Geologic Map of the Bailey 30’ × 60’ Quadrangle, 

North-Central Colorado

By Chester A. Ruleman, Robert G. Bohannon, Bruce Bryant, Ralph R. Shroba, and Wayne R. 

Premo

Pamphlet to accompany

Scientific Investigations Map 3156

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: 

Ruleman, C.A., Bohannon, R.G., Bryant, Bruce, Shroba, R.R., and Premo, W.R., 2011, Geologic map of the Bailey 30’ x 

60’ quadrangle, North-Central Colorado: U.S. Geological Survey Scientific Investigations Map 3156, scale 1:100,000, 

pamphlet, 38 p.

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ISBN 978-1-4113-3125-9
Contents

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

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

Artificial-Fill Deposits..................................................................................................................2

Alluvial Deposits...........................................................................................................................2

Water-Laid and Lacustrine Deposits........................................................................................4

Alluvial and Mass-Movement Deposits...................................................................................5

Mass-Movement and Glacial Deposits....................................................................................5

Glacial Deposits ...........................................................................................................................7

Eolian Deposits.............................................................................................................................8

Tertiary Sedimentary Rocks of the Front Range..............................................................................8

Tertiary Sedimentary and Volcanic Rocks of South Park ..............................................................9

Laramide and Post-Laramide Sedimentary and Volcanic Rocks of South Park and the

 Front Range.....................................................................................................................................11

Pre-Laramide Sedimentary Rocks of the Denver Basin and South Park ..................................13

Ordovician(?), Cambrian(?), or Neoproterozoic(?) Intrusive Rocks............................................17 

Proterozoic Intrusive Rocks..............................................................................................................17

Rocks of the Pikes Peak Batholith and Related Rocks of the Redskin Stock

 (Mesoproterozoic) .........................................................................................................................18

Rocks of the Pikes Peak Batholith (Mesoproterozoic).................................................................19

Intrusive Rocks of the Berthoud Plutonic Suite (Mesoproterozoic) ..........................................20

Intrusive Rocks of Uncertain Age (Mesoproterozoic and Paleoproterozoic) ..........................20

Intrusive Rocks of the Routt Plutonic Suite (Paleoproterozoic)..................................................21

Metasedimentary and Metaigneous Rocks (Paleoproterozoic).................................................22

Geologic History of the Bailey Quadrangle .............................................................................................23

Proterozoic History.............................................................................................................................23

Paleozoic and Pre-Laramide Mesozoic History ............................................................................24

The Laramide Orogeny.......................................................................................................................25

Post-Laramide Cenozoic History in the Bailey Quadrangle.........................................................27

East Side of Bailey Quadrangle...............................................................................................27

West Side of Bailey Quadrangle .............................................................................................28

Quaternary ..................................................................................................................................28

Acknowledgments.......................................................................................................................................31 

References Cited..........................................................................................................................................32

Figure

1. Structural setting of South Park in relation to the Elkhorn and Williams Range faults

 and the Ancestral Rockies ..................................................................................................................26

Table

 1. Correlation of alluvial deposits along the western margin of the Colorado Piedmont ......30
iv

Era Period or Epoch Age

 Subperiod

Divisions of the Cenozoic, Mesozoic, Paleozoic, and Proterozoic eras used in this report.

Ages of time boundaries are those of the U.S. Geological Survey Geologic Names Committee (2007) except those for the late-middle Pleistocene boundary and

middle-early Pleistocene boundary, which are those of Richmond and Fullerton (1986a). Ages are expressed in ka for kilo-annum (thousand years) and Ma for

mega-annum (million years).

 Holocene 0–11.5 ka 

Quaternary late 11.6 –132 ka 

 Pleistocene middle 133–788 ka

 early 789 ka–1.81 Ma

 Pliocene 1.82–5.33 Ma

Neogene

 Miocene 5.34–23.0 Ma

 Oligocene 23.1–33.9 Ma

Paleogene Eocene 34.0–55.8 Ma

 Paleocene 55.9–65.5 Ma

 Late 65.6–99.6 Ma

Cretaceous

 Early 99.7–145.5 Ma

 Late 145.6–161.2 Ma

Jurassic Middle 161.3–175.6 Ma

 Early 175.7–199.6 Ma

 Late 199.7–228.0 Ma

 Triassic Middle 228.1–245.0 Ma

 Early 245.1–251.0 Ma

 Lopingian 251.1–260.4 Ma

Permian Guadalupian 260.5–270.6

 Cisuralian 270.7–299.0 Ma

 Upper/Late 299.1–306.5 Ma

Pennsylvanian Middle 306.6–311.7 Ma

 Lower/Early 311.8–318.1 Ma

Mississippian 318.2–359.2 Ma

Devonian 359.3–416.0 Ma

Silurian 416.1–443.7 Ma (Absent in map area)

Ordovician 443.8–488.3 Ma

Cambrian 488.4–542.0 Ma

 542.1 –1000.0 Ma

 1000.0–1600.0 Ma

 

 

CENOZOIC

MESOZOIC 

PALEOZOIC

NEOPROTEROZOIC

MESOPROTEROZOIC

PALEOPROTEROZOIC 1600.0–2500.0 Ma
Geologic Map of the Bailey 30’ × 60’ Quadrangle, 

North-Central Colorado

By Chester A. Ruleman, Robert G. Bohannon, Bruce Bryant, Ralph R. Shroba, and Wayne R. Premo

Surficial Deposits

Descriptions of surficial units along the eastern flank of the Front Range on this 

map are based on those of Scott (1963a), Trimble and Machette (1979), and Kellogg and 

others (2008). Within South Park, distribution and description of surficial units were 

derived from reports by Stark and others (1949), Widmann and others (2005), Kirkham 

and others (2006), and the authors’ new mapping and field observations. Formal names 

for fluvial and pediment deposits are those established by Scott (1960, 1963a) in the 

Colorado Piedmont. South Park glacial, glaciofluvial, and alluvial deposits are based on 

inferred age relationships among geomorphic position in the landscape, soil development, 

and reports by Widmann and others (2005, 2006), and Kirkham and others (2006).

Surficial units were mapped primarily by interpretation of 1:40,000-scale, blackand-white aerial photographs, USGS black-and-white digital ortho-photoquads (1-meter 

resolution), Landsat Thematic Mapper images, and field analyses of deposits and landforms. Age assignments for surficial deposits within the map area are based chiefly on 

the relative degree of modification of their original surface morphology, relative heights 

above present stream channels, and degree of soil development and clast weathering. 

Soil-horizon designations are based on those of the Soil Survey Staff (1999) and Birkeland (1999). Some of the surficial deposits contain pedogenic calcium carbonate. Stages 

of pedogenic calcium carbonate morphology (referred to as stages I through IV) in Bk 

and K soil horizons follow the classification from Gile and others (1966) and Machette 

(1985). 

Grain sizes within unit descriptions follow those of the Soil Survey Staff (1951) and 

are as follows: sand (0.05–2 mm), silt (0.002–0.05 mm), and clay (<0.002 mm). Grain 

sizes are limited to field estimates. The term “clasts” is defined as granules and larger 

particles (>2 mm in diameter, visible without magnification), and the term “matrix” 

refers to sand and finer particles (≤ 2 mm in diameter).

Many of the surficial deposits are poorly exposed. Deposits that are of limited 

extent (commonly less than about 50 m wide) were not mapped, including 1) fill material 

in developed areas, 2) mine-and-mill waste and dredge tailings in mining areas, 3) thin 

mass-movement deposits above treeline (such as block fields and block streams), and 4) 

thin sheetwash deposits that locally mantle gently sloping map units. 

In addition to the relative age of deposits based on erosional, geomorphic, and 

weathering characteristics, we have inferred ages for some of the surficial units based on 

the marine oxygen isotope records of Lisiecki and Raymo (2005). These records reflect 

global ice volumes (for example, Shackleton and Opdyke, 1973, 1976). However, ice 

volume changes on continents may supercede oxygen isotope variation in marine cores 

by 500–3,000 years (Mix and Ruddiman, 1984).

Description of Map Units
2 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

Artificial-Fill Deposits

mw Mine waste (latest Holocene)—Unconsolidated to loosely compacted rubble 

composed of silt, sand, rock, waste rock, and locally deeply weathered 

mining equipment derived from mining operations in the Alma district. 

Angular rock fragments are primarily composed of a mixture of altered 

Proterozoic igneous and metamorphic rocks and Tertiary igneous rocks. 

Estimated thickness several meters to 10 m 

Alluvial Deposits

Qa Post-Piney Creek alluvium and Piney Creek Alluvium, undivided and 

youngest alluvium in mountain valleys and South Park (Holocene 

and late Pleistocene?)—Interbedded silt, sand, and gravel along major 

streams, tributaries, and washes. Unit is commonly coarser grained 

and cobbly proximal to source areas and mountain-piedmont junctions. 

Post-Piney Creek alluvium commonly lies within stream channels inset 

into Piney Creek Alluvium and consists primarily of silty to sandy 

overbank deposits and slightly coarser, pebbly sand channel deposits. 

Soils on post-Piney Creek alluvium commonly are very weakly 

developed and have A/Cu profiles. Piney Creek Alluvium commonly 

consists of sandy silt, silty sand, lenses of silty clay, and detrital organic 

matter in upper part, and mostly sand and lenses of pebble gravel 

in lower part. Unit forms one or two terraces less than 5 m above 

present streams, and locally forms alluvial fans and sheetwash aprons 

along margins of major valleys. Soils on Piney Creek Alluvium are 

slightly better developed. They commonly have A/Bw/C profiles and 

locally may contain weak stage I carbonate morphology. Piney Creek 

Alluvium forms much of the present floodplains with post-Piney Creek 

alluvium being the younger inset deposits that are more susceptible to 

seasonal flooding.

 Charcoal collected from an archeological site in Piney Creek 

Alluvium near Kassler, Colo. yielded an age of about 1,500 14C yr 

B.P. (carbon-14 years before present; Scott, 1962, 1963a). Radiocarbon ages of organic material in Piney Creek Alluvium near Colorado 

Springs range from about 1.1 to 2.2 ka (Madole and others, 2005). 

Unit may locally contain pre-Piney Creek alluvium (Scott, 1960, 1962, 

1963a). Thickness about 1–6 m 

Qay Young alluvium (late Pleistocene)—Beds and lenses of silt, sand, and pebbleto cobble-sized gravel deposited chiefly during Pinedale glaciation 

[oxygen isotope stage 2, 14-24 ka, (Lisiecki and Raymo, 2005]. Within 

South Park near the Mosquito Range, unit consists of glacial outwash 

composed chiefly of cobbly pebble gravel that underlies terraces less 

than 5 m above floodplains in major mountain valleys near and below 

the outer limit of glaciation. Unit may also contain younger alluvial fan 

and sheetwash deposits grading to top of unit. Depositional surface of 

deposits has well-expressed bar-and-swale morphology with less than 

1 m of relief. Gravel clasts are subrounded to well rounded and are 

well sorted. Soils formed in top of unit have thin (<20 cm) Bt horizons 

and commonly have A/Bw/C profiles. Locally they have Bk horizons 

with stage I to weak stage II carbonate morphology. Top of unit Qay is 

commonly inset into units Qao and Qai as much as 15 m below top of 

units Qai and Qao. Maximum thickness <10 m

Qai Intermediate alluvium (late middle? Pleistocene)—Moderately sorted to 

well-sorted sandy pebble-sized gravel and, locally, cobbly to bouldery 

gravels deposited during Bull Lake glaciation, which occurred about 

120–170 ka (Schildgen and others, 2002; Sharp and others, 2003; 
Description of Map Units 3

Pierce, 2004) [marine oxygen isotope stage 6, 130–150 ka (Lisiecki 

and Raymo, 2005)]. In South Park, Qai underlies outwash terraces 

grading to Bull Lake moraine complexes. Terraces are as much as 15 

m above present stream channels. Terrace treads have a smooth surface 

commonly underlain by eolian silt of variable thickness. Unit Qai

also includes alluvium of non-glacial origin. Soils formed in this unit 

commonly have a Bt horizon with blocky to prismatic structure and 

reddish hues. Pedogenic Bk horizons with stage II to III morphology 

are locally present. Thickness 3–10 m 

Qs Slocum Alluvium (middle Pleistocene)—Mapped only in northeast corner 

of quadrangle. Well-stratified, carbonate-cemented, pebbly to cobbly 

gravels and coarse sand fining to east of mountain front. Upper part of 

unit commonly consists of about 1-m-thick silty clay and probably is 

of eolian origin (Scott, 1963a). Clasts are deeply weathered and easily 

broken. Unit primarily consists of pediment and cut-and-fill terrace 

deposits about 20–30 m above floodplains. Madole (1991) assigned 

an age of 240 ka, based on a uranium-series age of 190 ± 50 ka near 

Canon City, Colo.; however, deposits of unit Qs may be as old as about 

390 ka (Kellogg and others, 2008, table 1). Thickness commonly less 

than 8 m

Qv Verdos Alluvium (middle Pleistocene)—Unit consists of two parts. Lower 

part is a well-stratified, boulder-to-pebble gravel grading upward 

into a coarse sand, with pebbles and cobbles; boulders are more 

common near Front Range mountain front. Planar cross-stratification 

is dominant bedform representing sheetflood deposits and deposition 

of sand as flow regimes waned (Scott, 1963a). Unit contains a silty 

clay cap up to 1 m in thickness of probable eolian origin (Reheis, 

1980). Soils formed in upper part of unit have A/Bt/Bk/K/Bk profiles 

with stage II–III and locally stage IV carbonate morphology. Most 

clasts are deeply weathered. Underlies gently sloping surfaces and 

consists of cut-and-fill terrace and pediment deposits about 55–75 m 

above modern floodplains. Contains beds and lenses of white, fine-tomedium sand, water-laid Lava Creek B tephra (about 640 ka; Lanphere 

and others, 2002). Tephra is locally present at base of older pediment 

deposits of unit Qv (Scott, 1963a), but is also present within or at top 

of older main-stream fluvial deposits of unit Qv (Hunt, 1954; Scott, 

1972; Baker, 1973; Van Horn, 1976; Kirkham, 1977). Based on age 

and stratigraphic position of the Lava Creek B tephra, older deposits of 

unit Qv have been assigned an age of about 640 ka; however, younger 

deposits of Verdos Alluvium may be 410–475 ka (Kellogg and others, 

2008, table 1). Thickness may be as much as 12 m underlying terrace 

treads; locally can be less than 1 m where deposits form a pavement of 

cobbles and boulders (Scott, 1963a)

Qao Old alluvium (middle and early? Pleistocene)—Poor to moderately sorted, 

sandy pebble- to boulder-sized gravel interbedded with red, yellow, 

and light-brown micaceous clay and sand. Unit is generally deposited 

on gently sloping aprons adjacent to questas, hogbacks, bedrock-cored 

upland surfaces, and modern drainage divides. Unit mantles pediment 

surfaces. Clasts are sub-angular and sub-rounded. Most are composed 

of Proterozoic igneous and metamorphic rocks and Tertiary volcanic 

rocks. Unit locally contains abundant chalcedony clasts, probably from 

the Maroon Formation (Kirkham and others, 2006). Locally gravel has 

an eolian-silt matrix (>20 percent). Many clasts exposed on surface 

show well-developed angular facets from eolian abrasion and are commonly deeply weathered. Most bedrock units beneath unit Qao are 

deeply weathered so that alluvium-bedrock contact is obscured. Soils 
4 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

formed in unit are similar to those formed in unit Qai, but the former 

have better developed carbonate morphology characterized by strong 

stage II to stage III Bk horizons, and locally K horizons with stage IV 

morphology. Unit underlies four sub-units of Como surface of Stark 

and others (1949). Deposit beneath youngest (lowest) surface has been 

correlated with Slocum Alluvium (Qs) in Colorado (Kratochvil, 1978). 

Unit Qao also includes unit Qa4 of Widmann and others (2005) and 

Kirkham and others (2006). Within Sulphur Mountain and Hartsel 

7.5-minute quadrangles, it underlies two surfaces topographically 

higher than middle Pleistocene deposits, indicating initial degradation and planation of older pre-glacial Neogene geomorphic surfaces. 

