U.S. DEPARTMENT OF THE INTERIOR MISCELLANEOUS FIELD STUDIES MAP MF–2347 U.S. GEOLOGICAL SURVEY Version 1.0 PAMPHLET

GENERALIZED SURFICIAL GEOLOGIC MAP OF THE DENVER 1° × 2° QUADRANGLE, COLORADO

Compiled by David W. Moore, Arthur W. Straub, Margaret E. Berry, Michael L. Baker, and Theodore R. Brandt

2001

CONTENT AND PURPOSE rock landslide deposit” and “stabilized dune sand.” Large areas (generally >1 km2; >0.4 mi2) of exposed This map locates surficial geologic deposits of bedrock, mostly devoid of surficial materials, are Quaternary age in the greater Denver area and mapped with a symbol “R.” surrounding region. Physical properties of the deposits are described. The physical properties PARTICLE SIZE AND COLOR include dominant grain size, composition, potential to expand upon sorption of water, and others. Origins TERMINOLOGY and ages of the deposits, and their inherent geologic hazards are identified. The properties of deposits and We used geological and soil textural terms to materials on and near the land surface affect its describe the size of particles in the surficial deposits. suitability for societal uses. Examples of such use Particle sizes ranged from microns to meters in include large-scale landfill sites, gravel pits and rock diameter. The combined use of geologic and soil quarries, large-scale residential developments, terminology resulted from a necessity to consult greenbelts, utility lines, and roads. Planners, zoning various data sources to characterize the materials. officials, builders, and interested citizens should have Clasts are particles larger than 2 mm in diameter. We a clear grasp of how the properties of surficial used geologic terms to describe clasts. Particles 2 mm deposits affect the growing urban sector within the or smaller in diameter make up the matrix of the Denver 1° × 2° quadrangle. This map provides basic deposits. Both geologic and agronomic terms are data that can be used to derive other maps that show used to describe the matrix. suitability of land for specific purposes. The clasts are divided into granules (>2 mm–4 Apart from its usefulness to urban planning, the mm; >1/12–1/6 in.), pebbles (>4–64 mm; >1/6–2.5 map can be a tool for the management and research in.), cobbles (>64–256 mm; >2.5–10 in.), and of natural resources. It can enhance ecological boulders (>256 mm; >10 in.) (see Bates and Jackson, surveys that were made by using satellite imagery. 1980). The matrix consists of three particle-size Certain biogeographical habitats, for example foothill classes: clay (<1/256 mm; <0.00016 in.), silt (1/256– woodlands and alpine forests, are influenced by soil- 1/16 mm; 0.00016–0.0025 in.), and sand (>1/16–2 related factors and correlate well with certain units on mm; >0.0025–0.08 in.). Terms like “sandy silt” and this map. Other map data such as the ages and origins “gravelly sand” reflect visual estimates by geologists. of the Quaternary surficial deposits are essential to We accepted some of these terms from previously any geomorphologic study of the Denver quadrangle. published reports and assigned some terms ourselves. For this purpose we compared samples from deposits to standard grain-size samples on an American MAP UNIT NAMES Geological Institute chart. We also used soil science terms to characterize The map unit names derive from a classification the matrix. For example, “clay loam” conforms to of geologic surficial materials. The names are based U.S. Department of Agriculture (USDA) terminology on prevalent grain size, minerals (or rock types), and in the Soil Survey Manual (Soil Survey Staff, 1951). geologic origin. Examples are “bouldery crystalline- These are soil textural class names (fig. 1), and they

1 describe the proportions of sand, silt, and clay. Soil METHOD OF MAPPING survey reports identified the soil textural class of the C horizon and parent materials in selected areas that We generalized the boundaries of some units on we mapped. this map from some geologic unit boundaries shown Our fieldwork required another source of grain- on the “Geologic Map of the Denver 1º × 2º size terms. This terminology used the USDA field Quadrangle” (Bryant and others, 1981). Many of our determination of soil textural class method (Soil unit boundaries, however, were drawn on the basis of Survey Staff, 1951, p. 212). This method requires our field observations and on our verification of the deformation of samples of the matrix by using the boundaries shown on other published, detailed fingers. The manner by which samples deform geologic maps. Also useful were agronomic soil indicates a basic soil textural class. maps on which a soil unit or units correspond to a Color terms describe clasts and the matrix of surficial geologic unit, because both soils and materials and deposits. Colors of clasts follow geologic units correspond closely to landforms. Our geologic convention. Some terms derive from geologic map unit descriptions incorporate color, published reports, and some we assigned from color grain size, thickness, and chemical data from the terms in the Rock Color Chart (Goddard and others, weathered zone (C soil horizon) immediately above 1970). Colors of the matrix were determined by bedrock, or non-soil, unconsolidated deposit. We did comparison with the Munsell soil color charts, which not incorporate properties of the solum (A and B are used by soil scientists and Quaternary geologists. horizons) of modern soils that are in the surficial deposits and materials. Diverse residual, colluvial, eolian, and alluvial UPLAND MATERIALS deposits and materials are present in the quadrangle. In general, the most widespread of these was mapped The origin of the names we apply to many in a polygon. Consequently, materials or deposits deposits, such as “stabilized dune sand,” are obvious. other than the prevalent one may be present in a However, we use two terms, “colluvium” and mapped area. Some users of the map may wish to “residuum,” which have several meanings. We define grasp the diversity of surficial deposits and materials them here for the purposes of our map. Colluvium in relatively small areas of the quadrangle. This may and residuum cover hillslopes and uplands, be accomplished by examining certain detailed respectively. Colluvium is unconsolidated rock geologic maps, for example those of Malde (1955) detritus and soil material that was very slowly and Machette (1977). transported and deposited on slopes, mainly by The user should not extend the generalized map gravity (some detritus also was removed by runoff). data into small areas without consulting supplemental Because colluvium creeps downslope, hillslopes data. Our map does not show the variation of deposits covered by it may be unsuitable and costly slope- at the scale of most home and commercial stabilization work may be required. Colluvium is construction sites. In general, soils engineers and mapped in areas dominated by hillslopes steeper than engineering geologists should investigate specific about an 8 percent (4° 34′) gradient and mapped in sites prior to construction. areas larger than 2 km2 (0.77 mi2). Residuum was derived primarily by in-place disintegration of exposed rock, and surface processes have not BASE MAP transported the material significantly. We mapped residuum on nearly level land that lacks widespread The Denver 1º × 2º topographic quadrangle map deposits of other origins. It was not mapped on slopes (scale 1:250,000, prepared by the Defense Mapping steeper than about 8 percent. Eight percent is a Agency Topographic Center, Washington, D.C., standard slope category boundary in the USDA land 1953, revised 1978) is the base for our digital classification (Soil Survey Staff, 1951, p. 163). That surficial geologic map. In making this digital map, system separates level and nearly level land from we scanned the base, printed as black lines on clear more sloping land. The system is consistent in the film, on a large-format scanner to make a raster USDA county soil reports (for example, Larsen and image (tiff file). The image file was georeferenced Brown, 1971), as is our distinction of colluvium and and imported into a computer-aided drawing program residuum on the surficial geologic map. to make the map layout. The original 1978 paper base map shows roads, streams, place names, and contours printed in different colored inks. The published paper map, thus, may be easier for some users to read than the all-gray scanned base.

