U.S. Department of the Interior

U.S. Geological Survey

Pamphlet to accompany

Scientific Investigations Map 3170

Geologic Map of the Eastern Half of the Vail 30’ × 60’ 

Quadrangle, Eagle, Summit, and Grand Counties, Colorado
COVER:

View north down the Blue River valley. The Gore Range is on the left and the Front Range is in 

the right distance. The town of Frisco and the edge of Dillon Reservoir are visible on the right 

side of the photo. (Photo by Karl S. Kellogg)
Geologic Map of the Eastern Half of the Vail 

30’ × 60’ Quadrangle, Eagle, Summit, and 

Grand Counties, Colorado

By Karl S. Kellogg, Ralph R. Shroba, Wayne R. Premo, and Bruce Bryant

Pamphlet to accompany

Scientific Investigations Map 3170 

U.S. Department of the Interior

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

KEN SALAZAR, Secretary

U.S. Geological Survey

Marcia K. McNutt, Director

U.S. Geological Survey, Reston, Virginia: 2011

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Suggested citation:

Kellogg, K.S., Shroba, R.R., Premo, W.R., and Bryant, Bruce, 2011, Geologic map of the eastern half of the Vail 

30’ × 60’ quadrangle, Eagle, Summit, and Grand Counties, Colorado: U.S. Geological Survey Scientific Investigations 

Map 3170, 49 p. pamphlet, 1 sheet, scale 1:100.000.
iii

Contents

Description of Map Units..............................................................................................................................1

Surficial Deposits ......................................................................................................................................1 

Artificial Fill Deposits ...........................................................................................................................1

Alluvial Deposits ...................................................................................................................................1

Alluvial and Mass-Movement Deposits............................................................................................2

Mass-Movement Deposits..................................................................................................................3

Glacial and Periglacial Deposits ........................................................................................................5

Mass-Movement and (or) Glacial Deposits .....................................................................................7

Sedimentary and Volcanic Rocks of the Middle Park, Blue River Valley,

 and Southern Gore Range ...........................................................................................................8

Tertiary Volcanic and Intrusive Rocks ...................................................................................................9

Late Cretaceous Intrusive Rocks..........................................................................................................12

Laramide Sedimentary Rocks ...............................................................................................................12

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

Proterozoic Rocks ...................................................................................................................................23

Proterozoic, Plutonic and Metamorphic Rocks .................................................................................23

Paleoproterozoic Metasedimentary and Metaigeneous Rocks......................................................25

Geologic History...........................................................................................................................................31

Proterozoic History .................................................................................................................................31

Paleozoic and Pre-Laramide Mesozoic history .................................................................................32

Late Cretaceous to Early Tertiary Laramide Mountain Building and 

 Synorogenic Sedimentation ......................................................................................................33

The Gore Fault Zone and Faults Cutting Proterozoic Rocks of the Gore Range ...........................33

Middle to Late Tertiary Development of the Northern Rio Grande Rift ..........................................35

Deposition of Upper Oligocene and Upper Miocene Troublesome Formation.............................36

Quaternary History..................................................................................................................................36

Resources .....................................................................................................................................................39

Gravel ........................................................................................................................................................39

Metals .......................................................................................................................................................39

Fluorite ......................................................................................................................................................40

Geologic Hazards.........................................................................................................................................40

Landslides.................................................................................................................................................40

Rockfall .....................................................................................................................................................40

Floods and Debris Flows ........................................................................................................................40

Seismicity .................................................................................................................................................41

Elevated Radon........................................................................................................................................42

Snow Avalanches....................................................................................................................................42

Acknowledgments.......................................................................................................................................42

Credits............................................................................................................................................................42

References....................................................................................................................................................43

Figures

1. East-facing view across Blue River, just north of Bushee Creek, showing 

a large active, or recently active, landslide deposit (Qyl)....................................................4

2. Unconsolidated till of Pinedale glaciation (Qtp).....................................................................5

3. Unconsolidated till of Bull Lake glaciation (Qtb)....................................................................6

4. Exposure of cobbly gravel at Mesa Cortina (QNmg)...........................................................8
iv

5. Moderately welded late Oligocene crystal-lithic tuff (:b) .................................................9

6. View to southwest from Elliott Ridge toward Piney Ridge, which is 

underlain mostly by olivine basalt flows and minor tuffs (N:bp)....................................10

7. Dike of 25–26 Ma (late Oligocene) hornblende latite porphyry at 

Haystack Butte............................................................................................................................11

8. View to east across Green Mountain Reservoir of black shales of the

lower Pierre Shale (Kpls).........................................................................................................13

9. View to north of lower Pierre Formation and Kremmling Sandstone ................................14

10. View to northeast across Blue River, just north of Spruce Creek, of 

upper Niobrara Formation.........................................................................................................14

11. Conformable contact between quartz sandstone of Lower Cretaceous 

Dakota Formation (Kd) and overlying black shale of Benton Group (Kb)........................16

12. Outcrop of Gartra Sandstone Member of Chinle Formation (dcg) on 

Elliott Ridge ..................................................................................................................................17

13. Outcrop of cross-bedded conglomeratic sandstone beds interlayered 

with thin-bedded, fine-grained sandstone of Maroon Formation (Phm).........................18

14. View to northwest of Gilman, now largely abandoned, and section 

of Paleozoic rocks ......................................................................................................................21

15. Clastic dike (|cd), consisting of about 99 percent well rounded, well 

 sorted quartz grains ...................................................................................................................23

16. Dark-gray, highly strained biotite diorite porphyry ...............................................................25

17. Amphibolite intruded by numerous thin sills of monzogranite, in turn, 

cut by small pegmatite dikes ....................................................................................................26

18. Migmatitic biotite gneiss (Xm)................................................................................................26

19. A wedge of overturned Dakota Formation (Kd) in fault contact across 

 Williams Range thrust................................................................................................................33

20. View to northwest of Dakota Sandstone (Kd) exposed in road cut along 

Interstate Highway 70 ................................................................................................................34

21. Steeply dipping beds of the Minturn Formation in fault contact (across 

Gore fault) with fractured Cross Creek Granite.....................................................................34

22. Early morning view to west from Ute Pass across Blue River Valley................................35

23. View to north, near crest of southern Williams Fork Mountains, 

of sackungen ...............................................................................................................................38

24. Road cut at Ute Pass in migmatitic biotite gneiss (Xm), showing well 

developed fractures ...................................................................................................................38

25. Large trench and berm built above houses in Vail to catch rocks falling 

from cliffs .....................................................................................................................................41

Table

 1. Sample or site locations shown on map.................................................................................29

Sheet

 1. Geologic map of the eastern half of the Vail 30° × 60° quadrangle, Eagle, 

Summit, and Grand Counties, Colorado
v

Multiply By To obtain

 Length

centimeter (cm) 0.3937 inch (in.)

millimeter (mm) 0.03937 inch (in.)

meter (m) 3.281 foot (ft) 

kilometer (km) 0.6214 mile (mi)

 Area

square kilometer (km2) 0.3861 square mile (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

Vertical coordinate information is referenced to the insert datum name (and abbreviation) here, 

for instance, “North American Vertical Datum of 1988 (NAVD 88)”

Horizontal coordinate information is referenced to the insert datum name (and abbreviation) 

here, for instance, “North American Datum of 1983 (NAD 83)”

Altitude, as used in this report, refers to distance above NFBOTFBMFWFM.

Period or 

Subperiod Epoch Age

Holocene 0–11.7 ka

Quaternary

late 11.7–132 kaPleistocene middle 132–788 ka

early 788 ka–2.58 Ma

Neogene

Pleistocene 2.58–5.33 Ma

Miocene 5.33–23.0 Ma

Oligocene 23.0–33.9 Ma

Paleogene Eocene 33.9–55.8 Ma

Paleogene 55.8–65.5 Ma

1

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2009)

Division of Quaternary, Neogene, and Paleogene time 

used in this report1

Conversion Factors; metric units to English units

SI to Inch

– 

Description Of Map Units

Surficial Deposits

Artificial Fill Deposits

af Artificial fill deposit (latest Holocene)—Unit consist of four types of deposits: (1) compacted fill material 

composed mainly of silt, sand, and rock fragments that forms roadbed for I–70 (Interstate Highway 70), 

(2) rip-rap (large angular boulders) and underlying compacted fill (cobbles and finer material) that forms 

dams for Dillon and Green Mountain Reservoirs, (3) river gravel along upper Blue River and Swan River 

that was dredged for gold in late nineteenth and early twentieth centuries (Parker, 1974) and subsequently 

bulldozed for residential and commercial development (mostly during the 1970s and 1980s), and (4) mine 

and mill waste, both reclaimed and unreclaimed, associated with Gillman Mine in Minturn 71⁄2-minute 

quadrangle (R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 2009) and tailings 

near Henderson Mill in Ute Peak 71⁄2-minute quadrangle

Alluvial Deposits

Qa Active alluvium (Holocene)—Mostly sand and gravel, with clast size as large as boulders. Clasts are subangular 

to rounded; most are subrounded or rounded. Includes overbank deposits in low terrace deposits less than 

about 2 m above present stream level; prone to infrequent flooding during exceptionally wet years. Locally 

includes some colluvium (Qc) and other mass-movement deposits, young fan deposits (Qfy), and glacial 

deposits (mostly Qtp) along valley margins, and organic-rich sediment (Qw) in bogs, marshes, and meadows. 

Estimated thickness as much as about 20 m along main channel of Blue River and Williams Fork, and as 

much as about 5 m along tributary streams 

Qw Wetland deposits (Holocene and late Pleistocene)—Dark-brown to black, organic-rich sediment (mostly silt 

and minor clay) in wetlands that commonly contain areas of standing water and a variety of grasses, sedges, 

and dense stands of willows. Estimated thickness as much as 5 m 

Qg1 Young stream-terrace alluvium (Holocene? and late Pleistocene)—Chiefly glacial outwash composed mostly 

of cobbly pebble gravel that underlies one terrace tread (locally two). Commonly about 6 to12 m above Blue 

River, Eagle River, Williams Fork, and tributary drainages. Amount and size of small boulders (commonly 

30–50 cm in diameter) and cobbles in deposits decrease downstream. Pebbly sand or silty overbank sediments less than about 50 cm thick locally overlie the gravel. Deposited during Pinedale glaciation, about 

12–30 ka (Nelson and others, 1979; Madole, 1986; Benson and others, 2004, 2005). Cosmogenic dating 

(based on measurement of isotopes 10Be and 26Al) of similar deposits 4–14 m above stream level in Boulder 

Canyon west of Boulder, Colo., yielded exposure ages of 10–32 ka (Schildgen and others, 2002). Unit Qg1

is a source for coarse aggregate. Estimated thickness 3–20 m

Qg2 Intermediate stream-terrace alluvium (late and middle Pleistocene)—Chiefly glacial outwash composed 

mostly of cobbly pebble gravel that underlies one terrace tread (locally two) about 24 m above the Blue 

River, Williams Fork, and tributary drainages downstream from lower limit of Pinedale glaciation. Unit 

locally contains large cobbles and boulders as long as 120 cm. Clasts are subangular to well rounded; most 

are subrounded and consist of granite, gneiss, and quartz sandstone. Proximity to till of Bull Lake glaciation 

(Qtb) suggests that the unit was deposited by glacial meltwater during Bull Lake glaciation, about 120–170 

ka (Sharp and others, 2003; Pierce, 2004). Recent cosmogenic dating (on 10Be and 26Al) of Bull Lake outwash, 16 m above stream level in Boulder Canyon west of Boulder, Colorado, yielded an exposure age of 

130 ka (Schildgen and others, 2002). Gravel is overlain by as much as 20 cm of loess or silty overbank 

sediment. Unit Qg2 is a source of coarse aggregate. Estimated thickness is 3–20 m

Geologic Map of the Eastern Half of the Vail 30’ × 60’ Quadrangle, Eagle, Summit, 

and Grand Counties, Colorado

By Karl S. Kellogg, Ralph R. Shroba, Wayne R. Premo, and Bruce Bryant
2 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Qg3 Old stream-terrace alluvium (middle and early? Pleistocene)—Probably glacial outwash and, locally, fluvial 

deposits of non-glacial origin that are composed chiefly of cobbly pebble gravel that underlies terrace remnants at five levels (about 35, 50, 70, 85, and 100 m) above Blue River and Williams Fork. Large pebbles 

and cobbles commonly are subangular to subrounded granite, gneiss, and quartz sandstone. Deposits locally 

contain a few boulders as long as about 50 cm. Gravel is locally overlain by as much as 30 cm of loess or 

silty overbank sediment. Unit likely deposited during two or more pre-Bull Lake glaciations. Estimated 

thickness is 3–15 m. Enigmatic gravel deposits, at and near the towns of Dillon and Silverthorne, are tentatively assigned to Qg3

. These deposits lack terrace scarps and treads and consist of light-yellowish-brown, 

unconsolidated, moderately rounded to well-rounded, mostly unstratified, matrix-supported pebble- and 

cobble-size gravel (Kellogg, 2002). Clasts are mostly Proterozoic gneiss and granite in a sandy and silty 

matrix that contains much decomposed granitic debris (grus) and clay. Unit locally contains iron-stained 

and partly weathered Proterozoic clasts. Gravel is overlain by a well-developed soil profile and is as high as 

100 m above present Blue River. Interpreted as “older outwash and pediment gravels” of Pleistocene age by 

Tweto (1973b). Thickness is locally greater than 30 m

Qg4 Oldest stream-terrace alluvium (middle or early Pleistocene)—Probably glacial outwash and, locally, fluvial deposits of non-glacial origin that are composed chiefly of cobbly pebble gravel that underlie terrace 

remnants at three levels (about 120, 135 and 150 m) above Blue River and Williams Fork. Large pebbles 

and cobbles commonly are subangular to subrounded granite, gneiss, and quartz sandstone. Deposits locally 

contain a few boulders as long as about 50 cm. Gravel may be locally overlain by thin loess or silty overbank 

sediment. Unit probably deposited during one or more pre-Bull Lake glaciations. Estimated thickness is 3–15 m

Ng High-level fan and stream gravel (Pliocene? and upper? Miocene)—Scattered deposits of coarse boulder 

gravel that mantle ridges and hills about 230 m above Blue River near Squaw Creek, about 215 m above the 

Williams Fork near northern boundary of map area, and about 240 m above Keyser Creek near the northeast corner of map area. Unit along the Blue River consists of nonsorted, nonstratified to crudely stratified, 

matrix-supported and locally clast-supported, bouldery gravel and slightly bouldery, coarse cobble gravel 

that locally contains lenses of boulder gravel, pebble gravel, and sand. Matrix material is slightly oxidized, 

slightly silty and clayey, coarse sand. Most of unit is alluvial, but locally may include debris flow deposits. 

Clasts are mostly subangular to subrounded and as long as about one meter. Biotite-rich clasts are partly or 

completely disintegrated. Unit along the west side of the Williams Fork consists of stratified, mostly subangular to subrounded cobbly pebble fluvial gravel. The gravel contains lenses of very fine to medium sand and 

coarse sand about 25–50 cm thick, and locally lenses of weakly consolidated, micaceous, silty, mostly very 

fine to medium sand as much as 6.5 m thick. Clasts consist of granite, gneiss, pegmatite, and sparse quartz 

sandstone of Dakota Formation (Kd). Cobbles are as large as 10 cm in diameter. Unit north of Keyser Creek 

consists chiefly of angular to well-rounded cobbly pebble fluvial gravel that contains abundant granules and 

small pebbles. Matrix is slightly silty very fine to very coarse sand. Clasts consist of granite, gneiss, pegmatite, and sparse quartz sandstone. Cobbles are as large as 25 cm in diameter. The gravel along the Williams 

Fork unconformably overlies Troublesome Formation and contains fossil beaver (Dipoides sp.) bones of 

Hemphillian (late Miocene or earliest Pliocene) age (Robinson, 1968; Kihm and Middleton, 1980). This 

deposit indicates that the course of the Colorado River and its tributaries were established after deposition of 

the Troublesome Formation during late Miocene or earliest Pliocene. Gravel along the Blue River is locally 

overlain by as much as about 50 cm of loess. Maximum thickness of unit Ng is about 30 m

Alluvial and Mass-Movement Deposits

Qac Alluvium and colluvium, undivided (Holocene to middle? Pleistocene)—Alluvium composed of typically 

poorly sorted silt to boulder gravel in narrow channels that are too small to map separately. Alluvium is 

flanked by colluvial deposits composed of mostly poorly sorted, angular clasts as long as one meter in a 

matrix of clay, silt, and sand derived from weathered bedrock and transported downslope. Colluvium is 

locally mantled by thin loess. Typically less than 10 m thick

Qfy Younger fan deposits (Holocene and late? Pleistocene)—Tan to grayish-brown, poorly to moderately well 

sorted, crudely stratified, sand and gravel in fan-shaped deposits. Deposits are both clast and matrix 

supported, suggesting deposition by both alluvial and debris-flow processes. Clasts mostly subangular to 

subrounded; some are as long as about 2 m, but most are considerably smaller. Unit grades to present stream 

level and probably began forming soon after deglaciation during the late Pleistocene. As much as 15 m thick
Description of Map Units 3

Qfo Older fan deposits (late? and middle Pleistocene)—Poorly to moderately well sorted, tan-weathering, crudely 

stratified sand and gravel that forms aprons on slopes east of the Blue River. Deposits are both clast and 

matrix supported, suggesting deposition by both alluvial and debris-flow processes. Clasts are mostly subangular to subrounded. They are as much as about 2 m long, but most are considerably smaller. Granitic and 

gneissic clasts, particularly those bearing significant biotite, are variably weathered. Two older fan deposits 

in southeastern part of the Squaw Creek 71⁄2-minute quadrangle grade to either unit Qg2 or unit Qg3, suggesting 

that unit Qfo is of Bull Lake age and older. Estimated thickness as much as 15 m 

Qp Pediment deposits (late? and middle Pleistocene)—Crudely stratified, matrix-supported small boulders to 

granules in a sandy, clayey silt matrix on gently sloping surfaces. Mapped at five levels on the west side 

of the Williams Fork in and near the Battle Mountain 71⁄2-minute quadrangle where deposits overlie the 

Troublesome Formation (N:t). The lower limits of unit Qp are about 12, 20–24, 30, 60–75, and 85–100 m 

above the Williams Fork. Much of the silt and minor clay in the matrix is chiefly derived from siltstone of 

the Troublesome Formation. Unit Qp was deposited chiefly by debris flows and ephemeral streams, probably 

under periglacial conditions during Pinedale and older glaciations. Large pebbles and cobbles commonly are 

angular to subangular and consist primarily of quartz sandstone, gneiss, and pegmatite. Sandstone boulders 

are commonly as long as 50 cm. Although sandstone clasts are identified from Dakota Formation (Kd), the 

source area is unknown. Estimated thickness 2–5 m

Mass-Movement Deposits 

Qc Colluvium (Holocene to middle? Pleistocene)—Unconsolidated to weakly consolidated, mostly non-stratified, 

non-sorted, dark-brown to light-gray-brown deposits that mantle surfaces that slope less than about 50 

degrees; clasts and fine-grained material deposited by downslope movement. Contains angular pebbles, 

cobbles, and boulders with compositions that reflect upslope bedrock and surficial deposits. Includes material transported by creep, landslide, debris flow, hyperconcentrated flow, and rock fall. Locally includes 

minor deposits of stream-channel and sheetwash alluvium as well as periglacial deposits above treeline 

(about 3,500 m elevation) formed chiefly by frost wedging and solifluction. Locally overlain by loess, as 

much as about 50 cm thick, composed of very fine sand, silt, and minor clay. Commonly underlies areas of 

open meadows, sagebrush, or aspen groves. Maximum thickness probably less than 10 m

Qdf Debris-flow deposits (Holocene to Middle? Pleistocene)—Lobate and fan-shaped masses of coarse material 

deposited by debris flows. Deposits are chiefly unsorted to very poorly sorted and unstratified to crudely 

stratified boulders to granules supported in a matrix of silty sand to slightly sandy clayey silt. Locally 

includes lenticular beds of poorly sorted, clast-supported, pebble-to-boulder-sized gravel. Clasts are commonly angular to subangular and randomly oriented. Low-lying areas of unit adjacent to stream channels 

are prone to periodic stream flooding and debris-flow deposition. Unit probably includes minor stream-flow 

deposits. Mapped only on west slope of Williams Fork Mountains where unit Qdf overlies Pierre Shale. 

Estimated thickness as much as 15 m

Qt Talus (Holocene to middle Pleistocene)—Angular and subangular cobble- and boulder-sized fragments that 

form near angle-of-repose deposits below steep valley walls or cliffs. Sandy matrix rarely is exposed. Boulders generally are as large as 2 m; locally they are as large as 10 m. Most talus deposits are on valley walls 

eroded by Pinedale glaciers, indicating talus is as old as late Pleistocene. Talus is less common below Paleozoic and Mesozoic units as compared to Proterozoic bedrock and Tertiary basalt, because the former units 

weather more readily to form sand and finer sediment, which commonly comprise colluvium (Qc) rather 

than talus. Thickness is as much as 20 m thick

Qly Active or recently active landslide deposits (Holocene)—Deposits are mostly earth slides and earth flows 

(terminology of Cruden and Varnes, 1996), characterized by hummocky topography, well formed, relatively 

steep, crescent-shaped, unvegetated headwall scarps, and downslope lobate toes. Several have documented 

historic movement. A large active earth slide in Vail just north of I–70 (location EE on map) was undercut 

by construction of the highway and is currently failing because of a combination of shallow earth sliding 

and deep rotation (Jurich and Miller, 1987). Another possibly active landslide, composed entirely of displaced Pierre Shale, is just east of the Blue River (fig. 1; mostly in sec. 23, T. 4 S., R. 78 W.). The surface 

of this deposit is very hummocky and numerous unvegetated scarps cut the crown area and zone of extension. Several large earth flow complexes just south of the intersection of I–70 and State Highway 24 in the 

northwest part of the Minturn 71⁄2-minute quadrangle have undergone historic movement (R.M. Kirkham and 
4 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

K.J. Houck, Colo. Geological Survey, written commun., 2009); these flows are on dip slopes of the Minturn 

Formation and have caused considerable road damage. Another large earth-flow deposit near Frisco, just 

north of I–70 (in secs. 22, 23, 26, and 27, T. 5 S., R. 78 W.), has hummocky topography and well-formed, 

unvegetated scarps in the crown region and zone of deflation (terminology of Cruden and Varnes, 1996) and 

may be undergoing very slow creep 

Qbf Boulder-field deposits (Holocene or late Pleistocene)—Hummocky deposits composed of angular boulders of 

Proterozoic granite as long as 4 m (most less than 2 m) that mantle large, nearly level to gently sloping areas 

in Pitkin Creek valley in Vail East 71⁄2-minute quadrangle (Kellogg and others, 2003). Matrix is absent at 

surface at most places. Up-valley end is at distal end of landslide deposit on east side of Pitkin Creek valley. 