Exposed thickness averages 4–6 m

QNr Rocky Flats Alluvium (early Pleistocene and late Pliocene?)—Massive 

to poorly stratified, reddish-brown, boulder-to-pebbly coarse sand, 

silty sand, and sandy clay. Unit is locally overlain by a thin mantle 

of silt and silty clay, possibly of eolian origin (Reheis, 1980; Shroba 

and Carrara, 1996). Near its type locality at Rocky Flats west of 

Denver, unit forms pediment veneer and paleovalley-fill deposits 

(Knepper, 2005). Many granitic clasts are deeply weathered. Soils 

commonly have an A/Bt/K profile with stage III to IV carbonate 

morphology overlying a soil with a Btk/K/Btk profile with stage 

I and stage III carbonate morphology. Dethier and others (2001) 

reported cosmogenic ages (10Be) for Rocky Flats Alluvium of about 

1.5 Ma. Soil characteristics and paleomagnetic data indicate an 

age of 1.4–1.6 Ma (Birkeland and others, 1996). The presence of 

buried soils suggests that the lower part of unit could be much older, 

possibly 2 Ma (Birkeland and others, 2003). Based on cosmogenic 

dating (10Be and 26Al), Riihimaki and others (2006) concluded that 

incision and dissection of the Rocky Flats surface began between 1–2 

Ma. Accordant beveled tops of Fountain Formation (P*f) and Dakota 

Group (Kd) hogback are considered to be remnants of the Rocky Flats 

surface (Scott, 1963a). Rocky Flats surface generally is 120–125 m 

above modern streams and low floodplains. North of quadrangle and 

south of Roxborough Park, surface can be projected into the Front 

Range to an elevation of 6,850 feet (Scott, 1963a). Thickness can be 

as much as 12 m near mountain front and less than 3 m near eastern 

boundary of map area, due in part to erosion

Water-Laid and Lacustrine Deposits

Qbc Bog clay and peat (Holocene and late Pleistocene)—Mapped only in northeastern part of map in small depressions. Light-gray to dark-gray and 

dark-brown clay and silty clay with interbedded peat layers. Deposits 

are present in deflation basins along western margin of the Colorado 

Piedmont. Scott (1963a) noted two other possible origins for the 

deposits, being 1) mammalian-generated erosion by stamping and wallowing, and 2) damming of stream channels by younger loess deposits 

(unit Qlo). Estimated thickness 2–5 m 

Qpl Playa deposits (Holocene and late Pleistocene)—Brown to light-brown and 

white, stratified, fine-grained silt, clay, and sand deposited in ephemeral 

lakes or ponds and shallow depressions produced by eolian deflation, 

mammalian-generated erosion (stamping and wallowing), and (or) 

probable collapse of subsurface evaporite deposits within South Park. 

Unit is locally overlain and interbedded with thin lenses and beds of 

pebble gravel and silty sand deposited sheetwash from adjacent ridges 

and stream alluvium. Estimated thickness 2–6 m, but may be much 

thicker in center of depressions
 

Description of Map Units 5

Alluvial and Mass-Movement Deposits

Qsw Sheetwash deposits (Holocene and late Pleistocene)—Poorly to moderately 

sorted, granule to boulder-gravel clasts in a silty sand to silty clay 

matrix. Unit is more commonly composed of pebble to cobble gravels 

in a silty sand matrix. Deposition due to downslope movement of soil 

and rock material by running water that is generally not confined to 

channels. Unit locally contains alluvium and colluvium undivided 

(Qac), debris flow deposits, and stream alluvium (Qa). Pedogenic 

surface soils commonly have A/Cu A/Cox, or A/Bw/Cox profiles, with 

weak and thin (<200 cm) argillic (Bt) horizons being more uncommon. 

Thickness 1–3 m

Qac Alluvium and colluvium, undivided (Holocene and late Pleistocene)—Unit 

typically consists of poorly to moderately sorted, sheetwash-alluvium 

deposits, perennial-stream deposits, and locally interfingered with fan 

deposits. Genesis of deposits is generally ambiguous and relative ages 

cannot be inferred. Alluvial component of unit locally consists of 

poorly to moderately sorted, pebble to cobble gravels with a silty clay 

to silty sand matrix in stream channels. Colluvial component of unit 

is commonly composed of an unsorted to poorly sorted, pebble gravel 

with angular to subangular clasts in a silty sand matrix. Unit locally 

contains material derived from debris flows, hyper-concentrated flows, 

sheetwash and rock fall. Thickness 2–5 m

Qf Fan deposits (Holocene and late Pleistocene)—Poorly to moderately sorted, 

weakly to moderately stratified, pebble and cobble gravel that contains 

thin beds and lenses of pebbly, silty sand. Deposits locally contain 

boulders. Deposited chiefly by sediment-charged avulsing streams, and 

locally by sheetwash and debris flows. Unit chiefly forms fan-shaped 

deposits at bases of steep slopes and gullies, where runoff is concentrated and channelized. Surface morphology of deposits is characterized by bar-and-swale micro-topography of less than 1 m in amplitude 

and minor stream dissection. Soils formed in deposits typically have 

A/Cox to A/Bw/Cox profiles. Deposit locally contains colluvium, 

sheetwash, and debris-flow deposits. Thickness 1–10 m

Mass-Movement and Glacial Deposits

Qc Colluvium (Holocene to middle? Pleistocene)—Unconsolidated deposits 

composed of clay, silt, sand, and angular to subrounded clasts ranging 

in size from granules to large boulders. Unit is generally composed 

of material eroded from local bedrock and (or) deposits beneath or 

upslope of colluvial deposit. Modes of transport and landforms associated with this unit indicate gravitationally driven, slow, downslope 

movement induced by frost creep, solifluction, and other periglacial 

processes, along with sheetwash, landslide, debris flow, hyperconcentrated flow, and rock fall. Estimated thickness 3–20 m

Qdf Debris-flow deposits (Holocene to middle? Pleistocene)—Lobate, hummocky, and fan-shaped deposits consisting of granules to boulders 

within a silty sand, clayey silt, and finer matrix. Deposits can be 

weakly stratified depending on existence of pore-water pressure 

and rate of deposition. Levees are common along margins of flows. 

Margins of flows tend to thin out to minimal thickness as flows spread 

across adjacent surfaces. Unit locally includes minor stream-flow and 

hyperconcentrated-flow deposits. Thickness 1–10 m
6 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

Qls Landslide deposits (Holocene to late Pleistocene)—Unsorted and unstratified 

to poorly stratified debris on slopes or at base of slopes. Generally 

consists of angular to subrounded, pebble- to boulder-sized clasts 

of various lithologies in a silty clay to silty sand matrix. Deposits 

generally have a lobate form. Micro-topography is hummocky with 

localized undrained depressions and has local relief of as much as 5 m. 

Landslides and landslide deposits are commonly translational processes 

sliding on a subsurface plane sub-parallel to parallel to land surface and 

forming on slopes that are underlain by shale, siltstone, and claystone 

overlying the Dakota Group along Front Range hogback belt or Dakota 

Sandstone in South Park (Kd). Unit locally includes deposits of 

rotational rock slides, rotational earth slides, and earth flows as defined 

by Varnes and Cruden (1996). Soils formed in deposits commonly 

have A/Cu, A/Cox, and A/Bw/Cox profiles. Landslide deposits 

are susceptible to reactivation due to natural processes (streams 

undercutting landslide toes or cuts excavated in toe of a landslide 

deposit) and human activities. Unit may locally contain thin sheetwash 

deposits. Estimated thickness 2–30 m

Qlso Older landslide deposits (late and middle? Pleistocene)—Generally 

characterized as unsorted to poorly sorted, unstratified to weakly 

stratified pebble- to boulder-sized clasts in a silty clay to silty sand 

matrix. Overall surficial expression and micro-topography of deposits 

is much more subdued than unit Qls. Soils formed in these deposits 

commonly have Bt horizons that are weakly to strongly developed 

blocky and prismatic structure. Thickness 3–30 m

Qt Talus deposits (Holocene to middle? Pleistocene)—Crudely sorted, angular 

rock fragments ranging from a few centimeters to several meters in 

diameter deposited along base of steep slopes and prominent cliffs as 

aprons, cones, fans, or pro-talus ramparts. Mechanisms of transport 

and deposition of unit includes rock fall, rock slide, and rock and 

snow avalanches. Locally contains debris-flow deposits where surface 

runoff has saturated and remobilized deposits. Tertiary intrusive rocks, 

Paleozoic sedimentary rocks, and Dakota Sandstone (Kd) are the main 

sources for talus deposits within region (Widmann and others, 2005), 

but in the Platte River, Kenosha, and Tarryall Mountains localized 

small deposits are composed of Proterozoic crystalline rocks. Thickness 2–10 m

Qrg Rock-glacier deposits (Holocene and latest Pleistocene)—Poorly sorted to 

unsorted, cobble- to boulder-sized, hummocky and lobate deposits 

along valley walls and on floors of alpine valleys. Unit consists of 

a veneer of matrix-free, angular cobbles and boulders that mantle a 

thicker mass of rock rubble containing a matrix of finer rock fragments. 

Lower part of unit locally contains interstitial ice, ice lenses, or an ice 

core. Valley walls commonly support lobate-shaped rock glaciers, 

whereas valley floors commonly contain tongue-shaped rock glaciers 

(Widmann and others, 2005). Rock fragments on and within rockglacier deposits are primarily derived from steep slopes and upslope 

cliffs chiefly by rockfall and locally by rock slide and avalanche. 

Orientations of curvilinear ridges and gullies on rock-glacier deposits 

indicate downslope motion. Most of the rock-glacier deposits in 

Colorado are of latest Pleistocene or early Holocene age (Meierding 

and Birkeland, 1980), but mantle relict ice bodies formed during 

former glacial episodes. Active rock glaciers in Front Range, north of 

Bailey quadrangle, have estimated rates of movement of 1–10 cm/yr 

(White, 1971, 1976). Thickness 20–50 m
Description of Map Units 7

Glacial Deposits

[Glacial deposits primarily consist of non-sorted and non-stratified till deposited by 

ice, as well as deposits of stratified sand and pebble gravel (stratified drift) deposited by 

meltwater streams and mass-movement deposits]

Qtp Till of Pinedale age (late Pleistocene)—Nonsorted and nonstratified, subangular to rounded boulders to granules in a silty sand matrix. Unit locally 

contains beds and lenses of stratified silty sand and pebble gravel. Surface morphology of deposit is characterized by partially buried large 

boulders surrounding depressions filled with beds and lenses of silt, 

sand, and clayey silt; fully exposed sub-angular to rounded cobbles are 

abundant on surface. Sharp crests of lateral moraines show little modification by erosion. Undrained depressions are common and locally are 

closely enough spaced to produce knob-and-kettle topography. Surface 

soils have A/Cox, A/Bw/Cox, and A/Bt/Cox profiles on moraine crests. 

Cambic (Bw) and weak argillic (Bt) horizons probably contain about 

1–5 percent more clay than underlying till (<2 mm-size fraction). Most 

of biotite-rich granite and gneiss clasts within soil are unweathered, 

and disintegrated clasts are rare. Unit locally may include till of Bull 

Lake and pre-Bull Lake age. Previous studies have found radiocarbon 

(14C) and cosmogenic-exposure ages of 12–30 ka for till of unit Qtp

(Nelson and others, 1979; Madole, 1986; Schildgen and Dethier, 2000; 

Benson and others, 2004, 2005). Estimated thickness 2–40 m 

Qtb Till of Bull Lake age (late middle Pleistocene)—Nonsorted and nonstratified, 

subangular to rounded boulders to granules in a silty sand matrix. 

Landforms underlain by unit Qtb are commonly rounded and more 

subdued than those formed by unit Qtp and are generally present 

further downvalley than unit Qtp (for example, Jefferson Creek in 

South Park). Surface morphology is characterized by very few exposed 

boulders, subdued, hummocky topography with gentle slopes, and 

generally no undrained depressions. Bull Lake age till is generally 

overlain by a thin loess mantle (<1 m) and generally supports more 

vegetation than Pinedale age till. Clasts in Bull Lake age till are more 

deeply weathered than those in till of Pinedale age, so that biotite-rich 

crystalline rocks can be easily disaggregated with a hammer or shovel. 

Pedogenic soils on surface of unit Qtb have A/Bt/Cox profiles and are 

thicker and more oxidized. Argillic (Bt) horizons have strong blocky, 

prismatic structure and probably contain at least 5 percent more clay 

than underlying till (<2 mm size fraction). North of map area near 

Nederland, cosmogenic 10Be and 26Al dating yields minimum ages 

for surface boulders on till of Bull Lake age of 101 ± 21 and 122 ± 

26 ka (Schildgen and others, 2002). Uranium-trend age estimates for 

Bull Lake age till near Allenspark, of 130 ± 40 ka, support these data 

(Shroba and others, 1983). K-Ar and 230Th/U analyses from Wind 

River Range and West Yellowstone, respectively, suggest that the onset 

of Bull Lake glaciation was before 167 ± 6.4 ka (possibly 190 ka) 

and culminated at about 122 ± 10 ka (Sharp and others, 2003; Pierce, 

2004). Estimated thickness 20–30 m

Qti Till of Pinedale, Bull Lake, and pre-Bull Lake age, undivided (late and 

middle Pleistocene)—Nonsorted and nonstratified, subangular to 

rounded boulders to granules in a silty sand matrix. Unit is mapped 

in areas where deposits of units Qtp and Qtb are indistinguishable 

or where Qtp overlies considerable deposits of Qtb and may locally 

include till of pre-Bull Lake age. Glacial deposits of pre-Bull Lake 

age have been identified along Tarryall Creek in the northwest part of 

Bailey quadrangle by Widmann and others (2005). Thickness 3–30 m
8 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

Eolian Deposits

Qlo Loess (late and middle? Pleistocene)—Nonstratified, well-sorted, winddeposited sandy silt and locally sandy, clayey silt derived by wind 

erosion from flood plains and possibly Cretaceous bedrock sources by 

westerly or northwesterly paleowinds. Soils formed in upper part of 

unit have A/Bt/Btk/Bk profiles with stage I–II carbonate morphology 

(Kellogg and others, 2008). Loess overlies deposits as young as 

Louviers Alluvium (mapped to north on Denver West quadrangle), 

but locally overlies Broadway Alluvium (Qb) along South Platte 

River about 70 km northeast of map area (Lindvall, 1980). Thin (≤50 

cm) layers of pebbly, clayey, sandy silt that locally mantle Slocum 

Alluvium (Qs) and older alluviums (Qao) in the Colorado Piedmont 

may consist in part of loess that contains clasts from underlying 

gravelly alluvium. Deposits of sheetwash locally eroded from unit Qlo

contain a minor amount of coarse sand and granules and a few pebbles.

Loess in northeastern Colorado records two episodes of deposition 

at about 10–13 ka and 14–20 ka (Muhs and others, 1999). Holocene 

loess is locally extensive in eastern part of Colorado (Madole, 1995), 

but none has been recognized within or near map area. Some loess 

near Littleton, Colo., near northeast corner of map area (older loess 

of Scott, 1962, 1963a), may be as old as 120–170 ka and may be 

correlative with eolian silt and sand about 90 km northeast of map 

area that yielded thermoluminescence age estimates of about 150 ka 

(Forman and others, 1995). Pre-Bull Lake till near Allenspark, about 

25 km north of Nederland, is locally mantled by about 15 cm of loess 

(Madole and Shroba, 1979). Some of the closed depressions on till of 

Pinedale age (Qtp, Qti) may contain thin (≤50 cm) deposits of loess 

or silty sheetwash deposits derived chiefly from loess. Dust deflated 

from flood plains in the Colorado Piedmont has influenced properties 

of soils downwind of flood plains (Reheis, 1980). Alpine soils in 

and near cirques on east side of the Continental Divide, north of map 

area, contain eolian dust (Birkeland and others, 1987, 2003; Muhs and 

Benedict, 2006). Thickness about 1.5–3 m 

Tertiary Sedimentary Rocks of the Front Range

Tg Gravel deposits (Pliocene? and Miocene?)—Boulder gravel deposits 

mantling ridges along major drainages on Elkhorn surface southwest of 

Kenosha Pass. Deposits are composed of subrounded to well-rounded 

boulder gravel with a coarse sand matrix. Some of the coarse sand 

in matrix may be due to in situ deep weathering and dissaggregation 

of granitic boulders to grus. Some boulders exceed 2 m in diameter. 