2 100

90 10

80 20

70 30 clay 60 40 percent silt

50 50 percent clay silty clay sandy 60 40 clay clay loam silty clay loam 70 30 sandy clay loam 20 80 loam sandy loam silt loam 10 90 loamy silt sand sand 100 100 90 80 70 60 50 40 30 20 10

percent sand

Figure 1. Chart showing the percentages of clay, silt, and sand in the basic soil textural classes (Soil Survey Staff, 1951).

3 ACKNOWLEDGMENTS 20–100 cm thick, to moderately or poorly cemented, interbedded, clayey This map is based in part on the mapping of sandstone, sandy claystone, and Bryant and others (1981), but it differs from the latter siltstone. Particles are mostly quartz, in that it focuses on the unconsolidated, geologically feldspar, granitic rock detritus; young, surficial deposits and materials. We thank andesite is abundant locally. In the Bruce Bryant for the loan of his original map and for Colorado Piedmont south of Denver, his discussions about rocks and surface deposits in and within 10 km of the east front of the Colorado Front Range. Glenn Scott, Dan Muhs, the mountains (especially in and Rich Madole kindly shared first-hand knowledge northwest El Paso County west of and unpublished information. We thank Ted Brandt Monument Creek), the unit includes for his assistance with computer applications, and remnants of intensely weathered, Diane Lane for advice in preparing the map layout. clayey and sandy gravel pediment and terrace alluvium at various high topographic levels above modern DESCRIPTION OF MAP UNITS streams. Many of the remnants are 150–300 m wide, and are too small to HOLOCENE be mapped. Within areas mapped are a few buttes. On the buttes sandstone afa Alluvial-fan deposit—Reddish-brown, boulders 0.5–6 m in diameter are brown, and brownish-gray, gravelly scattered on slopes below ledgy and clayey sand. Many subangular outcrops of pale-brown sandstone and pebbles, cobbles, and boulders, conglomeratic sandstone (where such mostly pink to reddish-tan, biotite or terrain predominates, a bouldery pyroxene-hornblende-biotite phase, unit cacb, was mapped). monzonite; locally sandstone, Moderately to steeply sloping uplands limestone, and dolomite. Clast include areas of class 1–2 stoniness lithologies reflect the source rock and class 2–3 rockiness (Soil Survey lithologies, which vary locally. Staff, 1951), a discontinuous lag of Torrentially crossbedded gravel is rounded granules, pebbles, cobbles, concentrated in the lowest beds; cut- and sparse boulders of granite, vein and-fill structures are common. High quartz, gneiss, schist, and 2–4 percent part of fan (apex and proximal part) is moderate-red arkosic sandstone. The moderately steep and bouldery, and it map unit includes gently sloping contains matrix-supported, poorly areas veneered by eolian silt and sand sorted, debris-flow deposits. Deposit (about 1 m thick). The eolian deposits was mapped if >600 m across. Many contain slowly permeable clayey deposits are a few tens to a few layers. Map unit includes channel and hundred meters in extent, too small to floodplain alluvium of intermittent map at the scale of this map. Alluvial- streams, mostly sandy clay to clayey fan deposits accumulate where turbid, sand, gravelly at the base. A dense rapid-flowing runoff debouches onto network of ravines in the Plum and nearly level land. Thickness near the Cherry Creek drainages (central center of a deposit may be as much as Douglas County) cuts the colluvium 5–10 m and sheetwash alluvium deposit. In cac Arkosic loamy colluvium and sheetwash the headwater reaches of a few alluvium—Light-gray, reddish- streams, the unit includes unmapped, brown, and olive-brown loam, sand, fluvial terrace gravel. Scattered, and sandy clay; locally gravelly, small, unmapped, inactive eolian sand including large boulders. Runoff has dunes are included in the unit. washed clay from the upper part Colluvium and sheetwash generally (upper 5–10 cm), concentrating are 0.3–1.5 m thick micaceous, fine- to coarse-grained cacb Bouldery phase of arkosic loamy quartz and feldspar sand. The colluvium—Yellowish-gray, pale- surficial sand grades downward brown, and grayish-brown, gravelly through oxidized colluvium (loam (10–30 percent) loam and clay loam and clay loam) and weathered rock containing weathered, very pale