Deposit probably formed as rock avalanche that flowed across a snowfield or glacial ice during the latter part 

of the Pinedale glaciation or during the Holocene. Thickness is probably less than 20 m

Ql Younger landslide deposits (Holocene to middle Pleistocene)—Deposits range from chaotic debris to almost 

intact slump blocks of bedrock. Unit includes earth slides, earth flows, rock slides, and debris slides (terminology of Cruden and Varnes, 1996). Surface of younger deposits commonly hummocky; relatively steep, 

crescent-shaped headwall scarp and downslope lobate toe are locally preserved. Many of the landslides and 

landslide deposits formed on unstable slopes that are underlain by shale, siltstone, claystone, and locally by

glacial till. Estimated thickness is 5–60 m 

QNl Older landslide deposits (middle? Pleistocene to Pliocene?)—Mostly angular fragments of Proterozoic rocks 

in grus-rich matrix partially altered to clay. Locally contains relatively unfractured gneiss blocks as long as 

30 m, suggesting at least some movement was by downslope creep. Mapped along west side of Williams 

Fork Mountains where source rock is in hanging wall of Williams Range thrust. All topographic evidence 

of breakaway zone and original hummocky landslide morphology eroded. Some valleys are incised as much 

as 50 m into these deposits, exposing underlying Cretaceous rocks. Deposits in many places cover trace of 

Williams Range thrust. Most deposits are thickly forested by conifers and aspen. Interpreted to include earth 

slides, earth flows, rock slides, and debris slides (terminology of Cruden and Varnes, 1996). In places, deposits 

were remobilized during late Pleistocene and Holocene. Inferred to be as thick as several hundred meters

Figure 1. East-facing view across Blue River, just north of Bushee Creek, showing a large active, 

or recently active, landslide deposit (Qyl; boundary outlined). The slide is composed of Pierre Shale, 

in part remobilized from older landslide deposits (QNl). The approximate trace of the buried Williams 

Range thrust is shown by the dashed line. Note the relatively smooth profile of the crest of the Williams 

Fork Mountains. Distance across landslide is about 2 km. Photograph by K.S. Kellogg, 2004.
Description of Map Units 5

Glacial and Periglacial Deposits

Qpg Periglacial deposits (Holocene to early? Pleistocene)—Deposits of angular blocks as long as about 3 m 

composed of Paleoproterozoic gneiss or granitic rock. Formed by periglacial processes (chiefly freeze-thaw 

wedging) in non-glaciated, alpine areas above about 3,600 m during glacial and periglacial episodes during 

the Pleistocene. Mapped chiefly along eastern side of Gore Range. Minor felsenmeer (rock field) formation 

may have occurred during the Holocene. At surface, blocks commonly have open spaces between them or 

contain a dark-brown gravelly soil-like material that is a combination of materials derived from mechanical 

weathering of blocks and smaller rock fragments and deposition of eolian silt (loess). Composition of blocks 

same as immediately underlying bedrock. Smaller, unmapped periglacial deposits are common along some of 

the higher ridges. Thickness is probably less than 5 m 

Qmd Moraine-dammed sediments (Holocene? and late Pleistocene)—Silt, sand, and gravelly alluvium in closed 

depressions behind moraines of Pinedale age. Mapped only in southern part of Minturn 71⁄2-minute quadrangle (R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 2009), although small, 

unmapped deposits locally occur elsewhere in or adjacent to moraines of Pinedale age. Estimated thickness 

less than 10 m

Qtp Till of Pinedale age (late Pleistocene)—Mostly non-sorted and non-stratified matrix supported. Matrix (less 

than 2 mm in diameter) comprises an estimated 20–40 percent of deposit and consists chiefly of poorly 

sorted sand and a minor amount of silt and clay. Most of the biotite-rich granitic and gneissic clasts within 

soil are unweathered; disintegrated clasts are rare. A typical outcrop of unit Qtp is shown in figure 2. On 

east side of Gore Range, clasts other than Proterozoic rock are very rare; on west side, till in Gore Creek 

valley contains sandstone, conglomerate, or limestone of Minturn and Maroon Formations, in addition to 

Proterozoic clasts. Unit locally includes (1) deposits of stratified drift, (2) younger tills of Satanta Peak and 

Triple Lakes advances of Benedict (1985) in cirques and at valley heads, dated at about 10–12 ka (Davis 

1988), (3) colluvium (Qc) and other mass-movement deposits; and (4) minor alluvium (Qa). Unit commonly 

forms large, very bouldery, sharp-crested lateral and end moraines that have distinct hummocky morphology. 

Deposits in some areas have well expressed knob-and-kettle topography (although earth movement by heavy 

equipment in urbanized areas of Gore Creek valley has muted many of these topographic features). Soil has a 

thin B horizon that is poorly developed with little or no clay accumulation. Glacier in Gore Creek valley was 

fed by ice caps in Vail Pass area; terminus of Pinedale glacier in Gore Creek valley terminates in the town of 

Vail, about 3 km west of western boundary of Vail East 71⁄2-minute quadrangle (Scott and others, 2002). Till 

of Pinedale age was deposited in most valleys on east side of Gore Range and in upper part of the Williams 

Fork valley. Pinedale-age glaciers flowing down Tenmile and North Tenmile Creeks deposited till almost as 

far east as Dillon Dam. Figure 2 shows typical exposure of till of Pinedale glaciation along Dillon Reservoir. 

Blue River valley in map area not glaciated; terminal Pinedale moraine in valley is just south of quadrangle 

boundary near the town of Breckenridge. Radiocarbon and cosmogenic-exposure ages from Front Range 

region indicate that till of Pinedale age is about 12–30 ka (Nelson and others, 1979; Madole, 1986; Benson 

and others, 2004, 2005). These ages are in agreement with ages of 16–23 ka from Pinedale deposits in type 

area in Wyoming (Chadwick and others, 1997). Estimated thickness is as much as 30 m

Figure 2. Unconsolidated till of 

Pinedale glaciation (Qtp) in 2-m 

high wave-cut exposure along 

Dillon Reservoir. Note minimal soil 

development and weak oxidation 

of the deposit. Photograph by 

K.S. Kellogg, 2005.
6 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Qtb Till of Bull Lake age ( late and middle Pleistocene)—Mostly non-sorted and non-stratified subangular to subrounded boulders as long as 4 m to granules in a silty sand matrix; clasts are matrix supported. Matrix (less 

than 2 mm in diameter) comprises an estimated 20–40 percent of deposit. Unit commonly forms prominent 

lateral moraines with rounded crests beyond lower limit of till of Pinedale age (Qtp). Surface boulders 

typically are less abundant on moraines of Bull Lake glaciation than on those of Pinedale glaciation. Bull 

Lake till (Qtb) is more weathered and its surface morphology is smoother and less hummocky than Pinedale till (Qtp). Boulders of Proterozoic gneiss and plutonic rock commonly have weathered rinds. On east 

side of Gore Range, unit Qtb is composed mostly of Proterozoic gneiss and granitic rocks with as much 

as 10 percent Dakota Formation sandstone clasts; west of Gore fault, locally contains clasts of sandstone, 

conglomerate, or limestone of Minturn Formation. Locally includes minor deposits of stratified drift. Soil 

formed in till of Bull Lake age has an orange-brown B horizon, typically 20–50 cm thick, of clay and iron 

accumulation. The B horizon overlies unweathered or slightly weathered till. Small areas of alluvium and 

colluvium locally overlie unit. Terminus of Bull Lake glacier in Gore Creek valley was about 4.5 km east of 

map boundary (Tweto and Lovering, 1977). A typical exposure of till of Bull Lake glaciation along Dillon 

Reservoir is shown in figure 3. Age of Bull Lake glaciation in type area in Wind River Range of Wyoming is 

95 ka to >130 ka (Chadwick and others, 1997). More recent K-Ar and 230Th/U dating near type area and ages 

of glacial deposits near West Yellowstone indicate that 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). 

These studies are in agreement with (1) 10Be and 26Al analyses of surface boulders on Bull Lake moraines 

near Nederland, Colo., about 40 km east of map area, that yielded minimum age estimates of 101±21 ka and 

122±26 ka (Schildgen and others, 2002), and (2) a uranium-trend age estimate of 130±40 ka for Bull Lake 

till (Shroba and others, 1983) near Allens Park, Colo., about 45 km northeast of map area. In most glaciated valleys, till of Bull Lake age extended farther down valley than till of Pinedale age (Qtp). Exceptions 

include Williams Fork valley, where till of Pinedale glaciation extends farther down valley than till of Bull 

Lake glaciation. Thickness may locally exceed 30 

Figure 3. Unconsolidated till of Bull Lake glaciation (Qtb) in 3-m high wave-cut exposure along Dillon 

Reservoir. Note soil development in the upper one meter and strong oxidation in the lower part of the 

exposure. Photograph by K.S. Kellogg, 2005.
Description of Map Units 7

Mass-Movement and (or) Glacial Deposits

Qr Rock-glacier deposits (Holocene and latest Pleistocene)—Bouldery, lobate, and tongue-shaped masses along 

valley walls and on valley floors that commonly have steep fronts and flanks. Deposits consist of a veneer 

of angular boulders that overlies a thick mass of rock rubble that contains finer interstitial rock fragments. 

Lower parts of deposits locally contain interstitial ice, ice lenses, or an ice core. Ice-cored rock glaciers 

commonly have depressions adjacent to headwall cliffs (where glacial ice melted), longitudinal meandering furrows, and collapse pits. Ice-cemented rock glaciers (containing only interstitial ice with no ice core) 

commonly lack these surface features (White, 1976). Rock fragments on and within rock-glacier deposits are 

derived from steep slopes chiefly by rockfall and locally by rock slide and avalanche. Unit locally includes 

protalus ramparts, minor talus deposits (Qt) displaced by post-depositional flowage, debris-flow deposits 

(Qdf), colluvium (Qc), and other mass-movement deposits. Unit also includes till of Satanta Peak and Triple 

Lakes advances of Benedict (1985), about 10–12 ka (Davis, 1988), as well as tills of Holocene age near 

cirque headwalls. Many of the rock-glacier deposits in Colorado are of latest Pleistocene or early Holocene 

age (Meierding and Birkeland, 1980). Rates of movement of active rock glaciers in Front Range are approximately 1–10 cm/yr (White, 1971, 1976). Estimated thickness is as much as 50 m

Qdy Younger diamicton (Holocene to middle? Pleistocene)—Unsorted and unstratified bouldery deposit containing 

clay- to boulder-size clasts; both clast supported and matrix supported deposits occur. Consists of probable 

till of Pinedale age (Qtp), colluvium (Qc), and landslide deposits (Ql). Topography smooth to hummocky. 

Poorly exposed in most places. Mapped only in and adjacent to Vail West 71⁄2-minute quadrangle (Scott and 

others, 2002), although unmapped deposits of younger diamicton occur elsewhere

QNd Older diamicton (middle Pleistocene to Pliocene?)—Unsorted and unstratified deposits containing clay-size 

material to boulder-size clasts. Deposits are deeply weathered and beveled smooth by erosion; only the tops 

of boulders are exposed. Matrix is orange-brown and clay rich. Proterozoic boulders (particularly those rich 

in biotite) have deeply weathered surfaces. Deposits on east side of Gore Range contain about 10–20 percent 

clasts of Dakota Formation, considerably higher amount than in nearby till of Pinedale age (Qtp) or till of 

Bull Lake age (Qtb). Typically mantled by loess; top of deposit has a well-developed soil profile. Genesis 

uncertain; may be pre-Bull Lake till and (or) debris-flow deposits. Mapped as pre-Bull Lake till along east 

flank of Gore Range (West, 1978) and in Minturn quadrangle (R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 2009), but sufficient uncertainty exists as to origin that non-genetic term 

“diamicton” is employed for these deposits. Total thickness unknown; estimated thickness is locally greater 

than about 80 m 

QNmg Bouldery gold-bearing gravels of Mesa Cortina and Gold Run (middle Pleistocene to Pliocene?)—Poorly 

sorted, poorly stratified to massive, subrounded to well-rounded, poorly consolidated, light-tan to grayishorange bouldery deposits underlying much of gently sloping surface (Mesa Cortina) in northwestern part of 

Frisco 71⁄2-minute quadrangle (fig. 4), and exposed along Gold Run in southern part of Frisco quadrangle 

(Kellogg and others, 2002). Gravels are deeply weathered and matrix supported. Clasts consist predominantly of Proterozoic gneiss and granite, Dakota Formation, and Tertiary monzogranite porphyry (the latter 

clast type common along Gold Run but is sparse at Mesa Cortina). Gravel at Mesa Cortina contains abundant 

purple clasts of pebbly sandstone from Maroon (*m) and Minturn (P*m) Formations. Proterozoic boulders 

at Mesa Cortina are as long as 8 m, and boulders of Dakota and Maroon are as long as 5 m (although most 

clasts are much smaller). Boulders in Gold Run deposit are as long as 2 m. Matrix in both places is weathered (clay-rich) grussy sand. Gravels at Mesa Cortina are poorly exposed on sloping, dissected surface as 

high as 330 m above Blue River, where they resemble old till. Enormous size of some clasts, their distance 

from source, and deeply weathered character of deposits suggests they may be either large debris flows or 

pre-Bull Lake till. Clast compositions in deposits at Mesa Cortina suggest source is either upper Blue River 

valley, south of quadrangle, or upper Tenmile Creek. In contrast, similar diamictons (QNd) along east flank 

of Gore Range were clearly derived from more local sources in the Gore Range. Gravels were previously 

mapped as older terrace gravels by Ransome (1911) and as Dry Union Formation of Pliocene and Miocene 

age by Tweto (1973b), but correlation with the mostly fluvial Dry Union Formation of Arkansas River valley near Leadville (Tweto, 1961) seems unlikely. Age unknown, but weathering and surface morphology of 

deposits suggest they may be as old as Pliocene. Total thickness unknown, but locally may be more than 100 

m. Gravels were first mined for gold by hydraulic methods starting in 1860 and were worked intermittently 

thereafter until 1934. Early miners called Mesa Cortina gravels “Buffalo Placers” (Parker, 1974)
8 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Sedimentary and Volcanic Rocks of the Middle Park, Blue River Valley, and Southern Gore Range

N:t Troublesome Formation (upper Miocene to upper Oligocene)—Light-gray, light-brown, and light-grayishorange tuffaceous siltstone; local white, water-laid tuff; and, near base of unit, light-tan and brown arkosic 

sandstone and conglomerate. Tuffaceous rocks are rich in expansive clay. Rocks are almost entirely 

terrestrial, as indicated by mammalian fossils and widespread root casts in growth positions described in 

Troublesome basin just north of quadrangle (Izett, 1968). The Troublesome Formation consists of two 

principal facies: a conglomerate facies that probably is more common along margin and base of basin, 

and a siltstone facies that probably is more common near the center of basin (Izett, 1968). The varicolored 

conglomerate facies was deposited in small (commonly less than 5 m wide) channels, and locally includes 

variable amounts of claystone, sandstone, and conglomerate. Conglomerate clasts consist mostly of Proterozoic rocks and Tertiary volcanics derived from local sources. Splintery petrified wood is locally present. 

Proterozoic rocks were derived chiefly from west of basin and volcanic clasts mostly, if not entirely, were 

derived from Rabbit Ears Volcanics (Izett, 1968). Rocks of siltstone facies are mostly poorly indurated, 

orangey gray, clayey, limy siltstones. Locally, they contain beds and lenses of claystones, sandstones, and 

pebble conglomerates. Unit also contains abundant ash beds, derived from either Rabbit Ears Volcanics or 

from more distant sources (Izett, 1968). Ash is mostly altered to expansive clay. Origin of much of silt-size 

component in Troublesome Formation is probably eolian (Shroba and others, 2010). Locally, silty limestone 

was deposited in ephemeral ponds, commonly in close proximity to lenticular beds of white chert that are 

probably altered ash deposits. Reported 40Ar/39Ar ages on tuffs in and near northeastern part of the map area 

range from 11.0±0.05 Ma to 23.5±0.06 Ma (late Miocene to late Oligocene) (Izett and Obradovich, 2001). 

Earlier reported K/Ar ages in lower part of Troublesome are 23–29 Ma (late Oligocene) (Braddock and Cole, 

1979). Hemingfordian (early Miocene) and Arikareean (early Miocene and Oligocene) mammalian fossils, 

including early horses (Merychippus sp.), beavers, small rodents, and camels further define age of formation 

(Lovering, 1930; Izett, 1968). The Troublesome is poorly exposed in most places, so thickness is difficult 

to determine, but may be at least 285 m thick just north of map area in Kremmling 71⁄2-minute quadrangle 

(Izett and Barclay, 1973), and is probably of similar thickness in map area. The Troublesome unconformably 

overlies Proterozoic basement or Mesozoic and Paleocene rocks

Figure 4. Exposure of cobbly gravel at Mesa 

Cortina (QNmg), about 30 m west of Interstate 

70, showing crude stratification and strong 

oxidation. The face was exposed by late nineteenth century hydraulic mining. Photograph by 

K.S. Kellogg, 2002.
Description of Map Units 9

N:st Tuffaceous sedimentary rocks (Miocene and Oligocene?)—Very pale orange to grayish-pink, poorly lithified 

to well-lithified calcareous silty sandstone and minor pebbly sandstone that contain minor volcanic ash and 

local pumice fragments. Pebbles consist of angular to subrounded limestone, chert, silicified siltstone, and 

pumice fragments as long as 3 cm. Massive to thin bedded. Mapped in Vail West quadrangle (Scott and others, 2002), where unit’s age is interpreted as Miocene. However, similarity with some beds in Troublesome 

Formation suggest unit is Miocene and (or) Oligocene in age. Locally deformed and tilted by normal faulting. Thickness is at least 60 m (Scott and others, 2002)

:b Basalt, rhyolite tuff, and sedimentary rocks of Blue River valley (Oligocene)—Interlayered mudstone, tuff, 

and conglomerate exposed in a section about 550 m thick overlying Pierre Shale between Palmer Knob and 

South Brush Creek (location A on map). Basal boulder conglomerate contains clasts of granite and migmatitic biotite gneiss. Tuffaceous mudstone, siltstone, pebbly tuffaceous sandstone, and a pale yellowish-white, 

moderately welded tuff (fig. 5) overlie conglomerate; clasts are all Proterozoic rocks. Tuff is about 2–3 m 

thick and contains small, abundant grains of obsidian, pumice, and quartz. Thin sections reveal about 60 

percent fine-grained matrix, 20 percent clear, angular to subrounded fragmental quartz as long as 4 mm, 10 

percent small (<1 mm) plagioclase, 10 percent sanidine, 5 percent opaque minerals, 2 percent biotite, and 

about 2 percent pumice fragments as long as 2 mm. Open pores lined with very fine grained feldspar crystals. Poorly exposed mudstone, siltstone, silty limestone, and calcareous tuff overlie the tuff and, in turn, are 

overlain by sanidine-bearing trachyandesite flows, tentatively correlated with basal flows of basalt of Piney 

and Blue Rivers (N:pb). Palynomorphs from about middle of section suggest a Miocene age (Holt, 1961), 

in disagreement with 40Ar/39Ar age on tuff of 26.89+0.06Ma (late Oligocene; Naeser and others, 2002). Estimated thickness greater than 550 

Tertiary Volcanic and Intrusive Rocks

N:bd Basalt dike (early Miocene? and late Oligocene?)—Dikes of weathered, greenish-gray olivine basalt or diabase 

that intrude Proterozoic and Mesozoic rocks, mostly in Mount Powell 71⁄2-minute quadrangle. Basalt contains 

small phenocrysts of labradorite, olivine, augite, and oxy-hornblende (Taggert, 1962) and have a diabasic 

texture. Dike in south part of sec. 28, T. 2 S., R. 80 W. (just east of Hoagland Reservoir) is about 30 m thick; 

all others are considerably thinner. Unit is undated, but probably includes feeders to some flows of early 

Miocene and late Oliogocene basalt of Piney and Blue Rivers (N:pb)

Figure 5. Moderately welded late Oligocene crystal-lithic tuff (:b); location A, 

near South Brush Creek, consists of about 60 percent very fine-grained matrix and 

contains crystals of quartz, biotite, plagioclase, and sanidine. Clasts consist of 

obsidian, felsic volcanic rocks, and Proterozoic gneiss. Hammer head 18 cm long. 

