lenses of granule-rich, medium to very coarse 

sand; (2) a minor amount of boulders about 25–120 cm in diameter composed of granite, pegmatite, and 

sandstone probably derived from the Dakota Group (Kd); (3) a few rounded rhyolite ($t) and red (10R) 

siltstone pebbles (possibly eroded from the Morrison Formation, (Jm); and (4) rare conglomerate clasts 

derived from the Troublesome Formation (N$t). Large pebbles and cobbles commonly are subangular and 

subrounded granite and gneiss as well as a minor amount of subangular pegmatite and sandstone probably 

derived from the Dakota Group. Cobbles are commonly as large as 25 cm in diameter. Matrix is poorly 

sorted, mostly coarse to very coarse sand that contains abundant granules; unoxidized matrix is pinkish gray 

(7.5YR 7/2). Unit consists of two terrace or stream-channel deposits about 145 m above the Fraser River 

about 1.5 km northeast and southeast of Winter Park (table 2). Thickness is about 2–12 m 

QNg Old gravelly stream alluvium (early Pleistocene or Pliocene)—Unit consists of two poorly exposed slightly 

bouldery deposits that overlie Proterozoic bedrock, and locally Troublesome Formation (N$t), on high 

ridges near the southeast corner of the map area. Deposits consist of poorly sorted, matrix-supported and 

clast-supported, slightly bouldery, cobbly pebble fluvial gravel. These deposits probably contain lenses of 

pebble gravel and sand. Matrix is mostly coarse to very coarse sand that contains abundant granules. Large 

pebbles and cobbles commonly are subangular and subrounded granite and gneiss. Boulders are composed 

chiefly of granite, gneiss, and pegmatite, as well as a minor amount of sandstone probably derived from the 

Dakota Group (Kd) that are commonly as long as 50–170 cm. Deposits contain a few rounded red (10R) siltstone pebbles probably eroded from the Morrison Formation (Jm). Top of unit is about 245–270 m above the 

Fraser River southeast of Winter Park (table 2). Unit QNg may be correlative with the boulder conglomerate 

of Pliocene(?) age in the nearby Trail Mountain quadrangle (Izett, 1974). Estimated thickness is 5–10 m

Alluvial and Mass-Movement Deposits

Qac Alluvium and colluvium, undivided (Holocene and late Pleistocene)—Chiefly undifferentiated valley-floor 

alluvium (Qva) along minor streams and colluvium (Qcc and Qcs) on adjacent slopes. Matrix of colluvial 

deposits is sandy (Qcs) in areas where deposits are derived chiefly from Proterozoic bedrock, and rich in silt 

(Qcc) where derived chiefly from the Troublesome Formation (N$t). Low-lying areas of the unit adjacent to 

stream channels may be subject to periodic stream flooding and debris-flow deposition; sloping areas outside 

of stream channels may be subject to periodic sheet flooding due to sheet flow. Estimated thickness is 1–5 m

Qf Fan deposits (Holocene and late Pleistocene)—Non-sorted to poorly sorted, fan-shaped masses composed 

chiefly of pebble-and-cobble gravel and silty sand southeast of Tabernash and bouldery, pebble-and cobblegravel northwest of the Winter Park Ski Area. Deposited chiefly by debris flow, stream flow, and probably by 

hyperconcentrated flow. Deposits southeast of Tabernash locally may include clayey-silt matrix colluvium 

(Qcc). Low-lying areas of the unit may be subject to debris-flow deposition and stream flooding. Estimated 

thickness is 3–15 m

Qp1 Lower pediment deposits (late Pleistocene)—Crudely stratified, matrix-supported small boulders to granules 

in a sandy, clayey silt matrix overlies northeast sloping surface formed on Troublesome Formation (N$t) on 

the west side of Crooked Creek southwest of Tabernash. Matrix makes up much of the unit, and is chiefly 

derived from siltstone of the Troublesome Formation. Unit probably deposited chiefly by debris flows, 

hyperconcentrated flows, and ephemeral streams under periglacial conditions probably during the Pinedale 

glaciation. Large pebbles and cobbles commonly are angular to subangular and consist of sandstone probably 

derived from the Dakota Group (Kd), gneiss, and pegmatite. Sandstone boulders are commonly as long as 

50 cm. Lower limit of unit is about 6 m above Crooked Creek. Estimated thickness is 2–5 m

Qp2 Higher pediment deposits (late and middle Pleistocene)—Crudely stratified, matrix-supported small boulders 

to granules in a sandy, clayey silt matrix that overlies a northeast sloping surface formed on Troublesome 

Formation (N$t) on the west side of Crooked Creek southwest of Tabernash. Matrix makes up much of the 

unit, and is chiefly derived from siltstone of the Troublesome Formation. Unit probably deposited chiefly by 

debris flows, hyperconcentrated flows, and ephemeral streams under periglacial conditions probably during

the Bull Lake glaciation. Large pebbles and cobbles commonly are angular to subangular and consist of 

sandstone probably derived from the Dakota Group (Kd), gneiss, pegmatite, and locally a minor amount of 

porphyritic andesite probably derived from the Middle Park Formation in the adjacent Bottle Pass quadrangle 

(Taylor, 1975) on the west side of the map area (fig. 1). Boulders composed of sandstone and locally volcanic 

porphyry are commonly as long as 40 cm. Lower limit of unit is about 12–18 m above Crooked Creek. Eastern margin of unit locally includes cobbly fluvial gravel (Qg2

). Estimated thickness is 2–5 m
Description of Map Units 9

Qgd Gravelly stream alluvium and diamicton, undivided (middle Pleistocene)—Unit consists of very poorly 

exposed slightly bouldery deposits that overlie the Troublesome Formation (N$t), and locally rhyolite tuff 

($t), on five high ridges and hills beyond the outer limit of till of the Pinedale glaciation (Qtp) a few kilometers south of Fraser. Deposits southeast of Fraser consist of fluvial gravel, and probably locally include 

bouldery debris-flow deposits. The fluvial gravel is poorly sorted, matrix-supported and clast-supported, 

slightly bouldery, cobbly pebble gravel. It probably contains lenses of pebble gravel and sand. Matrix is 

mostly coarse to very coarse sand that contains abundant granules. Large pebbles and cobbles commonly are 

subangular and subrounded granite and gneiss. Boulders composed of granite, gneiss, pegmatite, and sandstone probably derived from the Dakota Group (Kd) are commonly as large as 25–60 cm in diameter and as 

long as 40–100 cm. The gravel contains a few rounded red (10R) siltstone pebbles probably eroded from the 

Morrison Formation (Jm). Deposits south and southwest of Fraser consist of (1) fluvial gravel—possibly 

glacial outwash—deposited by Saint Louis Creek and Elk Creek, and (2) locally till-like deposits (diamictons 

composed of till or debris-flow deposits) that may have been deposited during one or more pre-Bull Lake 

glaciations. Fluvial gravel consists of poorly sorted, granite-rich, slightly bouldery, cobbly, pebble gravel. 

Matrix is mostly coarse to very coarse sand that contains abundant granules. Large pebbles and cobbles commonly are subangular; boulders are commonly as large as 25–40 cm in diameter and as long as 50–80 cm. 

Till-like deposits are locally present at elevations of about 2,830–2,920 m along Elk Creek. These deposits 

contain granite and gneiss boulders as long as 1–2 m. On slopes steeper than about 6 degrees, unit locally 

may include colluvium composed of clasts derived from fluvial gravel and till-like deposits, and matrix 

derived chiefly from siltstone of the Troublesome Formation. Some of the colluvium may be of late Pleistocene age. Top of unit is about 90–180 m above the Fraser River, about 85–110 m above Elk Creek, and about 

50–195 m above Saint Louis Creek. Estimated thickness of fluvial gravel is 5–15 m; till-like deposits 5–10 m

Mass-Movement Deposits

Qcc Clayey-silt matrix colluvium (Holocene to middle? Pleistocene)—Non-sorted, matrix-supported, boulders to 

granules in a light brown (7.5YR 6/4) very plastic, sandy, clayey to very clayey silt matrix. The matrix is 

derived chiefly from siltstone of the Troublesome Formation (N$t). Granules and larger clasts commonly 

are angular to subangular and make up about 20–60 percent of the deposits. Boulders composed of granite, 

pegmatite, and sandstone probably derived from the Dakota Group (Kd) commonly are as large as 25–90 

cm in diameter. Deposits locally contain angular to subangular clasts of rhyolite tuff ($t) as long 50 cm. 

Granules and larger clasts commonly derived from surficial deposits and bedrock farther upslope. Unit probably deposited chiefly by creep and commonly overlies siltstone of the Troublesome Formation. Estimated 

thickness is 1–5 m

Qcs Sandy matrix colluvium (Holocene to middle? Pleistocene)—Non-stratified to crudely stratified, mostly 

matrix-supported, boulders to granules in a matrix of very pale brown (10YR 7/3) slightly silty, poorly 

sorted, mostly medium to very coarse sand. Granules and larger clasts are commonly angular to subangular 

and make up about 20–75 percent of the deposits. Boulders are commonly as long as 50–100 cm. Clasts and 

matrix are derived chiefly from Proterozoic bedrock farther upslope. Unit probably deposited chiefly by 

creep, debris flow, and locally by sheet flow; commonly is downslope of, and overlies, Proterozoic bedrock. 