4 orange, arkosic, pebbly sandstone and ironstone (reddish-brown, brownish- conglomerate boulders. Boulders are black, and grayish-red siderite, abundant on hillslopes below ledgy calcite, and pyrite) and (or) limestone outcrops. Colluvium contains concretions and partly weathered abundant to common pebbles and shale bedrock, and shale-chip cobbles of gneiss, coarse-grained colluvium. The residuum is deeply pink granite, and, locally, minor pale- cut by ravines and arroyos in places. reddish-purple and light-gray welded Residuum grades downward to dark- rhyolitic tuff. Boulders are grayish-brown, olive-gray, weathered moderately to well cemented, 0.5–6 shale, silty or sandy claystone, clayey m across, and subround to blocky. thin-bedded sandstone, chalky shale, They cover 20–80 percent of silty blocky shale, weathered chalky hillslopes. Rockfall and toppling shale, or disintegrated platy failures of cliffs are a hazard. The limestone. In some places residuum is colluvium is subject to debris flow thin on silty, hard, very dark gray during torrential rainfall. Unit occurs shale that contains thin beds (1–10 on slopes of buttes, mesas, and cm thick) of bentonitic clay. Where uplands that are capped by sandstone these beds dip about 30° or more, or tuff. In the Castle Rock area, and they are susceptible to differential in southeast Douglas County and expansion toward the land surface north El Paso County, it locally (“heaving”), owing to sorption of includes minor angular gravel-size water by the clay minerals. Map unit pieces of rhyolitic vitric ash-flow tuff includes zones of ledge-forming (Wall Mountain Tuff, called “Castle platy, moderate-yellowish-orange Rock Rhyolite” in the dimension- limestone concretions as much as 2–4 stone building trade in the late m in diameter. Unit is approximately 1800’s). Thickness generally 2–4 m 0.5–1.5 m thick xch Clayey, calcareous disintegration residuum—Dark-grayish-brown, HOLOCENE AND LATE PLEISTOCENE light-yellowish-brown, olive to light- olive-brown, fine, platy shale Disintegrated deposits and periglacial residuum. Silt, clay, and minor sand deposits, bouldery till, and rock are present in variable proportions outcrop—Disintegrated, nonsorted, (clay loam and silty clay loam in the blocky deposits in high alpine and upper meter, and, at increasing depth, montane glaciated areas. Silt to silty clay and clay). The residuum is boulders; matrix commonly is sandy very calcareous, locally micaceous and micaceous. Angular rock and carbonaceous, and contains fragments form talus cones and limestone concretions. The surface aprons, protalus ramparts, rock- layer generally is pebbly, silty clay glacier deposits, block fields, loam and silty clay, 1–10 cm thick. solifluction or frost-heave deposits, Scattered, discontinuous veneers of colluvium, fan alluvium, and debris- quartz and feldspar sand granules, flow deposits. The deposits vary in rounded pebbles, and small cobbles genesis in short lateral distances. Map (chiefly granite and quartz) have unit includes some small, accumulated as a surface lag. The discontinuous deposits of bouldery residuum is firm or plastic when Pinedale till and scattered erratic moist, hard when dry. Unit is clasts on glacier-scoured bedrock. generally gypsiferous, and locally it Excludes till in large end and lateral contains small (0.2–1 cm) crystals of moraines (mapped separately as tb selenite (gypsum). The residuum may units). Clasts generally are angular, be saline and alkaline at shallow subangular, or subrounded; in some depth. Residuum is on nearly level to places boulders are larger than 5 m in moderately sloping land; locally, it is diameter. Map unit locally includes characterized by low hillocks, and it minor sheetwash alluvium and small includes ledgy bedrock outcrops. areas of lake silt and sand in glacially Locally, the residuum includes excavated lake basins. Map unit includes bare bedrock on some ridges

5 and massive peaks. Till, rock glacier boulders exceed 5 m in diameter. The deposits, and local lake deposits are surface soil on scree locally is dark- present in ice-carved basins at heads brown silt and sand 8–12 cm thick, of valleys. Talus accumulates on and enriched in organic matter. Below below steep valley sides. Colluvium, this is 20–30 cm of yellowish-brown solifluction deposits, and frost-heaved sandy gravel containing as much as deposits locally cover gently or 20 percent silt and clay. Deposits are moderately sloping uplands, high 1.5–8 m thick on fractured bedrock tundra-covered surfaces, and dbd Mixed-lithology-clast deposits— moderate slopes on valley sides. Disintegrated deposits, periglacial Locally, includes deposits of alpine deposits, and bouldery till that have debris flows and mudflows. mixed bedrock lithologies. Mapped in Snowmelt and heavy rain triggers the Mosquito Range (west boundary debris flows (Curry, 1966). Debris- of map area) where various lithologic flow deposits, composed of randomly source rocks produce deposits oriented, angular gravel supported in containing fragments of mixed a fine-grained matrix, form fans, lithology. Clasts are Precambrian lobate aprons, and concentric granite, gneiss, and migmatite, constructional ridges at the bases of Tertiary intrusive rock, and Paleozoic steep slopes. More extensive lichen sandstone, conglomerate, shale, cover, more well developed soil, and limestone, dolomite, and quartzite more pronounced rock weathering asa Alluvial sand, silt, clay, and gravel (post- phenomena characterize the older Piney Creek alluvium, Piney Creek disintegrated deposits that are Alluvium, and pre-Piney Creek included in this map unit. The alluvium of Hunt, 1954, and Scott, youngest (Holocene) deposits range 1960); Broadway Alluvium)— from unoxidized deposits lacking soil Underlies flood plains and low stream development, to oxidized deposits terrace surfaces (mapped only where that have weakly developed B soil width is >0.2 km). In the flood plain horizons. Thin loess covers some of of the South Platte River the deposit the older deposits. Locally, the unit overlies older alluvium (ags). Low includes disintegrated deposits that parts of mapped areas are susceptible accumulated during the Little Ice age to stream flooding. Following a (A.D. 1450–1850), the neoglacial general description of alluvium in the interval (younger than 6,000 yrs), and quadrangle, alluvium of the South early Holocene to late Pleistocene Platte River is described. (Richmond, 1986) General description: Light- dba Crystalline-clast deposits— yellowish-gray, grayish-brown, Disintegrated deposits, periglacial moderate-brown, locally reddish- deposits, and bouldery till that are brown, quartz sand and sandy gravel; composed mostly of Precambrian sand commonly includes feldspar and rock (granite, hornblende, biotite, and mica particles. Commonly dark-gray felsic gneiss, quartz monzonite, and and dark-grayish-brown granodiorite) or Tertiary intrusive noncalcareous silt and clay layers are rock (light-gray and white quartz thick and persistent in upper 0.4–2 m; monzonite and porphyritic rhyolite). grades downward to yellowish- Matrix is absent in coarse talus and is brown, calcareous, moderately present in most other deposits. Matrix alkaline loam and sandy loam at a generally is gray and grayish-brown depth of 2–4 m. Sandy rounded or yellowish-brown silt to coarse gravel or gravelly sand at base. In or sand; typically micaceous. Scree on near (as far as 15 km east of) the steep slopes above timberline on a mountains, 40–70 percent of the typical Tertiary intrusive mass (for alluvium is gravel; 10–30 percent of example, Red Mountain, south of the gravel typically is cobbles and Berthoud Pass) is composed of boulders (Trimble and Fitch, 1974). angular and subangular clasts, mostly The flood-plain and channel alluvium 10–30 cm in diameter; 2–5 percent of interfingers laterally with fan clasts are about 1 m across. A few