Deposits within the Pine Junction-Pine region contain clasts of the 

Mount Evans batholith. Deposit preserved along Michigan Hill, 

near Jefferson, Colo., could possibly be correlative to Wagontongue 

Formation (unit Tw, Miocene) mapped within Antero Reservoir region. 

Estimated thickness 20–120 m 

Tdg Gravel at Divide (Miocene)—Cobble to boulder gravel interbedded with 

weakly stratified sand, silt, and clay. Unit contains large clasts of hornblende andesite, described by Wobus and Scott (1977) to be derived 

from Thirtynine Mile volcanic field, intermixed with smaller pebbles 

and cobbles from Pikes Peak batholith and Proterozoic metamorphic 

and igneous rocks. However, Temple and others (2007) noted that 

volcanic clasts could also have been derived from Signal Butte, about 
Description of Map Units 9

18 km to the west. Exposure within quadrangle is in extreme southeast 

corner. Thickness <50 m

Tertiary Sedimentary and Volcanic Rocks of South Park

Tw Wagontongue Formation (Miocene)—Interbedded pebble-cobble gravel, 

sandstone, and siltstone. Clasts are subrounded to well rounded and 

consist of Proterozoic, Paleozoic, and Tertiary lithologies. Age determined from a jawbone from an upper Miocene equid (Stark and others, 

1949). Deposit is preserved in a syncline northeast of Antero Reservoir 

filling a paleovalley cut into Wall Mountain Tuff (Twm) and Antero 

Formation (Ta). Approximate thickness 150 m

Ts Sedimentary rocks of Fairplay paleovalley (Oligocene)—Interbedded 

mudstone, pebble conglomerate, sandstone, pebbly sandstone, and 

thin (1–3 cm) beds of tuff. Mudstones are reddish brown with bed 

thickness commonly less than 1–5 m. Pebble conglomerates form 

as lenses filling alluvial channels within mudstone. Kirkham and 

others (2006) reported that these sedimentary rocks, which crop out 

approximately 6 km south of Fairplay, filled a paleovalley called the 

Fairplay paleovalley. This unit consists of upper and lower parts. 

The principal difference between lower and upper parts of unit is that 

pebble conglomerate in lower part consists of clasts derived from upper 

Paleozoic units and has bimodal rounded and angular clasts, while 

conglomerate in upper part has a higher percentage of subrounded 

clasts of Tertiary intrusive rocks and Proterozoic metamorphic 

and intrusive rocks (Kirkham and others, 2006). Based on fossil 

correlations and stratigraphic position, Brown (1940) correlated 

these rocks to upper member of Oligocene Antero Formation (Ta). 

Estimated thickness 250–300 m

Tt Tuff (Oligocene)—Mapped approximately 6 km south of Fairplay. Light-gray 

to gray, crystal-lithic tuff with a fine-grained matrix consisting of glass 

shards and few small biotite and feldspar crystals. Tuff is comprised 

of 10–20 percent pumice clasts and locally as much as 40 percent 

(Kirkham and others, 2006). Contains sparse angular to subangular 

rock fragments composed of sandstone, mudstone, and igneous 

rocks. Kirkham and others (2006) reported a 40Ar/39Ar age of 33.47 ± 

0.15 Ma from a pumice clast within tuff. Unit has been correlated to 

Antero Formation (Brown, 1940; De Voto, 1964; Bryant and others, 

1981b). However, this unit may be correlative with Badger Creek 

Tuff or Grizzly Peak Tuff (Shannon, 1988; Fridrich and others, 1991; 

McIntosh and Chapin, 2004). Kirkham and others (2006) noted that 

size (1–10 mm) of pumice clasts indicates that this unit is proximal to 

its source. Maximum thickness is approximately 100–110 m (Kirkham 

and others, 2006)

Ta Antero Formation (Oligocene)—The Antero Formation was formed as fossiliferous, freshwater lacustrine and low-energy fluvial deposits within a 

inter-montane basin formed to west and north of volcanic breccia complex of Thirtynine Mile Andesite (35–36.5 Ma) (McIntosh and Chapin, 

2004). Based on tuffaceous sediments, tuff beds, and ignimbrites, McIntosh and Chapin (2004) determined a 40Ar/39Ar age of 34.3–33.4 Ma 

for the Antero Formation. Formation intertongues with Badger Creek 

Tuff and contains volcanic rocks associated with Mount Aetna cauldron. Formation consists of three informal members. Upper member 

is composed of interbedded, pebble-cobble conglomerate and lenticular 

sandstone beds. Middle member consists of a basal fine-grained tuff 
10 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

overlain by interbedded, thin, discontinuous, lenticular beds of shale 

and tuff, with rare discontinuous layers of limestone. Stark and others 

(1949) noted that a limestone bed approximately 6-m thick is within 

the middle member and extends almost entirely around the ancientlake basin. Lowest member consists of discontinuous and lenticular 

beds of tuff, sandstone, lacustrine limestone, pebble conglomerate, 

and thin beds of fissile shale. Limestone, which makes up lowest part 

of section, forms a more continuous deposit of algal beds and mounds 

along edge of Lake Antero (Stark and others, 1949). Exposures within 

quadrangle are limited and generally less than 5 m thick. Stark and 

others (1949) measured a maximum thickness of approximately 610 m 

south of quadrangle

Ttc Tallahassee Creek Conglomerate (Oligocene)—Polylithic, boulder-cobble 

conglomerate containing Proterozoic, Paleozoic, and Mesozoic clasts. 

Contains beds of tuff along Tarryall Creek. Maximum thickness about 

10 m 

Twm Wall Mountain Tuff (Eocene)—Gray, purplish-gray, dark-pink, and grayishpurple rhyolite ash-flow tuffs. Unit is locally vitric. Contains phenocrysts (<1 cm in length) of plagioclase, biotite, and sanidine. McIntosh 

and Chapin (2004) dated Wall Mountain Tuff at 36.69 ± 0.09 Ma by 

40Ar/39Ar. Weathers reddish brown to yellowish buff; resistant and 

well exposed in most places forming cliffs and ledges. Moderately to 

densely welded with black vitrophyre common. Mapped along crest of 

strike ridges northeast of Hartsel and within south-central South Park. 

Flow layering is common, but very irregular in orientation and not parallel or only sub-parallel to base of unit, suggesting post-depositional 

flowage. Thickness about 30 m

Tan Andesitic flow breccia, ash-flow tuff, and related intrusive rocks 

(Eocene)—Quartz-bearing, hornblende-biotite and biotite andesite 

flow breccia and ash-flow tuff mapped by Bryant and others (1981a), 

who after chemical analyses called it a rhyodacite (Bryant and others, 

1981b). These rocks are exposed within a paleovalley approximately 

2 km southeast of Kenosha Pass, beneath Wall Mountain Tuff (Twm) 

just northwest of Hartsel, and associated with a volcanic neck at 

Signal Butte in southeast part of quadrangle (Goss, 1985). Bryant 

and others (1981b) obtained a K-Ar age of 37.8 ± 0.9 Ma from biotite 

from flow beneath Wall Mountain Tuff and a zircon fission-track age 

of 36.4 ± 1.9 Ma for flow southeast of Kenosha Pass. They correlated 

the flow preserved in paleovalley with rocks of similar age derived 

from Montezuma stock or other nearby intrusive bodies. Signal Butte 

is a porphyritic hornblende andesite, emplaced as a series of four 

hypabyssal plugs (Goss, 1985) and has a sphene fission-track age of 

43.3 ± 3.9 Ma (Marvin and others, 1974) and a zircon fission-track age 

of 42.2 ± 2.1 Ma (Naeser, unpub, data). Thickness of paleovalley flow 

is approximately 100 m

Tpb Biotite quartz latite porphyry (Eocene)—Gray to light yellowish-gray latite 

porphyry of Baker Lake and Jefferson Hill. Unit consists of equant, 

euhedral phenocrysts of sanidine, andesine, and biotite in an aphanitic 

felsic matrix. Bryant and others (1981b) report a zircon fission-track 

age of 37.6 ± 1.5 Ma, showing time of cooling and crystallization. 

Unit is resistant, forming spheroidal outcrops and tors

Tqp Monzogranite of Boreas Mountain, Mount Guyot, and Bald Mountain 

(Eocene)—Gray to light yellow-brown porphyritic monzogranite 

of Bald Mountain, Montgomery Gulch, and Mount Guyot stocks, 

located in northwest part of quadrangle. Composition locally grades 

from granodiorite to monzogranite (Bryant and others, 1981b). K-Ar 
Description of Map Units 11

ages of biotite from Mount Guyot plug are 44.0 ± 1.5 Ma (Bryant and 

others, 1981b). Fission-track ages of zircon, sphene, and apatite range 

from 35.5 ± 1.4 Ma to 42.7 ± 8.2 Ma for rocks of this unit (Bryant and 

others, 1981b). Widman and others (2005) reported that cross-cutting 

relationships indicate that unit Tqp is younger than sills of unit Tmp

Tmp Monzogranite porphyry (Eocene)—Pale-green to light yellowish-brown 

porphyritic monzogranite with chloritized fracture surfaces. Unit 

composes sills and stocks within Montgomery Gulch-Mount 

Silverheels area. Sills appear to be contemporaneous to or younger 

than Montgomery stock in northwest part of quadrangle (Widmann and 

others, 2005). Rock consists of plagioclase, hornblende, and biotite 

phenocrysts in a light-gray to gray, aphanitic groundmass composed of 

plagioclase, quartz, biotite, augite, hypersthene, hornblende, magnetite, 

apatite, allanite, and zircon (Widman and others, 2005) 

Laramide and Post-Laramide Sedimentary and Volcanic Rocks of 

South Park and the Front Range

Txc Syntectonic conglomeratic unit (Paleocene)—Poorly sorted, subangular to 

subrounded, boulder- to cobble-sized conglomerate with interbedded 

coarse-grained sandy lenses ranging from 0.5–2.0 m in thickness. 

Most igneous clasts in unit are weathered. Clasts predominantly 

derived from unit Yg. Interpreted as a syntectonic conglomerate 

deposited coeval with displacement along the Elkhorn fault (Ruleman 

and Bohannon, 2008). Unit thickness variable, but generally less than 

100 m

South Park Formation (Paleocene and Upper Cretaceous)

Tsf Fine-grained arkosic member (Paleocene)—Interbedded and lenticular, pale 

pinkish-brown, greenish-gray, and gray, calcareous, mudstone, siltstone, silty sandstone, and conglomerate. Unit is highly friable, easily 

eroded and generally forms swales within South Park. Clasts primarily 

consist of Proterozoic igneous and metamorphic lithologies (Sawatzky, 

1967). At northern extent of exposure, the unit is in fault contact with 

Proterozoic rocks along Elkhorn fault. To the south, unit pinches out 

to where it unconformably overlies coarse, conglomerate member (unit 

Tsc) (Wyant and Barker, 1976). Bryant and others (1981b) report a 

biotite K-Ar age of 56.3 ± 1.3 Ma from a tuff bed in this unit. Thickness ranges from approximately 100 to 1,000 m (Barker and Wyant, 

1976; Wyant and Barker, 1976; Kirkham and others, 2006)

Tsl Link Spring Tuff Member (Paleocene)—Light yellowish-brown, light-gray 

to gray, laminated to thinly bedded tuff. Unit contains a few beds of 

brown and brownish-green volcaniclastic breccia and andesitic flows. 

Unit contains sparse leaf impressions (Wyant and Barker, 1976). 

Bryant and others (1981b) obtained a K-Ar age of 59.7 ± 2.0 Ma for 

Link Springs Tuff Member. Maximum thickness is approximately 

200 m

Tsc Conglomerate member (Paleocene)—Light yellowish-brown, brown, 

greenish-brown, and greenish-gray, conglomerate, silty sandstone, and 

mudstone. Bedding ranges from massive to thick (>1 m) lenticular 

beds. Weakly to moderately lithified and forms prominent strike ridges 

throughout South Park (Ruleman and Bohannon, 2008). Locally 

contains abundant silicified wood fragments and debris, which become 

less abundant to the west. Kirkham and others (2006) report that unit 

coarsens and thickens to west. Johnson (1935) noted that crossbeds 
12 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

within unit dip to the east indicating a source area to the west. Clasts 

consist of Paleozoic gray quartzite, chert-pebble conglomerate, red silty 

sandstone, and Cretaceous and Paleocene hypabyssal igneous rocks. 

Clast lithologies and ages indicate a west-northwest source area from 

the Mosquito Range as uplift and erosion occurred on the Sawatch 

anticlinorium (Wyant and Barker, 1976; De Voto, 1988; Kirkham 

and others, 2006). There is a distinct basal cobble conglomerate 

bed composed of quartzite and porphyritic igneous clasts. Bryant 

and others (1981b) obtained a K-Ar age of 65.5 ± 1.6 Ma on biotite 

from a tuff bed in unit Tsc. Kirkham and others (2006) obtained 

40Ar/39Ar ages of 66.6 ± 0.5 Ma and 66.4 ± 0.4 Ma on biotite from 

an intermediate to mafic volcanic clast and monzogranite clast, 

respectively. Kirkham and others (2006) noted that the lithology of a 

fine-grained igneous rock from Buckskin Gulch in the Mosquito Range 

having biotite with a K-Ar age of 63.6 ± 2.3 Ma (Bookstrom and 

others, 1987) was similar in composition to an igneous clast collected 

by Widmann and others (2005) from unit Tsc, having biotite dated by 40Ar/39Ar at 64.08 ± 0.11 Ma. This suggests that at least part of unit 

Tsc is younger than 64 Ma (Kirkham and others, 2006). Thickness 

ranges from approximately 440 to 1,700 m

TKsr Reinecker Ridge Volcanic Member (Paleocene and Upper Cretaceous)—

Consists of three parts. Upper and middle units are comprised of 

interstratified conglomerate and thin beds of light-gray to light 

yellowish-brown, crossbedded tuffaceous sandstone. Conglomerate is 

well cemented with iron oxide and consists of well-rounded cobbles 

in a tuffaceous sandy silt matrix. Clasts are comprised of various 

lithologies of extrusive, porphyritic rocks derived from flows of lower 

member. Lower part consists of dark-brown, dark-gray, greenish-gray, 

grayish-green, and purple porphyritic hornblende andesite flows and 

breccias. Widmann and others (2005) classified volcanic rocks as 

trachyandesite, andesite, and dacite. Flow breccias contain clasts of 

same composition as matrix (Kirkham and others, 2006). Sawatzky 

(1967) originally obtained a K-Ar age of 56 ± 2.6 Ma from a basal 

flow. However, Bryant and others (1981b) determined that sampled 

rocks were altered and yielded an erroneous young age. An 40Ar/39Ar 

age of 66.94 ± 0.25 Ma on hornblende from lowest exposed flow 

in Como 7.5-minute quadrangle considered to be minimum age of 

these flows (Widmann and others, 2005). Kirkham and others (2006) 

obtained an 40Ar/39Ar age of 68.8 ± 0.8 Ma from flows exposed within 

Fairplay East 7.5-minute quadrangle. Unit thickness is approximately 

100 to 300 m

Ksvs Lower volcaniclastic member (Upper Cretaceous)—Exposed in a small area 

on western slopes of Reineker Ridge, approximately 9 km southeast 

of Fairplay. It is composed of two lenses of volcaniclastic rocks 

(Kirkham and others, 2006). Unit consists of well-bedded, poorly 

sorted, medium- to coarse-grained, argillaceous, lithic sandstone 

ranging from reddish brown to greenish brown depending on the 

oxidation state of iron. Unconformable on Pierre Shale (Kp) (Kirkham 

and others, 2006). Thickness is approximately 140 m

TKda Dawson Arkose (Paleocene and Upper Cretaceous)—Gray, white, light 

yellowish-gray, reddish-brown, and light-brown, lenticular and 

crossbedded, conglomerate, sandstone, siltstone, and shale, grading 

from a basal conglomerate to an upper shale. Most clasts and grains 

are well rounded, loosely cemented, and have abundant pore space 

(Scott, 1963b). Leaf impressions are abundant within sandstone layers. 