Estimated thickness is 1–5 m

Qls Landslide deposits (Holocene to middle? Pleistocene)—Deposits of unsorted and unstratified debris that commonly display hummocky topography. Many of the landslides and landslide deposits form on unstable slopes 

that are underlain by either (1) weakly consolidated siltstone of the Troublesome Formation (N$t) along and 

near the major streams or (2) till (or stratified drift) of Pinedale age (Qtp) on the west side of Vasquez Creek. 

Sliding of the Troublesome Formation may be locally due in part to abundant clay formed by the alteration of 

volcanic ash. Younger landslide deposits are commonly bounded upslope by crescent-shaped headwall scarps 

and downslope by lobate toes. A few of the younger landslide deposits, such as the large landslide deposit 

on the east side of Saint Louis Creek, locally constrict flood plains and overlie valley-floor alluvium (Qva). 

Unit Qls locally includes material displaced chiefly by debris slides, and possibly by earth flows, and earth 

slide-earth flows as defined by Varnes and Cruden (1996). Some deposits probably are formed by translational slides and by 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 farther 

upslope. Landslide deposits are prone to continued movement or reactivation due to both natural and humaninduced processes. Deposits on gentle slopes locally include minor sheetwash (Qsw) and creep-deformed 

deposits. Estimated thickness is 5–50 m 
10 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

Qta Talus deposits (Holocene to middle? Pleistocene)—Unit consists of two deposits composed chiefly of angular tabular blocks of rhyolite tuff ($t), deposited chiefly by rock fall and rock creep, at the base of weakly 

welded rhyolite tuff about 0.5 km east and about 4 km southwest of Winter Park. Clasts commonly range 

in size from about 2 × 2 × 2 cm to as large as about 30 × 60 × 110 cm. The toe of the deposit east of Winter 

Park has ridges and furrows that formed where creep remobilized the lower part of the deposit. The long 

axes of some clasts near the toe of the deposit are nearly vertical. Black and green lichens cover exposed 

surfaces of clasts at the top of the deposit east of Winter Park; some clasts have impact scars. The upper part 

of this deposit near the toe has open voids that extend to a depth of 1.7 m. At a greater depth, the matrix is 

pink (7.5YR 7/4) sandy silt and locally coarse sand that contains granules and small pebbles. Coarse sand 

and larger fragments are composed of rhyolite tuff. Silt in the matrix may be derived from siltstone of the 

Troublesome Formation (N$t) or may be eolian silt derived from current or former flood-plain deposits. 

Estimated thickness is 3–10 m 

Glacial Deposits

Qtp Till of Pinedale age (late Pleistocene)—Mostly non-sorted and non-stratified, subangular and subrounded boulders to granules in a slightly silty to silty, sand matrix. Clasts are commonly angular to subangular and make 

up about 40–80 percent of the deposits. Boulders composed of granite, gneiss, and pegmatite commonly are 

1–2 m long; some are as long as 2.5–4.5 m. Most of the biotite-rich pebbles and cobbles composed of granite, gneiss, and schist within the surface soil and oxidized till are unweathered and disintegrated clasts are 

rare. Material less than 2 mm in diameter makes up about 20–60 percent of the deposits, and consists chiefly 

of medium to very coarse sand and probably about ≤ 20 percent silt and ≤ 5 percent clay. Unit is oxidized 

(10YR 6/3 or 10YR7/3) to a depth of about 2 m; unoxidized matrix is light gray (10YR 7/2). Unit commonly 

forms large prominent, sharp-crested lateral moraines that are very bouldery and have distinct constructional 

morphology. Unit locally includes deposits of stratified drift, mass-movement deposits, till of Bull Lake 

age (Qtb), and locally may include till of pre-Bull Lake age. Radiocarbon (14C) and cosmogenic surfaceexposure ages, some for deposits in nearby areas, 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). Subsurface 

deposits and locally some surface deposits of unit Qtp may be older than 30 ka, because uranium-series ages 

of travertine in the northern Yellowstone area suggest an advance of Pinedale ice occurred about 34–47 ka 

(Sturchio and others, 1994). Estimated thickness is 1.5–30 m

Qtb Till of Bull Lake age (late and middle Pleistocene)—Mostly non-sorted and non-stratified, subangular and 

subrounded boulders to granules in a slight silty to silty sand and, locally, in a clayey silt matrix. Clasts are 

commonly angular to subangular and make up about 40–80 percent of the deposits. Boulders composed of 

granite, gneiss, and pegmatite commonly are 1–2 m long. Near Ranch Creek, about 3.5 km east of Fraser, 

deposits locally contain Proterozoic clasts as long as 2.5–9 m. Many of the biotite-rich pebbles and cobbles 

composed of granite, gneiss, and schist within the surface soil are weathered and are partly or completely 

disintegrated. Material less than 2 mm in diameter makes up about 20–60 percent of the deposits. Unit Qtb 

has a slightly silty to silty sand matrix in areas where it is derived from Proterozoic bedrock, but locally 

has a clayey silt matrix, such as at Winter Park, where it contains abundant sediment eroded chiefly from 

the Troublesome Formation (N$t. Unit is oxidized (10YR 6/3 or 10YR7/3)) to a depth of greater than 3.5 

m; unoxidized matrix is light gray (10YR 7/2). Unit commonly forms prominent lateral moraines that have 

rounded crests beyond the outer limit of till of Pinedale age (Qtp). Unit Qtb locally includes deposits of 

stratified drift, mass-movement deposits, and till of Pinedale age (Qtp). Much of unit Qtb along Ranch 

Creek lacks morainal form and locally may include till of pre-Bull Lake age on the east side of Ranch Creek, 

about 25–840 m south of Little Cabin Creek. K-Ar and 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 near Nederland, Colo., yielded minimum age estimates of 101±21 and 122±26 ka (Schildgen and 

others, 2002). These age estimates are in accordance with a uranium-trend age estimate of 130±40 ka for till 

of Bull Lake age (Shroba and others, 1983) near Allens Park, Colo. Estimated thickness is 1.5–15 m
Description of Map Units 11

Qti Till of Pinedale age and till of Bull Lake age, undivided (late and middle Pleistocene)—Unit consists chiefly 

of till of Pinedale age (Qtp) and a minor amount of till of Bull Lake age (Qtb), along Ranch Creek and 

Cabin Creek, where till of Bull Lake age is of limited extent or is difficult to distinguish from till of Pinedale 

age. In some areas mapped as unit Qti, till of Bull Lake age may be mantled by till of Pinedale age and is not 

exposed. Unit Qti locally includes stratified drift and mass-movement deposits, and locally may include a 

minor amount of till of pre-Bull Lake age that lacks depositional morphology. Estimated thickness is 1.5–30 m

Bedrock Units

Post-Laramide Sedimentary and Volcanic Rocks

N$t Troublesome Formation (upper Miocene to upper Oligocene)—Poorly exposed outcrops and small manmade exposures of unit N$t in the Fraser basin (figs. 2, 3) consists chiefly of weakly consolidated siltstone, 

minor interbedded sandstone and conglomerate, and locally unconsolidated sand and gravel. Lithologic data 

in driller’s logs for water wells near Fraser and Tabernash suggest that the unit in the subsurface consists 

chiefly of siltstone (Bauch and Bails, 2003). Much of the Troublesome in the Troublesome basin in the Hot 

Sulphur Springs quadrangle, northwest of the Fraser quadrangle (fig. 2), is also composed of siltstone (Izett, 