6 alluvium and colluvium. Poorly to cobbles of granite, biotite-quartz well stratified, planar bedded and gneiss, quartz, ironstone, feldspar, crossbedded. Upper 0.5 m commonly welded tuff, and minor amounts of is gray, humic-rich (as much as 10 chert, sandstone, quartzite, altered percent by volume), silty, very fine to amphibolite, aplite, and pegmatite. fine sand, especially in flood plains of The alluvium is veneered by fine sand tributary streams (Simpson, 1973b). washed onto the terrace from small Gravel deposited by rivers that head side-valley drainageways. Thickness in the mountains typically is rounded of the alluvium is 5–7 m along Santa granitoid plutonic rock, including Fe Avenue in Littleton. Younger biotite and hornblende-biotite granite, Holocene alluvium generally is biotite gneiss, felsic gneiss, schist, chiefly pebbly quartz sand. Some and local quartzite, metaquartzite, older deposits of Holocene age have mafic-rich intrusive rock, minor accumulations of secondary amphibolite, and hornblende schist. calcium carbonate. Some deposits Granite and quartzite gravel is a contain sparse fossil bones of bison valued aggregate for concrete and and mule deer. The upper part of the road metal because the clasts are alluvium in modern flood plains is sound and in places the deposits humic and fine grained. contain little interstitial lime and few In Denver County, asa alluvium of or no reactive constituents (Trimble the South Platte River generally is 6– and Fitch, 1974). In deposits of rivers 12 m thick (it overlies thicker, older that drain glaciated terrain (for alluvium of similar composition), is example, Clear and Bear Creeks, and about half sand and half gravel, and the South Platte River), beds of the gravel is composed mostly of massive, clast-supported gravel granite, pegmatite, quartz, and minor alternate with thinly bedded sand and gneiss and schist. Thick interbeds of organic-rich sediment (Madole, clay are present locally. In small 1991). Deposits of rivers that drain tributary valleys, the alluvium is 2– unglaciated terrain contain mostly 4.5 m thick. Beneath the Monument pebbles of quartz, sandstone, Creek flood plain the alluvium is limestone, feldspar, calcium about 3 m thick carbonate (caliche), marl, and shale in ed Stabilized dune sand—Grayish-brown (at a sandy matrix. In the southwest surface), light-yellowish-brown, and corner of quadrangle (east and south yellowish-gray, very fine to medium of Fairplay), the gravel is mostly quartz sand and loamy sand (3/8–3/16 hypabyssal igneous rocks, felsic mm diameter, Muhs and others, porphyry, maroon arkosic sandstone 1996). In some places the deposit is and siltstone, scarce quartz and schist, 10–15 percent clay and 10–30 percent medium-gray limestone, and silt. The dune sand is moderately well aphanitic igneous rock. Locally, the sorted, subangular to rounded, quartz alluvium of major rivers includes and feldspar sand grains. Sand is discontinuous silty and clayey beds as calcareous below about 1–m depth; thick as 4 m. locally, the deposit contains white Alluvium of the South Platte River: calcium carbonate and sodium in Denver, Broadway Boulevard is on bicarbonate in soft masses. On the a Broadway alluvial terrace. The Great Plains in uncultivated areas, following is a description of dunes downwind from rivers and Broadway Alluvium, of late their major tributaries are stabilized Pleistocene age, exposed in a fluvial by vegetation (commonly sand sage, terrace 8–10 m above the river and Artemisia filifolia). Dune sand forms 0.8 km east of it, near Highway C– gently rolling land (3–20 percent 470 in Littleton: brown, grayish- slopes), stabilized parabolic dunes brown, and moderate-yellowish- (arms point northwest), and irregular brown (reddish-brown locally) hilly to hummocky terrain; dunelike pebbly, silty, micaceous sand. Fine to uplands having no external drainage very coarse sand, rounded granules commonly are 3–10 m higher than the (~30 percent), rounded pebbles and

7 adjacent river valley. Scattered deposits have characteristic large- blowout depressions and minor active scale crossbedding. The upper part of dunes indicate that the deposit is the eolian sand commonly contains a susceptible to wind erosion. Common well-developed Brown soil (formerly bedding types in the deposit are a Great Soil group of zonal soils massive bedded, planar bedded, and having brown surface and light- low-angle crossbedding. Dune sand colored subsurface zones over an locally contains buried discontinuous accumulation of calcium carbonate; calcium carbonate-cemented layers, now classified as Ustoll or Xeroll organic-rich silt layers, and (or) soil). With regard to the age of the buried paleosols. These features eolian sand, Scott and Lindvall suggest that intervals of dune (1970) stated that “we believe it was building were interrupted by periods deposited starting in Pinedale time of stability. Intervals of dune and ending when the ‘Altithermal’ formation may have been soil was formed ca. 4,500 yrs ago.” characterized by a copious supply of Eolian sand covers areas of nearly sand, persistent strong winds, and (or) level to very gently undulating land. relatively sparse vegetation. Recent It includes small areas of vegetated, studies in eastern Colorado indicate stabilized, dunes and blowout that there were several intervals of depressions (not mapped) and imparts dune building during the past 20,000 a large-scale (township in size) yrs. The presence of buried soils, landform grain that is elongated in a changes in fossil pollen, and ages northwest-southeast direction. The obtained by radiocarbon and deposits mantle uplands, mainly thermoluminescent dating techniques downwind from river valleys of the indicate reactivation of dunes about Colorado Piedmont and High Plains. 6,000, 4,500, and 1,000 yrs ago, and a Along the South Platte River the unit longer episode of dune formation overlies older Broadway Alluvium. In during the last glacial cycle extended general, grain size decreases from from 20,000–12,000 yrs ago (Forman northwest to southeast, grading to a and others, 1995). Reduced grass mantle of windblown silt and sandy cover associated with regional silt (elb) or thinning to scattered thin drought can cause reactivation of deposits that mix with, and are dunes. Other studies (Madole, 1994; separated by, sandy and silty Muhs and others, 1997) indicate that sheetwash alluvium and residuum. In upper parts of the dune deposits in the metropolitan areas, eolian landforms northeast part of the quadrangle and are leveled for building purposes. in Nebraska accumulated less than Where the deposits thinly cover the 1,000 yrs ago, possibly as recently as Denver and Dawson Formations during the last two centuries. Where (bedrock), excavations may encounter the deposit is thin, overlying the expandable materials (for example, Denver and Dawson Formations along Highway C–470 east of (bedrock), excavations may encounter Chatfield Reservoir in northernmost expansive clay minerals in underlying Douglas County). Thickness weathered claystone. Thickness generally less than 6 m; as thick as generally 3–10 m 12–15 m east of the South Platte es Eolian sand—Yellowish-brown, dark- River between Denver and Brighton, yellowish-brown, and light-olive- at north edge of map area brown, chiefly very fine to medium jea Slump-block, earthflow, and mudflow quartz sand; some coarse sand and, landslide deposits—Products of locally, silty and slightly clayey sand. gravitational downslope movement of Well to moderately well sorted. Less partly disaggregated sedimentary than 5 percent feldspar grains, 2–5 bedrock and (or) surficial materials. percent lithic grains, and about 2 Bedrock sources of most deposits are percent heavy mineral grains. Sand is poorly cemented shale, marl, clayey generally a sheetlike deposit. Locally, sand, mudstone, or claystone, it includes sand in stabilized dunes commonly interbedded with and degraded parabolic dunes. Some