Photograph by K.S. Kellogg, 2005.
10 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

N:pb Alkali basalt and andesite flows of Piney and Blue Rivers (early Miocene and late Oligocene)—The informal 

term Piney River volcanics was used by Taggert (1962), although Tweto and Lovering (1977) did not use the 

term in the northern part of the Minturn 15-minute quadrangle. Similarly, the informal term Blue River volcanics was used by Taggert (1962) for basalt flows west of Blue River primarily in Mount Powell 71⁄2-minute 

quadrangle. Both sets of volcanic rocks are part of a large, eroded volcanic field that extended from Blue 

River westward to at least the Colorado River near State Bridge, about 10 km west of map area (Tweto and 

Lovering, 1977; Kunk and others, 2002) and probably includes 23–10 Ma basalt flows of Flat Top volcanic 

field along northern part of White River uplift (Larson and others, 1975), as far as 80 km west of map area. 

Largest area of outcrop in map area is along Piney Ridge in western part of Piney Peak 71⁄2-minute quadrangle (fig. 6), where more than 25 flows, each 1.5–12 m thick, cover about 5 km2

. Thickest section on Piney 

Peak is about 250 m thick and consists of olivine basalt flows interlayered at one locality with pale orangegray friable calcareous tuff (Taggert, 1962). A basalt flow in northwest part of Vail West 71⁄2-minute quadrangle is nearly andesite in composition and is notable for containing significant potassium feldspar in the 

groundmass (Tweto and Lovering, 1977). Basalt southwest of Green Mountain Reservoir (in sec. 36, T. 2 S., 

R. 80 W.) contains scattered sanidine phenocrysts, “similar in size and composition to those in the porphyry 

intrusives at Green Mountain” (Taggert, 1962). Most basalt is dark gray to dark brownish-gray, vesicular, and 

aphanitic, except for a few zoned plagioclase phenocrysts (cores of labradorite and rims of oligoclase). Finegrained augite comprises about 25 percent of rock and fine-grained olivine occurs as scattered crystals. Both 

augite and olivine are strongly altered to iddingsite and chlorite. Fossil ancestral “dog”, oreodont, and Merychippus sp. (ancestral horse) bones from interlayered tuffaceous rocks 10 km west of map area suggest a late 

Miocene age (reported in Tweto and Lovering, 1977). However, potassium-argon ages of 22–25 Ma suggest 

an early Miocene or late Oligocene age (Mutschler and Larson, 1969; ages corrected for new decay constants 

of Steiger and Jäger, 1977). Eight 40Ar/39Ar age determinations on basalt flows from near State Bridge are all 

between 22.58±0.58 Ma and 23.97±0.29 Ma (Kunk and others, 2002). Three 40Ar/39Ar ages on sanidine from 

three trachyandesite flows on Otter Creek (location B on map) are statistically indistinguishable; their mean 

age is 23.62±0.06 (Naeser and others, 2002) 

:i Intermediate to felsic dikes, undivided (early Oligocene?)—Steeply dipping dikes as wide as about 15 m 

intrude Proterozoic rock in central Gore Range (Kellogg and others, 2003; Tweto and others, 1970). One 

northwest-striking fine- monzogranite porphyry dike in northwest part of Vail East quadrangle (Kellogg and 

others, 2003) is massive, fine-grained, pinkish-gray, containing about 15 percent pink, subhedral potassicfeldspar phenocrysts as long as 1.5 cm; 5 percent gray, rounded quartz phenocrysts as long as 3 mm; and 

Figure 6. View to southwest from Elliott Ridge toward Piney Ridge, which is underlain mostly by olivine 

basalt flows and minor tuffs (Nbbp). Photograph by K.S. Kellogg, 2005.
Description of Map Units 11

about 4 percent fine-grained biotite-rich clots as long as 4 mm. Matrix contains 20–30 percent quartz, 30–40 

percent sausseritized sodic plagioclase, 30–40 percent microcline, 2 percent muscovite, 2 percent biotite, 2 

percent opaque minerals, and a trace of garnet. Rock weathers reddish brown. Several felsic to intermediate 

dikes intrude along northwest-striking faults, parallel to faults or shear zones that were recurrently active 

since at least late Paleozoic time. Based on petrographic similarities, Tweto and Lovering (1977) suggested 

that latitic to dacitic dikes in area are related to Green Mountain intrusive center. The latter has an 40Ar/39Ar 

age of 31.51± 0.08 Ma (Naeser and others, 2002)

blt Latite and trachyte porphyry of Green Mountain and surrounding area (late and early Oligocene)–Range 

in composition from hornblende trachyte to augite latite. Most rocks contain large sanidine phenocrysts, 

abundant small andesine phenocrysts, and, locally, hornblende or augite phenocrysts. Hyalopilitic groundmass consists of feldspar microlites and brownish glass. Unit forms laccoliths, sills, and dikes that extend 

outward as far as 10 km from a small stock centered at Green Mountain (near location C on map). Rock in 

stock on Green Mountain is gray augite latite porphyry that contains andesine and augite phenocrysts as long 

as 12 mm and 2 mm, respectively (Taggert, 1962). Sills and laccoliths on west side of Green Mountain are 

light-gray hornblende-augite latite porphyry that contains phenocrysts of andesine, augite, and hornblende, 

and pale olive-gray hornblende trachyte porphyry containing sanidine, andesine, and hornblende phenocrysts. Includes a vent agglomerate that crops out along Blue River west of Green Mountain. Agglomerate 

is a mixture of brecciated black shale and latite porphyry together with well-rounded river gravel composed 

of Proterozoic rocks in a matrix of tuffaceous sand and silt (Taggert, 1962). 40Ar/39Ar age on sanidine from 

trachyte sill near Green Mountain Reservoir dam (location C on map) is 31.51±0.08 Ma (early Oligocene) 

(Naeser and others, 2002), in agreement with earlier zircon fission-track age of 29±2.8 Ma (Naeser and others, 

1973). Prominent vertical dike at Haystack Mountain (location D; fig. 7), about 4 km northeast of Green 

Mountain, is considerably younger than and chemically distinct from intrusive rocks on Green Mountain 

(E.H. DeWitt and T.L. Klein, U.S. Geological Survey, written commun., 2008), although is included in this 

unit. Rock at Haystack Mountain is composed of medium-gray hornblende latite porphyry with phenocrysts 

of hornblende, andesine, and sanidine. Recent 40Ar/39Ar ages on sanidine from two samples are 26.l±0.01 and 

25.2±0.1 Ma (late Oligocene; T.L. Klein and E.H. DeWitt, U.S. Geological Survey, written commun., 2008); 

the reason for the slightly disparate ages is unknown 

Figure 7. Dike of 25–26 Ma (late Oligocene) hornblende latite porphyry at Haystack Butte 

(location D). Dike is about 8 m thick. View to the southeast. Photograph by K.S. Kellogg, 2005.
12 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

:ms Quartz monzogranite porphyry of Swan Mountain (Eocene)– Light-gray, conspicuously porphyritic, massive, 

biotite-hornblende quartz monzonite. Orthoclase phenocrysts are 2 mm to 4 cm long (Lovering, 1934, reports 

phenocrysts as long as 8 cm) and comprise as much as 20 percent of rock. Partially resorbed and embayed 

quartz phenocrysts are 2 mm to 1.5 cm long and comprise 2–10 percent of rock. Abundant white, chalky 

plagioclase phenocrysts are as long as 1 cm. Euhedral biotite comprises 1–3 percent of rock and is much 

more abundant than hornblende. Groundmass is fine grained (< 0.5 mm). Weathers to buff or light brown, 

with flaggy to blocky fracture. It was not practical to differentiate two different phases of quartz monzonite 

porphyry based on variations in quartz and orthoclase phenocryst abundances (Ransome, 1911). Quartz monzonite porphyry at north end of main intrusive body underlying Swan Mountain yielded a potassium-argon 

age on biotite of 44.1±1.6 Ma (Marvin and others, 1989) and a rubidium-strontium whole-rock age of about 

44 Ma (Simmons and Hedge, 1978)

Late Cretaceous Intrusive Rocks

Kpp Pando Porphyry (Late Cretaceous)—Light-gray to white quartz latite porphyry; weathers orange-brown. Visible 

crystals are quartz, plagioclase, potassium feldspar, biotite (typically altered to chlorite), and muscovite 

(R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 2009). Deuteric alteration is 

ubiquitous (Tweto and Lovering, 1977). Porphyry forms two sills as thick as 30 m near base or middle of 

Belden Formation on east side of Eagle River canyon (K.M. Kirkham and K.J. Houck, Colo. Geological 

Survey, written commun., 2009), although only one sill as thick as 30 m at the base of the Belden is mapped 

in the quadrangle by Lovering and Tweto (1977) and Tweto and Lovering (1977). Upper sill extends only a 

short distance west of Redcliff; lower sill thins to north and pinches out or is concealed beneath Quaternary 

deposits about 3 km southeast of Minturn. The sills commonly have columnar joints. Contacts with Belden 

are sharp and contact-metamorphic zones in Belden are less than one meter thick. K-Ar date on Pando 

Porphyry is approximately 70 Ma (Pearson and others, 1962), although recent SHRIMP (Sensitive HighResolution Ion Microprobe) U-Pb zircon age is 64±1 Ma (W.R. Premo, U.S. Geological Survey, unpub. data, 

2009)

Laramide Sedimentary Rocks

:Kmu Middle Park Formation, upper member (Paleocene and Upper Cretaceous?)—Variegated gray, brown, 

maroon, and rusty orange siltstone, mudstone, sandstone, and conglomerate; dominantly micaceous and 

arkosic (Taylor, 1975). Largely coarse basin-fill deposits derived from various Proterozoic intrusive rocks 

and feldspathic gneiss shed during Laramide thrusting and uplift of Front Range. Lower 150 m contains 

as much as 50 percent clasts of porphyritic andesite, trachyandesite, and other porphyries. Well-rounded 

clasts of pink granite reported as large as 2 m in diameter (Izett, 1968). Siltstone and mudstone are potentially unstable, even where they form gentle slopes. Poorly exposed over large area in northeastern corner 

of quadrangle. Late Cretaceous? age is based on correlation with Upper Cretaceous and Paleocene Denver 

Formation on east side of Front Range, although fossil leaves and pollen collected from formation just north 

of map area indicate a Paleocene age (Izett, 1968). Underlying Windy Gap Volcanic Member of Middle Park 

Formation, of probable Late Cretaceous and (or) early Paleocene age (Izett, 1968), is not exposed in quadrangle but probably exists at depth. Maximum thickness of upper member in map area greater than 600 m, 

although thickness in much of the map area is probably considerably thinner

Pre-Laramide Sedimentary Rocks

Kp Pierre Shale, undivided (Upper Cretaceous)—Mostly dark-gray to black fissile shale and mudstone with thin 

interbeds of fine-grained sandstone. May have been as thick as about 2,600 m in region, but approximately 

1,000 m removed in upper part by erosion (Izett and others, 1971). Formal members below described by Izett 

and others (1971) 

Kpsh Shale member—Predominantly shale and minor sandstone mapped in stratigraphic intervals above Kremmling 

Sandstone Member (Kpk) or, south of Haystack Mountain, intervals above lower sandstone member (Kpl) 

that includes Kremmling Sandstone Member. Consists mostly of dark-gray and black silty shale and subordinate thin, brown, clayey, commonly ripple-laminated, fine-grained to very fine grained sandstone. Weathers 

to distinctive grayish-brown soil
Description of Map Units 13

Kpg Gunsight Pass Member—Very fine grained to fine-grained, gray, silty sandstone and silty shale; weathers 

brown. Lower contact with underlying shale gradational over about a meter; upper 15 m is locally cliff-forming sandstone; upper contact sharp with overlying dark-gray shale. Prominently exposed in tight recumbent 

syncline near summit of Ute Pass. Base of member about 1,415 m above base of Pierre Shale. About 

50 m thick

Kps Unnamed sandstone—Generally grayish-tan, fine-grained, thinly-layered, silty sandstone and minor siltstone. 

Mapped at several localities in the Dillon 71⁄2-minute quadrangle, north of the town of Dillon

Kph Hygiene Sandstone Member—Prominent cliff-forming yellowish-gray to olive-brown, fine-grained to very fine 

grained massive to thick-bedded sandstone in lower part; soft, thin-bedded sandstone in upper part. Weathers 

yellowish tan. Base of member about 940 m above base of Pierre Shale. Cliff of Hygiene Sandstone Member 

shown in figure 8. Thickness variable; as much as 40 m thick

Kpl Lower sandstone member, including Kremmling Sandstone Member—South of Haystack Mountain (location D on map), Kremmling Sandstone Member is less conspicuous and is included with a section of thinbedded, fine-grained gray sandstone and interbedded silty shale. Locally contains limestone concretions. 

Member as thick as 125 m

Kpk Kremmling Sandstone Member—Thin-bedded, ledgy to cliff-forming, gray-brown, shaly siltstone and finegrained sandstone (fig. 9). Upper part generally very fine grained flaggy sandstone. Weathers brown. A 

20-m-thick sandstone in northeast corner of Frisco 71⁄2-minute quadrangle is tentatively correlated with 

Kremmling Sandstone Member (Kellogg and others, 2002), although is correlated with Muddy Buttes 

Sandstone Member of Pierre of Izett and others (1971), which is about 30 m above Kremmling, in adjacent 

Keystone quadrangle to east (Widmann and others, 2003). About 20–22 m thick in northern part of map area

Kpls Lower shale member—Mostly black and dark-gray fissile, mostly non-sandy marine shale and mudstone with 

conchoidal fracture. Indistinct bedding in fresh outcrops; weathering induces a bedding-parallel fissility. 

Locally contains thin bentonite beds and, in lower 400 m, ironstone (orange sideritic concretions) and calcareous concretions. Upper part contains sparse, thin (less than 1 m), silty or very fine grained, dark-gray sandy 

beds. Calcareous shale in lowest 10–20 m; basal 3 m is transitional with underlying Niobrara Formation and 

consists of dark-gray calcareous shale. Large section of lower shale member shown in figure 8. Member 

approximately 450 m thick

Williams 

thrust (covered)

Range

Kpk

Figure 8. View to east across Green Mountain Reservoir of black shales of the lower Pierre Shale 

(Kpls). Prominent cliff part way up William Fork Mountains is Hygiene Sandstone Member (Kpk). 

Covered trace of Williams Range thrust shown by dashed line. Photograph by K.S. Kellogg, 2005.
14 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Kn Niobrara Formation (Upper Cretaceous)—Consists of two members that were not mapped separately. Uppermost is Smoky Hill Shale Member (fig. 10), which consists of gray, light-gray and platy-weathering, calcareous shale and shaly limestone that generally becomes more shaly upward; thickness in Dillon and Frisco 

area about 138 m (Robinson and others, 1974); in northern Blue River Valley thickness 145–175 m (Taggert, 

1962). Lower member, the Fort Hays Limestone Member, is blocky, gray, light-gray weathering, relatively 

resistant micritic limestone in beds 5–15 cm thick that commonly contain inoceramid bivalves (large oysters); 

member about 6–8 m thick; relatively resistant. Formation is conformable above Benton Group. Total thickness of Niobrara Formation near Hot Sulphur Springs, about 8 km north of quadrangle, is 165 m (Izett, 1968) 

Figure 9. View to north, from along 

State Highway 9, about 3.5 km south of 

northern map border, of lower Pierre 

Formation. Prominent upper sandstone 

cliff is Kremmling Sandstone Member; lower, smaller sandstone ledge is 

unnamed sandstone of lower Pierre 

Shale. Trace of Williams Range thrust 

shown by dashed line; hanging wall is 

highly fractured and oxidized monzogranite or granodiorite (Xgr). Photograph by K.S. Kellogg, 2005.

Figure 10. View to northeast across 

Blue River, just north of Spruce Creek, of 

upper Niobrara Formation (Kn), showing 

characteristic thin-bedded shaly limestone 

exposed in approximate 12-m-high bluff. 

Photograph by K.S. Kellogg, 2005.
Description of Map Units 15

Kbd Benton Group (Upper and Lower Cretaceous) and Dakota Formation (Lower Cretaceous), undivided—

Individual units described below. Mapped only adjacent to Dillon Reservoir where complex faulting makes 

map pattern complicated. Individual units shown on Kellogg and others (2002)

Kb Benton Group (Upper and Lower Cretaceous)—Uppermost 1.5 m is a thin-bedded, black to dark-gray, fetid, 

resistant, crystalline limestone that shows pinch-and-swell structures and contains thin, dark-gray, siliceous 

siltstone interbeds. It is correlative with Juana Lopez Member of Carlile Shale of Berman and others (1980). 

This uppermost limestone overlies about 5 m of dark-brown to gray calcareous, rusty siltstone and shale 

that, in turn, overlies about 3 m of resistant brownish-gray, fine-grained, rusty, arkosic sandstone that is 

bioturbated at base and locally contains chert pebbles as long as 2 cm. This unit is correlated with Codell 

Sandstone Member of Carlile Shale of Berman and others (1980). The Codell unconformably overlies 

mostly dark-brown to black, fissile, rusty shale that is more characteristic of lower part of Upper Cretaceous 

Mancos Shale as described west of area (for example, Merewether and Cobban, 1986). Approximately lower 

25 m, which is equivalent to Upper and Lower Cretaceous Mowry Shale of Wyoming (D.H. Dunkle, as 

cited in Izett, 1968; W.A. Cobban, U.S. Geological Survey, oral commun., 1998) consists of wavy-bedded 

black shale containing fish scales and, in lowest 3 m, thin (less than 5 cm) fine-grained, gray quartzite beds; 

sequence is conformable above Dakota (fig. 11). Term “Benton Shale” is well established to designate blackshale-dominated sequence between Dakota Formation and Niobrara Formation (for example, Tweto and 

others, 1978; Van Horn, 1976), but term Benton Group better encompasses correlation with disparate formations mapped regionally (Kellogg and others, 2008). Total thickness of unit estimated about 95–110 m, 

although thickness in Hot Sulphur Springs quadrangle, just north of map area, is about 135 m (Izett, 1968)

Kd Dakota Formation (Lower Cretaceous)—Generally consists of three informal members: an upper, hard, quartz 

sandstone member, a middle shaly member, and a lower, hard, quartz sandstone member. Upper quartz 

sandstone member is 6–20 m thick and contains light-gray, commonly cross-bedded sandstone in beds 10–30 

cm thick, with thin, black, carbonaceous shale interbeds (fig. 11). In many places, base of upper member is 

a massive, 2–10 meter thick, resistant sandstone bed. Joint surfaces contain red, orange, and yellow limonite 

encrustations. Middle shaly member consists of interbedded dark-gray to black, commonly carbonaceous 

shale and generally thin- to medium-bedded, medium-grained, equigranular, gray to light-gray quartz 

sandstone in beds as thick as about 2 m. Thickness of middle shaly member is 6–28 m . Lower sandstone 

member consists of thick (as much as 12 m), massive, medium-grained, grayish-white, equigranular quartz 

sandstone beds with thin, dark-gray shale interbeds; the sandstone is very rusty on joint surfaces. Chert-pebble 

beds occur locally in lower sandstone member, although they were not observed near Frisco and Dillon. 

Thickness of lower member is 20–26 m. Throughout Dakota Formation, sandstone beds are somewhat lenticular, undergoing lateral changes in thickness over short distances. Unit rests unconformably on Morrison 

Formation. Total thickness of Dakota Formation in Breckenridge area, just south of quadrangle, is 52–69 m 

(Lovering, 1934). Near Williams Fork Reservoir, thickness is about 70 m. Thickness of Dakota Formation 

in Harold D. Roberts Tunnel, which penetrates the unit east of Dillon Reservoir, is 66 m (Robinson and 

others, 1974)

Jm Morrison Formation (Upper Jurassic)—In Dillon and Frisco areas, Morrison is mostly light-gray and lightgreenish-gray, locally calcareous claystone; upper 4 m contains some maroon claystone. Lower half of 

formation contains several light-yellow to white medium-grained sandstone beds as thick as about 5 m 

that commonly contain Liesegang rings (rusty layers parallel to joint surfaces) and limonitic spots. One 

prominent 2- to 4-m-thick gray limestone bed occurs about 5–10 m from base of formation. On west side of 

Elliott Ridge in the Gore Range, southwest of Green Mountain Reservoir, a 7-m-thick massive, pale-orange, 

fine-grained sandstone occurs at top of Morrison and overlies greenish-gray and reddish-brown siltstone, 

gray limestone, and pale yellowish-brown and pale greenish-gray, locally conglomeratic sandstone (Taggert, 

1962). Thickness in Blue River Valley about 55–79 m (Holt, 1961); in spillway shaft of Dillon Dam, formation is about 70 m thick (Wahlstrom and Hornback, 1963). Morrison thickens to the northwest; a measured 

section on west side of Elliott Ridge is 111 m thick (Taggert, 1962). Just north of map area, near Williams 

Fork Reservoir dam, Morrison is 88 m thick (Izett, 1968)
16 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Jme Morrison Formation (Upper Jurassic) and Entrada Sandstone (Middle Jurassic), undivided—Morrison 

Formation described above comformably overlies Entrada Sandstone. Entrada Sandstone consists of lightgray to pale-yellowish-gray, well-sorted, fine- to medium-grained, commonly cross-bedded, carbonatecemented, quartzose sandstone. Quartz grains are frosted, reflecting eolian deposition. Small, brown limonitic spots are common in hand specimen. Locally ledge forming. Unit unconformably overlies Triassic red 

beds of Chinle Formation or, locally on Elliott Ridge, Upper Triassic Gartra Sandstone Member of Chinle 

Formation. Entrada Sandstone is recognized in Dillon and Frisco areas (Kellogg, 2002; Kellogg and others, 

2002), but was not recognized in northern part of map area by Taggert (1962). However, a 5-m-thick crossbedded, light-tan medium- to coarse-grained quartz sandstone, locally containing quartz pebbles, at base of 

Morrison Formation on Elliott Ridge, about 2 km southwest of Mahan Lake, is tentatively correlated with 

Entrada. A similar 12-m-thick sandstone exists at base of Morrison north of map area in Kremmling and Hot 

Sulphur Springs area, but is regarded as part of Morrison Formation (Izett, 1968, Izett and Barclay, 1973). 