1968). Sandstone and conglomerate probably are more common near the depositional margins of the unit, 

particularly where main streams entered the Fraser basin. Siltstone probably is more common in the rest 

of the basin. Limited lithologic data in driller’s logs suggest that sandstone and conglomerate locally form 

discontinuous channel-fill deposits of variable thickness (about 0.3–17 m) within more uniform intervals of 

fine-grained sediment (probably mostly siltstone). Siltstone exposed in near-surface, man-made exposures 

consists of white (10YR 8/2) and pink (7.5YR 7/4), massive, slightly clayey silt and sandy silt. Some of the 

clay is altered volcanic ash (Zielinski, 1982). Near the northern margin of the Fraser basin, near the northwest corner of the map area, massive siltstone locally contains lenses of interstratified fluvial conglomeratic 

sandstone and pebble conglomerate in stream channels commonly about 5 m wide and 3 m thick. The conglomeratic sandstone consists of carbonate-cemented, poorly sorted, very fine to very coarse sand that contains 

abundant granitic granules and pebbles commonly about 0.5–3 cm in diameter. Conglomerate consists of 

thinly (5–35 cm) bedded, carbonate-cemented, granitic pebble gravel (commonly 0.4–2 cm in diameter) that 

locally contains lenses of granule-rich, mostly coarse to very coarse sand and scattered cobbles. Cobbles 

are angular to subangular granite, quartz, gneiss, and pegmatite commonly as long as 10–20 cm. Siltstone 

near Maryvale, about 3 km southeast of Fraser, locally contains lenses of unconsolidated silty, very fine to 

fine sand and weakly consolidated sandstone. The siltstone locally contains lenses (about 3–200 cm thick) 

composed of unconsolidated (1) very fine to medium sand, (2) granule-rich, non-pebbly to pebbly, medium 

to very coarse sand, and (3) clast-supported, granitic gravel that range in size from small pebble (≤ 2 cm in 

diameter) to slightly cobbly, coarse pebble gravel. Gravel lenses locally contain subangular and subrounded 

clasts of rhyolite tuff ($t) as large as 15 cm in diameter. Weathered siltstone at and near the ground surface 

commonly is strong brown (7.5YR 5/6), clayey to very clayey silt that is massive or has very coarse, blocky 

soil structure. Siltstone locally contains thin (perhaps 2–20 cm) lenses of vitreous volcanic ash. Some of 

the ash in the unit has been altered to smectite (montmorillonite) clay (Zielinski, 1982). Probably all gradations exist between vitreous ash and ash that is completely altered to clay. Some of the lithologic logs of drill 

holes for water wells along the Fraser River near the town of Fraser suggest that the Troublesome near Fraser 

locally overlies shale, which may be Cretaceous marine shale (Benton Shale or Pierre Shale) exposed in the 

adjacent Bottle Pass quadrangle (Taylor, 1975). Interstratified sandstone and coarse pebble conglomerate of 

the Troublesome, penetrated in the lower part of these drill holes, probably are of fluvial origin. Cretaceous 

shale locally may underlie the Troublesome near the northwest corner of the Fraser quadrangle. The driller’s 

log for a water well, located about 2 km west of the northwest corner of the Fraser quadrangle, suggests that 

about 22 m of Troublesome overlies about 23 m of shale, which in turn overlies Proterozoic rock, probably granodiorite (Taylor, 1975). Unit N$t in the Fraser quadrangle has measured dips of 5–16°, is slightly 

tilted (Tweto, 1957), and is locally faulted. Siltstone in areas with steep slopes is prone to displacement by 

creep, sliding, and other mass-movement processes. Original thickness unknown, because top is everywhere 

eroded. Thickness based on outcrop and drill-hole data is at least 150 m near Fraser and about 255 m near 

Tabernash; probably is locally greater than 245 m thick in the adjacent Bottle Pass quadrangle (Taylor, 

1975); and is as much as 300 m or more in the nearby Granby quadrangle (Schroeder, 1995b) 
12 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

$t Rhyolite tuff (Oligocene)—Weakly welded, white, rhyolitic, crystal-lithic tuff and tuff breccia that underlies the 

Troublesome Formation (N$t) near the southern margin of the Fraser basin (Taylor and others, 1968). The 

largest exposure of the tuff, just east of Winter Park, contains conspicuous sanidine, plagioclase, and clear 

to smoky bipyramidal quartz phenocrysts in a white matrix of variably devitrified glass. Pumice fragments 

make up about 25 percent of the rock (Taylor and others, 1968). The tuff contains gas cavities commonly as 

large as 3 cm in diameter. Sanidine from welded tuff, about 0.5 km east of Winter Park, yielded an 40Ar/39Ar 

age of 27.3±0.1 Ma (Knox, 2005; corrected by E.E. Larson, U. of Colo., Boulder, written commun., 2009, 

based on new analytical standards). Talus deposits (Qta) locally form near the base of large outcrops of 

rhyolite tuff. Weathered rhyolite commonly consists of angular to subangular, blocky to tabular clasts about 

2–100 cm long; voids between clasts contain a matrix of pinkish white (7.5YR 8/2) slightly silty, very fine to 

very coarse rhyolite sand. Contacts of unit $t with other rock units are not exposed. Estimated thickness is 15 

to greater than 150 m

Pre-Laramide Sedimentary Rocks

Kd Dakota Group (Lower Cretaceous)—Unit consists chiefly of light-gray to tan, thin- to massive-bedded, finegrained sandstone and conglomeratic sandstone that formed during transgression of the Cretaceous Interior 

Seaway (Taylor, 1975); exposed only near the southwest corner of the Fraser quadrangle. Sandstone locally 

grades into orthoquartzite; cross bedding is common (Tweto, 1957). Sandstone in the upper part of the unit 

(Kd) contains a minor amount of carbonaceous siltstone and shale. Conglomeratic sandstone and chertpebble conglomerate are present near the base of the unit (Taylor, 1975). Near the southern margin of Middle 

Park Formation the Dakota disconformably overlies the Morrison Formation (Tweto, 1957). Thickness in the 

adjacent Bottle Pass quadrangle is about 75 m (Taylor, 1975)

Jm Morrison Formation (Upper Jurassic)—Unit consists chiefly of poorly exposed, mostly fluvial, light brownish-gray, greenish-gray, and grayish-red silty claystone and mudstone that contain interbedded fine-grained 

sandstone, south of the east-west-trending fault, near the southwest corner of the Fraser quadrangle. In the 

adjacent Bottle Pass quadrangle, unit Jm locally contains thin, discontinuous lenses of fine-grained limestone (Taylor, 1975). The Morrison is the basal Phanerozoic unit in this area. It overlies Proterozoic basement, and is disconformably overlain by rocks of the Dakota Group near the southern margin of Middle Park 

(Tweto, 1957). Thickness in the adjacent Bottle Pass quadrangle is about 45–75 m (Taylor, 1975)

Proterozoic Rocks

Yp Pegmatite of the Berthoud Plutonic Suite (Mesoproterozoic)—Muscovite-microcline-albite-quartz pegmatite, 

alaskite, and aplite. Contains scattered inclusions of country rock. Unit extends into the Fraser quadrangle 

from the south where it forms part of the northern margin of the Silver Plume batholith (Theobald, 1965) 

Ym Monzogranite and granite of the Berthoud Plutonic Suite (Mesoproterozoic)— Seriate-porphyritic biotite 

monzogranite and granite that show a prominent flow alignment of tabular microcline crystals (Silver Plumetype granite; Tweto, 1987); locally contains unoriented flakes of subsolidus muscovite. Includes small bodies 

of muscovite-microcline-albite-quartz pegmatite. Unit consists of two small rock bodies (one near the southeast corner of the quadrangle and one near East Elk Creek at the south boundary of the quadrangle) near the 

northern margin of the Silver Plume batholith (Theobald, 1965)

YXp Pegmatite (Mesoproterozoic or Paleoproterozoic)—Muscovite- or biotite-bearing pegmatite, aplite, and 

alaskite. Mapped outside the swarm of pegmatites (Yp) north of the Silver Plume batholith, near the southeast corner of the Fraser quadrangle 

YXg Granodiorite and monzogranite (Mesoproterozoic or Paleoproterozoic)—Biotite granodiorite and monzogranite that are medium to coarse grained and massive to foliated. Unit consists of small rock bodies mapped 

east of the Fraser River in the southeastern part of the quadrangle. Probably equivalent in part to Paleoproterozoic granitic rocks (Xgd; Routt Plutonic Suite of Tweto, 1987), but designated with equivocal age 

because some similar-appearing rocks in the region have produced Mesoproterozoic ages (Berthoud Plutonic 

Suite of Tweto, 1987; Kellogg and other, 2008)
Description of Map Units 13

Xgd Foliated granodiorite, tonalite, and diorite of the Routt Plutonic Suite (Paleoproterozoic)—Medium grained, 

dark gray, variably foliated plutonic rocks containing biotite or biotite and hornblende. Grain size generally 2–5 mm. Finer grained in more strongly foliated zones. A preliminary SHRIMP U-Pb zircon age of 

1,718.9±8.5 Ma was determined from a sample collected in a roadcut along U.S. Highway 40, just north 

of Winter Park Ski Area (W.R. Premo, U.S. Geological Survey (USGS), Denver, Colo., written commun., 

2008). These rocks are similar to widespread foliated granodiorite and related rocks of Paleoproterozoic age 

(Routt Plutonic Suite of Tweto, 1987) 

Xmg Migmatitic biotite schist and gneiss (Paleoproterozoic)—Biotite, sillimanite-biotite, garnet-biotite, and garnetsillimanite-biotite schist and gneiss that display irregular layers and lenses of variable granitic composition 

(leucosomes) bordered by darker layers (melanosomes) rich in biotite, sillimanite, and magnetite. Leucosomes commonly are 1–5 cm thick, but are locally as much as 5 m thick. They are interpreted to have formed 

as partial melts from adjacent rocks (Olsen, 1982). Garnet is pale-pink and typically 5 mm or less in diameter; sillimanite needles are as much as 1 cm long and locally form bundles as long as 2 cm. Some areas of 

unit Xmg in the Fraser quadrangle were incorrectly labeled “Porphyroblastic quartz-biotite-muscovite schist 

of White Ranch (Xbp)” in the Denver West quadrangle compilation (Kellogg and others, 2008)