8 sandstone or thin limestone. Colors movement of saturated material and and compositions of the deposits are slumping of coherent rock or soil those of the source rock and (or) masses combine to move masses surficial materials that were downslope slowly. Some deposits are transported. In some places, deposits chiefly reworked slump-block include clayey disintegration deposits; others are mostly reworked residuum and sheetwash alluvium, sheetwash alluvium or colluvium. and may include hard, rounded gravel Very recently active deposits derived from alluvial-terrace deposits. commonly have an abrupt scarp at Matrix variable, but commonly it is their upslope limits and crevassed, silty and clayey sand to sandy clay. bulbous, masses at the downslope Landslide deposits derived from limit. Thickness 1–5 m Cretaceous claystone and weathered Mudflow deposits—Chaotic, matrix- shale, locally contain expansive clay supported mixtures of clay, silt, and minerals and have high potential for sand and rock fragments. Emplaced swelling upon sorption of water by rapid flow of saturated, (Hart, 1974). Landslide deposits are unconsolidated soil, rock fragments, unstable, have low compressive and weathered shale or mudstone. strength, and are poor foundation Most mudflow deposits form lobate, materials for man-made structures. fanlike landforms in areas of Slump-block deposits—Masses of weathered shale, mudstone, and bedrock and unconsolidated materials interbedded sandstone, thin that have rotated or slid downslope as limestone, and shale. Thickness 1–5 a unit, with little or no flow; m properties of materials are not greatly ccd Calcareous clayey colluvium—Dark-gray altered, and original bedding, and olive-gray clay, silty and sandy textures, and sedimentary structures clay. Locally includes fragments of of the slumped materials are retained. white and light-gray chalky shale or On the flanks of North and South clayey limestone, sandstone gravel, Table Mountains (near Golden), and (or) discontinuous lag deposits of disoriented blocks of sandstone, crystalline rocks. In some areas, claystone, and mafic latite are mixed colluvium is 5–10 percent granules or with fine surficial material that is chips of grayish-brown shale and, predominantly silt- and clay-size locally, clayey limestone or marl, and particles. Scarps that may extend tens masses of secondary calcium or hundreds of meters across the carbonate. Moderately alkaline; slope commonly mark the upslope locally gypsiferous. Clay minerals limits of slump-block deposits. Low, include mixed-layer illite- hummocky ridges are separated by montmorillonite, montmorillonite or depressions that may contain bogs. smectite (20–40 percent), illite (15– Some deposits are stable and grass- 30 percent), kaolinite (10–15 covered. Some previously stable percent), and minor chlorite. slump deposits tend to move again Consistency of the colluvium matrix during wet periods. For example, in varies with moisture content; it is 1998, an El Niño year, paved streets, very hard when dry, sticky and plastic utilities, and several large, new when wet. Slow internal drainage. homes on the northwest flank of The map unit includes small, Green Mountain (18 km, or 11 mi unmapped areas of residuum (xch) on west of the State Capitol) were nearly level land. These nearly level severely damaged because an old areas also include scattered veneers of landslide deposit became active. In sheetwashed silty, fine sand. The ccd February 2000 these damaged homes unit grades downward to gray and were demolished and removed by olive-gray shale, claystone, and very work crews. Thickness 2–20 m clayey siltstone. In some places, Earthflow deposits—Heterogeneous swelling of the colluvium and mixtures of clay, silt, and sand heaving of claystone layers under the containing scattered clasts. Plastic colluvium have damaged homes,