Average thickness is approximately 450 m
Description of Map Units 13

Ka Thin Cretaceous Andesite Dikes (Upper Cretaceous?)—Thin, lens- to tabular-shaped bodies of grayish-brown, greenish-gray, to black andesite 

present in northeast part of quadrangle along South Platte River area. 

Generally weathers to brownish gray. Thickness ranges between <3 

cm to 3 m. Scott (1963b) notes that dikes intruding northwest-trending 

faults are sub-vertical and dikes intruding northeast-trending faults are 

sub-horizontal. He also notes that the andesite within the northwesttrending faults is intruded into fault gouge and brecciated by later faulting. These dikes may be associated with rocks eroded and deposited in 

Dawson Arkose (TKda) of the Denver Formation (Lovering, 1935)

Pre-Laramide Sedimentary Rocks of the Denver Basin

and South Park

Kl Laramie Formation (Upper Cretaceous)—Olive-gray to yellowish-brown 

shale; gray, medium-grained, sub-angular to sub-rounded, well-graded 

and well-sorted sandstone; reddish-brown chert-rich conglomerate; 

and lignitic coal beds. Sandstone is loosely cemented by both calcium 

carbonate and silica, and friable. Conglomerate is harder and more resistant. Yellow limonitic staining is common throughout formation and 

concentric iron concretions are common within sandstone layers. Lignitic coal beds range from 0.1–2 m in thickness. Formation contains 

abundant leaf impressions and wood fragments. Formation consists of 

fine-grained deposits of marine and fluvial origin apparently deposited 

within an estuarine environment. Thickness within Kassler 7.5-minute 

quadrangle approximately 200 m (Scott, 1963b) 

Kf Fox Hills Sandstone (Upper Cretaceous)—Subdivided into an upper 45–47 

m section consisting of olive-brown to light-gray friable sandstone, 

a middle, more resistant 8-m section of olive-gray to yellowish-gray, 

massive sandstone, and a lower section of soft, olive-brown sandy 

shale. Unit represents a sequence of nearshore trangressive and regressive phases between underlying, marine Pierre Shale (Kp) and overlying estuarine Laramie Formation (Kl). Locally, pelecypods are found 

within more resistant, middle sandstone. Thickness within Kassler 

7.5-minute quadrangle is estimated to be 56–60 m (Scott, 1963b) 

Kp Pierre Shale (Upper Cretaceous)—Consists of olive-gray and medium- to 

dark-gray, calcareous shale interbedded with few thin beds of finegrained, brownish-gray sandstone. Locally contains ironstone and 

limestone concretions. Prominent sandstone members are Hygiene 

Sandstone and Apache Creek Sandstone Members, which are possibly 

exposed along east side of upper Trout Creek (Kirkham and others, 

2006). Thin, bentonitic layers are common. Shale and bentonite 

beds are susceptible to swelling. Pierre Shale lies conformably above 

Niobrara Formation (Kn). In the Denver Basin, Scott (1963b) gave a 

total thickness of approximately 1,580 m. In South Park, estimated 

thicknesses range from about 820 m (Stark and others, 1949) to 

approximately 1,800 m (Barker and Wyant, 1976; Wyant and Barker, 

1976)

Kn Niobrara Formation (Upper Cretaceous)—Consists of Smoky Hill Shale 

Member (younger) and Fort Hays Limestone Member, which are 

not mapped separately. Smoky Hill Shale Member consists of pale 

to yellowish-brown, soft, thin-bedded, platy calcareous shale and 

interbedded thin limestone layers. Total thickness of 125 m and 

mapped together, herein. Weathers pale gray to white and contains 

many bentonite beds, forming a distinctive yellow soil (Widmann and 
14 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

others, 2005). Fort Hays Limestone Member is a gray, dense, micritic 

limestone with individual beds as thick as 2 m. Contains abundant 

large inoceramid bivalve (oyster) fossils in Denver Basin. Thickness of 

Fort Hays Limestone Member is about 105 m in South Park (Widmann 

and others, 2005)

Kb Benton Group (Upper Cretaceous)—In Denver Basin, consists of three 

formations in descending order: Carlile Shale, Greenhorn Limestone, 

and Graneros. These formations are not mapped separately. Carlile 

Shale consists of three members in descending order: grayishbrown, hard calcarenite containing abundant shell fragments (Juana 

Lopez Member); gray silty sandstone (Blue Hill Member); and 

yellowish-gray, soft calcareous shale (Fairport Member). The Carlile 

conformably overlies the Greenhorn Limestone, which in upper part 

consists of a gray, dense limestone and hard calcareous shale (Bridge 

Creek Member); middle part of gray, shaly calcareous sandstone 

(Hartland Shale Member); and lower part of grayish-brown, thin beds 

of hard calcareous sandstone and shale with a marker bentonite bed at 

base (Lincoln Member). Graneros Shale consists of dark-gray, hard, 

clayey shale and siltstone. 

In South Park, exposure of Benton Group is limited. It generally 

forms topographic swales and hummocky, slumped surfaces. Stark 

and others (1949) reported thick bentonite beds (0.5 m thick) within 

South Park region proximal to Reinecker Ridge. Widmann and others 

(2005) noted that shale is typically much darker colored than overlying 

Pierre Shale (Kp). In Kassler 7.5-minute quadrangle, Benton Group is 

approximately 182 m thick (Scott, 1963b), and in South Park thickness 

is approximately 75 m (Widmann and others, 2005) 

Knb Niobrara Formation and Benton Group, undivided (Upper Cretaceous)—

Mapped together in Sulphur Mountain vicinity due to limited and poor 

exposure. Mapping primarily in float and a few exposures in road cuts. 

Generally mantled by a thin veneer of surficial deposits too thin to 

map. Total thickness approximately 180 m

Ku Pierre Shale, Niobrara Formation, and Benton Group, undivided

(Upper Cretaceous)—Mapped together in South Park where exposure 

is poor or limited and contacts cannot be identified. Kirkham and 

others (2006) noted that areas where unit Ku is mapped are more likely 

underlain by more erosion-resistant Niobrara Formation (Kn). Total 

thickness approximately 2,030 m

Kd Dakota Group or Dakota Sandstone (Lower Cretaceous)—In Kassler 

area, this group consists of South Platte Formation and underlying 

Lytle Formation. South Platte Formation consists of five distinct 

units as described by Scott (1963b). From top to bottom they are 1) 

approximately 26 m of light-gray to medium-gray, tabular to massive, 

fine- to medium-grained, crossbedded sandstone; 2) a shale interval 

6 m thick with interbeds of gray to dark-gray and yellowish-gray, 

fine-grained sandstones; 3) Kassler Sandstone Member (Scott, 1963b) 

which consists of a basal chert and quartzite pebble conglomerate with 

overlying yellowish-gray to light yellowish-brown, fine- to coarsegrained, cross-laminated sandstones with an approximate thickness 

of 22 m; 4) an interval of gray to brown siltstone approximately 20 m 

thick with localized lenses of fine-grained sandstone; and 5) light-gray 

to yellowish-gray, fine-grained, crossbedded sandstone approximately 

17 m thick. Lytle Formation is comprised of yellowish-gray to light 

yellowish-brown, medium- to fine-grained sandstone and pebble 

conglomerate. Pebbles within conglomerate are well-rounded and 

composed of quartz, quartzite, chert, and petrified wood; conglomerate 
Description of Map Units 15

generally near base of unit. Lytle Formation is approximately 12 m 

thick (Scott, 1963b). Overall thickness of Dakota Group near Kassler 

is approximately 95–100 m.

In South Park, Dakota Group consists only of Dakota Sandstone. 

It is a light-brown to light-gray, fine- to coarse-grained, crossbedded, 

well-sorted sandstone and moderately well-sorted pebble conglomerate (Kirkham and others, 2006). Weathers brown to reddish brown. 

Kirkham and others (2006) estimated a maximum thickness of 76 m

Jm Morrison Formation (Upper Jurassic)—Mapped only in South Park, this 

unit grades from interbedded shale and fine- to medium-grained 

sandstone to interbedded green, red, and gray claystone to basal 

micritic limestones (Widmann and others, 2005). Claystones are 

generally bentonitic and deeply weathered. Formation thickness ranges 

from approximately 60–106 m (Stark and others, 1949; Barker and 

Wyant, 1976; Widmann and others 2005, 2006) 

Jmr Morrison Formation and Ralston Creek Formation, undivided (Upper 

Jurassic)—Mapped only within Kassler area following Scott (1963b). 

Unit consists of Morrison Formation in upper part and Ralston Creek 

Formation in lower part. Upper approximately 18 m of Morrison 

Formation consists of red to reddish-brown, variegated silty claystone 

with localized interbeds of limonitic, gray, medium-grained sandstone. 

The underlying 50 m is comprised of light-gray, locally limonitic 

siltstone and claystone. This section has interbedded, resistant, 

light-gray to gray limestone with fossilized algal mats and freshwater 

mollusks. Basal part of unit is comprised of a lower yellowish-gray to 

medium-gray, massive to thick-bedded, crossbedded, fine- to mediumgrained, calcareous sandstone that is approximately 13 m thick. This 

basal sandstone is overlain by approximately 13 m of interbedded 

medium-gray to reddish-brown siltstone; thin, discontinuous, sparitic 

limestone beds; and limonitic sandstone beds. Ralston Creek 

Formation consists of interbedded yellowish-gray, fossiliferous 

limestone, gypsum beds, silty calcareous shale, and fine-grained 

sandstone. Limestone beds contain freshwater gastropods (Scott, 

1963b). Freshwater gastropods, together with gypsum beds, indicate 

a coastal lagoonal depositional setting for the formation. Maximum 

thickness is less than 13 m (Scott, 1963b). Total thickness of combined 

Morrison and Ralston Creek Formations is approximately 55–60 m

KJdm Dakota Group (Lower Cretaceous) and Morrison Formation (Upper 

Jurassic), undivided—Unit mapped where limited exposure conceals 

precise location of contact between the two formations

^Pl Lykins Formation (Triassic? and Permian)—Mapped only within Denver 

Basin. Formation consists of five members deposited in a marine transgressive sequence (Scott, 1963b). Members are not mapped separately. 

Upper member, Strain Shale, consists of 90 m of reddish-brown, 

well-stratified, micaceous, fine-grained, silty sandstone and siltstone 

with some green siltstone layers. Middle member, Forelle Limestone, 

consists of 5 m of pink, wavy-laminated, sandy, marine, algal limestone. Lower member is about 40 m thick and includes undivided 

Bergen and underlying Harriman Shale, which consist of interbedded, 

reddish-brown and green siltstone, with a thin (<1 m), laminated, redweathering, gray and yellow crystalline limestone (Falcon Limestone 

Member) between them

Pl Lyons Sandstone (Cisuralian)—Mapped only in Denver Basin. Consists of 

yellowish-gray, white, reddish-brown to light-red, and pink, crossbedded, fine to medium-grained eolian sandstone (Scott, 1963b). Upper 

part of unit is coarse-grained sandstone and conglomerate containing 
16 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

large muscovite flakes (>0.5 cm) and lenticular beds of conglomerate at 

base (Scott, 1963b). Thickness is approximately 70 m

Pg Garo Formation (Permian)—Mapped only in South Park. Red to pink, and 

locally light-gray, medium- to well-sorted, fine- to medium-grained 

sandstone and conglomerate; calcareous; crossbeds as thick as 2 m 

are prominent; and ripple marks are locally preserved (Widmann and 

others, 2005; Kirkham and others, 2006). Previously mapped as conformably overlying and interbedded with Maroon Formation (P*m) 

(DeVoto, 1965a; 1965b), indicating coeval deposition. However, other 

workers have recognized probable unconformities both above and 

below formation (Singewald, 1942; Stark and others, 1949). Widmann 

and others (2005) and Kirkham and others (2006) assigned a Permian 

age based on Devoto’s work and additional supporting work by Lozano 

(1965). We have followed this age designation. Thickness of formation varies from approximately 20 to 70 m

P*f Fountain Formation (Cisuralian and Pennsylvanian)—Mapped only in 

Denver Basin (Scott, 1963b). Red to reddish-brown, arkosic, thin- to 

thick-bedded, coarse-grained sandstone and crossbedded fluvial conglomeratic sandstone with thin layers of dark reddish-brown, siltstone 

and shale. Coarse-grained, conglomeratic beds and lenses commonly 

fill shallow channels cut into surrounding sandstone. Sandstone locally 

bleached light tan to white. Formation contains thin, discontinuous, 

green siltstone and shale layers. Clasts in conglomerate consist of 

only Proterozoic crystalline rocks and are well rounded to subrounded. 

Scott (1963b) reported thicknesses ranging from 350 to 685 m, with 

thickness being thickest in Colorado Springs area and decreasing to 

south along eastern margin of Front Range

P*m Maroon Formation (Cisuralian to Middle Pennsylvanian)—Mapped only 

in South Park. Light-yellow to orangish-red, medium- to fine-grained, 

interbedded sandstone and thinly laminated siltstone and shale. Beds 

are tabular, trough cross-stratified, and finely laminated. Formation 

locally contains lenses of pebble- to cobble-conglomerate. Widmann 

and others (2005) mapped thin, discontinuous layers of dark-gray to 

reddish-gray, micritic limestone and limey siltstone. Conglomeratic 

facies are more prominent in north and fine-grained sandstone facies 

are more widespread to south (Widmann and others, 2005). Formation 

thickness is approximately 1,000 m

*m Minturn Formation (Middle Pennsylvanian)—Mapped only in South 

Park. Light yellowish-brown to reddish-gray, interbedded pebble- to 

cobble-conglomerate, reddish-brown siltstone, fine- to coarse-grained 

sandstone, and dark-gray, micritic limestone. Widmann and others 

(2005) noted that finer grained sandstone layers are planar, and trough 

cross-stratified beds are coarser. In addition, they noted that conglomeratic facies on southeast flank of Mount Silverheels, near margin 

of quadrangle approximately 9 km north of Fairplay, Colo., possibly 

represents a basin margin or paleovalley. Fossil fragments, or fossil 

hash, are preserved within some of the limestone beds. Kirkham and 

others (2006) mapped uppermost limestone, Jacque Mountain Limestone Member, within Fairplay East 7.5-minute quadrangle, being top 

of Minturn Formation. Thickness as much as 1,800 m

P*mm Maroon and Minturn Formations, undivided (Cisuralian to Middle

Pennsylvanian)—Unit mapped where exposures of contact between 

the two formations is limited

Mlw Leadville Limestone and Williams Canyon Formation, undivided

(Misissippian)—Exposed only within Manitou Park in the 

southeastern corner of quadrangle. Upper Leadville Limestone is 
Description of Map Units 17

chiefly composed of gray to light-brown, micritic limestone containing 

brachiopods, bryozoans, ostracodes, and conodonts (Hill, 1983). 

Mississippian age and classification of Leadville Limestone and 

Williams Canyon Formation follows that of Temple and others (2007). 

Leadville Limestone has a thickness of approximately 15 m. Basal 

Williams Canyon Member is chiefly composed of massive- to mediumbedded, calcitic sandstone, lime mudstone, dolomitic mudstone, and 

thin-bedded, lensoidal sandstone with a distinct basal medium- to 

coarse-grained, well-rounded quartzite bed (<30 cm in thickness) 

unconformably overlying Manitou Formation (Temple and others, 

2007). Lenses of sandstone have sub-rounded to well-rounded, frosted 

quartz grains. The total maximum thickness of the undivided unit is 

approximately 40–45 m

 Om Manitou Formation (Lower Ordovician)—Mapped only in Manitou Park 

area within southeast corner of quadrangle. Fine-grained, pink to 

light-gray, micritic limestone and dolomite. Temple and others (2007) 

described two members of formation. Upper member consists of lightpink to pinkish-gray, medium-grained, sparitic limestone and dolomitic 

limestone, locally containing thin chert layers, with a thickness of 

approximately 20 m. Limestone is locally bioturbated and fossiliferous, containing assemblages of trilobites, conodonts, and gastropods. 