Xhc Hornblende gneiss and calc-silicate gneiss (Paleoproterozoic)—Hornblende gneiss, amphibolite, calc-silicate 

gneiss, and a few layers of marble. Calc-silicate rocks contain various proportions of epidote, diopside, 

amphibole, plagioclase, and quartz. Adjoining areas to the south in the Berthoud Pass quadrangle were 

mapped as “hornblende gneiss” without conspicuous calc-silicate rocks (Theobald, 1965) 

Xig Interlayered gneiss (Paleoproterozoic)—Biotite schist and gneiss, quartzofeldspathic gneiss, quartz-rich gneiss, 

calc-silicate gneiss, biotite marble, and pegmatite. Layers are mostly 1–10 cm thick. Mapped along the northern border of the quadrangle, contiguous with heterogeneous interlayered gneisses in the adjacent Strawberry 

Lake quadrangle (Schroeder, 1995b) that are contained within a more extensive terrane of migmatitic biotite 

2 Report Title schist and gneiss (Xmg)

Table 2. Approximate height, in meters, of tops of gravelly stream alluvium (Qg and QNg) 

above the Fraser River and its major tributaries, Saint Louis Creek and Ranch Creek in the 

Fraser quadrangle. 

[..., no deposits along stream]

Fraser Saint Louis 

 Creek

near

Fraser

Ranch 

 Creek

northeast of

Fraser

 Map unit near

 Winter Park

River

near

Fraser

near

Tabernash

Qgl 6 and 12 6 and 12 6 6 6–9 

Qg2 24 24 12 and 18 12 18–24 

Qg3 ... 30 35 ... 30–35

Qg4 ... 50–55 ... ... 50–55 

Qg5 ... ... 65 ... 65

Qg6 ... 85–90 ... ... ...

Qg7 ... 105–115 ... ... ...

Qg8 145 ... ... ... ...

QNg 245–270 ... ... ... ...

Table 2. Approximate height, in meters, of top of gravelly stream alluvium (Qg and QNg) above 

the Fraser River and its major tributaries: Saint Louis Creek and Ranch Creek in the Fraser 

7.5-minute quadrangle.
14 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

Eolian Silt Contributions to the 

Troublesome Formation in the Fraser 

Basin

Sparse and widely separated outcrops, excavations, and 

water wells that have driller’s logs in the Fraser basin suggest 

that much of the upper 150 m of the Troublesome Formation 

(N$t) near Fraser and about 255 m of Troublesome Formation near Tabernash consists chiefly of siltstone. This observation is puzzling, because the Troublesome Formation was 

deposited in an intermontane basin (Izett, 1968), and many of 

basin-margin rocks are Proterozoic crystalline rocks that only 

produce a minor amount of silt upon weathering. 

In natural and man-made exposures in the Fraser basin, 

siltstone is massive and locally contains lenses and channelfill deposits composed of sandstone and conglomerate, and 

unconsolidated sand and gravel. The siltstone lacks distinctive 

lacustrine features (Feth, 1964; Picard and High, 1981), as 

well as diagenetic minerals and features commonly associated 

with saline playa deposits (Zielinski, 1982; Chapin and Lindley, 1986). The lack of distinctive lacustrine features may be 

due to deposition in shallow ephemeral lakes in which wave 

action or bioturbation destroyed or obscured primary sedimentary features. Although the Troublesome Formation in the 

Troublesome basin (fig. 2) contains a few beds of fresh-water 

limestone, it contains an exclusively terrestrial fauna and lacks 

fossil fish (Izett, 1968 and references within; Kron, 1988). 

 Most of the sand-size and coarser sediments, and probably much of the silt-size sediments that make up the Troublesome Formation, were eroded from basin margins and deposited in adjacent basins by alluvial processes (Tweto, 1957; 

Izett, 1968). The silt that makes up the abundant siltstone in 

the Fraser basin, as well as in the Troublesome and Granby 

basins, may be derived chiefly from either the deposition of 

(1) volcanic ash (primary airfall) and ash-rich alluvium, or (2) 

loess (windblown silt) and loess-rich alluvium. 

 It is unlikely that much of the substantial silt that accumulated in the Fraser basin was derived from the weathering 

and erosion of crystalline and sedimentary bedrock along the 

margins of the basin. The Proterozoic granitic rocks, schist, 

and gneiss are unlikely sources because they weather to form 

coarse-grained deposits, such as sandy matrix colluvium 

(Qcs). The latter contains abundant clasts, substantial amounts 

of very coarse sand to medium sand rich in quartz and feldspar, and only minor amounts of silt and clay. Marine deposits 

of the Benton Formation, Niobrara Shale, and Pierre Shale, 

and continental deposits of the Middle Park Formation along 

the western margin of the basin in the adjoining Bottle Pass 

quadrangle contain varying amounts of claystone, siltstone, 

shale, and mudstone (Taylor, 1975). However, it is unlikely 

that sediments derived from these bedrock units alone could 

account for the high proportion of siltstone in the Troublesome 

Formation of the Fraser basin, because they are so limited (in 

volume) here. 

Some of the silt in the lower part of the Troublesome 

Formation in the Fraser basin could be derived from Oligocene ash or eolian sediment that once blanketed the landscape 

and later was redeposited as silt-rich alluvium. For example, 

the thick remnant of crystal-rich lithic tuff near Winter Park 

beneath the Troublesome Formation indicates a nearby volcanic source at Red Mountain (fig 1; Shannon and others, 2004. 

Other possible sources include volcanic vents in the Rabbit 

Ears Range (Izett, 1968), the Never Summer Mountains 

(fig. 2; Cole and others, 2008), and an inferred buried source 

in the Fraser quadrangle (Tweto, 1957; Taylor and others, 

1968; Stein and Crock, 1990). Some of the very fine silt and 

clay in the siltstone could be (1) distal ash erupted from major 

volcanic fields nearby or in the Basin and Range province 

(Hansen, 1984; Luft, 1985; Buffler, 2003) hundreds of kilometers west of the Fraser basin, or (2) dust deflated from the 

extensive Chuska erg (Cather and others, 2008) about 400 km 

southwest of the Fraser basin. 

Massive bedding and limited grain-size data suggest 

that a major component of the siltstone in the Troublesome 

Formation of the Fraser basin is loess that is the distal facies 

of extensive deposits of eolian sand (Pye, 1987, 1995), or siltrich alluvium derived from loess deposits. The Troublesome 

Formation is coeval with extensive eolian sand sheets and 

loess deposits that accumulated in areas northwest, northeast, and south of the Fraser basin. On the west side of the 

Park Range, about 45 km northwest of Fraser basin (north of 

Gore Range; fig. 2), as much as 670 m of mostly very fine-to 

medium-grained, cross-bedded eolian sandstone in the Browns 

Park Formation accumulated between about 24.8 and 7.2 Ma 

(Luft, 1985; Buffler, 2003 ). Tuffs within the Browns Park thin 

from west to east suggesting transport by westerly or southwesterly winds (Luft, 1985; Buffler, 2003). On the high plains 

of eastern Wyoming and southwestern Nebraska, about 200 

km northeast of the Fraser basin, the uppermost lithostratigraphic unit of the “Upper Harrison” beds of Peterson (1907, 

1909) of latest Arikareean age (Tedford and others, 2004 ) 

consists of as much as 35 m of silty very fine to fine eolian 

sandstone (sandy loess) that accumulated during the early 

Miocene, probably under warm semiarid climatic conditions 

(Hunt, 1990) on a landscape dominated by C3

-grassland vegetation (Strömberg, 2002). In the Rio Grande rift of northcentral New Mexico, about 500 km south of the Fraser basin, 

about 350 m of mostly cross-bedded eolian Zia Sand was 

transported by westerly winds and accumulated between about 

22 and 16 Ma (Tedford and Barghoorn, 1999; Williams and 

Cole, 2007). 

Limited grain-size data for samples of clayey siltstone, 

siltstone, and silty sandstone collected from the Troublesome 

Formation near the town of Fraser indicate that most of the 

particles are 1–125, 16–250, and 31–500 mm in size for the 

three samples, respectively (table 1). By comparison, unweathered loess of Quaternary age is composed mainly of particles 

10–50 mm in size (Pye, 1987, 1995). Values for mean particle 

size and degree of sorting for the samples of clayey siltstone 

and siltstone (1) are similar to those of airborne dust in the 
Eolian Silt Contributions to the Troublesome Formation in the Fraser Basin 15

Mojave Desert and (2) suggest that these siltstones formed 

from loess or loess-rich alluvium (fig. 4). The relatively high 

clay content of the clayey siltstone, which is uncommon in 

modern dust, may be due in part to post-depositional, diagenetic alteration of volcanic ash to secondary clay-size clay 

minerals (Zielinski, 1982). 