9 pavement, and utilities that were built gravel and sand alluvium. Colluvium on or in these materials. Most of the locally contains rounded pebbles and recent damage has occurred in the few cobbles of quartzite, granitic urban corridor east of the Colorado rock, gneiss, and vein quartz. Front Range (in Douglas, Jefferson, Secondary calcium carbonate and Boulder Counties). The repairs commonly forms patchy coatings on have cost tens of millions of dollars clasts. Friable and soft, white veinlets (Noe, 1997). Where the colluvium and small masses of pedogenic has a high content of expandable clay calcium carbonate are common at a minerals, the colluvium expands depth of 2–3 m. Colluvium is slowly volumetrically as platy clay minerals permeable, calcareous, moderately and exchangeable ions take up water. alkaline, and gypsiferous in places. The closely related problem of Generally on moderately steep, heaving ground occurs in shallow eroded land; dissected by ravines near dipping, bedrock that underlies thin South Platte River. Generally about colluvium. Locally, expansive 1.5–2 m thick claystone can swell along certain caa Andesitic loamy colluvium and sheetwash beds as water is sorbed by the clay alluvium—Dark-grayish-brown, minerals following load removal (for olive-brown, and light-yellowish- example, after excavation of brown loam, sandy clay loam, and overburden for building purposes). clay. Commonly gravelly at base. The This is especially true where the dip volume of gravel in the colluvium of bedding exceeds 30°. East of the varies locally. Matrix is compact, Indian Hills area and the Dakota poorly sorted, clayey and silty, and Hogback (southwest of Denver in contains minor very fine to very north-central Jefferson County), an coarse quartz and lithic sand. area mantled by clayey colluvium is Colluvium is very sticky when wet divided into four zones: nonswelling, and very hard when dry, and dry slight-, moderate-, and high-swelling colluvium breaks into angular, coarse zones (Hart, 1974; Miller and Bryant, blocky pieces. Matrix contains soft 1976; Noe and Dodson, 1997; Noe masses and veins of calcium and others, 1997). In terms of carbonate, is moderately alkaline, and economic loss to urban infrastructure, shrinks and swells with sorption of swelling soil and heaving bedrock are varying amounts of water. Colluvium regarded by some to be the chief interfingers with residuum on summit geologic hazards in the Front Range surfaces and is veneered by loamy, urban corridor. Thickness of thinly bedded sheetwash alluvium on colluvium is 0.5–2 m gentle foot slopes. Map unit includes ccm Andesitic clayey colluvium—Yellowish- scattered remnants of pediment gravel brown, clayey, very fine to very (1–3 m thick), composed of coarse quartz sand in upper few subrounded pebbles and cobbles of centimeters. Grades downward andesite, granite, gneiss, and schist through 0.1–0.3 m of brown clayey washed by streams from the Green silt to silty clay (includes common Mountain Conglomerate and other feldspar and weathered volcanic-rock andesitic, tuffaceous, gravelly sand grains) to varicolored (olive- sandstone bedrock of Tertiary age. green, pale-yellow, and light-gray) The deposits are chiefly on hillslopes clay and silty clay, and to weathered, and toe slopes as aprons, dissected interbedded tuffaceous siltstone, pediments, and alluvial fans, and they claystone, and calcareous shale, veneer softer claystone and andesitic siltstone, andesitic sandstone, and and feldspathic sandstone bedrock. some conglomerate. Map unit The flanks of North and South Table includes areas of shale and soft Mountains near the summit cliffs are clayey sandstone outcrop that are strewn with latite (a mafic volcanic potentially expansive and corrosive to rock) boulders as much as 2 m in metal. Unit also includes thin, diameter; a rockfall hazard exists discontinuous, remnants of clayey there. Also, these slopes are unstable;

10 hummocky landslide deposits are amphibolite (Sims, 1964). Gravelly common, locally abundant. The sandy loam derived from quartz colluvium contains sparse white, monzonite covers much of Brewery light-gray, and light-olive-brown, Hill and Bald Mountain (south tuffaceous lithic sandstone clasts Summit County) and Boreas (litharenite). Colluvium grades to Mountain and Little Baldy Mountain pale-yellow-brown, olive-yellow- (north Park County). Map unit brown, and grayish-brown, includes colluvium made of volcanic weathered, clayey and silty sandstone rock in Clear Creek and Gilpin and conglomerate. Sand grains are Counties and biotite gneiss and angular, chiefly fine to medium sand granodiorite along the continental but including some coarse grains; divide between Gilpin and Grand grains of quartz and feldspar Counties. Map unit includes grus, predominate; minor lithic grains, colluvium, steep rock outcrop, and mica grains, and dark heavy-mineral local debris-flow deposits, and grains also are present. The colluvium includes alluvium on foot slopes, on and the underlying weathered flats, and in drainages. Types of sedimentary bedrock are firm to soft; materials vary locally; their generally easily excavated by earth- proportions in most mapped areas moving equipment. Thickness 0.5–5 generally are ~20 percent rock m outcrop, ~40 percent granitic blocky clx Clay loam colluvium—Light-yellowish- colluvium, ~30 percent grussified brown and brownish-yellow clay rock or saprolite, and ~10 percent loam, clay, and silty clay loam arkosic sandy, mixed colluvium, containing scattered shale or clayey debris-flow deposits and alluvium. limestone chips, locally abundant Grus is gravelly, sandy detritus limestone chips, and clusters of produced by in-place disintegration of selenite (gypsum) crystals < 5 mm in coarse-grained crystalline rock. Grus diameter. Locally, the colluvium is typically is brown and very pale micaceous and it contains pebble- orangish brown, fine to very coarse size, angular fragments of ironstone sand that includes granules and concretions and lag gravel of pebble-sized angular pieces, granitoid rock types derived from individual or aggregated, of quartz, older alluvial deposits. Strongly feldspar, biotite, and trace iron- calcareous, moderately alkaline. The magnesium minerals. Acidic at high colluvium erodes easily. Unit may be elevations; generally alkaline and 20–40 percent shale outcrop where calcareous below about 2,500 m. slope gradient is more than 25 Grus locally contains subangular percent. Grades downward to punky rock fragments and interbedded pale-brown and unweathered corestones 0.5–3 m in yellowish-gray, fine- to medium- diameter. The distribution of deposits grained sandstone and dark-gray, of thick (>10 m) grus is not carbonaceous or lignitic shale 0.5–2 predictable; the deposits are present m thick on some gently rolling uplands, and csk Grus, crystalline-clast colluvium, rock are exposed in roadcuts at Pine outcrop, and alluvium—Surficial Junction and south of Indian Hills (in detritus, commonly gravelly sandy Jefferson County). Blair (1975) noted loam, and bare rock in mountainous that depth of weathering (and areas where bedrock is mostly thickness of grus) derived from granite, felsite, granitoid plutonic crystalline bedrock in the Rampart rock, gneiss, schist, and (or) Range in Colorado generally is 6–30 migmatite. Published bedrock maps m on north-facing slopes and 3–4 m show many types of source rock. For on south-facing slopes, and it is example, in Clear Creek County concentrated along sets of joints in source rocks are chiefly biotite and the bedrock. Grus is easily excavated. amphibolite gneiss, locally Grus thickness generally is 1–10 m; granodiorite, gabbro, monzonite, and locally, it is as thick as 20 m.