Thickness of Entrada is highly variable; at Dillon Dam, it is about 50 m thick (Wahlstrom and Hornback, 

1962), although only 3 km to east, at a locality now under Dillon Reservoir, it is only about 11 m thick 

(Holt, 1961) 

^c Chinle Formation (Upper Triassic)—Thin-bedded, dark-red, pale-pink, and greenish-gray, locally mottled 

silty shale, siltstone, and fine-grained silty sandstone, with platy fracture; fluvial. Partial sections of Chinle 

exposed just west of Summit County High School in center of sec. 1, T. 6 S., R. 78 W. and along Snake River 

Arm of Dillon Reservoir. Beneath Dillon Dam, where Chinle is now entirely covered, unit is about 61 m 

thick and was called Lykins Formation (Wahlstrom and Hornback, 1963), a name more appropriate for a 

Triassic and Permian red-bed sequence on east flank of Front Range (for example, Broin, 1956). Just 

southwest of Boreas Pass, about 10 km southeast of map area, a 46-m-thick red shale and sandstone sequence 

underlying Entrada Sandstone is correlated with the Chinle Formation, a unit identified throughout western 

Colorado and eastern Utah (Poole and Stewart, 1964). Boreas Pass section includes basal Gartra Sandstone 

Member of Chinle Formation. Exposures of Gartra Sandstone Member near west side of Vail West 71⁄2-minute 

quadrangle (Scott and others, 2002) are included in unit Tac

Figure 11. Conformable contact between 

quartz sandstone of Lower Cretaceous Dakota 

Formation (Kd) and overlying black shale of 

Benton Group (Kb). Contact placed at top of 

approximately 0.5-m-thick yellowish sandstone 

bed. Lower Benton in upper part of image is 

equivalent to the Upper and Lower Cretaceous 

Mowry Shale in Wyoming. Outcrop briefly 

exposed in cut, now covered, immediately 

behind shopping center in Silverthorne. Note 

30-cm-long hammer for scale in lower part of 

image. Photograph by K.S. Kellogg, 2004.
Description of Map Units 17

^cg Gartra Sandstone Member—Forms lenses of white, pale orange or gray, fine- to coarse-grained, well-cemented, 

cross-bedded, quartz sandstone and quartz-pebble conglomerate (fig. 12). Contains moderately rounded to 

well-rounded quartz pebbles as long as 3 cm. Fragments of petrified wood are locally common. Called “Shinarump Conglomerate” by Taggert (1962), but later assigned to Gartra Sandstone Member, the basal member 

of Chinle Formation (Poole and Stewart, 1964). Well exposed over a small area on Elliott Ridge, about 2 km 

southwest of Mahan Lake, where the entire Chinle section above Gartra is missing; maximum thickness on 

Elliott Ridge about 6 m (Taggert, 1962), although most exposures thinner than 2 m. Also exposed in southwestern part of quadrangle in Vail West 71⁄2-minute quadrangle (included in unit ^c), where sequence of 

grayish-purple, dark-reddish-brown, orange-pink, and white sandstone and pebble conglomerate is as thick 

as 15 m (Scott and others, 2002). Gartra has not been identified in Frisco and Dillon area. Gartra is unconformable above State Bridge Formation

^*cm Chinle Formation (Upper Triassic) and Maroon Formation (Lower Permian to Middle Pennsylvanian), 

undivided—Chinle Formation described above. Combined unit mapped only in southern part of Frisco 

71⁄2-minute quadrangle where State Bridge Formation is absent. Chinle unconformably overlies a sequence, 

correlated with Maroon Formation, of red to orange-red, medium- to coarse-grained, micaceous, poorly 

sorted, locally conglomeratic, arkosic sandstone that locally contains interbeds of red sandy siltstone. Sand 

grains are angular to sub-angular. Maroon rests disconformably on Proterozoic basement in southeast corner 

of map area; a basal section is exposed along State Highway 9 about 7 km southeast of Frisco. Maroon is 

estimated to be as much as 180 m thick in Frisco quadrangle (Kellogg and others, 2002) but thickens considerably to south; near Boreas Pass, about 10 km southeast of map area, combined Chinle and Maroon Formations is more than 1,200 m thick (Taranik, 1974)

Figure 12. Geologist points to outcrop of Gartra Sandstone Member of Chinle Formation 

(^g) on Elliott Ridge. Gartra section here is about 6 m thick and includes sandstone on skyline. Gartra Sandstone is unconformably overlain by Morrison Formation (Jm) at 

this location. Note cross bedding and white calcareous efflorescence. Photograph by 

K.S. Kellogg, 2007.
18 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

^Pgs Gartra Sandstone Member of Chinle Formation (Upper Triassic) and State Bridge Formation (Lower 

Triassic and Permian), undivided—Gartra Sandstone Member described above and State Bridge Formation 

described below. Combined unit mapped in Piney Peak 71⁄2-minute quadrangle along south side of Piney 

Ridge and at two small outcrops along Elliott Ridge

^Ps State Bridge Formation (Lower Triassic and Permian)—Thin-bedded, micaceous, brick-red, shale, siltstone, 

and very fine grained to fine-grained calcareous sandstone; grains well rounded. One intermittent bed of 

gray, arkosic conglomerate as thick as 5 m occurs in middle of formation. Well exposed on west side of 

Elliott Ridge. Similar to Chinle Formation, as mapped near Frisco, but apparent presence of overlying Gartra 

Sandstone Member of the Chinle identifies red beds on Elliott Ridge as State Bridge. Lower part rests on 

Proterozoic basement or is unconformable above Maroon Formation (Freeman, 1971). Ranges from about 

100 m thick at its northern exposures on Elliott Ridge to a vanishing point on east side of Gore Range about 

2 km southwest of Hoagland Reservoir. At type section at McCoy, about 12 km west of map area, formation 

is 160 m thick (Donner, 1949) 

P*m Maroon Formation (Lower Permian to Middle Pennsylvanian)—Grayish-red, pale-reddish-brown, and 

orange-red, coarse-grained to medium-grained, micaceous, calcareous sandstone, muddy sandstone, pebbly 

to cobbly sandstone, and conglomerate. Contains minor lenses of red shale and shaly limestone. Some finergrained beds are gray-green. Pebbles and cobbles are commonly quartz and granite; less commonly pegmatite, and rarely biotite schist and gneiss. Cobbles are subrounded and as much as 10 cm in diameter. In one 

outcrop capping unnamed peak near southern border of Vail East quadrangle, basal 10 m is mostly red, micaceous, locally cross-bedded sandy shale in beds 1–10 cm thick. About 40 m above base at this locality, unit 

contains an approximately 10-m-thick, cross-bedded, ledge-forming pebbly arkosic sandstone bed (fig. 13); 

pebbles include limestone and chert. Sequences of massive, siliceous, generally reddish, siltstone-dominated 

rock interpreted as loessite (Johnson, 1989; Tramp and others, 2004). Maroon overlies Jacque Mountain 

Limestone Member of Minturn Formation (included in units *m or *mu); contact locally an unconformity. 

Regionally, formation coarsens and thins significantly eastward and is absent east and north of Gore fault. 

Deposited by streams along western flanks of ancestral Front Range uplift. About 750 m of section exposed 

along west border of map area. Maximum thickness in southwestern part of map area estimated as 1,280 m 

(Tweto and Lovering, 1977); maximum thickness in Vail West 71⁄2-minute quadrangle 1,070 m (Scott and 

others, 2002)

Figure 13. Outcrop of cross-bedded 

conglomeratic sandstone beds interlayered with thin-bedded, fine-graind 

sandstone of Maroon Formation (P*m), 

about 50 m above Jacques Mountain 

Limestone Member of Minturn Formation, 

on unnamed peak 2.5 km south of confluence of Booth and Gore Creeks. Note 

hammer for scale. Photograph by K.S. 

Kellogg, 2003.
Description of Map Units 19

*ev Eagle Valley Formation (Middle Pennsylvanian)—Pale-reddish-brown, pale-red, light-gray, greenish-gray, and 

light-brown shale, siltstone, fine-grained to very fine grained sandstone, and dark- to light-gray micritic to 

finely crystalline limestone. Locally contains gypsum beds, particularly in lower part of unit. Lower gypsum 

beds transitional into and mostly overlying the Eagle Valley Evaporite, a separately mapped evaporate facies 

that includes gypsum, anhydrite, and halite as thick as thousands of meters, and is exposed extensively 

just west of map area in a more axial, deeper part of Central Colorado trough, which formed in a shallow, 

saline sea in Middle Pennsylvanian time. A small outcrop of Eagle Valley Evaporite is mapped in Vail West 

71⁄2-minute quadrangle (Scott and others, 2002), but is included here with the Eagle Valley Formation. Interbedded pinkish-gray to light-greenish-gray, very coarse grained to pebbly and conglomeratic sandstone may 

represent tongues of Minturn Formation (*m). Upper part intertongues with lower part of Maroon Formation. Unit first mapped as fine-grained clastic rocks transitional between coarse-grained clastic rocks of 

Minturn and Maroon Formations and evaporitic rocks of Eagle Valley Evaporite (Tweto and others, 1978)

*m Minturn Formation, undivided (Middle Pennsylvanian)—Arkosic sandstone, conglomerate, siltstone, shale, 

and volumetrically small but stratigraphically important beds of limestone and dolomite. Conglomerate 

is common and contains subrounded to subangular clasts of mostly Proterozoic rocks as long as 30 cm. 

Limestone beds locally contain abundant fossils, including foraminifera, brachiopods, and mollusks (Tweto 

and Lovering, 1977). Distinctive pale-gray, locally oolitic Jacque Mountain Limestone Member (not mapped 

separately) marks top of formation. Deposited by streams near sea level and multiple marine incursions. 

Informal members mostly follow those recognized by Tweto and Lovering (1977), Kellogg and others 

(2003), and R.M. Kirkham and K.J. Houck (Colo. Geological Survey, written commun., 2009). Undivided 

Minturn Formation mapped where uncertainty exists in placement into specific members. Type section in 

northeastern part of Minturn 71⁄2-minute quadrangle, near Minturn, is 1,923 m thick (Tweto and Lovering, 1977)

*mu Upper sandstone and conglomerate member and Jacque Mountain Limestone Member, undivided—Mostly 

orange-red to maroon, arkosic conglomerate, sandstone, siltstone, shale, and sparse gray limestone. Color 

locally greenish-gray, especially in finer-grained horizons. Beds typically 1–10 m thick and lenticular, so that 

lateral extent is limited. Cross bedding and channel fill common. Coarser grained, thicker beds commonly 

form ledgy outcrops on hillsides. Clasts are moderately rounded to well-rounded, as long as about 30 cm, and 

consist of quartz, chert, Proterozoic felsic plutonic rocks (including pegmatite), and rare mafic plutonic rock. 

Silty and shaly beds are micaceous. Equivalent to clastic unit H and overlying Jacque Mountain Limestone 

Member of Tweto and Lovering (1977). Jacque Mountain Limestone consists of pale to medium-gray, fossiliferous, locally oolitic, stylolitic limestone, sandy limestone, and calcareous sandstone; 6–9 m thick and in 

places forms prominent ledges. Jacque Mountain locally removed by erosion before deposition of overlying 

Maroon Formation, but where it exists, it forms the marker bed that defines the top of the Minturn Formation. Thickness of map unit on south side of Gore Creek about 235 m (Kellogg and others, 2003); about 

256 m thick at type section (Tweto and Lovering, 1977).

*mw White Quail Limestone and Elk Ridge Limestone Members and sandstone and shale member, undivided—

Mostly pinkish-gray, grayish-brown, gray-green, and mottled maroon and gray-green conglomerate, sandstone, siltstone, shale, and minor limestone. Clast size smaller than in unit *mu, generally less than 

5 cm. Top of unit marked by White Quail Limestone Member of Tweto and Lovering (1977) and Kellogg and 

others (2003), which is mostly gray, fine-grained limestone as much as 10 m thick in beds 1 cm to 5 m thick; 

massive within each bed. White Quail weathers pale yellowish brown to medium gray and is a prominent 

ledge-forming unit. White Quail not identified west of Middle Creek or in northwest corner of Vail West 

71⁄2-minute quadrangle (Scott and others, 2002). About 60 m below top of unit, a ledgy, dark-gray, locally 

sandy limestone bed as thick as 10 m correlated with Elk Ridge Limestone Member of Tweto and Lovering 

(1977). Lower part of sandstone and shale member contains flaggy, brown, medium-grained, arkosic, micaceous sandstone containing about 20 percent interbedded dark-gray micaceous fissile shale. Total thickness 

south of Gore Creek about 230–300 m; about 60 m thick north of Gore Creek and inferred to pinch out 

against Gore fault

*mr Robinson Limestone Member— Consists of at least four sequences, each as thick as 20 m, of gray to yellowish-gray, fine-grained to medium-grained, locally fossiliferous, medium-bedded to thick-bedded marine 

limestone and dolomitic limestone. Each carbonate sequence is interbedded with pinkish-tan and light-tan, 

cross-bedded, arkosic, micaceous pebbly sandstone and light-grayish-pink sandy siltstone and shale. Ratio 

of carbonate to clastic rocks is about 1:2.5. Most limestone and dolomitic limestone beds are 0.5 cm to 10 m 

thick; many beds extend for several kilometers. A lower, 18-m-thick, gray-weathering limestone contains 

distinctive laminated, knobby beds and probably represents a reef facies. A few other thick carbonate beds 

(as much as 10 m thick) may also represent reefs and are massive, vuggy, and grayish-orange weathering. 
20 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Dolomitic limestone generally weathers to pale yellowish brown; limestone weathers pale gray. Clastic beds 

typically weather in rounded forms, as compared to relatively angular weathered forms in carbonate beds. 

Thickness north of Gore Creek about 120 m; on south side of Gore Creek as thick as about 180 m (Kellogg 

and others, 2003). In northwest part of Vail West 71⁄2-minute quadrangle, where at least four prominent limestone or dolomite beds are mapped individually as comprising Robinson Limestone Member (Scott and others, 

2002). Combined thickness of sequence that includes carbonate beds and interbedded clastic rocks is about 

270 m. As correlation of carbonate beds with unit *mr is uncertain, this sequence is included in Minturn 

Formation, undivided ( *m). Thickness of Robinson Limestone Member 270 m at type section (Tweto and 

Lovering, 1977)

*mm Middle member—Pinkish-gray, grayish-brown, gray-green, and mottled maroon and gray-green arkosic 

conglomerate, sandstone, siltstone, and shale; more reddish hues nearer top. Similar to unit *mw. About 

400 m of section exposed on west side of Pitkin Creek beneath Robinson Limestone Member. Mapped as 

“lower member of Minturn Formation” in Vail East 71⁄2-minute quadrangle (Kellogg and others, 2003), 

different from lower member of this report (*ml) which is missing north of Gore Creek due either to erosion 

or nondeposition during Middle Pennsylvanian time. Middle member forms channels in Sawatch Quartzite 

about 0.6 km southwest of Bald Mountain, just north of Gore fault, where basal bed is a granite-clast, matrixsupported, massive conglomerate bed as thick as 5 m 

hml Lower member—Interbedded conglomerate, sandstone, siltstone, shale and minor dolomite and dolomitic 

limestone. Uppermost 150 m consists mostly of gray and tan sandstone and conglomerate (largest pebbles 

about 5 cm long) and dolomite mounds (reef facies) as thick as 15 m. Top of upper sequence marked by reef 

dolomite of Lionshead of Tweto and Lovering, (1977), a lenticular reef facies as thick as 10 m. A prominent 

outcrop above the town of Minturn, called “The Lionshead,” is the namesake for this informal unit. Middle 

180 m is reddish, maroon, gray, tan, and greenish quartz-rich sandstone and conglomerate in beds about 

2.5–4 m thick (“red zone” of R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 

2009). Lower 120 m consists mostly of dark-gray shale and subordinate gray and tan quartz sandstone. 

Contains rare carbonate beds as thick as 10 cm and sparse conglomerate containing pebbles as long a 1 cm. 

Weathers to relatively gentle slopes. Lower member mapped on slopes immediately east of Eagle River 

(R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 2009); eroded or was never 

deposited north of Gore Creek. Coarse quartz sandstone of lower member rests conformably above black 

shale and tan dolomite of Belden Formation along U.S. Highway 24. Lower member 454 m thick at type 

section of Minturn Formation near Minturn (Tweto and Lovering, 1977)

*b Belden Formation (Middle Pennsylvanian) and Molas Formation (Lower Pennsylvanian), undivided—

Belden Formation is interbedded dark-gray to black, relatively fossiliferous marine shale, limestone, dolomite 

and sandstone. Most beds are dark-gray to black, thin-bedded shale with interbedded, thin-bedded black 

limestone and thin beds of dark-gray, fine-grained sandstone and sandy mudstone. In most places, upper 

contact with gray and green, thick-bedded, channeled sandstone of Minturn Formation is gradational. Sharp, 

unconformable lower contact with either Leadville Limestone (or Dolomite) or regolith of Molas Formation. 

Thickness as much as 61 m. Well exposed along U.S. Highway 24 north of Gilman. Molas Formation is thin, 

gray, yellow, brown silt and clay containing abundant chert fragments; represents a regolith (ancient soil) as 

thick as 3 m, principally infilling sink holes (paleokarst), caves, and channelways in Leadville Limestone (or 

Dolomite). The paleokarst served as a dust trap for oldest known loessite, consisting of coarse-grained quartz 

siltstone, in North America (Evans and Reed, 2007). Neither Belden nor Molas observed north of Gore Creek 

due to erosion of thick Pennsylvanian and Mississippian strata along flanks of ancestral Front Range during 

initial uplift of range during Middle Pennsylvanian time (Tweto and Lovering, 1977; DeVoto, 1980)

Ml Leadville Limestone (Dolomite) (Lower Mississippian)—Dark-gray, fine- to medium-grained, locally cherty, 

dense, tan-weathering dolomite and limestone; chert mostly black and concentrated in lower half of formation. 

Thick-bedded and massive in upper part; medium-bedded in lower part. Locally has distinctive “zebra 

banding,” alternating bands as thick as 5 cm of black to dark-gray fine-grained dolomite and white coarsegrained vuggy dolomite. Upper contact with Molas Formation is an unconformity with irregular karst 

topography. Boundary between limestone and dolomite is approximately at Cross Creek (Tweto and Lovering, 

1977). Southeast of boundary Leadville is dolomitic; to the northwest it is limonitic. Dolomitization has been 

interpreted as result of hydrothermal alteration (Tweto and Lovering, 1977), but origin of the dolomite is 

debated (see Radabaugh and others, 1968). Leadville not present north of Gore Creek, where it apparently 

was eroded during uplift of ancestral Front Range during late Paleozoic. Contact with underlying rocks of the 

Chaffee Group is an unconformity. The Leadville Limestone and underlying Paleozoic units near Gilman are 

shown in figure 14. Thickness of Leadville is as much as 43 m
Description of Map Units 21

M_g Chaffee Group (Lower Mississippian? to Upper Devonian), Harding Sandstone (Middle Ordovician), 

Taylor Pass Member of Manitou Formation (Upper Cambrian), and Dotsero Formation (Upper 

Cambrian), undivided—Combined unit mapped near Gilman and along U.S. Highway 24 on east side 

of Eagle River canyon where individual units too narrow in map view to show. Chaffee Group consists, in 

descending order, of Lower Mississippian? and Upper Devonian Gilman Sandstone, Upper Devonian Dyer 

Dolomite, and Upper Devonian Parting Formation. Following description of Chaffee Group follows Tweto and 

Lovering (1977) and R.M. Kirkham and K.J. Houck (Colo. Geological Survey, written commun., 2009). Gilman Sandstone is interbedded yellow-gray sandstone, sandy and cherty dolomite, and breccia; thickness as 

much as 15 m but pinches out to west. Not present north of Gore Creek. Dyer Dolomite is thin-bedded gray 

to black marine dolomite and subordinate reddish to orange chert breccia; as thick as 24 m. Not present north 

of Gore Creek. Parting Formation is white to pale green, coarse-grained, inequigranular, well-indurated, 

locally cross-bedded quartzite and pebbly quartzite and interbedded green shale; pebbles entirely white 

quartz and quartzite; thickness as much as 20 m. Quartz sandstone of Parting Formation can be distinguished 

from that in Sawatch Quartzite or Harding Sandstone by inequigranular grain size and clear quartz grains (as 

compared to white quartz grains in Sawatch and Harding). Thinly bedded sequence of dolomitic sandstone, 

sandy dolomite, and shale is tentatively assigned to Middle Ordovician Harding Sandstone and underlying 

Upper Cambrian Taylor Pass member of the Manitou Formation of Myrow and others (2003) (R.M. Kirkham 

and K.J. Houck, Colo. Geological Survey, written commun., 2009), although the Manitou Formation is not 

recognized in quadrangle by Tweto and Lovering (1977). The Manitou was formerly considered entirely 

Lower Ordovician in age (for example, Tweto and Lovering, 1977), but the Taylor Pass Member has been 

reassigned to the Upper Cambrian by Myrow and others (2003). The upper, Ordovician part of the Manitou 

apparently eroded from the area (Myrow and others, 2003) prior to deposition of the Harding Sandstone. 