Lower member is about 15 m thick and is composed of pink to light 

grayish-red, fine-grained, micritic limestone and dolomitic limestone. 

Total thickness of formation is approximately 35–40 m

_s Sawatch Formation (Upper Cambrian)—Purplish-pink, red to light-brown, 

and white, medium- to coarse-grained, crossbedded quartzite. Bedding 

thickness averages about 1 m. Unit generally fines upward with basal 

beds being coarse grained with lenses of quartz pebble conglomerate in 

a medium-grained quartzite matrix. Grains are well sorted and rounded 

to well rounded with frosted surfaces. Quartzite is locally glauconitic. 

Forms sandstone dikes along South Platte River corridor near Bailey, 

Colo. Weathers to a mottled light-brown color. Temple and others 

(2007) defined four informal members of formation: a red to maroon, 

porous, less-resistant sandstone; grading into two gray to light-brown 

quartzite members; and a basal conglomerate. Estimated thickness is 

15–20 m

M_ Leadville Limestone and Williams Canyon Formation (Mississippian), 

Manitou Formation (Lower Ordovician), and Sawatch Formation 

(Upper Cambrian), undivided—Mapped where limited exposure 

prevents precise location of contacts between formations or where subhorizontal bedding makes outcrop widths too narrow to depict at map 

scale (Temple and others, 2007)

Ordovician(?), Cambrian(?), or Neoproterozoic(?)

Intrusive Rocks

OZd Diabase—Black to reddish-brown, deeply weathered potassic feldspar-bearing 

augite diabase forming dikes between South Park and Tarryall Creek 

drainage 

Proterozoic Intrusive Rocks

Radiometric-age determinations by several methods show that Proterozoic intrusive 

rocks in Bailey quadrangle were emplaced during three principal time intervals. These 
18 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

major intrusive episodes were recognized and formalized by Tweto (1987) as rocks of 

1,074–1,092 Ma (Unruh and others, 1995) Pikes Peak batholith and associated intrusive 

bodies, including Redskin stock, Mesoproterozoic Berthoud Plutonic Suite (roughly 

1,400 ± 25 Ma), and Paleoproterozoic Routt Plutonic Suite (roughly 1,700 ± 25 Ma). In 

order to more clearly represent age and composition and plutonic suite, the following 

map unit label scheme was created following that of Kellogg and others (2008):

Age: X=Paleoproterozoic (2,500–1,600 Ma), Y=Mesoproterozoic (1,600–900 Ma)

Composition: g=granite, gd=granodiorite, qd=quartz diorite, and gm=gabbro 

and monzogranite; in addition, where appropriate, textural and (or) mineralogic 

variants that are mapped separately within an intrusive complex are symbolized 

with an additional lower-case letter (for example, p=porphyritic, f=fine-grained, 

fp=fine-grained porphyritic).

Pluton: P=Pikes Peak batholith and R=Redskin stock. 

Thus, the symbol for the coarse-grained porphyritic phase of the Pikes Peak batholith is YgPcp; the symbol for the gabbro and monzonite associated with the Redskin 

stock is YgRm.

Classification of intrusive rocks follows that of Streckeisen (1976), in which 

“granite” includes syenogranite and monzogranite.

Rocks of the Pikes Peak Batholith and Related Rocks

of the Redskin Stock (Mesoproterozoic)

YgRf Redskin granite fine-grained facies—White to pink, massive, fine-grained 

granite forms core of stock and Boomer and China Wall cupolas. This 

facies is associated beryllium-bearing greisen deposits (Hawley and 

others, 1966). Rock is a two-mica granite containing quartz, microcline, and albite in approximately equal proportions. Boomer cupola 

is formed by a fine-grained aplitic, quartzose muscovitic two-alkalifeldspar granite (Hawley and others, 1966)

YgRp Redskin porphyritic granite facies—White, orangish-pink, and pink, 

massive, medium-grained seriate porphyritic granite forming a zone 

surrounding core of main Redskin stock. Porphyritic facies is a twomica granite with quartz, microcline, and albite in equal proportions 

(Hawley and others, 1966). Hydrothermal alteration has created 

beryllium-bearing greisen deposits (Hutchinson, 1964; Hawley and 

others, 1966)

YgRg Redskin equigranular granite facies—White to pink, medium-grained, 

equigranular granite composed of biotite, quartz, albite, and perthitic 

microcline. Quartz, microcline perthite, and albite are in proportions 

of 33:43:24 (Hawley and others, 1966). Granular facies forms outer 

zone facies of Redskin stock

YgRm Gabbro and monzogranite associated with Redskin stock and Pikes 

Peak batholith—Black to green, coarse-grained labradorite-olivinehypersthene-ilmenite gabbro forms outer ring of a small pluton at 

southern margin of Redskin stock. Core of this body is composed of 

a light-pink to brownish-white monzogranite ranging in texture and 

composition from a coarse-grained hornblende-biotite monzogranite 

to a aphanitic-porphyritic biotite monzogranite (Hawley and others, 

1966). Hawley and others (1966) noted that discordant contacts and 

funnel-shape of intrusive body indicates that it is a hypabyssal intrusion 

younger than unit Yg, and older than Redskin stock 
Description of Map Units 19

Rocks of the Pikes Peak Batholith 

(Mesoproterozoic)

YgP Pikes Peak Granite—Pinkish-orange to light-gray, medium- to coarsegrained, subequigranular biotite- and biotite-hornblende granite with 

feldspar crystals as long as 2.5 cm. Forms large, slabby or rounded 

outcrops and weathers to orange-brown grus that contains abundant 

feldspar and mica fragments. U-Pb zircon ages range from 1,074 ± 3 to 

1,092 ± 2 Ma, with late stage, hypabyssal pegmatites in west and south 

ranging from 1,059 ± 2 to 1,062 ± 2 Ma (Unruh and others, 1995). A 

more recent age for eastern side of batholith has a U-Pb zircon age of 

1,085.6 ± 2.5 Ma (Smith and others, 1999). Unit surrounds coarsegrained porphyritic facies (unit YgPcp) within batholith (Hawley and 

Wobus, 1977)

YgPcp Pikes Peak coarse-grained porphyritic phase—Light-pink to pink, seriate 

porphyritic biotite granite with tabular microcline phenocrysts as long 

as 5 cm. Intruded by fine-grained, porphyritic granite and has diffuse, 

gradational boundaries with coarse-grained unit YgP (Wobus and 

Scott, 1977)

YgPf Pikes Peak fine-grained granite—Pink, fine-grained equigranular biotite 

granite. Locally grades into fine-grained, porphyritic granite (YgPfp)

YgPfp Pikes Peak fine-grained porphyritic granite—Pink to brownish-red, 

holocrystalline, fine- to medium-grained porphyritic biotite granite 

with phenocrysts of spherical gray quartz, and subhedral microcline 

and oligoclase are abundant. Fluorite is a common accessory mineral. 

Resembles Windy Point Granite of Pikes Peak batholith mapped south 

of quadrangle, and represents late phase, rapidly cooling intrusive 

bodies within batholith. U-Pb zircon ages range from 1,074–1,092 

Ma (Unruh and others, 1995). Appears to have intruded Pikes Peak 

Granite (unit YgP) (Wobus, 1976b), but age difference is too small 

to be detected by U-Pb zircon age determinations (Unruh and others, 

1995). Unit is dominantly deeply weathered. Breaks into angular 

masses and platy pieces

YgPqg Pikes Peaks monzogranite to granodiorite—Medium- to coarse-grained 

porphyritic hornblende-biotite monzogranite to granodiorite. Locally 

contains xenocrysts of sodic labradorite with albite rims

YgPs Syenite and fayalite granite of Pikes Peak batholith—Fine- to coarsegrained pegmatitic syenite and fayalite granite comprising youngerphase intrusive centers within Pikes Peak batholith. Fayalite granite 

is primarily composed of a medium-grained granite with minor 

amounts of fayalite. Dominant minerals are pink, perthitic feldspar 

and ferrorichterite amphibole (Beane and Wobus, 1999). Accessory 

minerals include fluorite, zircon, and chevkinite. Wobus (1976b) 

stated that interlocking tabular fabric in fayalite granites makes the 

rock very resistant to weathering and erosion. Syenite granite tends 

to be coarser-grained than fayalite granite and contains green albiteoligoclase feldspars and dominant mafic mineral is ferrohastingsite 

(Wobus, 1976b). Syenite body near Lake George contains large, 

elongate inclusions of alkali gabbro as much as 0.5 m long. Ring dikes 

surrounding Lake George pluton are composed of quartz syenite and 

fayalite granite. An 40Ar/39Ar age of 1,074 ± 1 Ma has been determined 

by Unruh and Premo (unpub. data, 2008)
20 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

Intrusive Rocks of the Berthoud Plutonic Suite

(Mesoproterozoic)

Yg Monzogranite and granite—Yellow-orange, light-pink, and pinkbrown, biotite-muscovite monzogranite, biotite monzogranite, and 

leucogranite. Unit contains aplite and pegmatite. Locally foliated. 

Garnet and sphene are accessory minerals. SHRIMP U-Pb zircon 

ages taken from samples collected at Elkhorn Ranch (UTM NAD 

27: E0431568, N4337945) and Kenosha Pass (UTM NAD 27: 

E0435390, N4365565) yielded ages of 1,391 ± 27 Ma and 1,429 ± 

15 Ma, respectively (reference needed). Rocks mapped in this unit 

are very similar to those mapped as Silver Plume Granite in Denver 

West quadrangle (unit YgSP of Kellogg and others, 2008). In some 

previous work all such rocks were called Silver Plume Granite. 

Because of uncertainty whether they formed from the same magma or 

intrusive episode, the term Silver Plume is not applied to these rocks in 

accordance with the recommendation of Tweto (1987)

Intrusive Rocks of Uncertain Age 

(Mesoproterozoic or Paleoproterozoic)

YXp Pegmatite and aplite—Coarse-grained to very coarse grained, white to lightpink, inequigranular plagioclase-microcline (or perthite)-quartz-mica 

rock that forms irregular-shaped pods, commonly zoned dikes, and 

intrusive bodies cutting monzogranite and granite (Yg) and all older 

rocks; relatively few pegmatites cut Pikes Peak Granite (YgP). Some 

pegmatites contain microcline crystals more than 1.0 m long. Mica is 

mostly biotite, but locally includes or is entirely muscovite. Accessory minerals include tourmaline, beryl, garnet, and magnetite. 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 pinkishlight brown, fine- to medium-grained, leucocratic, equigranular rock

YXmr Mixed metamorphic and igneous rocks—Unit consists of at least two of the 

following units in equal percentages: monzogranite (Yg), biotite gneiss 

and schist (Xb), and felsic gneiss (Xf). Xenoliths of units Xb and 

Xf within unit Yg show diffuse boundaries and represent intense and 

profuse intrusion, and initial assimilation of the metamorphic rock into 

monzogranite (Yg)

YXqgm Mixed metamorphic and igneous rocks—Unit consists of estimated equal 

proportions of unit Xqg intruded into country rock consisting of biotite 

gneiss and schist (unit Xb) and hornblende gneiss (unit Xh) with diffuse bodies of unit Yg and YXp. Contacts between igneous component and metamorphic rocks are diffuse and indicate profuse intrusion 

and assimilation of metamorphic rocks into igneous bodies. Samples 

from Observatory Rock 7.5-minute quadrangle yielded SHRIMP U-Pb 

zircon ages ranging from 1,430 ± 15 Ma to 1,437 ± 6 Ma and probably 

reflect metamorphism related to intrusion of unit Yg (W.R. Premo, 

unpub. data, 2008)

YXgd Granodiorite and monzogranite of unknown age—Gray, medium- to coarsegrained, equigranular to porphyritic, massive to weakly foliated quartzplagioclase-microcline-biotite-hornblende tonalite, granodiorite, and 

monzogranite. May include a younger, medium-grained monzogranite 

phase and an older, medium- to coarse-grained granodiorite phase
Description of Map Units 21

Intrusive Rocks of the Routt Plutonic Suite 

(Paleoproterozoic)

Xqg Monzogranite and granodiorite—Light-pink to gray, fine- to coarse-grained, 

massive to strongly foliated monzogranite and granodiorite. Unit is 

locally intruded by monzogranite of unit Yg. Plutonic body of this 

suite mapped south of Hartsel, yielded a SHRIMP U-Pb zircon age of 

1,705 ± 7 Ma (W.R. Premo, unpub. data, 2008). A correlative plutonic 

body mapped south of Singleton, along North Fork of South Platte 

River yielded SHRIMP U-Pb zircon ages cores with inherited cores 

ranging from ≈1,650 to 1,750 Ma and an imprecise age on high-U 

magmatic rims at 1,461 ± 33 Ma (W.R. Premo, unpub. data, 2008). 

Younger age is most likely related to 1,425– to 1,445–Ma plutonic 

event within Front Range. One possible interpretation is that age of 

this plutonic body is ≈1,665 Ma and that it was metamorphosed at 

≈1,445 Ma (W.R. Premo, written comm., 2008)

Xbm Mixed metamorphic and igneous rocks—Mapped southwest of Pine Junction, unit consists of unit Xb (described below) as well as abundant 

light-gray to white leucosomes, commonly ranging from 0.1 to 10 cm 

in thickness, but locally exceeding 1 m in thickness. It is uncertain 

whether formation of leucosomes was from in situ partial melting, or 

from profuse injection of granitic bodies. Two U-Pb zircon dates from 

melt phase in similar migmatitic gneiss from Clear Creek, approximately 30 km north of the quadrangle boundary, are 1,693 ± 35 Ma and 

1,698 ± 3 Ma (Kellogg and others, 2008); one date from a leucosome 

in migmatite sampled along Elk Creek in the Pine, Colo. 7.5-minute 

quadrangle is 1,692 ± 6 Ma and 1,780 ± 9 Ma sampled from a melanosome (W.R. Premo, unpub. data, 2005). These data suggest peak 

metamorphism and partial melting in region at ≈1,695 Ma

Xqd Quartz diorite—Mapped in Badger Mountain-Puma Hills region. Gray 

to dark-gray, fine- to medium-grained, massive to strongly foliated, 

locally porphyritic, biotite and biotite-hornblende quartz diorite. 

Contains abundant locally derived xenoliths of country rock, mainly 

biotite gneiss and schist (Xb). Primarily forms border phase around 

larger granodiorite plutons and is intruded by granodiorite (Wobus, 

1976b). Locally forms small stocks and intrusive bodies

Xgg Granitic gneiss—Light- to medium-gray and pinkish-gray, fine- to mediumgrained, weakly to strongly foliated monzogranite, granodiorite, and 

trondhjemite, with lesser amounts of biotite, hornblende, and muscovite. Within Mount Logan region (north of Singleton), unit locally 

contains abundant xenoliths of biotite gneiss (Xb). Age has been 

inferred by Sheridan and others (1972) to be same as that of Boulder Creek Granodiorite mapped to north in Denver West quadrangle 

(Kellogg and others, 2008). North of map area, a preliminary U-Pb 

zircon age from a foliated granodiorite on Mount Morrison in Morrison 

7.5-minute quadrangle is 1,771 ± 11 Ma, and a foliated monzogranite 

from near Deer Creek in southern Harris Park 7.5-minute quadrangle is 

1,776 ± 10 Ma (W.R. Premo, unpub. data, 2005)

Xpgd Porphyritic biotite granodiorite gneiss—Biotite granodiorite gneiss 

containing potassic feldspar grains as much as 2 cm in length. 