11 Colluvium of the csk map unit is outcrop is common elsewhere in mostly a mixture of sand, granules, canyons and above timberline but is and blocks of crystalline rock not distinguished within this map deposited by gravity on steep (about unit. Extensive bare bedrock is 30–70 percent; 17º–35º) hillslopes. mapped with the symbol “R.” On moderately steep hillslopes (15– Alluvium in map unit csk is 30 percent), generally it is finer gravelly, quartzose and feldspathic grained and has a matrix of brown, sand and silt or sandy, crystalline- micaceous, silty, fine to coarse- rock gravel in stream valleys, on flats, grained quartz and lithic sand and and in small fans. The alluvium on granules containing 5–35 percent upland meadows and in valleys of angular crystalline-rock clasts. On tributary streams is too small in area relatively gentle hillslopes (less than to map separately. Alluvium 15 percent), colluvium commonly generally is 1–4 m thick grades laterally to sandy alluvium in csm Tuffaceous colluvium—Light-brownish- fans and aprons. Quartzose silt, 0.5–2 gray, light-gray, and light-grayish- m thick, mantles some toe slopes on orange silty clay, clayey silt, clayey fractured crystalline rock. Gravelly, fine sand, and tuffaceous, micaceous, matrix-supported debris-flow deposits feldspathic, quartz sand; locally occur commonly on steep valley and gravelly and (or) ashy. Grades canyon sides of most major east- downward to interbedded tuffaceous flowing streams in the Front Range or ashy (volcanic), arkosic, and (James Soule, oral commun., 1999). chalcedonic siltstone, claystone, The matrix of the deposits commonly sandstone, and conglomerate beds of is silty sand, and clasts range from valley-fill origin composed of granite, granules to boulders in random quartz-feldspar gneiss, pegmatite, orientation. Debris flows are a schist, and andesite. Includes pale- significant geologic hazard in the brown deposits of sand, silt, and Front Range because they have a subordinate clay and gravel. Unit may consistency of wet concrete, carry contain swelling clay and thin beds of large clasts, and can travel large clayey silt and sandy alluvium. Near distances (Jeff Coe, written commun., mountain fronts, colluvium contains 2000). boulders 1–2 m in diameter. Some Rock outcrop includes ledges, boulders are dark-gray, greenish-gray, cliffs, and pinnacles of gray, grayish- or purplish-gray andesite and brown (weathers brownish-yellow), trachyandesite derived from volcanic pale-red, reddish-brown, and pink laharic bedrock. In the Vasquez crystalline rock. Rock may contain Mountains in the extreme northwest soil-filled crevices. Intersecting corner of the quadrangle, boulders of joints, shear zones, layering, and granitic and metamorphic rock 1.5–3 foliation cause the rock to break into m in diameter are present. Crystalline blocks and crude rhombohedral clasts generally present in the pieces, centimeters to several meters colluvium include granodiorite, across. The tendency to break along quartz monzonite, pegmatite, aplite, planes of weakness allows engineers related granitoid intrusive rocks, and to predict where block slides, rock feldspathic gneiss. In the southwest falls, and other types of landslides part of the quadrangle, in Park may occur. In lower Turkey Creek County near Jefferson and Milligan Canyon (southwest of Denver and in Lakes, prominent cobble-strewn central Jefferson County) rock sliding ridges are present. The cobbles are (unit jbc) has occurred in response to tuff, silicified wood, porphyry, construction of Highway 285. quartzite, chert-pebble conglomerate, Undercutting of foliation planes in and sandstone. Colluvium commonly layered granitic gneiss and is 1–3 m thick; less than 1 m thick on amphibolite bedrock on steep slopes moderately steep slopes has caused sliding of rock masses cgh Arkose-clast loamy colluvium—Pale-red (Miller and Bryant, 1976). Rock and reddish-brown, pebbly, sandy

12 loam and coarse sandy loam predominant gneiss and granite, clasts colluvium containing 10–35 percent include quartz, Silver Plume Quartz subangular to subround granules, Monzonite, Pikes Peak Granite (south pebbles, and small cobbles of arkose part of quadrangle), and some or arkosic sandstone. Near Woodland porphyry; some boulders are as large Park, at the south-central edge of the as 5 m in diameter. On the upper half quadrangle, as many as half of the of Green Mountain, clasts (listed as clasts may be pink, coarse-grained most to least abundant) are felsic granite (Pikes Peak Granite). In some gneiss, granite, pegmatite, and places, the pebbly sandy loam grades biotite-quartz-plagioclase schist. downward to sandy loam that is Rock types that are less than 10 sticky to slightly sticky and plastic to percent of the clasts include slightly plastic when wet. The hornblende-plagioclase gneiss, colluvium forms sloping fans and amphibolite, mica schist, white, very valley slope deposits derived from hard quartzite and vein quartz, olive- weathered granite. Cliffs and rounded yellow and grayish-brown andesite, monolithic outcrops of reddish porphyritic and aphanitic volcanic arkosic sandstone make up 20–40 rocks, and a trace of red sandstone percent of local areas within the map and conglomerate. On the flanks of unit. Colluvium grades downward to, the lower half of Green Mountain, or abruptly overlies, moderate- clasts are mostly andesite, which reddish-brown, iron-stained arkosic disintegrate easily. Small areas of alluvium and sandstone. Thickness dark-brown, stony, micaceous silty 0.2–3 m sand residuum are present in 5–20 cgc Gneiss- and granite-clast gravelly percent of the mapped area on Green colluvium—Brown, very cobbly Mountain. This map unit includes sandy loam. Pale-red, brown, white, scattered landslide deposits, some of and gray, rounded to subangular which are active (Scott, 1972b). pebbles, cobbles, and boulders in a Colluvium is generally less than 1– dark-grayish-brown and light- 1.5 m thick; locally, it is 3 m thick at yellowish-brown sandy loam, clay the bases of slopes, in slump-formed loam, or sandy clay matrix. Locally, depressions, and in fans the colluvium has a sandy clay matrix cbh Blocky sedimentary-rock colluvium— that is hard when dry, sticky and Grayish-red, moderate-red, reddish- plastic when wet. The colluvium brown, pale-brown, white, and gray commonly contains clay minerals that blocky loam, clay loam, and silty clay can exert swelling pressures of more loam derived from sandstone, arkosic than 2,500 pounds per square foot sandstone, shale, mudstone, and (Scott, 1972c). Relatively young limestone. Locally, the colluvium colluvium has weathered from, and includes fragments of silty limestone, grades downward to, poorly anhydrite, and gypsum. In areas of cemented conglomerate, sandstone, well-cemented, steeply sloping and claystone bedrock of Paleocene bedrock, more or less continuous rock age that underlies Green Mountain outcrops make up 10–35 percent, (16 km west-southwest of downtown locally as much as 60 percent, of area Denver). The colluvium also has mapped. Southeast of Fairplay (in the developed on sandy gravel of Tertiary southwest part of quadrangle), unit is age (Bryant, 1976) that caps some mainly pale-red to grayish-pink, drainage divides in the Front Range blocky sandstone colluvium on the and probably was deposited by flanks of Red Hill. Locally, the map ancestral Clear and Bear Creeks and unit includes small, unmapped by the ancestral North Fork of the deposits of talus, fan alluvium, South Platte River. Other cgc deposits debris-flow fan deposits, and are mapped near the Jefferson landslide deposits. Grades downward County–Park County line, 11–12 km to, or abruptly overlies, various kinds north of Evergreen (western Jefferson of sedimentary bedrock. Thickness County). In addition to the generally 0.10–1 m on steep slopes