Harding Sandstone of Tweto and Lovering (1977) is white, gray, and green, well-indurated sandstone and 

Figure 14. View to northwest of Gilman, now largely abandoned, and section of Paleozoic rocks, 

taken from state highway 24 (1.2 km south of Eagle Mine, point CC on map). p_ = Proterozoic basement rocks; _s = Sawatch Quartzite; _d = Dotsero Formation; O_hm = Harding Sandstone? and 

Taylor Pass Member of Manitou Formation; Dp = Parting Formation; MDgd = Gilman Sandstone and 

Dyer Dolomite of Chaffee Group; Ml = Leadville Dolomite; *b = Belden Formation; *m = Minturn 

Formation. Photograph by K.S. Kellogg, 2010.
22 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

quartz sandstone, and interbedded green shale; thickness reported as much as 24 m, but may include part 

of section assigned to Upper Cambrian Taylor Pass Member of Manitou Formation by R.M. Kirkham and 

K.J. Houck (Colo. Geological Survey, written commun., 2009), which is thinly bedded tan, pale green, 

and reddish-purple dolomitic sandstone, sandy dolomite, and shale that lies unconformably above Dotsero 

Formation. Thickness of Harding Sandstone and Manitou Formation changes abruptly where combined unit 

crosses some faults (Allen, 2004; R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 

2009), and neither formation is recognized north of Gore fault. Dotsero Formation is brick-red and subordinately brown, green, and tan, dolomitic, bioturbated, micaceous mudstone, poorly sorted, fine-grained, 

locally glauconitic sandstone, and flat-pebble conglomerate; thickness as much as about 30 m but changes 

abruptly across small faults in the Eagle River Canyon. Contains the Clinetop Bed at top, a distinctive, 

approximately one-meter-thick stromatolite bed (Myrow and others, 2003). The Dotsero was previously mapped 

as Peerless Formation (Tweto and Lovering, 1977), but more recently correlated with Dotsero Formation in 

the Glenwood Canyon area (Myrow and others, 2003). Thickness about 6 m at one locality north of Gore 

Creek (Kellogg and others, 2003) and about 30 m in Eagle River canyon (R.M. Kirkham and K.J. Houck, 

Colo. Geological Survey, written commun., 2009). Conformably overlies Sawatch Quartzite

Dd Dyer Dolomite (Upper Devonian)—See description above. Mapped on dip slope of Sawatch Range in southwestern corner of quadrangle

Dp Parting Formation (Upper Devonian)—See lithologic description above. Unit Dp mapped on dip slope of 

Sawatch Range in southwestern corner of quadrangle and at two localities north of Gore Creek in Gore fault 

zone in Vail East 71⁄2-minute quadrangle: (1) just southwest of Bald Mountain, where a 20-m-thick section 

of Parting Formation overlies Proterozoic rocks and pinches out abruptly to north (Kellogg and others, 

2003); (2) just southeast of Bald Mountain, where rocks of Parting Formation unconformably overlie Upper 

Cambrian Dotsero Formation (mapped as combined unit D_p). Harding Sandstone and Manitou Formation 

eroded from Gore fault zone area prior to deposition of Parting Formation

D_p Parting Formation (Upper Devonian) and Dotsero Formation (Upper Cambrian), undivided—Unit D_p

mapped at one location southeast of Bald Mountain, north of Gore fault, where Parting Formation (described 

above) unconformably overlies a 6-m-thick section of red dolomitic mudstone and sandstone of Dotsero 

Formation, which conformably overlies Sawatch Quartzite

|cd Clastic dike (lower Paleozoic?)—Quartz-sandstone dikes intrude Proterozoic rocks at three localities in map 

area: (1) fine-grained, tan, equigranular, very well indurated 0.5–20-m-wide lens-shaped body of quartz 

sandstone that fills a west-striking fault for about 200 m just north of trace of Williams Range thrust in 

northwest King Creek 71⁄2-minute quadrangle; (2) fine-grained, maroon, equigranular, very well indurated 

0.5–1.0-m-wide tabular body of quartz sandstone that fills a north-northwest-striking fault just northwest 

of unnamed lake in upper Piney River drainage in northwest Vail East 71⁄2-minute quadrangle (Kellogg and 

others, 2002) (location HH; fig. 15); dike can be traced for about 100 m; and (3) black (tinged red), mediumgrained, equigranular, undulatory quartz grains in hematitic matrix over about 100-wide area on crest of 

Williams Fork Mountains, just east of head of Acorn Creek, in southern part of Ute Peak quadrangle; rock 

intrudes mix of granitic rock and migmatitic biotite gneiss in irregular veins; observed only on float boulders. 

In all three places, 97–99 percent of grains are quartz, 1 percent calcite, and in a few places 1–2 percent 

hematite. Grains well rounded, unbroken, although some pressure solution exists around grain boundaries. 

Dikes and (or) veins cut by numerous, thin (less than 1 mm), irregular microbreccia zones and margins of 

dikes are sheared, indicating recurrent faulting prior to and after dike emplacement. Speculatively, the dikes 

formed during the lower Paleozoic by sand moving down cracks from overlying Sawatch Quartzite , before 

diagenesis. The Sawatch was eroded from the area prior to late Paleozoic uplift of ancestral Front Range. 

Age of clastic dikes unknown, but lack of grain deformation suggests fluidized injection of sand occurred 

before complete diagenesis of parent unit, which may be Upper Cambrian Sawatch Quartzite. If so, clastic 

dikes also predate Late Paleozoic uplift of ancestral Front Range, when Sawatch Quartzite was eroded from 

area, and are probably Late Cambrian age 

O_r Harding Sandstone (Middle Ordovician), Taylor Pass Member of Manitou Formation (Upper Cambrian), 

and Dotsero Formation (Upper Cambrian), undivided—See descriptions above. Mapped on northeastern 

dip slope of Sawatch Range in southwestern corner of map area (R.M. Kirkham and K.J. Houck, Colo. 

Geological Survey, written commun., 2009)

_s Sawatch Quartzite (Upper Cambrian)—Vitreous, white to pale-pinkish-white, well-sorted, medium-grained, 

well-indurated quartz sandstone and pebbly quartz sandstone; color locally gray or greenish-white; quartz 

grains well rounded. Accessory detrital minerals sparse, but include muscovite, biotite, chert, potassium 

feldspar, sphene, green tourmaline (in all examined thin sections), and zircon (Tweto and Lovering, 1977). 
Description of Map Units 23

Pebbles in lower 5 m as long as 1 cm composed entirely of white quartz or quartzite. Grain size decreases 

upward. Sandstone in upper 15 m locally dolomite cemented. Medium to thick bedded; typically a prominent 

cliff former. About 67 m thick in southwestern part of map area near Eagle River. North of Gore Creek, well 

exposed at one locality north of Gore fault about 700 m southeast of Bald Mountain, where a greenish-white 

section less than 40 m thick, resembling Parting Formation but underlying Dotsero Formation, overlies 

Proterozoic rock. Elsewhere north of Gore Creek, sporadic exposures of Sawatch in fault-bounded blocks 

along Gore fault identified only in float 

Proterozoic Rocks

[Classification of plutonic rocks after Streckeisen, 1976]

YXu Proterozoic rocks, undivided (Meso- and Paleoproterozoic)—Shown only on cross section

Proterozoic, Plutonic and Metamorphic Rocks

YXp Pegmatite and aplite (Meso- and Paleoproterozoic)—Pegmatite is coarse-grained to very coarse grained, white 

to light-pink, inequigranular quartz-feldspar-mica rock that forms irregularly shaped, commonly zoned dikes 

and intrusive bodies cutting all Proterozoic rocks in map area. Microcline crystals may be longer than 0.5 m 

in some pegmatites; mica is mostly biotite, but locally includes or is entirely muscovite. Accessory minerals 

include tourmaline, garnet, and opaque minerals. Unit consists predominantly of pegmatite, which commonly 

grades into and is intimately mixed with aplite, which also forms separate dikes and bodies. Aplite is similar 

in composition to pegmatite but is a pinkish-tan, fine- to medium-grained, leucocratic, equigranular rock. 

Pegmatite and aplite may be late-stage intrusions associated with rocks of either Routt Plutonic Suite (about 

1,700 Ma) or Berthoud Plutonic Suite (about 1,400 Ma) of Tweto (1987), although most pegmatites in map 

area are probably related to late stages of Routt Plutonic Suite. Two uranium-lead (U-Pb) SHRIMP ages on 

zircons from pegmatite intruding migmatitic biotite gneiss are 1,697±Ma (location E on map) and 1,601±50 

(location F on map) (W.R. Premo, U.S. Geological Survey, unpub. data, 2007). Muscovite-±tourmalinebearing varieties probably related to intrusions of Berthoud Plutonic Suite 

Xap Aplite (Paleoproterozoic)—Gray to pinkish-gray, hypidiomorphic, fine-grained to medium-grained, equigranular 

to weakly foliated, leucocratic monzogranite. Contains about 25 percent quartz, 30–40 percent both sodic 

plagioclase and microcline, 1–5 percent biotite, 0–2 percent muscovite, 1 percent opaque minerals, and trace 

Figure 15. Clastic dike |cd, consisting of about 99 percent well rounded, well sorted quartz 

grains, at location HH. Hammer is 30 cm long. Photograph by K.S. Kellogg, 2000.
24 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

of zircon, apatite, and garnet (Kellogg and others, 2003). Forms small irregular-shaped intrusive bodies in 

Cross Creek Granite (Xgc). One large body mapped in northwest part of Vail Pass 71⁄2-minute quadrangle, 

but found locally throughout Cross Creek Granite

Xgc Cross Creek Granite (Paleoproterozoic)— Medium to coarse-grained, gray to pinkish-gray, texturally diverse 

granitic rock, mostly monzogranite and granodiorite containing stringers, pods, lenses, and dikes of pegmatite and aplite, and inclusions of migmatitic biotite gneiss (Xm), biotite-sillimanite schist (Xsb), and diorite 

(Xd) with sharp to gradational contacts; some partially assimilated inclusions form diffuse bodies with 

gradational contacts. Hypidiomorphic, xenomorphic, equigranular, inequigranular, and porphyritic textures 

are all common. Contains 35–50 percent oligoclase (commonly altering to sericite), 20–30 percent undulatory quartz, 15–30 percent microcline, 8–15 percent biotite, 0–2 percent muscovite (commonly as flakes 

in biotite), and 0–1 percent garnet. Accessory and trace minerals are opaque minerals, zircon, and apatite. 

Locally contains undeformed pink microcline crystals as long as 4 cm which contain deformed biotite and 

plagioclase crystals commonly aligned parallel to regional foliation, a relationship that led Tweto and Lovering 

(1977) to suggest microcline crystals were porphyroblasts rather than phenocrysts. Contact with country rock 

mostly gradational but locally sharp. Forms large batholith in southern part of Gore Range and northern part 

of Sawatch Range. Zircons strongly altered with significant lead loss, so SHRIMP U-Pb ages on several 

samples are imprecise, but indicate age about 1,700±20 Ma (D. M. Unruh and W.R. Premo, U.S. Geological 

Survey, unpub. data, 2007) 

Xgr Monzogranite and granodiorite (Paleoproterozoic)—Gray to light-gray, medium- to coarse-grained, hypidiomorphic to xenomorphic, equigranular to porphyritic, massive to slightly foliated plagioclase-microclinequartz-biotite intrusive rock in scattered, irregular-shaped stocks in Williams Fork Mountains, granodiorite 

near northern border of King Creek quadrangle in hanging wall of Williams Range thrust, and several small 

stocks in Vail Pass quadrangle that may be outliers of Cross Creek batholith (Xgc); Vail Pass rocks were 

speculatively interpreted as Paleoproterozoic “Boulder Creek Granite” or Mesoproterozoic “Silver Plume 

Granite” (Bergendahl, 1969), but age of all mapped bodies interpreted as Paleoproterozoic. Also include 

several granodiorite bodies along eastern border of map area. Accessory minerals are zircon, opaque minerals, 

apatite, + muscovite (locally as much as 5 percent), and + epidote. At map location S in northeastern Dillon 

quadrangle, a SHRIMP age on zircon of 1,725+2 Ma (W.R. Premo, U.S. Geological Survey, unpub. data, 

2008) on an equigranular monzogranite stock is interpreted as the crystallization age; rock here contains 

about 25 percent quartz, 40 percent plagioclase(~ An25), 20 percent microcline, 15 percent chloritic biotite, 

1 percent opaque minerals, and trace epidote, muscovite, and zircon. Granodiorite near northern border of 

King Creek quadrangle contains about 2 percent hornblende

Xt Tonalite (Paleoproterozoic)—Several large bodies of tonalite mapped along crest of Williams Fork Mountains, 

where rock is dark gray, coarse grained, inequigranular and massive; protoclastic texture. Two thin sections 

contain 25 percent quartz, 45–60 percent plagioclase, 1–2 percent microcline (as large as 5 mm), 15–20 

percent biotite, 2–3 percent opaque minerals, 1–2 percent muscovite, 1 percent hornblende, and trace zircon 

and apatite; one hand specimen contained about 3 percent pink garnet; contains numerous inclusions of 

hornblende gneiss and amphibolite, and is abundantly intruded by pegmatite and aplite. Strongly fractured 

in hanging wall of Williams Range thrust. SHRIMP U-Pb zircon age at map location T is 1,711±7 Ma (W.R. 

Premo, U.S. Geological Survey, unpub. data., 2008). Unit also includes a dark-pinkish-gray, weakly to 

moderately foliated, medium-grained tonalite body in northern Sheephorn Mountain quadrangle; here, rock 

contains 25 percent quartz, 47 percent plagioclase, trace microcline, 20–25 percent biotite, 3-5 percent muscovite, 1 percent opaque minerals, and trace apatite and zircon

Xgb Gabbro (Paleoproterozoic)—Gray, dark-gray, and dark-greenish-gray, medium- to coarse-grained, massive, 

generally equigranular intrusive rocks ranging from melagabbro to pyroxene diorite. Composed mostly 

of calcic plagioclase (bytownite), clinopyroxene, bronzite, magnetite, ilmenite, and variable amounts of 

secondary amphibole, mica, and opaque minerals (Taylor and Sims, 1962). Hornblende surrounds most 

pyroxene crystals. One thin section contained about 3 percent large (~2 mm) quartz crystals. Forms a large, 

irregular-shaped body, Upson Creek pluton of Taylor and Sims (1962), in southeastern corner of Sylvan 

Reservoir and northeastern corner of Ute Peak quadrangles. Preliminary SHRIMP U-Pb zircon date is 

1,706±9 Ma (location R on map) (W.R. Premo, U.S. Geological Survey, unpub. data, 2007)

Xd Quartz diorite and diorite (Paleoproterozoic)—Dark-gray to black, medium- to coarse-grained, inequigranular, 

massive to moderately foliated, biotite-plagioclase-quartz diorite and hornblende diorite; generally has 

a “salt-and-pepper” texture (defined by white plagioclase and black mafic minerals). Includes several large 

dioritic plutons in and south of Cross Creek Granite batholith in southern Gore Range, locally strongly 

foliated diorite porphyry in hanging wall of Williams Range thrust in northeastern Squaw Creek 71⁄2 minute 
Description of Map Units 25

quadrangle (fig. 16), altered body of hornblende diorite in southern part of Sylvan Reservoir 71⁄2-minute 

quadrangle, and several small plutons in southern Williams Fork Mountains. Three thin sections from 

irregular-shaped diorite masses in Cross Creek Granite in southern Gore Range contains 10–15 percent 

undulatory quartz, 40–55 percent andesine (An 35–44), 0–10 percent microcline, 20–30 percent biotite, 0–10 

percent hornblende, 1–5 percent opaque minerals, 0–2 percent apatite (as long as 0.2 mm), 0–1 percent garnet, 

and trace sphene. These bodies in Cross Creek Granite commonly have gradational contacts with surrounding 

granite. Dark-gray, coarse-grained hornblende diorite pluton in southern Sylvan Reservoir quadrangle contains 40 percent plagioclase, 40 percent chlorite after hornblende, 10 percent hornblende, 5 percent biotite, 

1–2 percent quartz, and 1–2 percent opaque minerals. An imprecise U-Pb zircon age of 1733±35 Ma was 

obtained from coarse-grained porphyritic quartz diorite from the Willow Lakes quadrangle (W.R. Premo, 

U.S. Geological Survey, unpub. data, 2009) 

Xgg Granitic gneiss (Paleoproterozoic)—Light- to medium-gray, fine- to medium-grained, weakly to strongly foliated monzogranite, granodiorite, tonalite, and trondhjemite. Composed mostly of 25–30 percent undulatory 

quartz, 35–60 percent oligoclase, trace-25 percent microcline, 5–15 percent biotite, 1–2 percent opaque 

minerals, 0–3 percent hornblende, 0–1 percent muscovite, and trace amounts zircon, apatite, and garnet. 

Granitic gneiss is intimately interlayered with amphibolite in most places (fig. 17) and contains numerous 

amphibolitic inclusions (probably ductily deformed and dismembered basaltic dikes) indicating igneous 

origin, although protolith of some granitic gneiss is uncertain (Braddock, 1969). Bergendahl (1963) called 

granodioritic to tonalitic gneiss in the canyon of Tenmile Creek “granulite,” a name reflecting the 

quartzofeldspathic nature of the rock, not its metamorphic grade, and interpreted it as oldest rock in area. 

One SHRIMP U-Pb zircon age from mapped granitic gneiss in Tenmile Creek is 1,769±9 Ma (location W on 

map). Two additional dates from granitic gneiss interlayered with sparsely migmatitic biotite gneiss, mapped 

as migmatitic biotite gneiss (“banded gneiss” of Bergendahl, 1963) are 1,758±6 Ma (location Q on map), and 

1,754±4 Ma (location X on map) (W.R. Premo, U.S. Geological Survey, unpub. data, 2008). Just to the south 

of the map area in the Montezuma 15-minute quadrangle, four localities in the abandoned (Tweto, 1977) 

Swandyke Hornblende Gneiss of Lovering (1935), considered part of the older granitic gneiss terrane, are 

about 1,770 Ma (W.R. Premo, U.S. Geological Survey, unpub. data, 2009). Dates for granitic gneiss in the 

map area are very similar to three preliminary SHRIMP U-Pb zircon ages from foliated monzogranite and 

granodiorite, mapped as granitic gneiss (Xgg) by Kellogg and others (2008), collected between 30 to 60 km 

east of map area that are between 1,766±9 Ma and 1,771±11 Ma (W.R Premo, U.S. Geological Survey, unpub. 

data, 2007)

Paleoproterozoic Metasedimentary and Metaigneous Rocks

Xm Migmatitic biotite gneiss—Mostly gray to dark-gray biotite gneiss (Xb) containing numerous light-colored 

layers and lenses (leucosomes) typically 0.110 cm thick, although locally may be much thicker. In most 

cases, leucosome bodies form less than half of rock and have sharp contacts with host rock. Leucosomes are 

composed of equigranular, massive to weakly foliated, white to light-gray microcline-plagioclase-quartz rock 

Figure 16. Dark-gray, highly strained biotite 

diorite porphyry (phase of quartz diorite and 

diorite unit, Xd) near crest of Williams Fork 

Mountains, 1.5 km west of Ute Pass, showing 

strong foliation and elongation of biotite-rich 

inclusions in shear plane. Pencil is 12 cm long. 

Photograph by K.S. Kellogg, 2006.
26 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

containing less than 5 percent biotite; accessory minerals are muscovite, opaque minerals, sphene, apatite, 

garnet, and zircon. Leucosome layers show much pinch and swell and in some places are strongly folded. A 

typical migmatitic biotite gneiss outcrop is shown in figure 18. Near margins of mapped Cross Creek Granite 

(Xgc) in Vail Pass 71⁄2-minute quadrangle, migmatitic biotite gneiss intruded by numerous small unmapped 

bodies of leucocratic Cross Creek Granite. Formation of leucosome may be due to either injection or in-situ 

partial melting (anatexis) (Olsen, 1982; Johannes and Gupta, 1982); in the latter case, host rock (melanosome) adjacent to leucosome layers commonly has biotite-rich selvages representing refractory material 

remaining from partial melting. Unit includes “injection gneiss” of Bergendahl (1969) in Vail Pass 71⁄2-minute 

quadrangle, a mixture of granitic rocks of Cross Creek batholith (Xgc) and blocks of migmatitic biotite 

gneiss. Injection gneiss of Bergendahl is common around margin of Cross Creek Granite, where the gneiss 

is partially resorbed by granite so that only wispy aggregates of biotite remain. Mapped contacts between 

areas of migmatitic biotite gneiss and biotite gneiss (Xb), which contains little (less than about 10 percent) 

or no leucosome, such as in northwestern corner of map area (Sheephorn Mountain 71⁄2-minute quadrangle), 

are highly subjective. Metamorphic grade in this relatively non-migmatitic area increases to the southeast; 

most biotite gneiss to southeast of Sheep Mountain is migmatitic (Xm). Eight new SHRIMP U-Pb zircon 

ages (W.R. Premo, U.S. Geological Survey, unpublished data, 2009) from various rocks mapped as migmatitic 

biotite gneiss, collected in the Vail Pass, Willow Lakes, and Mount Powell quadrangles, are listed in table 1 

and their locations (G, L, M, N, O, P, X, and Y) shown on the map. The ages are between about 1,750 Ma 

and 1,785 Ma, which represents the minimum provenance age of the zircons, or maximum deposition age of 

Figure17. Amphibolite intruded by numerous 

thin sills of monzogranite, in turn, cut by small 

pegmatite dikes. The granitic phase is foliated 

and is part of the granitic gneiss unit (Xgg) 

in Tenmile Canyon, although entire outcrop is 

mapped as unit Xgg. Photo faces north and 

was taken near locations E and Q on map, 

where unit Xgg (granitic phase) was dated 

at about 1,755 Ma. Knife is about 8 cm long for 

scale. Photograph by K.S. Kellogg, 2006.