Commonly strongly foliated. Forms an irregular plutonic body and 

concordant stocks near Pine Gulch (south of Pine Junction). Contacts 

with biotite gneiss (Xb) and mixed metamorphic rocks (Xbm) are 

diffuse. Rb-Sr age determination on similar foliated plutonic rocks 

in Front Range indicate an age of 1,710 Ma (Hedge, 1969); however, 

this age could either be whole-rock or mineral isochron ages due to 

possible metamorphic resetting 
22 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

Metasedimentary and Metaigneous Rocks

(Paleoproterozoic)

Xb Biotite gneiss—Gray to black, fine- to medium-grained, equigranular, wellfoliated, biotite-quartz-plagioclase gneiss and schist. Locally contains 

abundant sillimanite (<30 percent). Locally garnetiferous with average 

size of crystals <0.5 cm. Unit includes a few lenses of hornblende 

gneiss (Xh) and calc-silicate rock. Near younger intrusive bodies 

(Yg and YgP), unit contains abundant lenses, pods, sills, and dikes of 

pegmatite (YXp) and rocks similar in composition to Yg and YgP. 

Near Pine, a melanosome of biotite gneiss in migmatite yielded a 

U-Pb zircon age of 1,780 ± 9 Ma (W.R. Premo, unpub. data, 2005). In 

northwestern parts of Tarryall Mountains, unit contains sillimanitecordierite-quartz gneiss and sillimanitic biotite-muscovite gneiss

Xbh Interlayered biotite gneiss and schist, hornblende-plagioclase gneiss, and 

amphibolite—Mapped south of Shaffers Crossing and Pine Junction. 

Gray to black, fine- to medium-grained, biotite-plagioclase gneiss and 

schist, amphibolite, hornblendite, and hornblende gneiss. Unit contains 

minor amounts of calc-silicate rock. Commonly contains abundant 

dikes, sills, and pods of units YgP and Yg. Biotite-plagioclase gneiss 

and amphibolite make up 10 to 20 percent of unit 

Xbs Biotite-sillimanite gneiss and schist—Mapped in Farnum Peak region, 

Kenosha Mountains, and north-northwest of Pine Nook in northeast 

part of quadrangle. Light-gray to black, fine- to coarse-grained, wellfoliated, biotite-quartz-plagioclase-sillimanite gneiss. Commonly 

contains muscovite as a secondary mineral. Unit is locally interlayered 

with biotite gneiss (Xb) and hornblende gneiss (Xh). Locally contains 

garnets >1 cm in diameter 

Xca Calc-silicate gneiss, marble, and amphibolite—Mapped north-northeast 

of Jefferson Lake. Fine- to medium-grained, interlayered dark-green 

calc-silicate gneiss, white and opaque marble, and black amphibolite. 

Calc-silicate dominantly consists of clinopyroxene, hornblende, calcite, 

epidote, scapolite, and sphene. Marble contains clinopyroxene, hornblende, and other silicates (Barker and Wyant, 1976). Unit contains a 

few lenses of medium- to coarse-grained quartzite

Xf Quartz-feldspar gneiss—Mapped in northeast and northwest corners of quadrangle, in Elkhorn region, and along Tarryall Creek. Gray, dark-gray, 

white, pinkish-gray, and tan moderately foliated to well-foliated, layered quartz-plagioclase-microcline-biotite gneiss. Ranges from fine to 

coarse grained, but is chiefly medium grained. Proportion of minerals 

ranges widely. Layers typically 10 cm to several tens of meters thick 

and commonly are wavy. Locally contains layers of foliated monzogranite and granodiorite. Contains minor thin layers of hornblende 

gneiss and amphibolite. U-Pb date on weakly foliated rock of monzogranitic composition in northwestern part of Indian Hills 7.5-minute 

quadrangle is 1,776 ± 4 Ma (Kellogg and others, 2008). Weathers 

light-yellow-brown to pink-yellow-brown in rounded, spheroidal outcrops 

Xfh Quartz-feldspar gneiss and hornblende gneiss—Mapped in Georgia Pass 

region. Interlayered dark-gray and light-gray, well-foliated quartzfeldspar gneiss (Xf) and hornblende-plagioclase gneiss and amphibolite (Xh) in approximately equal proportions. Unit locally contains 

very few layers and lenses of calc-silicate rock

Xh Hornblende-plagioclase gneiss and amphibolite—Mapped in Georgia Pass 

region, proximal to Webber Park along Tarryall Creek, and near 

Pine Notch in northeast corner of quadrangle. Dark-gray to black, 
Geologic History of the Bailey Quadrangle 23

greenish-black, and white, weakly to strongly foliated, layered, fine- to 

coarse-grained, hornblende-plagioclase gneiss and black to greenishblack amphibolite containing variable amounts of biotite, quartz, 

and augite. Layering ranges from less than 1 cm to greater than 1 

m. Commonly has black-and-white mottled texture due to weathered 

plagioclase (white) and hornblende (black). Amphibolite contains >50 

percent amphibole

Xhc Hornblende gneiss and calc-silicate gneiss—Black to dark-gray, greenishblack, greenish-white, and greenish-gray, interlayered hornblendeplagioclase gneiss and amphibolite (Xh), with lesser amounts of 

interlayered calc-silicate gneiss (<20 percent). Unit locally contains 

layers of white to light-gray marble of thickness less than 1 m. 

Mapped primarily in Webster-Kenosha Pass region in northwest part of 

quadrangle and southeast of Farnum Peak in south-central part of map, 

near bodies of Paleoproterozoic rocks of igneous origin (Xgg, Xqd, 

and Xqg)

Xhq Amphibolite, marble, and quartzite—Interlayered amphibolite. Quartzite, 

coarse-grained, locally siliceous, white to light-gray marble, quartzrich calc-silicate gneiss and schist, and quartz-sillimanite-muscovitebiotite gneiss and schist. Marble and quartzite layers locally as thick as 

10 m

Geologic History of the Bailey 

Quadrangle

The Bailey, Colo. 1:100,000-scale quadrangle lies 

within two physiographic and geologic provinces in central 

Colorado: 1) the Front Range and 2) South Park. Most of 

the Front Range is composed of Proterozoic rocks ranging 

in age from 1,790 Ma to 1,074 Ma. Along the eastern flanks 

and within the Denver Basin, sedimentary rocks ranging from 

Pennsylvanian to Cretaceous are deformed and steeply tilted 

to the east. Upper Cretaceous through Paleocene rocks were 

deposited in the foreland (that is, the Front Range eastern 

flank) and hinterland (that is, South Park) of this thrust and 

reverse fault system developed during the Late Cretaceous 

to Paleocene Laramide orogeny. Within South Park, rocks 

range in age from Pennsylvanian to Miocene with Quaternary 

deposits indicating tectonic subsidence of the basin. These 

rocks record five major geologic episodes: 1) the Paleozoic 

Anasazi uplift that formed the Ancestral Rockies, 2) the Late 

Cretaceous to Paleocene Laramide orogeny, 3) widespread 

Eocene to Oligocene volcanism, 4) Oligocene–Quaternary 

tectonics, and 5) Quaternary glacial episodes.

Proterozoic History

The Proterozoic history recorded in the Front Range 

begins with a sequence of gneiss and schists dating back 

to approximately 1,790 Ma. These metasedimentary and 

metaigneous rocks consist of biotite gneiss and schist (Xb), 

calc-silicate (Xca), amphibolite (Xh), and felsic gneiss (Xf). 

These rocks may represent a package of interlayered pelitic, 

clastic, and volcanic rocks deposited within back-arc basins 

and along the southwestern margin of the Archean North 

American craton. The margin of the Archean craton was 

just north of the southern Wyoming stateline. Syntectonic 

deformation of these deposits occurred as accretion of 

successive island arc complexes and associated rocks 

developed along the continental margin (Reed and others, 

1987). 

Coeval to back-arc deposition and accretion of the island 

arcs, widespread igneous activity took place from approximately 1,776 ± 10 Ma to 1,646 ± 13 Ma (W.M. Premo, unpub. 

data, 2006) (Xqg and Xgg). Composition of these plutonic 

bodies is chiefly granodiorite and monzogranite. Profuse 

intrusion of plutonic rocks into the surrounding country 

rocks induced a migmatization event and created mixed rocks 

mapped as migmatite (units Xbm and Xqgm). North of the 

quadrangle, this migmatization event has been dated at 1,750 

Ma to 1,697 Ma (Reed and others, 1993; W.M. Premo, unpub. 

data, 2005; Kellogg and others, 2008).

During the Mesoproterozoic, plutonic bodies of predominantly monzogranite composition (Yg) were emplaced and 

locally bodies of mixed rock (that is, migmatite) (YXmr) were 

formed in the adjacent country rock. These Mesoproterozoic 

monzogranites have been dated at 1,429 ± 15 Ma to 1,391 ± 

27 Ma, contemporaneously with the Silver Plume Granite of 

the central and northern Front Range dated at 1,424 ± 6 Ma 

(W.M. Premo, unpub. data, 2006). One of the Mesoproterozoic plutons was derived from partial melting of the lower 

crust and emplaced at depths of 8 to 9 km (Anderson and 

Thomas, 1985), and perhaps that model is applicable to the 

others. Regionally, the country rock was hotter than 300o C 
24 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

geography, a fact that is commonly observed in other uplifts of 

that age (Kluth, 2007; Nesse, 2007). Nesse (2007) proposed 

the term Anasazi uplifts for these uplifts of Mississippian to 

Pennsylvanian age to avoid the misconception of coincidence 

with later uplifts and we adopt that term herein. The relative 

roles of denudation versus non-deposition are poorly 

understood beneath the Denver Basin, but denudation is 

the most represented geomorphic process within the Bailey 

quadrangle where similar sections of Upper Paleozoic rocks 

are preserved on either side of the eroded zone. 

Anasazi-uplift geography changed in the Pennsylvanian 

and Permian with the deposition of the Pennsylvanian Minturn 

(*m), Pennsylvanian and Permian Belden (not within map 

area), Fountain (P*f), and Maroon (P*m) Formations, and 

the Permian Garo (Pg) Formation, which were derived from 

western parts of the earlier uplift and covered eastern parts 

of it. About 500 m of mostly fluvial red and pink, arkosic 

sandstone and conglomerate of the Fountain Formation (P*f) 

were shed eastward from the present-day Front Range to cover 

the northeastern extent of the earlier area (the part now buried 

beneath the Denver Basin). Similar fluvial deposits of the 

Belden, Minturn, Maroon, and Garo Formations in western 

South Park and the adjacent Mosquito Range (Kluth, 2004) 

were shed west off the narrowed erosional area. The fluvial 

deposits are para-conformable on Mississippian limestone 

where the Leadville Limestone and older units have not been 

eroded, otherwise they rest directly on Proterozoic rocks. That 

fact and the preponderance of sandstone over coarser grained 

sedimentary rocks suggest that relief and depth of denudation 

were only modest at best. The presence of marine limestone 

horizons interbedded with the Pennsylvanian clastic rocks on 

the west side of the uplift (Widmann and others, 2004; 2005) 

indicates that the uplift might not have progressed far above 

sea level at any time. Thus, the Anasazi uplifts within this 

region are actually more related to differential subsidence than 

anything else. Nonetheless, the large volume of the redbeds 

suggests considerable erosion over a wide area. 

Permian eolian deposits of the Lyons Sandstone (Pl) and 

Permian and Triassic fluvial and near-shore deposits of the 

Lykins Formation (^Pl) overlie the thick clastic sequences 

of the Fountain Formation along the east side of the Front 

Range. By the Late Jurassic, the present Front Range area 

was mostly covered by fluvial and lacustrine deposits of the 

Morrison Formation (Jm) and the Ralston Creek Formation 

in the Bailey area. Near the end of the Early Cretaceous, 

major subsidence coeval with a rise in sea level caused 

transgression of the Western Interior seaway over the entire 

area of the present Front Range. Deposition commenced as 

shoreline deposits of the Dakota Group (or Sandstone) (Kd), 

were followed by more than 2 km of marine shale and minor 

amounts of sandstone and limestone (Benton Group [Kb], 

Niobrara Formation [Kn], and Pierre Shale [Kp]).

The Western Interior seaway began to withdraw from 

the quadrangle area after 69 Ma, the age of the youngest 

ammonite zone in the Pierre Shale (Scott and Cobban, 1965; 

Cobban, 1993). This age is based on 40Ar/39Ar dating of 

and locally hotter than 550o C as indicated by 40Ar/39Ar studies 

(Shaw and others, 1999).

The final event of the Proterozoic occurred with the 

intrusion of the Neoproterozoic Pikes Peak batholith between 

1,092–1,074 Ma (Unruh and others, 1995). Two pulses of 

igneous activity forming the Pikes Peak batholith occurred at 

≈1,090–1,080 Ma and ≈1,075–1,070 Ma. Individual plutonic 

bodies and intrusive centers, such as that at Lake George, were 

generated from alkali basaltic magma derived from the mantle 

(Wobus and Anderson, 1978). Beane and Wobus (1999) suggested that the parallel alignment of Neoproterozoic faults and 

linear, plutonic bodies indicated that the Pikes Peak batholith 

was emplaced within an extensional tectonic regime, possibly 

being rift-related. Together, these suggest a tectonic setting 

involving post-orogenic, extensional collapse. However, constraints on this tectono-magmatic model are limited and the 

tectonic setting of the Neoproterozoic is uncertain.

In additon to Proterozoic faulting, linear sandstone and 

quartzite dikes cut Proterozoic rock throughout the quadrangle. These dikes are chiefly composed of well-sorted finegrained sand, varying from brown to red in color and locally 

spotted white (Vitanage, 1954; Scott, 1963b; Peterson, 1964). 

In the southeastern corner of the quadrangle within Manitou 

Park, these dikes contain brecciated sandstone indicating a 

polyphase deformational cycle (Temple and others, 2007). 

Peterson (1964) notes within the South Platte 7.5-minute 

quadrangle, the dikes are encased in brecciated Proterozoic 

country rock indicating faulting of the country rock, healing of 

the fault, reopening of the same fault, and finally, filling of the 

fissure with sand. Vitanage (1954) suggested the source of the 

sand was the Sawatch Quartzite before lithification occurred, 

but this is still highly debatable.

Paleozoic and Pre-Laramide Mesozoic History 

From Cambrian through Mississippian times, quartz-rich 

sand, shale, and carbonate deposits accumulated regionally in 

fluvial plains and shallow seas, forming a thin continental platform section that extended across much of the Southern Rocky 

Mountains, including all of South Park and the Front Range. 

Remnants of the platform section are preserved in a broad 

open syncline in the southeast corner of the quadrangle near 

Mount Deception (Temple and others, 2007), as well as the 

Colorado Springs area. The same section is preserved in its 

entirety throughout the Mosquito Range, which borders South 

Park west of the Bailey quadrangle (Tweto, 1974; Widmann 

and others, 2004; 2005), but it is absent everywhere northeast 

of South Park in a belt that extends into western Nebraska 

(Rocky Mountain Association of Geologists, 1972). 

The missing section is the result of widespread Late 

Mississippian to Early Pennsylvanian erosion or nondeposition in an uplifted area that is typically referred to 

as part of the Ancestral Rocky Mountains. Its northeast 

orientation, which crosses the present-day Front Range, shows 

that it had little in common with the present Rocky Mountain 
Geologic History of the Bailey Quadrangle 25

tuffs outside the quadrangle but in the same ammonite zone 

(Obradovich, 1993). The Upper Cretaceous-lower Tertiary 

rocks overlying the Pierre Shale record the uplift history, 

starting with the regressive shoreline deposits of the Fox Hills 

Sandstone, followed by coastal-plain deposits that formed 

the sandstones and coal beds of the Laramie Formation 

(from which the Laramide orogeny is named). The Laramie 

Formation is in turn overlain by the fluvial conglomerates, 

sandstones, and claystones of the Arapahoe and Denver 

Formations (not within map area) (Raynolds, 1997, 2002). 

Uplift in this area was geologically rapid; only a few million 

years (≈5 m.y.) separate the ages of the uppermost marine 

deposits of the Pierre Shale and the earliest conglomerates 

of the terrestrial Arapahoe Formation, which contains clasts 

derived from Proterozoic basement rocks. During this short 

period, the newly formed Rocky Mountains rose from the 

sea, and more than 2 km of upper Paleozoic and Mesozoic 

sedimentary rocks were eroded from the core of the Front 

Range. Peterson (1964) mapped mafic dikes of probable 

Cretaceous or early Tertiary age. These dikes are fine-grained, 

dark gray to greenish black and are composed chiefly of 

plagioclase, pyroxene, and magnetite with a diabasic texture. 