The deposition of eolian sediment in the Troublesome 

Formation and extensive coeval regional eolian units suggests 

that semiarid climatic conditions prevailed regionally during 

the late Oligocene and Miocene. However, limited pollen data 

suggest that climatic conditions during the deposition of the 

Troublesome Formation (about 28.4–11 Ma) may not have 

been much drier than those of the present (table 3). The pollen 

assemblages in two samples of fine-grained carbonaceous 

sediment from the Troublesome Formation, collected about 11 

and 13 km northeast of Kremmling, Colo. (U.S. Geological 

Survey Botanical localities D1905 and D3493, respectively; 

stratigraphic positions not specified), are dominated by pine 

(D3493) and by pine and subordinate spruce (D1905) (E.B. 

Leopold, USGS, Denver, Colo., written commun., 1962 and 

1964). Except for the presence of scarce pollen from broadleaf 

trees (Ulmus (elm), Carya (hickory), and Juglans (walnut)), 

now exotic in Colorado, these fossil pollen assemblages 

resemble the present pollen rain in the area (E.B. Leopold, 

unpublished data; cited by Weber, 1965). The present natural 

geographic ranges of these broadleaf trees are limited to areas 

of the United States that have a mean annual precipitation of at 

least 35–55 cm (Thompson and others, 1999), which is greater 

than that of the Kremmling area (table 3). 

Very fine

sand

Coarse

silt

Medium

silt

Fine

silt

Mojave

Desert

dust

Northeastern

Colorado

loess

SORTING (IN PHI UNITS)

4.0

3.5

3.0

2.5

2.0

1.5

1.0

3

(125 mm)

5

(31 mm)

6 

(16 mm)

7

(8 mm)

4

(63 mm)

Very poorly

sorted

Poorly 

sorted

MEAN PARTICLE SIZE

(IN PHI UNITS AND MILLIMETERS)

Silty

sandstone

Siltstone

Clayey

siltstone

Figure 4. Plot of mean particle size and degree of sorting (standard deviation of the mean 

particle size) of silty sandstone, siltstone, and clayey siltstone collected from the Troublesome 

Formation near Fraser, Colorado (triangles). Data for modern airborne dust in the Mojave Desert 

in southern California (green), and late Pleistocene and early Holocene loess in northeastern 

Colorado (yellow) are from Muhs and Benedict (2006, their fig. 5).

Table 3. Mean annual precipitation near Kremmling, 

Granby, and Fraser, Colorado. 

Weather Approximate Mean annual 

station name elevation (m) precipitation (cm)

Kremmling1 2,250 29

Granby2 2,500 36

Fraser1 2,610 47

1

Values for Kremmling (1908–2009) and Fraser (1908–1974) are 

from http://www.wrcc.dri.edu.

2

Value for Granby (1959–1974) is from Hansen and others (1978).

Table 3. Mean annual precipitation near Kremmling, Granby, 

and Fraser, Colo.
16 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

Structural Setting of the Fraser 

7.5-minute Quadrangle

The Fraser 7.5-minute quadrangle is located along the 

western boundary of the Laramide Front Range basement 

uplift (Tweto, 1957), near the southeastern margin of a physiographic feature known as Middle Park (fig. 2). Middle Park 

and North Park (North Park is north of Rabbit Ears Range, see 

fig. 2) lie within an intermontane basin that formed between 

the Laramide Medicine Bow Mountains (north of Never Summer Mountains; fig. 2) and Front Range on the east and the 

Park and Gore Ranges on the west (Tweto, 1980). The basin is 

filled mostly with Upper Cretaceous(?) to Paleocene sedimentary and volcanic rocks, which are cut by a complex array of 

both thrust and normal faults in the Bottle Pass quadrangle, 

immediately west of the Fraser quadrangle. The fault geometry is complex, and is chiefly the result of southwest-directed 

Laramide thrusting (Taylor, 1975).

Three north- to northwest-trending, post-Laramide basins 

formed near the eastern and western margins of the Laramide 

North Park-Middle Park structural basin. The upper Oligocene 

to upper Miocene Troublesome Formation occupies the 

central parts of the Troublesome, Granby, and Fraser basins 

and, locally, unconformably overlies Proterozoic to Paleocene rocks (fig. 2). These basins consist of paleovalleys and 

structural lows (Izett, 1975); the latter probably formed during 

extensional deformation associated with the Rio Grande rift 

system in northern Colorado (Tweto, 1978, 1979b; Chapin and 

Cather, 1994). The Fraser quadrangle encompasses much of 

the Fraser basin, which contains sediments of the Troublesome 

Formation (N$t). 

Two major faults are inferred to traverse parts of the 

quadrangle based on geologic relations in adjacent areas (Kellogg and others, 2008). The northeast-trending Vasquez Pass 

fault is inferred beneath the Vasquez Creek valley, but there is 

no evidence it displaces the Troublesome Formation. A northtrending branch of the generally northeast-trending Berthoud 

Pass fault zone (a complex array of brittle faults and fractures 

that disrupts some older mylonite) passes through the extreme 

southeast corner of the Fraser quadrangle. A minor fault with 

youngest known displacement in the quadrangle is the short 

northwest-trending structure near Maryvale, about 1 km north 

of Winter Park, which displaces beds of the Troublesome 

Formation. 

The Saint Louis Lake shear zone is one of a series of 

zones of ductile deformation formed during the Proterozoic 

(Tweto and Sims, 1963; Taylor, 1971; Shaw and others, 2002). 

It is part of a major northeast-trending zone in central Colorado 

referred to collectively as the Homestake shear zone (Tweto 

and Sims, 1963). Where best exposed, at Saint Louis Lake 

in the Byers Peak quadrangle 8.5 km southwest of the Fraser 

quadrangle, ductilely deformed rocks in the Saint Louis Lake 

shear zone contain monazite grains that are interpreted to 

indicate medium-temperature recrystallization in mylonites at 

about 1.4 Ga (McCoy and others, 2005). 

Geologic History of the Fraser 

7.5-minute Quadrangle

The geologic history of the quadrangle can be divided 

into three major periods: (1) Proterozoic history, (2) Paleozoic 

through Laramide history (Late Cretaceous to early Eocene), 

and (3) post-Laramide Cenozoic history. Some of the geologic 

history below is summarized from Kellogg and others (2008).

Proterozoic History

The history of the basement rocks in the Fraser quadrangle may extend as far back as about 1,800 Ma (Reed and others, 1987; Aleinikoff and others, 1993). The oldest metasedimentary rocks formed chiefly from marine shale and siltstone 

(Cole and Braddock, 2009), sparse mafic volcanic flows and 

tuffs, and a few thin beds of limestone. None of the layered 

metamorphic rocks in or near the Fraser quadrangle have been 

dated. SHRIMP (Sensitive high-resolution ion microprobe) 

analyses of zircon in similar rocks, about 50 km southeast 

of the Fraser quadrangle, indicate that at least some of the 

metamorphosed volcanic rocks are as old as 1,770–1,780 Ma 

(Premo and others, 2007). Some of the metavolcanic rocks 

dated by Premo and Van Schmus (1989) in the Sierra Madre 

of southern Wyoming are of similar age. 

The marine sediments were folded and metamorphosed 

concurrent with or after intrusion by granodiorite plutons 

(Xgd). Evidence for this is the concordance of pluton contacts 

and foliation in the plutonic rocks with the layering and foliation of the adjacent metasedimentary rocks. Rock of the main 

granodiorite pluton, collected in a roadcut along U.S. Highway 

40 just north of Winter Park Ski Area, has a preliminary 

SHRIMP U-Pb zircon age of 1,718.9±8.5 Ma (W.R. Premo, 

USGS, written commun., 2008). Eppinger and others (1985) 

reported that small bodies of plutonic rock, correlative with 

the larger, dated pluton in the Fraser quadrangle are folded. 

Foliation and compositional layering in the Proterozoic 

rocks are generally parallel, except in rare outcrops exposing a 

fold hinge, where foliation penetrates compositional layering. 

Trends of foliation and compositional layering are generally 

east to east-northeast. In the southern part of the Fraser quadrangle, the Proterozoic rocks have a more consistent foliation 

trend and dip than elsewhere. Lineation formed by aligned 

minerals, stretched-mineral trains, and rodding have a consistent southeast trend. 

Strands of the Saint Louis Lake shear zone cut Proterozoic 

rocks in the southern part of the Fraser quadrangle. At Saint 

Louis Lake in the Byers Peak quadrangle, mylonite along the 

shear zone is interpreted to have formed at temperatures of 

350–450oC at 1.42 Ga, and thin zones of ultramylonite contain 

monazite dated at 1.34 Ga (McCoy and others, 2005). 
 Geologic Histroy of the Fraser 7.5-minute Quadrangle 17

Paleozoic through Laramide History

Lower Paleozoic sedimentary rocks probably were deposited in the Fraser area but were removed during Pennsylvanian 

and later time when the Ancestral Front Range was uplifted 

(DeVoto, 1980). In the Fraser quadrangle and the Bottle Pass 

quadrangle to the west, the Upper Jurassic Morrison Formation (Jm) was deposited directly on the Proterozoic basement 

rocks following uplift and prolonged erosion (Taylor, 1975). 