Figure 18. Migmatitic biotite gneiss (Xm) on 

east side of crest of Williams Fork Mountains, 

1 km north southern boundary of Ute Peak 

quadrangle. Leucosome grades into coarsegrained pegmatitic rock, suggesting they are 

coeval. Pencil for scale. Photograph by K.S. 

Kellogg, 2006.
Description of Map Units 27

the sedimentary protolith. These ages compare with those obtained from two samples of migmatitic biotite 

gneiss about 50 km east of map area of 1,773±18 Ma and 1,780±9 Ma (Kellogg and others, 2008). The zircon 

cores from the non-melt phase (melanosome) at location Y (table 1) is 1,784±11 Ma,, although an age of 

about 1,700 Ma from the zircon rims is highly discordant and probably represents resetting by the adjacent 

1,706±Ma gabbro of Upton Creek (W.R. Premo, U.S. Geological Survey, unpub. data, 2007). A SHRIMP 

U-Pb zircon age from the melt phase (leucosome) from a sample at location H (table 1) is 1,698±4 Ma, 

which is virtually identical with ages between about 1,692±6 and 1,698±3 Ma obtained from the melt phase 

from three samples about 50 km east of map area (Kellogg and others, 2008). These ages suggest regional 

peak metamorphism and partial melting occurred at about 1,695 Ma. This is also the age determined from a 

pegmatite (YXp) at location E (1,693±2 Ma), suggesting partial melting and migmatization coeval with at 

least some pegmatite formation

Xb Biotite gneiss—Gray, medium-grained, equigranular, well-foliated, non-migmatitic or sparsely migmatitic (less 

than about 5 percent leucosome) gneiss typically containing approximately 25–50 percent quartz, 20–30 

percent plagioclase (approximately An20–30), 0–30 percent microcline, 10–15 percent biotite, 0–15 percent 

muscovite, 0–5 percent hornblende, 1–2 percent opaque minerals, and traces of zircon, apatite, monazite, 

and secondary chlorite (after biotite). As discussed above for migmatitic biotite gneiss (Xm), the age for the 

biotite-gneiss melanosome is between about 1,750 Ma and 1,785 Ma. These ages indicate that the protolith, 

interpreted as shaly sandstone and sandy shale, was derived from igneous sources as young as about 1,750 

Ma and as old as about 1,785 Ma. Locally includes minor amounts of biotite-hornblende gneiss, hornblende 

gneiss, and amphibolite. Mapped only in northwestern part of map area in northern Gore Range where partial 

melting was minimal 

Xbh Biotite-hornblende gneiss—Medium-gray to dark-gray, medium-grained, layered to massive, moderately foliated gneiss consisting chiefly of plagioclase, hornblende, biotite, quartz, and magnetite. Mapped only in 

Front Range near east margin of map. Typically occurs as thinly layered gneiss interlayered with hornblende 

gneiss, migmatitic biotite gneiss, and amphibolite. Protolith probably greywacke sandstone. U-Pb zircon 

dates of 1,690±13 Ma on hornblende gneiss (location U on map) and 1714±7 Ma on biotite gneiss (location 

V on map), both included in unit Xbh, are metamorphic ages (W.R. Premo, U.S., Geological Survey, unpub. 

data., 2008), probably related to intrusion of the adjacent Upton Creek gabbro pluton of Taylor and Sims (1962) 

Xsb Biotite-sillimanite schist—Gray, well-foliated schist and gneiss containing about 25–30 percent quartz, 20–25 

percent biotite, 10–20 percent sillimanite, 10–15 percent plagioclase, 10–15 percent microcline, 10 percent 

muscovite, 1 percent opaque minerals, and trace zircon. Schist has characteristic fibrous texture from aggregates as wide as about 1 cm of light-colored sillimanite needles. Strongly migmatitic at all localities. Occurs 

in large area in and near Williams Fork valley. Protolith shales and clay-rich rocks. Unit undated, but considered generally coeval with deposition of protolith of biotite gneiss (Xbg) at or after about 1,750 Ma

Xq Quartzite—Very-fine-grained, light-greenish-gray, massive very fine grained quartzite, mostly brecciated and 

recrystallized. Crops out in a narrow (about 30-m wide) belt near Dice Hill (center of Sheephorn Mountain 

71⁄2-minute quadrangle) in northwest part of map area, where cataclasis suggests that body lies along a 

northeast-striking (Proterozoic?) shear zone. Quartzite also crops out in several small outcrops in east-central 

part of Dillon 71⁄2-minute quadrangle. Relation to biotite gneiss (Xbg) indicates quartzite is no older than 

about 1,750 Ma

Xc Calc-silicate gneiss—Dark greenish-gray to almost black, well foliated, medium-grained to coarse-grained, 

inequigranular gneiss containing variable amounts of hornblende, epidote, calcite or dolomite, diopsidic 

clinopyroxene, quartz, and opaque minerals. Foliation typically very convoluted. Protolith calcareous 

sandstone and argillite. Occurs in several small bodies in south-central and northeast Ute Peak 71⁄2-minute 

quadrangle and in the eastern Vail Pass 71⁄2-minute quadrangle (Bergendahl, 1969)

Xf Quartz-feldspar gneiss— Gray, dark-gray, white, pinkish-gray, and tan, moderately foliated to well-foliated, 

layered, fine- to coarse-grained (mostly medium grained) quartz-plagioclase-microcline-biotite gneiss. 

Proportion of minerals ranges widely; biotite content generally less than 5 percent. Layers typically 10 cm 

to several tens of meters thick and commonly wavy. Locally contains layers of foliated monzogranite and 

granodiorite. May contain minor, thin layers of hornblende gneiss and amphibolite. Only two occurrences 

mapped: northern Gore Range (Taggert, 1962) and southern Williams Fork Range in Dillon 71⁄2-minute 

quadrangle (Kellogg, 2002). Undated in map area, but SHRIMP U-Pb zircon date from quartz-feldspar 

gneiss from about 70 km east of map area (in Indian Hills 71⁄2-minute quadrangle) is 1,776±4 Ma (Kellogg 

and others, 2008). Weathers tan to pinkish tan in rounded outcrops. Protolith interpreted as felsic volcanic 

rock (Braddock and Cole, 1979; Aleinikoff and others, 1993; Reed and others, 1993), although E.H. DeWitt 

(U.S. Geological Survey, verbal commun., 2009) suggests protolith is a fine-grained intrusive granitic rock
28 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Xh Hornblende-plagioclase gneiss and amphibolite—Dark-gray to black, weakly to strongly foliated, layered, 

mostly medium grained, hornblende-plagioclase gneiss and amphibolite, locally containing variable but 

small amounts of biotite, quartz, and augite. Commonly has black-and-white mottled texture due to weathered plagioclase (white) and hornblende (black). Amphibolite contains >50 percent amphibole. Intimately 

interlayered in places with more felsic gneissic rocks, so many small unmapped occurrences are included 

with other units (particularly Xbh, Xb, Xm, and Xc). Protolith interpreted as mafic volcanic and intrusive 

rocks that was emplaced during several episodes. For example, amphibolite interlayered with migmatitic 

biotite gneiss in places may be younger than about 1,740 Ma. Amphibolite intruded by granitic gneiss (Xgg) 

in Tenmile Canyon (Frisco and Vail Pass 71⁄2-minute quadrangles) is older than about 1,770 
Description of Map Units 29

Table 1. Sample or site locations shown on map. All Proterozoic ages are U-Pb zircon ages performed on SHRIMP facility in Menlo 

Park, California by W.R. Premo (USGS).—Continued.

[Abbreviations: bt, biotite; hb, hornblende; qz, quartz; plag, plagioclase; feld, feldspar; AFT, apatite fi ssion track. UTM, datum North America 1927].

Letter

on map

Field number Location

(UTM)#

Age (no ref. if 

unpub. date) Description

A 

B

C

D

E

F

G

H

I

J

K

L

M

N

O

P

Q

R

S

T

U

V

W

X

SC08–232–2341

K98–9–21A,B1

K98–9–21C–E1

K98–9–21F1

---

VP95–3A1

 DI95–2A1

UP09–469A1

UP09–469B1

WRP–03–242

VP–143

WL–123

VP–8A3

VP–93

VP–103

VP–113

VP–123

VP95–3B1

WRP–06–042

DI96–1801

SC07–1531

UP07–1611

UP07–1131

F98–5611

VP95–3D1

0396294E

4405442N

0390500E

4409150N

0386350E

4414900N

0389260E

4418920N

0402100E

4377440N

0412685E

4389260N

0409821E

4401023N

0409821E

4401023N

0390908E

4387273N

0397212E

4378474N

0393451E

4389595N

0397028E

4377182N

0400053E

4372948N

0401981E

4385581N

0402031E

4385781N

0402632E

4386143N

0402090E

4377405N

0412652E

4412789N

0411800E

4398610N

0396651E

4413707N

04013708E

4412221N

0411228E

4413156N

0404550E

4380700N

0402000E

4377295N

26.89±0.06 Ma

(Naeser and

others, 2002)

23.62±0.06 Ma

(Naeser and

others, 2002)

31.51±0.08 Ma

(Naeser and

others, 2002)

26.1±0.01 Ma

25.2±0.13 Ma

1697±8 Ma

1601±50 Ma

1760 Ma*

1698±4 Ma

1700±16 Ma

1693±16 Ma

1733±35 Ma

1774±16 Ma*

1733±19 Ma*

1749±9 Ma*

1720 Ma*

1758±9 Ma*

1758±6 Ma

1706±9 Ma

1725±2 Ma

1711±7 Ma

1690±13 Ma

1714±7 Ma

1769±9 Ma

1754±4 Ma*

Brush Creek near Palmer Knob (N:b); tuff, interbedded with shale, conglomerate; 

Squaw Creek quadrangle; AFT date

Otter Creek; Three trachyandesite fl ows (N:pb); Mount Powell quadrangle; AFT 

date

Green Mountain trachyte porphyry sill (N:lt); King Creek quadrangle; AFT date

Haystack Mountain; hb latite porphyry (N:lt) (T.L. Klein and E.H. DeWitt, 

USGS, unpub. data, 2009); King Creek quadrangle; 40Ar-39Ar dates on hb

Pegmatite; Offi cers Gulch (in Xm); Vail Pass quadrangle

Pegmatite; West of Eisenhower Tunnel; Dillon quadrangle

Migmatitic bt gneiss melanosome; Ute Peak quadrangle

Migmatitic bt gneiss; leucosome: Ute Peak quadrangle

Cross Creek Granite (Xgc); Vail East quadrangle

Cross Creek Granite (Xgc); Vail Pass quadrangle

Cross Creek Granite (Xgc); Willow Lakes quadrangle

Bt-feld-qz rock; mapped as Xm; intruded by Xgc; Vail Pass quadrangle

Bt-plag-qz granofels; mapped as Xm; Vail Pass quadrangle

Migmatitic bt gneiss (Xm); Vail Pass quadrangle

Migmatitic bt gneiss (Xm); Vail Pass quadrangle

Migmatitic bt gneiss (Xm); Vail Pass quadrangle

Tonalite gneiss (Xgg); Offi cers Gulch; Vail Pass quadrangle

Upton Creek gabbro of Taylor and Sims (1962) (Xgb); Ute Peak quadrangle

Bt granite (Xgr); Dillon quadrangle

Bt tonalite (Xt); Squaw Creek quadrangle

Hb gneiss (Xh); (included in unit Xbh); metamorphic age; Ute Peak quadrangle

Bt gneiss; non-migmatitic (interlayered with Xbh); probably reset by Upton Creek 

gabbro; Ute Peak quadrangle

Granitic gneiss (Xgg); Frisco quadrangle

Bt gneiss; migmatitic (Xm); Offi cers Gulch; Vail Pass quadrangle
30 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Table 1. Sample locations shown on map. All Proterozoic ages are U-Pb zircon ages performed on SHRIMP facility in Menlo Park, 

California by W.R. Premo (USGS).—Continued.

[Abbreviations: bt, biotite; hb, hornblende; qz, quartz; plag, plagioclase; feld, feldspar; AFT, apatite fi ssion track. UTM, datum North America 1927].

Letter

on map

Field number Location

(UTM)#

Age (no ref. if 

unpub. date) Description

Y

Z

AA

BB

CC

DD

EE

FF

GG

HH

UP07–2201

SM08–4131

No fi eld no.

No fi eld no.

No fi eld no.

No fi eld no.

No fi eld no.

No fi eld no.

No fi eld no.

VE99–2511

0409347E

4408742N

0376978E

4423889N

0397560E

4394560N

0406460E

4391500N

0380500E

4376750N

0390610E

4423090N

0384200E

4389000N

0387100E

4389300N

0398407E

4393346N

0385989E

4396066N

1784±11 Ma* Migmatitic bt gneiss (Xm); Ute Peak quadrangle

Tupps Creek shear zone; Sheephorn Mtn. quadrangle

Boss Mine; Willow Lakes quadrangle

Hammer fl uorspar deposit (in unit Ql); Dillon quadrangle

Eagle Mine, Gilman mining district; Minturn quadrangle

Blue Ridge mining district; King Creek quadrangle

Active landslide; Vail East quadrangle

Site of rock-fall mitigation; Vail East quadrangle

“Gouge boils” of Tweto and others (1970)

Clastic dike; Vail East quadrangle

1

K.S. Kellogg fi eld number

2

W.R. Premo fi eld number

3

Bruce Bryant fi eld number

#

Datum: North America 1927

*Minimum provenance (maximum depositional) age
Geologic History 31

Geologic History

The eastern half of the Vail quadrangle encompasses 

almost the entire Gore Range, the Blue River Valley from 

Frisco north almost to the confluence of the Blue River with 

the Colorado River, and most of the Williams Fork Mountains, 

which define the western margin of the more extensive Colorado 

Front Range. The cores of the Gore and Front Ranges are 

composed of basement rocks as old as 1,780 Ma that were 

uplifted during the Laramide orogeny, which fundamentally 

changed the mountain landscape during the Late Cretaceous 

to early Tertiary, approximately 70–50 Ma (Tweto, 1975). The 

Blue River valley is a complex half graben, bounded on the 

west by the northwest-striking Blue River normal fault that 

lies along the base of the abrupt northeast flank of the Gore 

Range. Just east of the Blue River valley, a major Laramide 

thrust fault, the Williams Range thrust, bounds the west side of 

the Front Range and places Proterozoic rocks over Upper 

Cretaceous marine sedimentary rocks that underlie much of 

the Blue River valley. The geologically complex Gore fault 

on the west side of the Gore Range also underwent major 

movement during the Laramide orogeny, but has a history that 

extends at least as far back as the late Paleozoic. The northeastern margin of the Central Colorado trough, containing a 

thick section of upper Paleozoic, mostly clastic rocks shed 

generally southwestward off the late Paleozoic ancestral Front 

Range, occupies the southwestern corner of the map area.

The geologic history of this area can be divided into 

five major chapters: (1) Proterozoic history, (2) pre-Laramide 

Paleozoic and Mesozoic history, (3) Late Cretaceous to early 

Tertiary Laramide mountain building and basin development, 

(4) middle to late Tertiary basin development, and (5) Quaternary history. Much of the geologic history herein is summarized from Kellogg (2002) and Kellogg and others (2000, 

2002, 2004 and 2008).

Proterozoic History

Details of the history of the Proterozoic rocks in Colorado 

are only locally well known (see, for example, Braddock and 

Cole, 1979; Bickford and others, 1986, 1989; Aleinikoff and 

others, 1993; Reed and others, 1993; Premo and Fanning, 

2000; Premo, Kellogg, and Bryant, 2007; Premo, Kellogg, 

Castineiras, and others, 2007). Minor marine sediments and 

mafic and felsic volcanic rocks, deposited locally in the Front 

Range region (including the map area) between about 1,740 

and 1,780 Ma (Premo, Kellogg, and Bryant, 2007), were generally metamorphosed to amphibolite grade and intruded and 

deformed by mostly calc-alkalic granitic rocks during a long 

orogenic episode that lasted about 110 m.y. These rocks are 

part of a Paleoproterozoic terrane called the Colorado province 

(Bickford and others, 1986) that is generally interpreted to 

have formed following accretion of island arcs and back-arc 

basins to the southern margin of the Wyoming craton, which 

marks the southern edge of the Archean continent of Laurentia 

(Reed and others, 1993; Aleinikoff and others, 1993; 

Chamberlain, 1998). This 1,780–1,740 Ma suturing event is 

called the Medicine Bow orogeny by Chamberlain (1998). 

Recent alternative models argue that island arc rocks are not 

present in the Colorado province and that the rocks were 

derived from extension of pre-existing crust of an unknown 

age (Hill and Bickford, 2001; Bickford and Hill, 2007; Bickford and others, 2008).

Proterozoic supracrustal rocks of the Front Range include 

complexly interlayered and folded quartz-feldspar gneiss, 

biotite-sillimanite gneiss and schist, amphibolite and hornblende-plagioclase gneiss, and minor quartzite and calc-silicate 

gneiss (metamorphosed calcareous sediments). Granitic gneiss 

(Xgg) may represent part of a deformed approximate 1,770 

Ma basement onto which subsequent sedimentary basins, 

containing some volcanic rocks, were developed.

Zircons from biotite gneiss in the map area and from the 

Front Range to the east are 1,740–1,785 Ma (Kellogg and 

others, 2008; W.R. Premo, U.S. Geological Survey, unpub. 

data, 2008), which is the age of the source rock from which 

the zircons were derived and represents the maximum age of 

deposition. Minor amounts of Trans-Hudsonian (1,800–1,88- 

Ma) and Late Archean zircons are also found in some of the 

biotite gneiss in the Front Range east of the map area (Premo, 

Kellogg, Castineiras, and others, 2007), although none thus far 

has been found for the gneisses in the map area.

Amphibolite (part of unit Xh) and quartz-feldspar gneiss 

(Xf) may represent original volcanic complexes, although 

locally both the metasedimentary and metavolcanic packages 

are complexly interlayered and tectonically interleaved on a 

regional scale. Alternatively, hornblende-plagioclase gneiss 

and amphibolite (Xh) may represent oceanic crust, and quartzfeldspar gneiss (Xf) may be deformed granitic intrusive rock 

(E.H. DeWitt, U.S. Geological Survey, oral commun., 2009). 

The rocks were metamorphosed to high-temperature, 

low-pressure, upper amphibolite assemblages; the more felsic 

metasedimentary rocks are mostly partially melted (migmatitic), although the grade appears to decrease toward the 

northwest, such that biotite gneiss in the northwest part of 

the map area is largely non-migmatitic. Peak metamorphism, 

presumably during the Medicine Bow orogeny, is thought 

to have occurred about 1,750 Ma (Reed and others, 1993), 

although new single-crystal, U-Pb zircon results from several 

localities in the Front Range indicate that a period of partial 

melting (anatexis) and formation of migmatites also occurred 

about 1,697 Ma (Premo, Kellogg, and Bryant, 2007; Kellogg 

and others, 2008).

Coeval with or closely following the approximately 

1,750 Ma metamorphic and deformational event, extensive 

batholiths and smaller bodies of mostly granodiorite and 

monzogranite, referred to as the Routt Plutonic Suite (Tweto, 

1987), intruded the older layered rocks. The Cross Creek 

Granite, dated by strongly discordant zircons from three 

localities sampled by Bruce Bryant (locations not shown) at 

about 1,700 Ma (D.M.Unruh, 1998, and W.R. Premo, U.S. 

Geological Survey, unpub. data, 2007), forms a large batholith 
32 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

that is a major member of this suite. Additional intrusive 

rocks ranging in composition from gabbro to monzogranite, 

intruded between 1,710–1,725 Ma, approximately coeval with 

emplacement of the large Boulder Creek Granodiorite batholith of the Front Range (Premo, Kellogg, and Bryant, 2007). 

Paleoproterozoic basement of the Front Range was 

extensively modified by widespread intrusions of the Berthoud 

Plutonic Suite (Tweto, 1987), regional heating, and local 

deformation during a regional orogenic event about 1,400 Ma. 

This event is represented by extensive plutonism immediately 

east of the map area (Kellogg and others, 2008), although no 

known rocks of this age, except possibly some pegmatites, are 

represented in the eastern half of the Vail quadrangle. Most 

pegmatites in the map area are late-stage intrusives with U-Pb 

zircon ages of about 1,700 Ma.

The Proterozoic rocks of the Front Range are transected 

by a number of northeast- to east-trending, discontinuous, 

en-echelon shear zones consisting of steeply dipping fabric 

exposed in mylonitic and nonmylonitic rocks. In the Sawatch 

Range, these shears are collectively called the Homestake 

shear zone (Tweto and Sims, 1963), although we employ this 

name for generally northeast-trending shear zones throughout 

the basement in the eastern half of the Vail quadrangle. Recent 

study of the Homestake shear zone, including northeasttrending shear zones at several places in the Front Range, 

shows that a zone of steeply dipping, highly strained but nonmylonitic rock formed between about 1,710–1,630 Ma (electron microprobe monazite ages), during or following the main 

period of intrusive activity of the Routt Plutonic Suite (Shaw 

and others, 2001, 2002). A second period of shearing along 

the same general shear zones produced localized mylonites 

and ultramylonites 1,380–1,450 Ma (also monazite ages), 

approximately during emplacement of plutons of the Berthoud 

Plutonic Suite (Shaw and others, 2001, 2002; McCoy and 

others, 2005). Mineral lineations in mylonites along most of 

these shear zones are steep. The vertical Tupps Creek shear 

zone (location Z in Sheephorn Mountain 71⁄2-minute quadrangle) 

contains well-defined steep lineations with kinematic indicators 

that demonstrate south-side up movement with a slight rightlateral component.