Dikes are generally 2–3 meters in width and have sharp 

contacts with the country rock. The age of these dikes is still 

debatable, but could be related to Laramide deformation.

The Laramide Orogeny

The Laramide orogeny was a widespread period of lithospheric buckling, in the form of arches and uplifts, that lasted 

approximately 25 m.y. during the Late Cretaceous and Paleogene (75–50 Ma) (Tikoff and Maxon, 2001). The deformation 

affected most of the eastern Rocky Mountains and large parts 

of the adjacent mid-continent region where later burial has 

obscured the ancient uplifts (Tikoff and Maxon, 2001). 

This time period (20 (25?) m.y.) coincided with widespread volcanism in Colorado. Most of the igneous activity 

occurred northwest, south, or southwest of the Bailey quadrangle, but some volcanism spatially overlaps parts of the buckled 

and uplifted regions within the quadrangle. Several thin, 

discontinuous volcanic flows, lahars, and tuffs of Laramide 

age are present in the South Park region, which make up the 

two formal members of the South Park Formation (Tsl and 

TKsr). The igneous activity is commonly included as part of 

the Laramide orogeny, but there is no reason to assume it had 

anything to do with the mechanical arching or uplifts. Rather, 

it is more logically a product of subduction-related melting 

that would have occurred even in the complete absence of the 

structural deformation that can be more strictly defined as 

being Laramide. 

Laramide uplift is evident throughout the Bailey 

quadrangle between South Park and the eastern range front 

near Kassler in the northeast corner of Bailey quadrangle, and 

Manitou Park in the southeast corner of Bailey quadrangle. 

These areas contain early Tertiary age and older sedimentary 

rocks that exhibit steep dips related to folding associated with 

Laramide basement uplifts. Proterozoic rocks are exposed 

everywhere in this area except where Paleozoic strata are 

preserved in a faulted syncline in the Mount Deception and 

Dakan Mountain 7.5-minute quadrangles. A sedimentary 

apron, now largely eroded, lapped onto the flanks of the uplift 

by the end of the Eocene. Conglomerate and arkose deposits 

are still preserved east of the present range front at the latitude 

of the Bailey quadrangle (Dawson Arkose, unit TKda) and 

are commonly cited as evidence for Front Range uplift and 

erosion during the latest Cretaceous through early Eocene. 

Since the structural dip of the Dawson is less than that in 

older units along the range front, a large amount of structural 

relief is assumed to have formed just prior to and during its 

deposition. Conglomerate of similar age occurs in South 

Park on the west side of the uplift (South Park Formation). 

Within South Park, however, the older units in the South 

Park Formation coarsen and thicken to the west (Kirkham 

and others, 2006), away from the uplift, and crossbeds are 

consistent with easterly transport away from a western 

source (Johnson, 1935). Clasts of Paleozoic sedimentary 

and Cretaceous and Paleocene intrusive rock types further 

suggest a source in the Mosquito Range or beyond (Wyant 

and Barker, 1976; De Voto, 1988; Widman and others, 2005). 

Coarse-grained and immature sand components of the South 

Park Formation probably had a source in a nearby terrain of 

granitic rock. Basement rocks in the Mosquito Range were 

covered by a thick section of Phanerozoic sedimentary rocks 

in the Paleocene, leaving the Laramide uplift to the east as the 

only nearby source of exposed granitic rock. Thus, there was 

probably mixing of east- and west-draining river systems that 

transported clastic material into the South Park Basin. 

The eastern edge of the Laramide uplift corresponds to 

the present-day range front near Kassler. It is primarily a zone 

of moderate-to-steeply east-dipping strata that forms a large 

east-facing monocline. Typical cross sections drawn across 

the uplifted margin, for example those of Nesse (2007), depict 

as much as 4 km of structural relief. The relief is largely due to 

folding, but partly to overthrusting on faults that are now buried. Along the eastern flank of the Front Range in the Kassler 

7.5-minute quadrangle, the Rampart Range fault cuts out the 

Paleozoic and Mesozoic sections.

The western structural edge of the Laramide uplift in 

South Park is delineated by the Elkhorn fault (fig. 1). The 

Elkhorn fault is a high-angle thrust or reverse fault with a 

north-northwest orientations and it is well exposed in two 

areas. In the northwest corner of Bailey quadrangle at 

Georgia Pass, a steep-to-moderate, east-dipping fault contact 

between mixed Proterozoic crystalline rocks (east side) and 

a 44-Ma monzogranite stock that intrudes Cretaceous marine 

strata (west side) is interpreted to be the southern branch of 

the Williams Range thrust fault (fig. 1). North of the Bailey 

quadrangle this branch is quasi-continuous with the Williams 

Fork thrust fault (Kellogg and others, 2008). A southern 

branch is partially exposed between Elkhorn Ranch and Muley 

Gulch in the east-central part of South Park where it dips 
26 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

FOOTWALL–

Triassic and younger 

rocks on basement

FOOTWALL–

Paleozoic and 

Mesozoic rocks on

basement

Quaternary

rocks

Quaternary

rocks

UPLIFTED AREA/HANGING WALL–

Mostly various Proterozoic 

crystalline rocks

WILLIAMS RANGE

THRUST FAULT

Buried edge of

Anasazi (Ancestral

Rocky Mountain) 

uplift

NORMAL FAULT

DEPOSITIONAL CONTACT

 (Triassic rocks on basement)

Broad, open

synclinal basin

ELKHORN FAULT

[southern branch]

Elkhorn 

Ranch

Muley Gulch

Georgia

Pass

S O U T H

P A R K

39°15’

39°30’

106°45’

0 5 10 KILOMETERS
Geologic History of the Bailey Quadrangle 27

moderately east (fig. 1). In this segment, the hanging wall is 

comprised of a variety of Proterozoic rocks that structurally 

overlie a footwall of steeply folded to overturned Jurassic 

to Paleocene beds that include the South Park Formation. 

Folding accounts for much of the structural relief across this 

part of the deformational front. Actual fault displacement is 

hard to quantify, but we suggest that it is not much (<200–300 

m). Displacement appears to lessen near the southern end of 

the fault exposure where older footwall rocks are exposed at 

the fault contact. 

The downdropped South Park half-graben intervenes 

between the exposed southern branch of the Williams Range 

thrust at Georgia Pass and the Elkhorn fault (fig. 1). A 

young normal fault bounds the east side of the half-graben in 

northern South Park where it acts as the structural boundary 

between uplifted and downdropped areas. How and where the 

two branches of the Elkhorn connect across the graben, or if 

they even do, is the subject of interpretation. Most workers 

(for example, Barker and Wyant, 1976; Bryant and others, 

1981a) directly connect the two branches across South Park by 

proposing a buried branch, or numerous ones, between the two 

exposed areas. Buried branches, which are attractive because 

of the alignment of the two segments, are feasible but they 

are hard to document and do not satisfy the basic Elkhornfault relations as understood in the two exposed areas. Where 

exposed, the Elkhorn consistently has Proterozoic rocks in the 

hangingwall to its east and Mesozoic to lower Tertiary strata 

or plutonic rocks in the footwall to its west (this is also true of 

the Williams Fork thrust to the north). Any buried branch that 

cuts across South Park would have to be entirely within the 

Mesozoic and Tertiary strata that are preserved in that graben, 

resulting in very little hangingwall/footwall distinction. 

This would locally change the basic character of the fault 

and require a corresponding local redefinition of its role as 

an uplift-defining structure. Other than the apparent desire 

to connect nearly aligned segments, there is no compelling 

justification for any buried connection since the geology of 

northern South Park is no better explained as a result. The 

obvious alternative is two discrete faults, the Williams Range 

thrust fault and the Elkhorn fault, whose displacements die 

along strike in the flanks of the broad open fold structure of 

northern South Park. This is our preferred interpretation. 

South of Muley Gulch, the southern branch is also inferred to 

die out, as only broad open folds have been identified in the 

southern parts of the quadrangle. 

Thick conglomerate and breccia deposits of the South 

Park Formation (Txc) occur near the trace of the Elkhorn fault 

trace in the vicinity of Elkhorn Ranch. The deposits form low, 

wooded hills of modest exposure and they contain boulders so 

large that they can easily be mistaken for outcrops of basement 

rocks. Clasts, which are as large as 10 m or more in diameter, 

are chiefly light-colored granitic rocks that were probably 

derived almost exclusively from Middle Proterozoic unit Yg. 

The deposits rest unconformably on Proterozoic gneiss east 

of the Elkhorn fault and on the conglomerate member of the 

South Park Formation (Tsc) west of the fault. The underlying part of the South Park Formation dips steeply to vertically below the subhorizontal unconformity at the base of the 

conglomerate indicating considerable fault-related folding 

of the older unit prior to emplacement of the conglomerate 

and breccia. It is not clear whether, or to what extent, the 

Elkhorn fault displaced the conglomerate and breccia. The 

size of the boulders and near complete lack of bedding suggest the conglomerate and breccia originated as either large 

rock slides from a nearby high-relief source area or as highly 

fractured hangingwall rock that was emplaced just in front of 

the advancing thrust. Any high relief was probably the result 

of activity on the Elkhorn fault, but all evidence of it is gone 

today, if it ever existed at all. For this reason we prefer the 

fractured-hangingwall interpretation. 

In figure 1 the position of the western edge of the 

Laramide uplift at the Elkhorn fault is shown relative to the 

west edge of the older Anasazi uplift. The edge of the Anasazi 

uplift is buried beneath central South Park (cross section 

C–C’). Pennsylvanian and Permian rocks are exposed along 

the west edge of the Bailey quadrangle and the older Paleozoic 

rock units are present a few kilometers west of the quad 

border, all forming a section that dips uniformly east. Thus, 

the uplift boundary lies east of this dipping section. However, 

between the Paleozoic exposures to the west and the Elkhorn 

fault to the east there is a zone 8–9 km wide where Upper 

Jurassic Morrison Formation is the oldest known rock unit 

above the Proterozoic basement, the entire Paleozoic rock 

section having been removed by erosion during growth of 

the Anasazi uplift. Clearly this part of the Elkhorn footwall, 

which extends north-northwest throughout central South Park, 

was within the area affected by Anasazi uplift and erosion. 

Because the contact is concealed, the nature of this ancient 

uplift boundary, whether structurally controlled or purely 

erosional, is not known.

Post-Laramide Cenozoic History in the

 Bailey Quadrangle

East Side of Bailey Quadrangle

By the close of the Laramide orogeny, during the early 

Eocene (about 50 Ma), debris eroded from the Laramide 

Front Range uplift formed a sedimentary apron, now largely 

eroded, that lapped onto the flanks of the range. By the end 

of the Eocene (about 34 Ma), a widespread erosion surface 

cut on Precambrian rock had formed across the Front Range 

(Epis and Chapin, 1975), due in part to the relatively stable 

base level, unusually warm climate, and deep weathering 

(Chapin and Kelly, 1997). The erosion surface generally 

Figure 1 (facing page). Structural setting of South Park in relation 

to the Elkhorn and Williams Range faults and the Ancestral 

Rockies.
28 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

Manitou Park formed as a post-Miocene graben. Landslides, 

disturbed topography, lineaments, and anomalous drainage 

patterns within the Tarryall Mountains would also suggest 

plausible evidence for Neogene tectonic activity. The Chase 

Gulch fault has late Pleistocene displacement (Shaffer, 1980; 

Shaffer and Williamson, 1986; Kirkham and Rogers, 1981; 

Colman, 1985). These faults align with one to the south 

that has demonstrable Quaternary displacement southwest 

of Kenosha Pass along the Elkhorn fault, so deformation 

apparently has continued into the Quaternary Period within 

South Park. 

Quaternary

Surficial deposits in the Bailey quadrangle record 

alluvial, mass-movement, glacial, and eolian processes during 

the Quaternary and late Neogene. Quaternary deposits and 

landforms in the quadrangle are the products of earth-surface 

processes during global climate-driven glacial-interglacial 

cycles during the past 1.8 million years. Global climatic 

cooling during the later part of the Pliocene continued into the 

Pleistocene and intensified after 900 ka (Clark and Pollard, 

1998). The stratigraphic record suggests that there were at 

least 12 glaciations in the mountains of the Western United 

States during the Quaternary (Richmond and Fullerton, 

1986b, chart 1). Pleistocene glacial deposits in Colorado 

and elsewhere in the Rocky Mountains are commonly 

correlated with Pinedale, Bull Lake, and pre-Bull Lake 

glaciations (Meierding and Birkeland, 1980; Pierce, 2004). 

Blackwelder (1915) named the Pinedale (last major) and Bull 

Lake (penultimate) glaciations for younger and older sets 

of moraines and glacial deposits 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 have scattered surface boulders; they are locally preserved 

just beyond the outer limits of Pinedale and Bull Lake ice. 

These deposits have been identified in a few areas north of the 

quadrangle (Meierding and Birkeland, 1980). 

Glacial deposits of pre-Bull Lake age are also locally 

exposed within the map area. They consist of till that underlies 

very subdued landforms about 2 km downstream along 

Tarryall Creek from subdued moraines of Bull Lake ages 

approximately 4 km northwest of Como, Colo. (Widmann and 

others, 2005). The till overlies poorly sorted boulder gravel 

with a silty sand matrix. Due to erosion and local burial by 

deposits of younger and more extensive glacial advances 

(Meierding and Birkeland, 1980) or glacial outwash, many 

of these older glacial deposits are difficult to distinguish. 

These older glacial deposits are included with younger tills 

in unit Qti. Glacial deposits of early Pleistocene age are 

especially likely to be buried by younger glacial deposits 

because cold (glacial)/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). Also, marine oxygen 

had low relief, but locally subdued ridges and summits stood 

as much as a few hundred meters above it (Epis and others, 

1980). Although absolute elevations are not known, there 

is no compelling reason to think that this erosion surface 

was particularly high in elevation, because there might not 

have been much difference in topographic relief between the 

present-day plains and mountains at the close of the Eocene. 

MacGinitie (1953) provided paleobotanical evidence from the 

Florissant fossil beds (≈34 Ma), approximately 10 km south 

of the quadrangle within the Front Range that suggested that 

elevation could not have been higher than 900 m (2,950 ft).

The present topographic relief along the eastern margin 

of the Front Range is due to post-Eocene erosion (Leonard 

and Langford, 1994), which removed 300–400 m of material 

from the Denver Basin area (Kelley and Chapin, 1995). This 

erosion occurred in response to profound, regional uplift that 

affected plains and mountains more-or-less equally (Kelley 

and Chapin, 1995). The present high elevations result chiefly 

from this late Cenozoic epeirogenic uplift. 

West Side of Bailey Quadrangle

Upper Eocene, Oligocene, and Miocene rock units in the 

South Park area are chiefly ash flows, ash falls, and conglomerates. Some lacustrine beds are also present. Some of these 

units may have been widespread at one time, but they are now 

confined to discontinuous and isolated exposures. 

Remnants of at least one of the ash flows, the Wall 

Mountain Tuff (Twm), are preserved on both sides of the trace 

of the Elkhorn fault, a strong suggestion that most of the relief 

associated with that structure was leveled by 36 Ma (McIntosh 

and Chapin, 2004) when the tuff erupted. The thickness of 

tuff is highly variable and contorted internal flow structures 

suggest that the ash piled up in paleovalleys where late-stage 

flow in the still-hot accumulations took place. The paleovalleys must have crossed the trace of the Elkhorn fault. 

Tuffaceous lacustrine deposits of the Oligocene Antero 

Formation (Ta) are preserved in eastern and southern South 

Park. They are thin and poorly exposed equivalents of the 

Florissant lake beds to the east. The lakes formed where thick 

volcanic deposits dammed streams on the Eocene erosion 

surface (McGookey, 2002). Exposures along the southern 

quadrangle border near Hartsel are on average 100 m lower 

in elevation than the more northerly outcrops north of Muley 

Gulch. Assuming a single level lakebed, this suggests at least 

that much differential vertical tectonism over a distance of less 

than 15 km in this area since Oligocene time. Several postAntero normal faults with northwest-southeast orientations 

are present in the area and may account for such an elevation 

difference. 