Nearby, to the north, in the Strawberry Lake and Granby quadrangles sedimentary rocks overlying the basement belong to 

the Triassic Chugwater Formation (Schroeder, 1995a, 1995b).

During Late Cretaceous the area was beneath a shallow, 

inland sea in which more than 2 km of Upper Cretaceous 

Pierre Shale was deposited (Izett, 1968). Near the end of the 

Cretaceous, uplift and erosion related to the Laramide orogeny 

began (Tweto, 1957, 1975, 1980). This deformation established the outlines of the main basement uplifts of the Front 

Range and Vasquez Mountains (fig. 2). West of the Fraser 

quadrangle, Laramide-age reverse faults deformed rocks as 

young as the Paleocene and Upper Cretaceous(?) Middle Park 

Formation. 

Locally, on the west side of the Front Range, much of the 

Pierre Shale was eroded before mafic volcanic conglomerate 

in the lower part of the Middle Park Formation was deposited (Tweto, 1957). The Middle Park Formation consists of 

volcaniclastic and arkosic sandstone and conglomerate that 

are about 2.2 km thick in the Hot Sulphur Spring 15-minute 

quadrangle (Izett, 1968) northwest of the Fraser quadrangle. It 

was faulted and folded, probably during the late Paleocene or 

early Eocene (Tweto, 1957, 1975, 1980). 

Apatite fission-track cooling ages of 67–48 Ma along 

Clear Creek Canyon, about 35 km southeast of the Fraser 

quadrangle, suggest Laramide denudation in the eastern part 

of the Front Range during latest Cretaceous to middle Eocene 

time (Chapin and Kelley, 1997). 

Post-Laramide Cenozoic History 

After a period of erosion during the Eocene and early 

Oligocene time, three northwest- to north-trending basins 

were formed by tectonic processes that probably were similar 

in character and age to those in the Rio Grande rift in central 

and southern Colorado (Tweto, 1978, 1979b; Kellogg, 1998). 

These basins contain rocks and locally weakly consolidated 

sediments assigned to the upper Oligocene to upper Miocene 

Troublesome Formation (N$t). The Troublesome Formation probably was deposited in one irregular basin that is now 

separated into three basins by later erosion and some normal 

faulting (Izett, 1968). The western basin is the Troublesome 

basin (Tweto, 1957; Izett, 1968; Izett and Barclay, 1973). Sediments in the two eastern basins are preserved in the relatively 

low-lying land surrounding Granby and Fraser (Taylor and 

others, 1968). The boundary between the Granby and Fraser 

basins is near the north-trending ridge of Proterozoic bedrock 

(Tweto, 1957), where the locally exposed Troublesome Formation is narrowest (fig. 2). The boundary does not appear 

to be structurally controlled, but rather is located where thin 

Troublesome Formation onlaps a low topographic divide in 

the underlying basement rocks (see Schroeder, 1995a).

The eroded top of the Troublesome Formation in the Fraser basin ranges in altitude from about 2,990 m near the eastern boundary of the Fraser quadrangle, southeast of the town 

of Winter Park, to about 2,715 m near the northern boundary 

of the Bottle Pass quadrangle (Taylor, 1975) west of Tabernash. Sparse remnants of Troublesome are preserved on top of 

Proterozoic basement at about 2,800 m in the Strawberry Lake 

quadrangle to the north (Schroeder, 1995a). Widely spaced 

outcrops, excavations, and water wells with driller’s logs near 

Tabernash indicate that the northern part of the Fraser basin 

contains about 255 m of Troublesome Formation not removed 

by post-Troublesome erosion (fig. 2). 

Weakly welded porphyritic rhyolite tuff ($t), with an 40Ar/39Ar sanidine age of 27.3±0.1 Ma (Knox, 2005; corrected 

by E.E. Larson, U. of Colo., Boulder, written commun., 2009, 

based on new analytical standards) underlies the Troublesome 

Formation in the southern part of the Fraser basin (Taylor and 

others, 1968), about 0.5 km east of Winter Park. The tuff has 

not been identified north of this locality in the Fraser quadrangle or anywhere in the Granby basin in the Granby and 

Trail Mountain quadrangles (Schroeder, 1995b; Izett, 1974). 

Contacts of the tuff with younger and older bedrock units are 

not exposed. Map relations indicate that, at least locally, the 

tuff overlies Proterozoic basement rocks. In other places the 

tuff is topographically higher than adjacent deposits of the 

Troublesome Formation, which locally contains clasts of similar rhyolite tuff in gravel lenses. These relationships indicate 

that the tuff is older than the Troublesome Formation, and that 

erosional remnants of the tuff may have locally formed hills 

around which, and probably over which, the Troublesome 

Formation was deposited. 

The rhyolite tuff near Winter Park may have erupted from 

either (1) an eroded volcanic complex associated with the Red 

Mountain intrusive system, about 15 km south of the Fraser 

quadrangle (fig. 1; Stein and Crock, 1990); (2) a local source 

now concealed by the Troublesome Formation or Quaternary 

deposits (Tweto, 1957; Taylor and others, 1968; Stein and 

Crock, 1990); or (3) the Braddock Peak intrusive-volcanic 

complex (Cole and Braddock, 2009) in the Never Summer 

Mountains about 60 km north of Winter Park. The latter possibility was suggested by Kellogg and others (2008). 

The rhyolite intrusive rocks of the Red Mountain magmatic system beneath Red Mountain (fig. 1) yield 40Ar/ 39Ar 

ages in the range 29.9±0.3 Ma to 27.0±0.3 (Geissman and 

others, 1992) that overlap the age of the rhyolite tuff near 

Winter Park (although the Red Mountain pluton most similar 

to the rhyolite tuff near Winter Park yielded an older age). 

Studies indicate that the plutons at Red Mountain were 

emplaced at a depth of about 1.6 km (Geraghty and others, 

1988); magma associated with the intrusive system may have 

vented (Taylor and others, 1968). By 27.6 Ma, the intrusions 
18 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

in the core of the system were about 1.4 km below the present 

top of Red Mountain, and they had cooled below 280oC based 

on 40Ar/ 39Ar dating of biotite (Geissman and others, 1992). 

These age data alone are inconclusive about a correlation of 

rhyolite intrusive rocks at Red Mountain to the rhyolite tuff 

near Winter Park (analytical procedures of the two investigations have not been directly compared). However, the 

rare-earth patterns of the tuff near Winter Park and the older 

rhyolite intrusions at Red Mountain are almost identical, suggesting a common magma source (Stein and Crock, 1990). In 

addition, the considerable thickness, high crystal content, and 

texture of the tuff near Winter Park indicate that it was erupted 

from a source not far away. 40Ar/39Ar ages for volcanic rocks of the Braddock Peak 

intrusive-volcanic complex (53 km north of the map area) 

indicate activity from about 29.2±0.2 to 28.0±0.1 Ma (Knox, 

2005; corrected by E.E. Larson, U. of Colo., Boulder, written 

commun., 2009, based on new analytical standards). Rareearth patterns of the rhyolite tuff of Thunder Mountain (Cole 

and others, 2008) of the Braddock Peak intrusive-volcanic 

complex (Cole and Braddock, 2009) differ considerably from 

both those of the rhyolite tuff near Winter Park and the rhyolite at Red Mountain (Stein and Crock, 1990; Knox, 2005). 

Thus, age and geochemical data, as well as the considerable 

distances involved (greater than 60 km), indicate that the 

Braddock Peak complex is an unlikely source for the tuff near 

Winter Park (E.E. Larson, U. of Colo., Boulder, written commun., 2009).

We favor the interpretation that the rhyolite tuff near 

Winter Park was erupted from either the Red Mountain area 

(Stein and Crock, 1990) or a concealed local source (Tweto, 

1957; Taylor and others, 1968; Stein and Crock, 1990). 

The range in altitude (2,672–2,977 m) at which the rhyolite tuff crops out near Winter Park suggests that either the tuff 

partly fills paleovalleys, similar in depth to the present valleys 

of the Fraser River and its tributary, Elk Creek, or it was displaced by subsequent faulting. There is no evidence that major 

faults displace the Troublesome Formation in the Fraser quadrangle however, deep pre-Miocene paleovalleys have been 

documented elsewhere in the northern Front Range and other 

nearby mountain ranges (Evanoff, 1990; Cole and others, 

2008; Cole and Braddock, 2009). Topographic and geologic 

relations suggest that (1) the tuff near Winter Park occupies 

paleovalleys, and (2) there was significant topographic relief 

prior to the deposition of the Troublesome Formation. The 

latter interpretation is in accord with previous studies in the 

Troublesome basin near Kremmling and in the Granby basin 

north of Granby that indicate that the Troublesome unconformably overlies a surface of considerable relief (Izett, 1968, 

1974; Braddock and Cole, 1990). 