Paleozoic and Pre-Laramide Mesozoic History

Sometime during the latest Proterozoic, a widespread 

and poorly understood period of uplift and erosion exposed 

deep levels of the basement rocks across a nearly planar erosional surface that extended over much of what is now North 

America. Shallow seas covered this surface during most of 

the time between deposition of the Upper Cambrian Sawatch 

Quartzite and Middle Pennsylvanian Belden Formation. A 

brief period of emergence followed deposition of the Mississippian Leadville Limestone, resulting in karsts and caves in 

the upper Leadville and soil formation of the Lower 

Pennsylvanian Molas Formation.

The Proterozoic Homestake shear zone was reactivated 

between the Middle Ordovician and Late Devonian, as shown 

by abrupt changes in thickness of Cambro-Ordovician rocks 

across northeast-striking faults with apparent short strike 

length in the Eagle River canyon, but rocks of the overlying 

Upper Devonian Parting Formation are unchanged across 

these faults (Allen, 2004; K.M. Kirkham and K.J. Houck, Colo 

Geological Survey, written commun., 2009). 

Uplift of the ancestral Front Range, one of many late 

Paleozoic uplifts in the western and central United States, 

began during the Middle Pennsylvanian (DeVoto, 1980). 

Coarse sediment was shed southeastward toward the flanking 

Central Colorado trough, forming coarse alluvial fans and 

fluvial deposits of the Minturn and Maroon Formations. Clastic 

sedimentation was interrupted periodically by transgressive 

marine incursions and deposition of limestone, much of which 

was subsequently dolomitized. Grain size of the clastic deposits 

decreased toward the axis of the trough or basin, where clastic 

sediments interfinger with finer-grained clastic sediments 

and interbedded gypsum of the Eagle Valley Formation, 

and, farther into the basin, thousands of meters of evaporite 

deposits of the Eagle Valley Evaporite (DeVoto, 1980), which 

is exposed extensively immediately west of the map area. 

The Gore fault zone played an important role in the 

development of the Central Colorado trough. The fault zone is 

actually a complicated array of intersecting high-angle faults 

that defines the approximate eastern boundary of the trough. 

The zone has had a protracted tectonic history beginning at 

least as early as the early Paleozoic, and perhaps even the 

Precambrian (Tweto and Lovering, 1977). Recurrent movements along the fault zone during Ordovician, Mississippian, 

and Pennsylvanian time are documented by removal of the 

Harding Formation, most of the Chaffee Group, Leadville 

Limestone, Molas Formation, and Belden Formation adjacent 

to the fault. Dramatic thinning of the Minturn and Maroon 

Formations toward the Gore fault zone also strongly suggest 

that the fault was active along the west margin of the ancestral 

Front Range during much of Late Pennsylvanian and Permian 

time.

Positive topography and exposed basement rock existed 

until the Jurassic Period when the region once again was 

eroded to a near-sea-level planar surface and the near-shore 

fluvial and lagoonal deposits of the Upper Jurassic Morrison 

Formation covered the entire region. The subsequent mature 

sandstones and carbonaceous shales of the Lower Cretaceous 

Dakota Formation represent deposits along beaches, slowmoving rivers, and in swamps. Gradual sea-level rise and (or) 

tectonic subsidence beginning in the late Early Cretaceous 

caused transgression of a vast Late Cretaceous Western Interior Seaway that inundated much of the central part of North 

America. Several thousand meters of marine strata, mostly 

shale and subordinate limestone and sandstone (Benton Group, 

Niobrara Formation, and the thick Pierre Shale) were deposited in the seaway. 
Geologic History 33

Late Cretaceous to Early Tertiary Laramide 

Mountain Building and Synorogenic 

Sedimentation

The Laramide orogeny was an approximate 20-m.y. 

period of crustal contraction, uplift, faulting, igneous activity, 

and erosion that marked a major period of building the present 

Rocky Mountains. Its early stirrings were marked by renewed 

uplift of the Front Range region in the Late Cretaceous and 

by the initiation of magmatic activity. Laramide activity in the 

region of the Sawatch Range and eastern Front Range is generally considered to have begun about 70 Ma (Tweto, 1975). 

The Western Interior Seaway began to withdraw from 

the region after 69 Ma, the age of the youngest ammonite 

zone in the Pierre Shale on the eastern side of the Front Range 

(Scott and Cobban, 1965; Cobban, 1993; Obradovich, 1993). 

A sequence of upward-coarsening rocks as young as 64 Ma 

documents this initial, rapid uplift and is dramatically exposed 

near Golden on the eastern side of the Front Range (Kellogg 

and others, 2008, and references contained therein). 

The only rocks in the map area that record Laramide 

uplift are the fine- to very coarse-grained clastic sediments 

of the thick Paleocene and Upper(?) Cretaceous Middle Park 

Formation (:Kmu) , which were shed mostly off the rising 

upper plate of the Laramide Williams Range thrust (Izett, 

1968) and locally off the Mount Bross fault, which is just 

northeast of the map area (see regional structure map). This 

relationship suggests that deformation and erosion started 

before the end of the Cretaceous and continued into the Paleocene, in agreement with evidence from the eastern margin of 

the Front Range. Clasts in the upper member of the Middle 

Park Formation are mainly derived from Proterozoic basement rocks, indicating that the basement was exposed during 

deposition. Exploratory drilling on the crest of the Blue Ridge 

anticline, which transects the northeastern corner of the map 

area (see regional structure map), revealed that Cretaceous 

rocks as old as the Dakota Formation were thrust laterally at 

least 2 km over the Middle Park Formation (Taylor, 1975). 

The poorly constrained age of this thrusting is Paleocene or 

early Eocene. The physiographic area referred to as Middle 

Park includes both the low-lying region between Kremmling 

and Hot Sulphur Springs and the Granby and Fraser basins 

to the east, underlain largely by the Troublesome and Middle 

Park Formations (refer to regional structure map).

The Laramide Williams Range thrust defines the western 

structural boundary of the Front Range. The thrust is a lowangle structure that, at the latitude of Frisco, has a minimum 

of 9 km of horizontal movement, as demonstrated by a 

window in the thrust near Keystone, about 4 km east of the 

map area (Kellogg and others, 2004). Figure 19 is a view of 

the Williams Range thrust, showing migmatitic biotite gneiss 

thrust above Dakota Sandstone (Kd), and figure 20 shows a 

small satillitic thrust in Dakota Sandstone (Kd), several km 

west of the main trace of the Williams Range thrust. The age 

of the thrust is suggested by its on-strike relationship with the 

Elkhorn thrust to the south, although the possible connection 

is obscured by Tertiary intrusive rocks and overlying surficial 

deposits. The Elkhorn thrust is as young as late Paleocene 

(Bryant and others, 1981). 

The Gore Fault Zone and Faults Cutting 

Proterozoic Rocks of the Gore Range

Although the Gore fault zone was active during and 

perhaps prior to the Paleozoic, most of the displacement 

along the Gore fault zone probably took place during the Late 

Cretaceous and early Tertiary Laramide orogeny. En-echelon, 

right-stepping faults along the zone also suggest a left-lateral 

component of movement (Bergendahl, 1969; Kellogg and 

others, 2000).

Figure 19. White rock in background 

is part of a wedge of overturned Dakota 

Formation (Kd) in fault contact across 

Williams Range thrust, 3.4 km south-southeast of Ute Pass; trace of thrust shown 

by line. Hanging-wall rock is migmatitic 

biotite gneiss (Xm). Dakota, in turn, is in 

fault contact with Niobrara Formation (Kn) 

along another imbricate thrust of Williams 

Range thrust (about 20 m to left of the 

image). Photograph by K.S. Kellogg, 2007.
34 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

The Gore fault zone was previously interpreted as having 

overall normal fault movement (Tweto and Lovering, 1977; 

Tweto and others, 1978). Recent mapping, however, shows 

that most faults in the zone are high-angle reverse faults 

(Kellogg and others, 2003). Beds of the Minturn Formation 

dip steeply or are overturned immediately south of the fault 

zone, and thin layers of quartzite (either Upper Cambrian 

Sawatch Quartzite or Upper Devonian Parting Formation) are 

entrained as fault-bounded blocks. Figure 21 shows a particularly well exposed fault contact between steeply dipping 

Minturn Formation (Robinson Limestone Member, *mr) and 

fractured Cross Creek Granite (Xgc) in Booth Creek, in the 

Vail East 71⁄2-minute quadrangle.

The north-northwest-striking faults that cut the Proterozoic 

basement of the Gore Range had a very long history, with as 

many as five possible periods of movement: (1) Precambrian 

movement, because the faults are parallel to the general foliation trends (Tweto and Lovering, 1977), (2) movement related 

to late Paleozoic uplift of the ancestral Front Range (Tweto 

and Lovering, 1977), (3) uplift related to Laramide orogeny 

(Tweto, 1975), (4) strike-slip movement related to a poorly 

understood mid-Tertiary period of deformation (Erslev and 

others, 1999), and (5) extension related to late Oligocene and 

younger normal faulting, crustal heating, and uplift along the 

Blue River fault zone (Tweto, 1979; Kellogg, 1999; Naeser 

and others, 2002). 

Figure 20. View to northwest of 

Dakota Sandstone (Kd) exposed in 

road cut along Interstate Highway 

70, 3 km west of trace of Williams 

Range thrust. Small west-directed 

thrust is satellitic to the Williams 

Range thrust. Note development of 

prominent hanging-wall anticline. 

Distance across road cut about 20 m. 

Photograph by K.S. Kellogg, 1998.

Figure 21. View, facing west from 

across Booth Creek, of steeply 

dipping beds of the Minturn Formation (Robinson Limestone Member, 

*mr) in fault contact with fractured 

Cross Creek Granite. Trace of Gore 

fault shown by the white line. Photograph by Bruce Bryant, 1999.
Geologic History 35

Another set of east-northeast-striking faults cut the Proterozoic basement of the Gore Range, and are probably related 

to Tertiary differential uplift of the Range. A particularly large 

down-to-the-northwest normal fault system just north of Mt. 

Powell separates the high, rugged terrane south of the fault 

from the more gentle, lower topography of Elliott Ridge.

Middle to Late Tertiary Development of the 

Northern Rio Grande Rift

Following the Laramide orogeny, which persisted until 

the early Eocene, a period of crustal extension and increased 

volcanism dominated the tectonics of the region. The 

north-striking grabens of the Rio Grande rift, which extend 

southward from Colorado through New Mexico, are the largest manifestations of this period. The oldest dated volcanic 

rocks interbedded with syn-rift sedimentary deposits in the 

rift are about 26 Ma (Thompson and others, 1991; Chapin 

and Cather, 1994), the approximate time of initial rifting. The 

west-tilted Blue River half-graben, underlying the Blue River 

valley, marks the northern end of the rift, although normal 

faults extend as far north as southern Wyoming (Tweto, 1979; 

Mears, 1998). Unlike other parts of the rift, the Blue River 

half-graben is relatively shallow and narrow, and Tertiary 

basin-fill deposits are not as thick as elsewhere in the rift. A 

few hundred meters of basin-fill deposits (sandstones, gravels, 

and volcanic rocks of unit :b and mafic volcanic flows of 

unit N:pb) as old as Oligocene are mostly covered by a thick 

mantle of late Tertiary and Quaternary gravels and Quaternary 

glacial deposits. 

Many of the Mesozoic rocks of the Blue River valley, 

dominated by the over-1,500-m-thick Pierre Shale, are cut by 

several down-to-the-east normal faults, especially in the southern part of the valley (Kellogg, 2002). The Blue River normal 

fault along the east side of the Gore Range (fig. 22) offsets the 

Proterozoic-Phanerozoic contact by at least 1,200 m (West, 

1978). The consistent down-to-the-east displacement of faults 

in the Blue River valley suggest that they may have soled 

into and exploited the Laramide Gore fault (Kellogg, 1999). 

Movement along the normal fault has been interpreted to be 

as young as Holocene (Tweto and others, 1970; Unruh and 

others, 1993) although West (1978) interpreted the movement 

to be no younger than late Pleistocene. Refer to the section on 

seismicity for a further discussion of possible recent faulting. 

Oligocene to early Pliocene apatite fission-track ages of 

rock in the uplifted flanks of the Blue River graben mark the 

time the rocks cooled through the approximate 100°C isotherm. 

These ages are considerably younger than annealing ages 

recorded farther from the graben and indicate that uplift along 

the flanks of the graben post-dates uplift in adjacent areas 

(Naeser and others, 2002).

Figure 22. Early morning view to west from Ute Pass across Blue River Valley 

toward the valley of Slate Creek and the crest of Gore Range. The Blue River normal 

fault lies along the eastern base of the Gore Range, behind the large, rounded, 

forested hill in foreground. This hill is underlain mostly by diamicton (QNd), which 

locally consists of pre-Bull Lake till (West, 1978). Lower, largely grass- and sagebrush-covered hill in sunshine (adjacent to shadow in the foreground) is underlain 

by till of Bull Lake glaciation. Arcuate ridges west of this hill underlain by till of 

Pinedale glaciation. Rusty-red color of lodgepole-pine forest due to widespread 

infestation by pine-bark beetles. Photograph by K.S. Kellogg, 2006.
36 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Deposition of the Upper Oligocene to Upper 

Miocene Troublesome Formation

The tuffaceous siltstones, sandstones, and conglomerates 

of the Troublesome Formation occupy a generally northnorthwest-trending, sagebrush- and grass-covered basin (Izett, 

1968) that extends southward into the north-central part of 

the map area. The basin is part of an area of relatively low 

topographic relief referred to as Middle Park. Middle Park is 

bounded on the west by the Gore Range (the northern part of 

which is commonly called the Park Range), on the north by 

the Tertiary sedimentary, volcanic, and intrusive rocks of the 

Rabbit Ears Range, and on the east by the Front Range. The 

Blue River valley is a southern physiographic extension of 

Middle Park. The Troublesome Formation was deposited

during a period of widespread intermontane basin filling, and 

is contemporaneous, in part, with the Browns Park, North 

Park, and Arikaree Formations in adjacent or nearby basins 

(Izett, 1968).

The Troublesome Formation in the map area occupies the 

western (and largest) of three basins. Two eastern basins northeast of the map area (see regional structure map on map sheet) 

near Granby and Fraser are also filled by rocks of the Troublesome Formation (Shroba and others, 2010). The western basin 

and two eastern basins are separated by a topographic divide 

underlain by Proterozoic rocks and rocks of the Upper(?) 

Cretaceous and Paleocene Middle Park Formation. The generally northwest to north trend of the three basins suggests that 

their geometry is controlled by Tertiary normal faults (Izett, 

1968), so these basins may be considered part of the northernmost Rio Grande rift system (Tweto, 1979), which began 

forming in the early Oligocene about 26 Ma (Thompson and 

others, 1991; Chapin and Cather, 1994).

Fossil remains of Hemphillian age (early Pliocene to late 

Miocene) in fluvial sediments deposited by the Williams Fork 

on Troublesome beds just north of the map area (Izett, 1968; 

Kihm and Middleton, 1980) further constrain the age of the 

Troublesome. This relation also suggests that the courses of 

the Williams Fork and Colorado River were established after 

deposition of the Troublesome, during the late Miocene or 

early Pliocene. 

Quaternary History 

Quaternary deposits and landforms reflect the influence 

of earth-surface processes in concert with global climatedriven, glacial-interglacial cycles during the past 2.58 million 

years. Global climatic cooling, beginning about 2.5 Ma, continued into the Pleistocene and accelerated after 900 ka (Clark 

and Pollard, 1998). 

The Pleistocene glacial deposits in Colorado and elsewhere in the Rocky Mountains are commonly correlated with 

Pinedale, Bull Lake, and pre-Bull Lake glaciations of Wyoming 

(Meierding and Birkeland, 1980; Pierce, 2004), although 

the stratigraphic record suggests that there were at least 12 

glaciations in the mountains of western North America during the Quaternary (Richmond and Fullerton, 1986, chart 1). 

Blackwelder (1915) named the Pinedale (last major) and Bull 

Lake (penultimate) glaciations for younger and older sets of 

moraines on the eastern and western flanks of the Wind River 

Range, Wyo. Glacial deposits that predated the Bull Lake 

glaciation typically lack morainal form and commonly are 

locally preserved just beyond the lower limits of Pinedale and 

Bull Lake ice. 

At least some of the deposits mapped as Pliocene to 

middle Pleistocene diamicton (QNd) and bouldery gold-bearing 

gravels of Mesa Cortina and Gold Run (QNmg) may be preBull Lake till. The latter deposits are particularly interesting 

because of the enormous clast size (some clasts are at least 

8 m long) and gold content. The gravels were locally mined 

hydraulically, mostly in the 1860s and 1870s (Parker, 1974). 

Glacial deposits of early Pleistocene age are mostly 

buried by younger glacial deposits because cold (glacial) and 

warm (interglacial) climatic cycles prior to 900 ka (marine 

oxygen isotope stage (MIS) 22) were of lower amplitude 

(lower global ice volume) and of much shorter duration (about 

40 percent as long) than those after 900 ka (Clark and Pollard, 

1998). Marine oxygen isotope records show only two glacial 

episodes (MIS 12 and 16; about 424–475 ka and 621–675 ka, 

respectively; Lisiecki and Raymo, 2005) as severe (in terms 

of temperature and global ice volume) as those during the 

Bull Lake and Pinedale glaciations. This suggests that glacial 

deposits formed during pre-Bull Lake glaciations in the Rocky 

Mountains prior to MIS 16 tend to be less extensive than those 

formed during the Bull Lake and Pinedale glaciations.

The oldest unequivocal glacial deposits exposed in the 

map area are till and, locally, stratified drift, which form subdued moraines of the Bull Lake glaciation. Bull Lake deposits 

in their type area in Wyoming are dated at 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 (130–70 ka, Lisiecki 

and Raymo, 2005; Pierce, 2004). Mapped deposits of the 

Bull Lake glaciation may locally include small, unrecognized 

deposits of pre-Bull Lake glaciations. 

Till and minor amounts of stratified drift of the Pinedale 

glaciation form well- preserved moraines that are widespread 

in valleys along the east flank of the Gore Range and in the 

Williams Fork valley. Pinedale glacial deposits in Colorado 

are dated at about 30–12 ka (Nelson and others, 1979; Madole, 

1986; Schildgen and others, 2002; Benson and others, 2004, 

2005). They accumulated during a major cold climatic episode 

of MIS 2 (35–14 ka; Lisiecki and Raymo, 2005).

Glaciers on the east side of the Gore Range during the 

Bull Lake and Pinedale glaciations were as long as 17 km, as 

found along Slate Creek. The glacier that occupied the valley 

of Gore Creek during the Bull Lake glaciation was about 25 

km long. Elevations of terminal moraines are as low as about 

2,500 m. No glaciers occupied the Blue River valley in the 

map area; terminal moraines of the Bull Lake and Pinedale 
Geologic History 37

glaciations in the Blue River valley are near the town of 

Breckenridge, about 2 km south of the map boundary. Large 

Pinedale and Bull Lake glaciers flowed down the valleys 

of Tenmile and North Tenmile Creek and formed terminal 

moraines almost as far north as Dillon Dam. Extensive glacial 

outwash and Pinedale-age till underlie the town of Frisco. 

A glacier of Bull Lake age in the Snake River valley barely 

extended into the map area. A terminal moraine of Pinedale 

age is in the Williams Fork valley, and a till of Bull Lake age 

is mapped about 0.5 km farther down the valley. 

The youngest glacial deposits in cirques of the Gore 

Range are tills that probably correlate with deposits in the 

Front Range that are Holocene (Benedict, 1985) and latest 

Pleistocene in age (about 12–10 ka; Davis, 1988). These tills 

lie above present treeline near cirque head walls, typically 

above an elevation of about 3,350 m. They were deposited 

during minor glacial advances after the Pinedale glaciation, 

chiefly during MIS 1 (14–0 ka; Lisiecki and Raymo, 2005), 

and form sharp-crested moraines. 

Snow lines during the Pleistocene in the western United 

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

others, 1983). Fossil beetles near Denver (at 1,731 m elevation), dated at 14,500 14C yr B.P., suggest that during full- or 

late-glacial climatic conditions, mean July temperatures were 

10°–11°C colder than present, and mean January temperatures 

were 26°–30°C colder than present (Elias, 1996). Relict permafrost features in Wyoming suggest that temperatures could 

have been 10°–13°C colder than they are at present (Mears, 

1981). 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 

and Gore Ranges. These processes form such features as block 

fields and block streams on interfluves above and beyond the 

limit of glacial ice. Increased infiltration of precipitation may 

have locally promoted deep-seated rock creep on steep slopes 

in periglacial environments. 

Streams draining from glaciers within the map area produced broad, gravelly glacial outwash deposits, such as those 

underlying Frisco, particularly during times of significantly 

greater sediment yield during deglaciation (Church and Ryder, 

1972). Some of the glacial outwash likely is slightly younger 

than corresponding tills, because fluvial deposition lagged 

(perhaps by several thousand years) the onset of the climatic 

change that affected the transition from glacial to interglacial 

conditions (Church and Ryder, 1972; Hancock and Anderson, 

2002). Maximum grain size and the surface slope of glacial 

outwash deposits decrease significantly within a distance of 

about 10 km downstream of former glacier fronts (Church and 

Ryder, 1972; Ritter, 1987). 

Outwash deposits and till of the Pinedale glaciation in the 

Front Range are correlated with young stream-terrace alluvium 

(Qg1); deposits of the Bull Lake glaciation are correlated with 

intermediate stream-terrace alluvium (Qg2). Older streamterrace alluvium is correlated with pre-Bull Lake glaciations. 