Geomorphic evidence suggests Neogene to Quaternary 

tectonic activity within South Park and the Front Range. 

The Miocene gravel at Divide (Tdg) lies on the crest of the 

Rampart Range, east of Manitou Park, in the southeast corner 

of the quadrangle. This deposit contains clasts of volcanic 

rocks with a South Park provenance. This indicates that 
Geologic History of the Bailey Quadrangle 29

isotope records show only two glacial episodes (marine 

oxygen isotope stages MIS 12 and 16; about 424–475 ka and 

621–675 ka, respectively; Lisiecki and Raymo, 2005) that 

were as severe (in terms of temperature and global ice volume) 

as those during the Bull Lake and Pinedale glaciations. This 

suggests that deposits of pre-Bull Lake glaciations prior to 

MIS 16 are likely to be less extensive than those of the Bull 

Lake and Pinedale glaciations. Bull Lake deposits in their 

type area in Wyoming are dated at about 170–120 ka (Sharp 

and others, 2003; Pierce, 2004). 

Till and minor amounts of stratified drift of the Pinedale 

glaciation form well- preserved moraines that are widespread 

in the upper part of glaciated valleys of the Mosquito Range 

and near canyon mouths in South Park. Pinedale glacial 

deposits (Qtp) in Colorado are about 30–12 ka (Nelson and 

others, 1979; Madole, 1986; Schildgen and Dethier, 2000; 

Benson and others, 2004, 2005). Till and other ice-contact 

deposits of the Pinedale glaciation may locally include or 

overlie glacial deposits of early Wisconsin age (MIS 4, 70–55 

ka; Lisiecki and Raymo, 2005), which are identified in a few 

areas of the Western United States (Pierce, 2004).

During the Bull Lake and Pinedale glaciations, glaciers 

in the quadrangle were as long as 5–9 km, and typically 

descended to elevations of about 3,000–3,200 m. These 

glaciers formed on the east side of the Continental Divide 

and descended canyons on the eastern flank of the Mosquito 

Range. Mount Logan along the North Fork of the South Platte 

River, along the northern boundary of the quadrangle, also 

supported small alpine glaciers. Glacial deposits have not 

been identified within the Platte River, Kenosha, and Tarryall 

Mountains, which are lower in elevation than the Mosquito 

Range and Mount Logan. Sharp-crested moraines produced 

by post-Pinedale glacial advances of Holocene and latest 

Pleistocene age are locally present within cirques along the 

western boundary of the quadrangle (Davis, 1988).

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 (1,731 m), 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). 

Increased 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 Mosquito 

Range, the Front Range, and the hogback belt on the west 

side of the Colorado Piedmont. Much of the coarse debris, 

which forms features such as block fields and block streams 

on interfluves above and beyond the limit of glacial ice in 

the Mosquito Range and the Front Range, probably formed 

chiefly under periglacial conditions. Some of the landslide 

deposits in glaciated valleys formed after glaciers retreated 

and glacial ice no longer provided lateral support to weakly 

consolidated material on steep, unstable slopes. Increased 

infiltration of precipitation may have locally promoted deepseated rock creep on steep mountain slopes in periglacial 

environments.

Streams draining from glaciers within the quadrangle 

produced broad, gravelly glacial outwash in mountain valleys 

and in South Park, particularly during times of significantly 

greater sediment yield during deglaciation (Church and Ryder, 

1972). Some of the glacial outwash may be slightly younger 

than corresponding tills, because fluvial deposition lagged 

(perhaps by a few to several thousand years) the onset of the 

climatic change from glacial to interglacial climates (Church 

and Ryder, 1972; Hancock and Anderson, 2002). Glacial outwash deposits within South Park are mapped as intermediate 

(Qai) and young (Qay) alluvium of late middle Pleistocene 

and late Pleistocene respectively. These deposits grade upvalley to coeval glacial deposits. 

Glacial outwash (Qay) and till of the Pinedale glaciation 

(Qtp) on and near the east flank of the Mosquito Range and 

coeval alluvium of non-glacial origin (Qay) in South Park are 

correlated with the Broadway Alluvium (Qb on Denver West 

quadrangle) in the Colorado Piedmont. Likewise, older glacial 

outwash (Qai) and till of the Bull Lake glaciation (Qtb) on 

and near the east flank of the Mosquito Range and coeval alluvium of non-glacial origin (Qai) in South Park are correlated 

with the Louviers Alluvium (Ql on Denver West quadrangle) 

(Scott, 1975; Madole, 1991). These correlations (table 1) are 

based chiefly on the morphology of surface soils formed in 

these deposits (Birkeland and others, 2003) and heights of 

glacial outwash and piedmont alluvial deposits above present 

streams. Recent cosmogenic dating supports these correlations (Schildgen and Dethier, 2000). About 50 km north of 

the map area, 10Be and 26Al ages of Pinedale (30–11 ka) and 

Bull Lake (≈130 ka) glacial outwash in Boulder Canyon east 

of Nederland (Schildgen and others, 2002) and their heights 

above stream level, support the concept that Pinedale outwash 

(8–12 m) is equivalent in age to Broadway Alluvium (Qb on 

Denver West quadrangle), and Bull Lake outwash is equivalent 

in age to Louviers Alluvium. Alluvial deposits of Holocene 

age are in present flood plains and form terrace deposits less 

than 5 m above present streams in the Colorado Piedmont. 

They are difficult to correlate with specific climatic episodes, 

but some may reflect past climatic conditions that were colder 

and possibly more moist (or due to more effective precipitation) than at present. These latter climatic episodes promoted 

glacial and (or) periglacial activity in the Front Range during 

the Holocene (Scott, 1975). 

The depositional chronology, as well as the climatic 

(glacial or interglacial) and fluvial conditions that prevailed 

during the deposition of most of the pre-Louviers (pre-Bull 

Lake) alluvial deposits in the Colorado Piedmont near Denver, 

can only be inferred. Pre-Louviers alluvial deposits were 

transported and deposited by streams that headed in glaciated, 

as well as nonglaciated drainage basins in the Front Range. 

Many of these deposits probably accumulated chiefly during 

glacial episodes, particularly during maximum glaciation or 
30 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

deglacial phases of glacial episodes, when climatic (glacial 

and periglacial) and fluvial conditions, as well as abundant 

sediment supply, promoted increased stream discharge and 

sediment load (Church and Ryder, 1972; Sinnock, 1981; 

Ritter, 1987; Madole, 1991). However, events unrelated 

to glaciation, such as stream capture and minor periods of 

cutting and filling unrelated to major climatic events (such as 

lateral migration of the South Platte River and its influence on 

tributary streams), likely accounted for deposition of some of 

these deposits (Ritter, 1987; Reheis and others, 1991). 

Comparison of the isotopic ages for deposits of the 

Pinedale glaciation (30–12 ka) and Bull Lake glaciation 

(170–120 ka) with cold and warm climatic episodes of the 

marine oxygen isotope record (for example, Shackleton and 

Opdyke, 1973, 1976; Lisiecki and Raymo, 2005) suggest that 

outwash deposits of the Pinedale (Qtp) and Bull Lake (Qtb) 

glaciations and the correlative fluvial deposits in the Colorado Piedmont (Broadway Alluvium and Louviers Alluvium, 

respectively) were deposited chiefly during major cold climatic episodes, and in part during succeeding warm climatic 

episodes. These relations suggest that pre-Louviers alluvial 

deposits in the Colorado Piedmont may have accumulated 

under somewhat similar climatic conditions.

 Younger (topographically lower) and older (topographically higher) deposits of Slocum Alluvium (Qs) are locally 

present near the northeast corner of the quadrangle. These 

deposits are about 20 and 30 m, respectively, above flood 

plains of Plum Creek and the South Platte River. The younger 

deposits may have accumulated between about 300 and 220 ka 

and older deposits between about 390 and 320 ka (Kellogg and 

others, 2008, table 1).

Table 1. Correlation of alluvial deposits along the western margin of the Colorado Piedmont near Denver, Colo. with

alluvial deposits in South Park and glacial deposits in the Rocky Mountains. Modified from Kellogg and others (2008).

Deposits in

Colorado 

Piedmont

Deposits in

South Park

Age or 

age estimate

(ka)

Correlative glaciation

in

Rocky Mountains

Post-Piney Creek Alluvium

and

Piney Creek Alluvium

Valley-floor alluvium

(Qa)

0–4a Minor post-Pinedale

ice advances

Broadway Alluvium Young alluvium

(Qay)

12–30b Pinedale

No known deposits No known deposits 55–70c Early Wisconsin

Louviers Alluvium Intermediate alluvium

(Qai)

120–170b Bull Lake

Slocum Alluvium Old alluvium

(Qao)

240–390d Pre-Bull lake

Verdos Alluvium

older deposits

Old alluvium

(Qao)

610–675e Sacagawea Ridge

Rocky Flats Alluvium No known deposits 1,40 –2,000f No known episode

a

Age of alluvial deposits based on limited radiocarbon analyses (Scott, 1962; 1963a; Madole, 1976; Lindsey and others,

 1998; Madole and others, 2005).

b

Age of till and outwash of the Pinedale glaciation is based on radiocarbon and cosmogenic isotopic ages (Nelson and 

 others, 1979; Madole, 1986; Schildgen and Dethier, 2000; Schildgen and others, 2002; Benson and others, 2004; 2005).

 Age of till and outwash of the Bull Lake glaciation is based on cosmogenic isotopic ages (Schildgen and Dethier, 2000;

 Schildgen and others, 2002; Sharp and others, 2003; Pierce, 2004).

c

Age estimate is for marine oxygen isotope stage 4 (Lisiecki and Raymo, 2005).

d

Age estimate derived from uranium-series dating (Madole, 1991; Kellogg and others, 2008).

e

Age estimate for alluvial deposits based on stratigraphic position of 640-ka Lava Creek B tephra and tentative correlation

 with marine oxygen isotope stages (Kellogg and others, 2008).

f

Age estimate derived from soil characterisitics and paleomagnetic data (Birkeland and others, 1996) and could be as old as

 2 Ma (Birkeland and others, 2003; Riihimaki and others, 2006; Kellogg and others, 2008).
Acknowledgments 31

Younger (topographically lower) and older 

(topographically higher) deposits of Verdos Alluvium 

(Qv) are also locally present near the northeast corner of 

the quadrangle. These deposits are about 55 and 75 m, 

respectively, above the present flood plain of Plum Creek. 

The age of older deposits of Verdos Alluvium is fairly well 

constrained, because these deposits locally overlie, contain, 

or are overlain by water-laid deposits of the Lava Creek B 

tephra (640 ka) (Machette, 1975; Machette and others, 1976; 

Van Horn, 1976), which was deposited when glaciers were 

retreating, during the transition from glacial to interglacial 

climatic conditions (Dethier and others, 2001; Fullerton, D.S., 

oral commun., 2006). This indicates that older deposits of 

Verdos Alluvium were deposited near the end of a major cold 

climatic episode during MIS 16 (675–621 ka; Lisiecki and 

Raymo, 2005), and possibly in part during the early part of 

MIS 15. These deposits are similar in age to till and outwash 

of the Sacagawea Ridge glaciation near the type area for this 

glaciation on the eastern flank of the Wind River Range, Wyo., 

where water-laid Lava Creek B tephra is present in the top of 

the outwash (Jaworowski, 1992; Chadwick and others, 1997). 

The stratigraphic position of the water-laid tephra with respect 

to older pediment and fluvial deposits of the Verdos Alluvium 

(Qv) suggests that the older pediment deposits are younger 

than the older fluvial deposits (Machette, 1975). The younger 

deposits may have accumulated between about 475 and 410 ka 

(Kellogg and others, 2008, table 1). Younger deposits of the 

Verdos Alluvium may be correlative in part with shoreline and 

nearshore deposits that formed during a high stand of Lake 

Alamosa in the San Luis Basin in southern Colorado at about 

450 ka (Machette and others, 2007). If the age estimate for 

younger deposits of Verdos Alluvium is correct, it suggests 

limited fluvial deposition during cold (glacial) climatic 

conditions of marine oxygen isotope stage 12 (about 475–425 

ka) in the Colorado Piedmont near Denver. Alternatively, 

younger deposits of Verdos Alluvium may locally include 

alluvium deposited during marine oxygen isotope stage 14 

(about 570–533 ka) as well as during stage 12. 

 Cosmogenic dating (on 10Be and 26Al) of alluvialfan deposits that are typically ≤5 m thick (Knepper, 2005) 

that constitute the upper part of the Rocky Flats Alluvium 

(QNr) at Rocky Flats indicates that they date from about 1.5 

Ma (Dethier and others, 2001) or about 1–2 Ma (Riihimaki 

and others, 2006). However, at the Rocky Flats type section 

(Scott, 1960; Birkeland and others, 1996), the buried soil 

formed on underlying valley-fill deposits, as much as 25 m 

thick (Knepper, 2005), suggesting that the lower part of the 

unit may be much older. Morphologic and paleomagnetic 

properties of buried soils at a site about 3 km south of the 

Rocky Flats plant support the cosmogenic ages and suggest 

that the Rocky Flats Alluvium dates from at least 1.6–1.4 Ma 

(Birkeland and others, 1996), and possibly is about 2 m.y. 

old (Birkeland and others, 2003). Although the Coal Creek 

drainage basin (the source of the Rocky Flats Alluvium on 

Rocky Flats) was never glaciated, the Rocky Flats Alluvium 

may have been deposited during one or more glacial episodes. 

Considering the age constraints for the Rocky Flats Alluvium 

and the fact that no evidence has been found for a major 

glaciation in the United States between 1.5 Ma and 900 ka 

(Richmond and Fullerton, 1986a; 1986b, chart 1), the Rocky 

Flats Alluvium may have been deposited after 2.5 Ma but prior 

to 1.5 Ma, the onset of major northern-hemisphere glaciations 

(Shackleton and others, 1984; Prell, 1984; Thompson, 1991; 

Ravelo and others, 2006).

Eolian deposits are widespread east of the mountain 

front in the Colorado Piedmont northeast of the quadrangle 

(Trimble and Machette, 1979). Thin, unmapped deposits 

locally mantle many of the older alluvial surfaces and deposits 

in South Park. Age assignments, spatial distribution, and 

downwind fining of the particle size of eolian sand and loess 

in the Colorado Piedmont suggest a close relation between the 

genesis of these deposits and active aggradation of sparsely 

vegetated flood plains, chiefly during the Pinedale and Bull 

Lake glaciations. Some of the loess, and possibly some of the 

eolian sand northeast of the quadrangle, were derived directly 

from bedrock sources (Scott, 1962; Madole, 1995; Aleinikoff 

and others, 1999). Probably much of the loess within the map 

area was deposited during two episodes during the Pinedale 

glaciation (about 20–14 ka and 13–10 ka; Muhs and others, 

1999). Some accumulation may have occurred during the 

Bull Lake glaciation (about 150 ka; Forman and others, 1995). 

Episodes of loess deposition probably were coeval with 

episodes of flood plain aggradation.

Acknowledgments

Previous work by and collaboration with geologists of 

the Colorado Geological Survey (CGS) greatly improved 

our understanding of the map area. Discussion in the field 

and office, and geophysical modeling by Lyndsay Ball and 

Jonathan Caine (U.S. Geological Survey, USGS) clarified 

and facilitated our interpretations of the structure of South 

Park, in particular, the Elkhorn thrust and its subsidiary 

faults and associated folds. We benefited from comments by 

participants during a one-day field trip in South Park led by 

Ruleman, Bohannon, and Kirkham (consulting geologist) for 

the Colorado Scientific Society. This report was improved 

by comments by John C. Reed, Jr. (USGS) and Jay Temple 

(CGS). We thank Jeremy Workman and Paco Van Sistine 

(USGS) for preparing the GIS database. And finally, we thank 

Gayle Dumonceaux for her expertise and exhaustive efforts in 

performing the final edit and graphic design of this work.
32 Geologic Map of the Bailey 30’ × 60’ Quadrangle, North-Central Colorado

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