In the Trail Mountain quadrangle at the north end of the 

Granby basin, several basalt flows are interbedded with sediments in the lower part of the Troublesome Formation (Izett, 

1974) and pinch out southward toward Granby (Schroeder, 

1995b). The second lowest basalt flow has a corrected wholerock K-Ar age (using decay constants of Steiger and Jäger, 

1977) of 26.7±0.9 Ma (Izett, 1974; Marvin and others, 1974). 

At the north end of the Granby basin a latite porphyry intrusion, with a zircon fission-track age of 28.4±2.3 Ma, probably 

intrudes Troublesome Formation beds beneath the lowest 

basalt (Izett, 1974), and locally is unconformably overlain 

by beds of the middle part of the Troublesome Formation 

(Braddock and Cole, 1990). These results indicate that the 

lower part of the Troublesome Formation in the north part of 

the Granby basin is Oligocene in age. South of the town of 

Granby, the Troublesome Formation contains vertebrate fossils 

of late Oligocene age (Kron, 1988; Schroeder, 1995b). The 

Troublesome Formation is very poorly exposed in the Fraser 

quadrangle, and the only isotopic-age control for the Troublesome Formation in this area is the 27.3-Ma rhyolite tuff that 

underlies it near Winter Park. In contrast, the Troublesome 

Formation is better exposed in the basin near Kremmling (Izett 

and Barclay, 1973) where it contains 30 or more tuffs and 

diagnostic vertebrate fossils that range in age from latest 

Arikareean to early Clarendonian (Izett and Obradovich, 

2001). Eight of these tuffs have 40Ar/39Ar sanidine ages that 

range from 23.5 Ma to 11.0 Ma (latest Oligocene to late Miocene; Izett and Obradovich, 2001).

Locally, the Troublesome Formation is broadly folded 

and faulted. In the northern part of the Granby basin in the 

Trail Mountain quadrangle, the Troublesome Formation is 

folded into a broad, asymmetric syncline with dips as much 

as 29o on the west limb and 20o on the east limb (Izett, 1974). 

The syncline is cut by normal faults of small displacement 

(Izett, 1974). In the Granby quadrangle dips in the west part of 

the basin are as much as 27o, although most are less than 20o. 

Cross-sections for the Granby quadrangle (Schroeder, 1995b) 

show an interpreted fault with an implied 650 m displacement 

of the basal Troublesome Formation; surface and drillhole data 

do not seem to require a fault of such magnitude. The Troublesome Formation in that part of the Granby basin was deposited 

on a faulted, hilly terrane (Izett, 1968). 

In the Fraser quadrangle, structures and bedding attitudes 

in the Troublesome Formation are poorly constrained, because 

much of the unit is very poorly exposed and is commonly 

mantled by colluvium and other surficial deposits. Poor exposure is also due, in part, to dense forest cover above an altitude 

of about 2,600 m. The few measured dips and one observed 

normal fault that cuts the Troublesome Formation suggest that 

Troublesome beds are deformed, but not as much as those 

in the Granby basin. The fault is about 0.6 km southeast of 

Maryvale, trends N. 30o W., dips 70o to the northeast, and displaces a white, tuffaceous, siltstone bed. The lateral extent and 

total displacement of this fault are not known. The Troublesome Formation overlies Proterozoic basement in many areas, 

but limited subsurface lithologic data near Fraser and Tabernash suggest that in some areas it overlies Mesozoic shale, 

which is not exposed in the Fraser quadrangle. 

The inferred fault beneath the valley of Vasquez Creek 

might extend into the Fraser basin beneath Quaternary deposits 

and, if so, may have played a role in the formation of the 

basin. This Vasquez Pass fault intersects a complex of local 
Geologic History of the Fraser 7.5-minute Quadrangle 19

faults that offset the Oligocene Red Mountain intrusive system 

south of the Fraser quadrangle (Eppinger and others, 1985; 

Geraghty and others, 1988). Tilting of as much as 25° appears 

to have occurred since latest Oligocene time (Geraghty and 

others, 1988; Geissman and others, 1992). 

Following the deposition of the Troublesome Formation 

during the late Oligocene to late Miocene, the rest of the 

Miocene and the Pliocene appear to have been chiefly a time 

of widespread erosion in upland areas of the Front Range and 

stream incision along the flanks of the Front Range and in the 

adjacent basins. This erosion and stream incision was probably 

due in part to climate change (Chapin, 2008) as well as ongoing epeirogenic uplift of the Southern Rocky Mountain region 

(Eaton, 2008). 

 Hills on the west side of the present course of the 

Williams Fork, near its confluence with the Colorado River 

(fig. 2), are capped by at least 30 m of weakly consolidated 

cobbly gravel and finer-grained fluvial sediments deposited 

by the Williams Fork (Izett, 1968) in a paleovalley cut into the 

Troublesome Formation (R.R. Shroba, USGS, unpublished 

mapping, 2008). Fossil remains of Hemphillian age (late 

Miocene and early Pliocene; Robinson, 1968, 1972; Kihm and 

Middleton, 1980) near the top of these fluvial sediments suggest that the courses of the Williams Fork and Colorado River 

in the Troublesome basin, and likely the Fraser River in the 

Granby and Fraser basins, were established after the deposition of the Troublesome Formation during the late Miocene or 

early Pliocene. Prior to the development of through-flowing 

drainage across the Gore Range at Gore Canyon (fig. 1), the 

Fraser River probably flowed on or near the depositional top 

of the Troublesome Formation above an altitude of 2,990 m in 

the southern part of the Fraser basin, and above 2,800 m near 

the southern part of the Granby basin. The Fraser River has 

subsequently cut the narrow, deep canyon (locally as much 

as 300 m) in Proterozoic bedrock between Tabernash and 

Granby, and removed much of the Troublesome Formation 

from the Fraser and Granby basins. The reach of the river cut 

in the resistant Proterozoic bedrock, rather than in adjacent 

more easily eroded sedimentary rocks, suggests that the Fraser 

River is a superimposed stream. 

Erosion during the Pliocene may have been fostered by 

climatic conditions in the western United States that were 

significantly wetter (more effective precipitation) and stormier 

than those of today, particularly about 4.5–3.5 Ma (Forester, 

1991) and 3.2–2.8 Ma (Smith and others, 1993). At about 

3 Ma there were significant increases in the magnitude of 

temperature ranges during cold (glacial)/warm (interglacial) 

climatic cycles (Morrison, 1991), which culminated in the 

development of major northern hemisphere ice sheets at about 

2.7–2.4 Ma (for example, Shackleton and others, 1984; Balco 

and others, 2005), or possibly as early as 3.2 Ma (Prell, 1984) 

or 3.6 Ma (Mudelsee and Raymo, 2005). Erosion rates on 

upland surfaces in Front Range canyons and in adjacent basins 

are likely linked to, and were likely enhanced by, erosional 

processes that were intensified by global cooling during the 

Pliocene and Pleistocene. 

The oldest surficial deposits in the Fraser quadrangle are 

bouldery stream gravel (QNg), of early Pleistocene or Pliocene age, that mantle two ridges about 2 km northeast and 

3 km north of the Winter Park Ski Area. Their age is uncertain, 

but they may be correlative with the boulder conglomerate 

of Pliocene(?) age in the nearby Trail Mountain quadrangle 

(Izett, 1974). 

Quaternary deposits and landforms within the Fraser 

quadrangle reflect the influence of earth-surface processes in 

concert with global climate-driven glacial-interglacial cycles 

during the past 1.8 m.y. Global climatic cooling during the 

Pliocene continued into the Pleistocene and intensified after 

about 900 ka (Clark and Pollard, 1998). Pleistocene glacial 

deposits in Colorado and elsewhere in the Rocky Mountains 

are commonly correlated with Pinedale, Bull Lake, and preBull Lake glaciations (Meierding and Birkeland, 1980; Pierce, 

2004). Blackwelder (1915) named the Pinedale (last major) 

and Bull Lake (penultimate) glaciations for younger and 

older sets of moraines and corresponding tills on the eastern 

and western flanks of the Wind River Range in northwestern 

Wyoming. Glacial deposits that predated the Bull Lake glaciation typically lack morainal form, and are locally preserved 

just beyond the outer limits of Pinedale and Bull Lake deposits 

(Meierding and Birkeland, 1980), such as those near Lake 

Granby, about 20 km north of the Fraser quadrangle (Meierding, 1977; Cole and Braddock, 2009). 

The youngest glacial deposits near the Fraser quadrangle 

are tills of latest Pleistocene age (about 10–12 ka; Davis 

1988) and Holocene age (Benedict 1985; Benson and others, 

2007). These tills typically form sharp-crested moraines 

above present treeline (about 3,350 m), within roughly 1 

km of cirque head walls. They were deposited during minor 

glacial advances after the Pinedale glaciation, during marine 

oxygen isotope (MIS) stage 1, (0–14 ka; Lisiecki and Raymo, 

2005). Some of the older, post-Pinedale moraines probably 

correlate with those of the Satanta Peak advance of Benedict 

(1973, 1985) in the Arapahoe Pass area, in the Monarch Lake 

7.5-minute quadrangle, about 8 km northeast of the Fraser 

quadrangle. Deposits of the Satanta Peak advance have 

minimum-limiting radiocarbon ages of 9,915±380 yr B.P. and 

9,700±215 yr B.P. (Benedict, 1973), and probably date from 

12 to 10 ka B.P. (Benedict, 1985). They 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). 