These correlations are based chiefly on the (1) morphology of 

surface soils formed in these deposits (Birkeland and others, 

2003) and (2) height of glacial outwash and piedmont alluvial 

deposits above present streams. Recent cosmogenic dating of 

similar deposits on the east side of the Front Range supports 

these correlations (Schildgen and others, 2002; Kellogg and 

others, 2008, table 1). Alluvium of Holocene age is present in 

flood plains and in terrace deposits less than about 5 m above 

present streams (included in unit Qa). These deposits are 

difficult to correlate with climatic episodes, but some of them

may reflect past climatic conditions that were colder than at 

present, and possibly more moist (or marked by more effective 

precipitation) than at present. These recent climatic episodes 

promoted minor glacial and (or) periglacial activity in the 

Front Range during the Holocene (Scott, 1975). 

Most large landslide deposits probably formed chiefly 

under periglacial conditions during the Pleistocene, although 

some of these landslide deposits were recurrently remobilized 

into the Holocene. Some large landslide deposits in glaciated 

valleys along the east side of the Gore Range formed after 

glaciers retreated and glacial ice no longer provided lateral 

support to weakly consolidated material on steep, unstable 

slopes. 

The large landslide deposits along the west side of the 

Williams Fork Range are composed almost entirely of fragments and blocks of Proterozoic rock as long as tens of meters 

derived from the strongly fractured upper plate of the Williams 

Range thrust. Most of the landslide complex is highly dissected 

and no longer retains a landslide morphology, indicating that 

the last significant movement may be as old as Pliocene. In 

addition, the crest of the Williams Fork Mountains, although 

glaciated, is relatively smooth, rounded, and maintains a 

nearly constant altitude of about 3,700 m. Sackungen, deep 

fractures with uphill-facing scarps induced by gravitational 

spreading of the entire mountain range, are common along and 

near the crest of the range (fig. 23). This subdued topography 

is in stark contrast to that along the rugged crest of the Gore 

Range, where rocks are relatively unfractured, although sackungen do occur in several localized, strongly fractured areas 

near the Blue river normal fault zone (West, 1978).

The strong fracturing and sackungen developed in the 

Williams Fork Mountains suggest a tectonic origin for the 

large landslide deposits on the west side of the range (Kellogg, 

2001): during Laramide thrusting, the attitude of the Williams 

Range thrust changed from relatively steep at depth, where 

Precambrian rocks occupied both the lower and upper plates, 

to a much more gentle attitude where lower-plate rocks were 

mostly Cretaceous shale. The relatively brittle upper-plate 

rocks were shattered (fig. 24) by the proposed steep-to-flat 

flexure in the thrust plane. Starting in the late Neogene, as the 

Blue River and Williams Fork incised their valleys, the shattered Precambrian rocks were undercut, causing much of the 

relatively weak upper-plate rocks to adjust gravitationally by 

spreading, forming the sackungen along the crest of the range 

and extensive landsliding along the western flank (Kellogg, 2001).
38 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Figure 23. View to north, near crest of southern Williams Fork Mountains, of sackungen, 

or deep-seated extension fractures and uphill-facing scarps caused by gravitational 

spreading. Photograph by K.S. Kellogg, 2004. 

Figure 24. Road cut at Ute Pass in migmatitic biotite gneiss (Xm), showing well 

developed fractures and oxidation of rock fragments in hanging wall of Williams Range 

thrust. The trace of the thrust is about 40 m west of road cut, so gently dipping thrust 

plane probably less than 25 m below level of road. Distance across base of photo is 

about 10 m. Photograph by K.S. Kellogg, 2001.
Resources 39

Resources

Gravel

The principal gravel resources in the quadrangle are in 

alluvium (Qa) and younger terrace deposits (Qg1) along the 

Blue River northwest of Dillon, and dredge tailings derived 

from alluvium (Qa) and younger terrace deposits (Qg1) along 

the Blue and Swan Rivers south of Dillon Reservoir. Additional resources were exploited along Gore Creek, although 

these deposits, like many of those along the Blue River, are 

now covered by or adjacent to extensive residential and commercial development and thus unavailable for further commercial exploitation. Alluvium in unit Qac along the Piney River 

may be an additional potential source of gravel.

Metals 

The most widely exploited metal deposits in the map area 

are near Frisco in the northern part of the Breckenridge mining 

district (Ransome, 1911; Lovering, 1934; Kellogg and others, 

2000) and in the Gilman district near Minturn (Lovering and 

others, 1978; Tweto and Lovering, 1977). 

Some of the richest gold deposits in the Breckenridge 

district were in the gravels between Frisco and Breckenridge, 

where placer dredging was focused in areas within the Swan 

and Blue River drainages (Parker, 1974). Lesser activity (both 

dredging and hydraulic mining) took place in a drainage south 

of the Swan River (Gold Run Gulch) and along bluffs west of 

Dillon dam (Mesa Cortina). Placer dredging in the map area 

lasted for one hundred years, from 1859 to 1959, although,

most production occurred from 1906 to 1924. Total value of 

gold from the placers was more than $15.5 million at thencurrent prices (Parker, 1974).1

Hard-rock mining in the Breckenridge district commenced in 1884. Mining in the map area was concentrated 

in three small areas south of the Swan River and east of the 

Blue River (summarized in Kellogg and others, 2000). Total 

gold production in this area is estimated between 90,000 and 

170,000 ounces (Kellogg and others, 2000). Deposits are confined to a pervasively sericitized quartz monzonite porphyry 

laccolith, radial dikes surrounding the laccolith, and peripheral 

sills, all of which intrude Pierre Shale. Mineralization is essentially restricted to the porphyry, which contains a stockwork 

of gold-bearing, iron-oxide-quartz veinlets. Accompanying the 

quartz in the epigene (oxidized) zone are, in decreasing order 

of relative abundance: massive goethite, earthy yellow limonite, 

red hematite, jarosite, sericite, and, rarely, visible native gold. 

Exploratory drilling indicates that the oxidized zone extends 

to depths as much as 25 m below the ground surface. Deeper 

hypogene (unoxidized) sulfides include, in decreasing order 

of abundance, pyrite, sphalerite, and galena. Ransome (1911) 

considered the presence of galena to be a particularly favorable indication of gold. 

The Gilman mining district (Radabaugh and others, 1968; 

Lovering and others, 1978; R.M. Kirkham and K.J. Houck, 

Colo. Geological Survey, written commun., 2009), specifically 

the Eagle Mine (location CC on map), was a major source of 

zinc and has also produced relatively large amounts of silver, 

copper, lead, and gold. Total production until 1972, when mining 

was in decline (production ceased in 1984), was 393,000 troy 

ounces Au, 66,000 troy ounces Ag, 105,000 tons Cu, 148,000 

tons Pb, and 858,000 tons Zn. Production was almost entirely 

from replacement deposits in the dolomitized Leadville Limestone. The ore occurs in mantos (blanket-like deposits) in the 

Leadville Limestone, with funnel-shaped chimneys descending into rocks of Mississippian and Devonian age, although 

sparse veins that typically contained more gold and silver than 

zinc and lead extend down as far as the Cross Creek Granite. 

Significant economic sulfide minerals are chalcopyrite, 

sphalerite, tetrahedrite (argentiferous), galena, sulfosalts, and 

telluride minerals; gangue minerals include quartz, barite, and 

fluorite. Rocks above the Leadville Limestone (or Dolomite) 

are almost unmineralized. Dolomitization of carbonate rocks 

and the emplacement of the approximately 65 Ma Pando Porphyry occurred prior to mineralization and was accompanied 

by argillic and sericitic alteration. Tweto and Lovering (1977) 

point out that if the Eagle River had not cut a deep canyon 

through the ore zone, the ore deposits might never have been 

discovered. As a result of the extensive mine workings and 

associated tailings piles of the Eagle Mine, the district has 

been designated as a superfund site (Colorado Department of 

Public Health and Environment, 2008).

The Gore Range contains several small prospects or 

mines for lead, copper, zinc, and silver mined from quartz and 

quartz-carbonate veins that were exploited during the latter

part of the nineteenth and early twentieth centuries (Tweto 

and others, 1970). The largest of these, the Boss Mine on 

the eastern side of the Gore Range (Tweto and others, 1970; 

location AA on map), was mined for argentiferous galena and 

sphalerite from east-dipping quartz veins in the footwall of the 

Blue River normal fault.

Several small mines in the Tenmile Range produced gold, 

silver, lead, and copper from mixed-sulfide quartz veins cutting Proterozoic rock (Bergendahl, 1963). 

A small but rich deposit of sphalerite and argentiferous 

galena was mined between 1937 and 1950 at the Four Star 

Mine, about 200 m east of Green Mountain dam (near location 

C on map), from fracture fillings in sandstone of the Dakota 

Formation (Taggert, 1962). The mineralization probably 

accompanied emplacement of the early Oligocene latite and 

trachyte porphyry of Green Mountain.

A small copper deposit, consisting of chalcopyrite in 

quartz-filled fracture fillings in migmatitic biotite-sillimanite 

gneiss, was mined along Copper Creek in the Blue Ridge 

mining district (Neubert, 1994). The mining district is on the 

eastern side of the Williams Fork Mountains in the eastern 

King Creek 71⁄2-minute quadrangle (location DD on map). 

Malachite coats many of the rocks on the mine dump.
40 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Fluorite

Blocks of economically marginal fluorite in a brecciated 

matrix of Dakota Formation sandstone, called the Hammer 

fluorspar deposits (Tweto and others, 1970), were mined from 

a landslide deposit in the north-central Dillon quadrangle 

(location BB on map). This occurrence was interpreted as a 

hot-spring deposit (Tweto and others, 1970), but probably was 

transported a significant distance to the east by landsliding 

from a now-buried, faulted and mineralized Dakota Formation 

sandstone. 

Geologic Hazards

Steep mountain landscapes are settings in which geologic processes are particularly active and will likely affect 

humans more aggressively as urban development continues. 

The steeper slopes of the increasingly developed Blue River 

and Gore Creek valleys are particularly susceptible to landslides, particularly where underlain by Cretaceous shales and 

Pleistocene till. Rockfall hazards pose a threat in steeper areas 

underlain by cliff-forming Proterozoic and Paleozoic rocks. 

Additional hazards include floods and debris flows, seismic 

hazards, expansive soil, elevated radon, and snow avalanches.

Landslides 

All mapped landslides are considered to be earth flows 

and earth slides, as well as rock slides and debris flows 

(terminology of Cruden and Varnes, 1996). Conditions that 

contribute to landslides commonly include: (1) oversteepening 

of slopes by such processes as fluvial erosion of the base of 

slopes and human undercutting of slopes (Varnes and others, 

1989), (2) bedding oriented parallel to slope, (3) deforestation 

by logging, fires, and (or) other human activity, (4) high water 

content (by intense or prolonged rainfall, or rapid or protracted 

snowmelt), and (5) vertical contrast in density or plasticity of 

geologic materials (Cruden and Varnes, 1996).

Most landslides in the map area appear to be inactive, 

although several (Qly) are active or recently active. A large 

active earth slide in Vail just north of I–70 (location EE on 

map) was undercut by construction of the highway and is 

currently failing due to a combination of shallow earth sliding 

and deep rotation (Jurich and Miller, 1987). Several other 

landslides have had recent movement, or possibly are currently 

active. Landslide deposits, some with hummocky topography suggesting Holocene movement, underlie large areas of 

the Vail ski area (Kellogg and others, 2003); some of these 

deposits (mapped as younger landslide deposits, Ql) may be 

unstable and susceptible to future movement. Another recently 

active landslide, composed entirely of displaced Pierre Shale, 

is just east of the Blue River (mostly in sec. 23, T. 4 S., R. 78 W.). 

The surface of the slide is very hummocky and numerous 

unvegetated scarps cut the crown area and zone of extension. 

Another large landslide deposit near Frisco, just north of I–70 

(in secs. 22, 23, 26, and 27, T. 5 S., R. 78 W.), has hummocky 

topography and well-formed, unvegetated scarps in the crown 

region and zone of deflation (terminology of Cruden and Varnes, 1996) and may be undergoing very slow creep.

Several large landslide complexes just south of the 

intersection of I–70 and State Highway 24 in the northwest 

part of the Minturn 71⁄2-minute quadrangle are primarily earth 

flow deposits (R.M. Kirkham and K.J. Houck, Colo. Geological Survey, written commun., 2009). Most of these landslides 

formed on dip slopes on the Minturn Formation. Portions of 

these landslides have undergone historic movement that has 

caused damage to infrastructure in transportation and utility 

corridors. Several studies of these landslides are summarized 

by R.M. Kirkham and K.J. Houck (Colo. Geological Survey, 

written commun., 2009). 

Rockfall

Rockfalls occur in areas of steep slopes and cliffs. In 

1983, 1986, and 1987, large boulders fell from cliffs composed 

of the Robinson Limestone Member of the Minturn Formation 

(*mr) east of lower Booth Creek in Vail, just north of I–70 

(fig. 25; location FF on map), and damaged several residences 

(Stover, 1988). A combination of weak shale beds, thick 

sandstone and limestone beds, joints in the rocks forming the 

cliffs, and a regional dip towards the valley all make this cliff 

particularly susceptible to rockfall. A large berm and trench 

were bulldozed along the side of the hill below the cliffs in an 

attempt to mitigate this hazard.

In the high country of the Gore Range, rockfalls from 

cliffs in Paleoproterozoic rocks occur frequently, especially 

in the spring when water from daytime melting of snow 

freezes at night in cracks in rocks. Repeated freeze-thaw 

cycles tend to dislodge rocks from cliffs. Talus deposits (Qt) 

are the products of rockfall from higher steep outcrops.

Floods and debris flows

Intense summer rainstorms or rapid melting of deep 

snowpack during unusually warm spring thaws may cause 

flooding or debris flows that locally can cause damage to 

roads and structures. Flooding during such conditions may be 

particularly severe along developed sections of Gore Creek in 

the Vail area and along Straight Creek east of Dillon, where 

prehistoric overbank flood deposits currently underlie some 

roads and structures. Debris flows generally form fan deposits (Qfy and smaller unmapped fan deposits) where steep 

tributaries join major drainages. They can obstruct portions of 

roadways, including I–70. With the completion of Dillon Dam 

in 1962, flow of the Blue River is now closely regulated and 

the potential for damaging floods adjacent to the river channel 

downstream of the dam is greatly reduced. 
Geologic Hazards 41

Seismicity

Although Colorado is not generally regarded as an area 

with substantial seismic risk, large historic earthquakes have 

occurred in the Front Range region (Kirkham and Rogers, 

1981). An earthquake of inferred Richter magnitude 6.5 

caused considerable damage in the northern Front Range in 

1882 (Spence and others, 1996), and scattered earthquakes 

of smaller magnitude periodically shake the region. The Blue 

River valley defines the northern terminus of the Rio Grande 

rift, an active zone of crustal extension, elevated heat flow, 

and late Tertiary flank uplift (Naeser and others, 2002). The 

faults in the Blue River Valley are not historically active, 

and the timing of youngest movement along normal faults in 

the valley, including the Blue River normal fault, is somewhat controversial. Tweto and others (1970) believed that 

the prominent scarp that defines the Blue River normal fault 

along the east side of the Gore Range indicated Holocene 

movement. As one indication of Holocene, perhaps currently 

active movement, Tweto and others (1970) note “gouge boils” 

above the trace of the Blue River normal fault (location GG 

on map) in treeless areas containing mounds as high as 12 m 

of fine-grained “fault gouge,” interpreted as extruded upward 

along the fault zone and shouldering aside bouldery material. 

Alternatively, these fine-grained hummocky deposits may be 

the result of young, possibly currently active, landsliding. 

A study subsequent to that of Tweto and others (1970) 

suggested that movement on the normal fault was probably no 

younger than Pliocene (West, 1978). More recently, Unruh and 

others (1993) identified features along the northern part of the 

normal fault, at Cataract Creek and Otter Creek, that suggested 

late Quaternary movement, with a slip-rate range of 0.05–0.5 

mm/yr (O’Connell and others, 2005). In addition, low (less 

than about 2 m), subtle fault scarps west of I–70, just northwest of Frisco, cut old gravel deposits (QNmg) of middle 

Pleistocene to Pliocene(?) age (Kellogg and others, 2000).

LIDAR (Light Detection and Ranging) imagery acquired 

and generously shared by L.W. Anderson and L.A. Piety (U.S. 

Bureau of Reclamation, written commun., 2008; Anderson and 

Piety, 2009) to evaluate the seismic stability of Green Mountain Reservoir dam (a Bureau of Reclamation facility), has 

revealed subtle scarp-like features that cut deposits as young 

as till of the Pinedale glaciation (Qtp), but further study is 

needed to verify the origin of these features. These LIDARdefined scarps, suggesting middle Pleistocene to possible 

Holocene fault movement, are identified in red on the map. 

Down-to-the-northeast scarps of suspected late Pleistocene age along the west side of the Williams Fork valley 

displace Troublesome Formation against Proterozoic basement 

rocks (R.M. Kirkham, written commun. 2008). These relationships suggest ongoing seismicity during late Pleistocene time, 

and possibly as late as Holocene time. Slip rates in the northern 

part of this fault system are estimated at 0.02–0.3 mm/yr 

(O’Connell and others, 2005).

The overall paleoseismic evidence suggests that damaging earthquakes in the Blue River and Williams Fork valleys, 

although possible, are considered to be unlikely in the next 

few hundred years. 

Figure 25. Large trench and berm built above houses in Vail to catch rocks falling from cliffs 

above. Photograph by K.S. Kellogg, 2002.
42 Geologic Map of the Eastern Half of the Vail 30’ x 60’ Quadrangle, Colorado

Elevated radon

Most of Colorado has elevated radon values compared 

to other parts of the country (Otton and others, 1993). One 

in three homes in Colorado has radon values greater than 4 

picocuries per liter (pCi/l), the maximum allowable value for 

household radon determined by the EPA; mitigating action is 

recommended for values higher than 4 pCi/l (Environmental 

Protection Agency, 1993), because elevated radon in homes 

increases the risk for contracting lung cancer. Granite and 

felsic gneiss are relatively radiogenic compared to most other 

rocks, so surficial units, such as alluvium (Qa), till (Qtp,

Qtb), and other deposits derived from felsic Proterozoic 

bedrock (and locally Qdf and Qc) may have elevated radon 

values (Otton and others, 1993). The hazard increases with 

increased permeability, so younger, less weathered surficial 

units may have higher radon risk. Testing for radon is relatively easy and inexpensive, and steps can be taken to mitigate 

the hazard (Environmental Protection Agency, 1993).

Snow Avalanches

Snow avalanches may occur where (1) slopes are steeper 

than about 25o (90 percent of avalanches are on slopes 

between 30o–45o), (2) snow accumulates to a sufficient depth, 

(3) a weak layer or layers develop at depth in the snow, and 

(4) a trigger initiates the snowslide (Colorado Avalanche 

Information Center, 2008). Prevailing westerly winds cause 

cornices and snowpack to accumulate mostly on the eastern 

sides of ridges and, thus, most avalanches occur on this side. 

Triggers might be a skier, snowmobile, animal, or a sonic 

boom, but most avalanches are caused simply by the weight 

of accumulated snow; avalanches occur when shear stress 

exceeds shear strength along a weak snow layer. Backcountry 

winter travelers need to be particularly aware of the indications of high avalanche danger. Avalanche tracks or run-out 

zones are commonly devoid of large trees and may be littered 

with broken logs and tree limbs. However, tracks that have 

not run for many decades may be overgrown with aspen and 

conifers, which may obscure evidence for potential avalanche 

hazards. Although too small to map, debris cones and aprons, 

somewhat similar to fan deposits (Qfo and Qfy), composed 

of unsorted and unstratified rock and minor wood fragments, 

commonly lie at the base of avalanche tracks. In particular, 

many of the fan deposits along the south side of Gore Creek 

are avalanche runout zones (Mears, 1979); building has been 

prohibited in most of these runout zones on the valley bottom 

in and near avalanche runout zones. Large avalanches may 

cross a valley and move hundreds of meters up the opposite 

valley side. During times of unusually high snowfall, avalanches may run into mature forests, uprooting and shearing 

off trees and creating new avalanche tracks.

Acknowledgments

Outstanding reviews by J.C. Reed, Jr., and M.C. Reheis 

(both with the U.S. Geological Survey, Denver, Colo.) 

significantly improved the map and explanatory text. Proprietary LIDAR data generously provided by L.W. Anderson and 

L.A. Piety (U.S. Bureau of Reclamation) proved invaluable 

for mapping surficial deposits, including glacial deposits, 

landslides, and particularly subtle scarps that may represent 

Holocene movement along the Blue River normal fault. 

R.M. Kirkham and K.J. Houck, contract geologists with the 

Colorado Geological Survey, kindly shared unpublished 

mapping for the Minturn 7 1⁄2 minute quadrangle.

Credits

Karl Kellogg wrote most of the report, compiled previous 

geologic mapping where necessary, and completed a considerable amount of new mapping (see index to sources of geological data). R.R. Shroba wrote the section on late Tertiary 

(Neogene) and Quaternary history and some of the map-unit 

descriptions for Quaternary and Neogene deposits, revised 

some of the text, and mapped the Quaternary and Neogene 

deposits in the Battle Mountain and Sylvan Lake 71⁄2-minute 

quadrangles. Except where indicated, all uranium-lead age 

analyses of Proterozoic rocks were performed by W.R. Premo 

using the SHRIMP facility at Stanford University, Menlo Park, 

Calif. Much earlier mapping in the Gore Range was performed 

by Bruce Bryant, who also collected many of the samples for 

which U-Pb zircon ages were measured.
References 43

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Publishing support provided by: 

Denver Publishing Service Center

Editing and map layout by Lisa J. Binder

Pamphlet layout by Mary A. Berger

For more information concerning this publication, 

 contact:

Center Director, USGS Geology and Environmental 

Change Science Center

Box 25046, Mail Stop 980

Denver, CO 80225

(303) 236-5344

Or visit the Geology and Environmental Change 

 Science Center Web site at:

http://esp.cr.usgs.gov/

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