Except for a few scattered till-like deposits (diamictons in 

unit Qgd) along Saint Louis Creek and Elk Creek, about 4–5 

km southwest of Fraser, there are no known exposed glacial 

deposits of pre-Bull Lake age within the Fraser quadrangle. 

However, much of the till of Bull Lake age (Qtb) along Ranch 

Creek lacks morainal form and locally may include till of preBull Lake age. The sparse distribution of glacial deposits of 

pre-Bull Lake age in the Front Range and near the margins of 

adjacent basins probably is due, in part, to erosion and subsequent burial by deposits of younger and more extensive glacial 

advances (Meierding and Birkeland, 1980). Glacial deposits 
20 Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado

of early Pleistocene age are likely to be buried by younger 

glacial deposits, because only two glacial episodes (12 and 

16, Lisiecki and Raymo, 2005) were as severe – in terms of 

temperature and global ice volume – as those during the Bull 

Lake and Pinedale glaciations. This suggests that deposits of 

pre-Bull Lake glaciations other than those that accumulated 

during MIS 12 and16 are likely to be less extensive than, and 

are probably buried by, deposits of the Bull Lake and Pinedale 

glaciations. Some of the coarse cobbly stream gravel in unit 

Qgd as well as some of the older deposits of gravelly streamterrace alluvium (Qg3-6 and Qg8

) may be outwash gravel 

deposited during pre-Bull Lake glaciations. 

The oldest glacial deposits in the Fraser quadrangle that 

have preserved morainal morphology are till and, locally, 

minor deposits of stratified drift, which form subdued 

moraines of the Bull Lake glaciation. Bull Lake deposits in 

their type area in northwestern Wyoming are about 170–

120 ka (Sharp and others, 2003; Pierce, 2004). These deposits 

accumulated during one or more major cold climatic episodes 

during MIS 6 (190–130 ka; Lisiecki and Raymo, 2005) and 

probably during the early part of MIS 5 (Pierce, 2004). Till 

and minor stratified drift of the Pinedale glaciation form well 

preserved moraines in glaciated valleys in and near the Fraser 

quadrangle. Pinedale glacial deposits in Colorado are about 

30–12 ka (Nelson and others, 1979; Madole, 1986; Schildgen 

and Dethier, 2000; Benson and others, 2004, 2005). They 

accumulated during a major cold climatic episode during MIS 

2 (35–14 ka; Lisiecki and Raymo, 2005). Till and other icecontact deposits of the Pinedale glaciation locally may include 

or overlie glacial deposits of early Wisconsin age (MIS 4, 

70–55 ka; Lisiecki and Raymo, 2005), which have been identified in a few areas of the western United States (Pierce, 2004). 

Till briefly exposed in a deep excavation at the Mary Jane site 

(in the Winter Park Ski Area, just south of Fraser quadrangle; 

fig. 1), is older than 30,480 and 30,050 14C yr B.P. (Nelson and 

others, 1979). It may be early Wisconsin in age (Richmond, 

1986; Richmond and Fullerton, 1986b) or may have been 

deposited during an early advance of Pinedale ice (Sturchio 

and other, 1994). This till is overlain by three thin (30–50 cm) 

till-derived debris-flow deposits that accumulated during the 

Pinedale glaciation as well as by interstratified silty lake sediments, all of which are older than 13,740 14C yr B.P. (Short 

and Elias, 1987). 

The aerial and altitudinal distribution of till in and near 

the Fraser quadrangle suggests that glaciers during the Bull 

Lake and Pinedale glaciations were about 8–18 km in length, 

typically descended to elevations of about 2,610–2,745 m, 

and probably were at least 120–245 m thick. By comparison, 

glaciers during the Bull Lake and Pinedale glaciations throughout the Front Range typically attained lengths of 10–20 km, 

terminated at elevations between 2,500 and 2,700 m, and were 

180–350 m thick (Benson and others, 2004). 

Snow lines during the Pleistocene in the western United 

States were roughly 1,000 m lower than present (Porter and 

others, 1983). Fossil beetles at the Mary Jane site (2,882 m), 

dated at 13,200 and 12,800 14C yr B.P., suggest that during 

full- or late-glacial climatic conditions mean July temperatures 

were 3–4 °C colder than present and January mean temperatures were 19–21°C colder than present (Elias, 1996; table 4 

this report). 

More effective precipitation and vigorous freeze-thaw 

action likely accompanied expanded periglacial environments 

during glacial episodes, and would have promoted slope instability and intensified mass-movement processes in the Front 

Range and the adjacent Fraser basin. Weakly consolidated 

material, such as siltstone in the Troublesome Formation, 

was particularly prone to mass-movement processes such as 

creep and solifluction. Much of the coarse debris, which form 

features such as block fields and block streams on interfluves 

above present treeline in areas adjacent to the Fraser quadrangle, probably formed chiefly under periglacial conditions. 

Increased infiltration of atmospheric precipitation may have 

locally promoted deep-seated rock creep beneath steep slopes 

in periglacial environments.

Streams draining from glaciers within and near the 

Fraser quadrangle produced broad, gravelly deposits of glacial 

outwash, particularly during times of significantly greater 

sediment yield during deglaciation (Church and Ryder, 1972). 

Some of the glacial outwash may be slightly younger than 

corresponding tills, because significant fluvial deposition may 

have lagged (perhaps by a few to several thousand years) the 

transition from glacial to interglacial climates (Church and 

Ryder, 1972; Hancock and Anderson, 2002). Large deposits 

of glacial outwash in the Fraser quadrangle, such as units Qg1

 

and Qg2 in the valley of the Fraser River, locally extend 12 

km or more downstream of former glacier fronts. Maximum 

grain size and the slope of the depositional tops of glacial 

outwash deposits decrease significantly within a distance of 

about 10 km downstream of former glacier fronts (Church and 

Ryder, 1972; Ritter, 1987). 

Alluvium of Holocene age in present stream channels 

and in flood plains less than 5 m above present streams are 

difficult to correlate with climatic episodes, but some episodes 

of alluvial deposition may reflect past climatic conditions that 

were colder than, and possibly more moist (or had more effective precipitation) than the present climate. Cooler climatic 

episodes during the Holocene promoted glacial and (or) periglacial activity in the Front Range (Scott, 1975). 4 Report Title

Table 4. Mean January and July temperatures near Fraser, Colorado and 

estimated mean January and July temperatures near the Mary Jane site, 

about 9 km southeast of Fraser

Weather station

or site name

Fraser1

.

Approximate

elevation

(m)

2,610

Mean January 

temperature

(°C)

-11.0

Mean July 

temperature

(°C)

12.2

Mary Jane site2 2,882 -12.8 10.4

1

Value for Fraser (1959–1974) are from Hansen and others (1978). 

2

Values for the Mary Jane site were estimated by decreasing the mean January and 

July temperatures for Fraser by 1.8°C, assuming an environmental lapse rate of 6.5°C per 

1,000 m (Meierding, 1982).

Table 4. Mean January and July temperatures near Fraser, 

Colo., and estimated mean January and July temperatures near 

the Mary Jane site, about 9 km southeast of Fraser.
References Cited 21

One deposit of loess, too small to show on the map, 

mantles gravelly stream-terrace alluvium of middle Pleistocene age (Qg3

), just south of Tabernash. Thin loess deposits 

locally may overlie, or were redeposited, as silty overbank 

deposits that mantle some of the other deposits of gravelly 

stream-terrace alluvium. Thin (≤ 50 cm) deposits of reworked 

loess (clayey silt) are locally present in wet alpine meadows 

and bogs above treeline, south and southwest of the Fraser 

quadrangle (Retzer, 1962). Geochemical composition and 

grain-size data suggest that alluvial silt in Middle Park and 

possibly North Park is are a likely source for the eolian silt 

that is present in A horizons of alpine soils in the Indian Peaks 

Wilderness Area, about 6 km east of the Fraser quadrangle 

(Muhs and Benedict, 2006). 

Acknowledgments

This report was much improved by comments by Paul 

E. Carrara and James C. Cole, U.S. Geological Survey. Cole 

provided recalculated radiometric ages for several rock units 

listed in Marvin and others (1974), adjusted to the revised 

decay constants of Steiger and Jäger (1977). We thank Tom 

Kalan of the Winter Park West Water and Sanitation District 

for providing subsurface data for the district’s water wells near 

Fraser, Colo. 

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Shroba—Geologic Map of the Fraser 7.5-minute Quadrangle, Grand County, Colorado—Scientific Investigations Map 3130