13 and 1–3 m elsewhere; in local aprons lahars and volcanic breccia. Map unit colluvium is as thick as 10 m includes landslide deposits, especially cbm Carbonate-clast loamy colluvium—Brown where bedrock is shale. Thickness 2– and grayish-brown, stony loam 4 m colluvium containing 35–50 percent xlu Feldspathic loamy disintegration pebble- to cobble-size angular and residuum—Light-gray and olive- subrounded fragments of gray brown sandy silt and clay, clayey limestone, dolomite, and minor sand, and pebbly clayey sand. Clay sandstone and quartzite. Matrix has loam at surface grades downward to granular structure, is friable, slightly weathered, stratified clayey sticky, and slightly plastic when wet. sandstone, sandy claystone, and Grades downward to weathered and siltstone. Residuum commonly is fractured bedrock. Map unit locally micaceous, feldspathic quartz-grain includes outcrops of limestone, sandy loam on very gently rolling dolomite, and minor sandstone and land surfaces. Sparse to common, quartzite. Thickness of colluvium rounded pebbles of granitic rock, vein generally less than 0.5 m on steep quartz, gneiss, schist, and slopes, 0.5–1.5 m elsewhere approximately 5 percent maroon and cbs Rhyolite- and andesite-clast colluvium— red arkosic sandstone (Fountain Bouldery sandy colluvium containing Formation) are scattered on the land abundant (50 percent or more) surface; locally, cobbles and a few angular, granule-, pebble-, cobble- boulders are on the surface. In some and boulder-size clasts of dark-gray, places the residuum is an expansive pink, or white andesitic and rhyolitic soil hazard. During construction of volcanic rock. Along Reinecker highway E–470 (in west Arapahoe Ridge, in the southwest corner of the County), engineers determined that quadrangle, unit includes clasts of surficial materials were so expansive tuff, hypabyssal porphyry, andesitic that the subgrade required excavation sandstone, breccia, and conglomerate and replacement with nonexpansive and, near the west edge of quadrangle material. This costly remediation (northwest Park County), includes work extended from about 5 mi (8 monzonite clasts. Colluvium grades km) southeast of Cherry Creek downward to volcanic rocks and Reservoir, northward toward Denver volcaniclastic sedimentary rocks, International Airport. Included in the including fine-grained and map unit are unmapped scattered porphyritic tuffs, glassy and areas of small eolian sand dunes brecciated volcanic rocks, stabilized by vegetation (unit ed). feldspathoid-rich, fine-grained rocks Locally, near some streams, unit xlu in dikes and plugs (phonolite); includes scattered, thin terrace underlying bedrock locally is laharic deposits of sand and gravel that were and tuffaceous claystone, siltstone, mapped by other workers as Slocum volcanic sandstone, and (or) arkosic Alluvium. Residuum is 1–2 m thick volcanic-clast conglomerate. The xlv Loamy disintegration residuum, eolian composition of clasts in the sand, and sheetwash alluvium— colluvium may include these kinds of Pale-brown, very pale brown, light- rock in varying proportions. yellowish-brown, and light-gray Thickness 2–4 m loam, sandy loam, and loamy sand. cbu Mixed-lithology blocky colluvium— Surficial materials cover upland Grayish-brown, light-brownish-gray, slopes that are less than about 5 and varicolored blocky colluvium percent. Locally, the materials are characterized by rock clasts mixed in slightly dissected by drainageways. various proportions. The matrix The deposits generally are alkaline commonly is slightly clayey, cobbly and highly calcareous; locally, the sand. Clasts are chiefly sandstone, sediments contain calcium carbonate hornblende gneiss, schist, granite, and splotches and veinlets. Hard when monzonite; locally, clasts are dry, friable when moist. Locally andesitic volcanic rocks derived from contains gravel size clasts. Grades

14 downward to interbedded fine- to are locally common on the surface. medium-grained sandstone and dark- Residuum is 0.5–2 m thick gray carbonaceous shale. Map unit xgb Latite-clast disintegration residuum— locally includes areas of stream Dark-gray, brownish-gray, grayish- alluvium and reworked wind brown, and yellowish-brown, deposits. Mapped in northeast corner subrounded granules, pebbles, of quadrangle. Thickness generally is cobbles, and scattered boulders of more than 2 m, locally 4 m mafic latite in a poorly sorted sand, xsa Feldspathic quartz-sand disintegration silt, and clay matrix. The residuum is residuum—Pale-yellow, white, very on dark-gray, porphyritic latite light gray, and medium-gray, coarse- caprock of South Table Mountain and grained sand containing scattered North Table Mountain near Golden, yellowish-gray and grayish-orange- in Jefferson County. This residuum pink sandstone pebbles, cobbles, and generally is of two types boulders. Within mapped areas there (intermediate types occur locally): (1) are conglomeratic sandstone ledges noncalcareous, gravelly or stony loam from which fall blocks that are 0.3–3 and sandy loam and (2) calcareous, m in diameter. Upper 5–15 cm of clay and clay loam; hard when dry, residuum is pinkish-gray, feldspathic sticky when wet. The residuum and granitic, coarse sand. On nearly contains sand grains of volcanic rock, level land, at a depth of 0.2–0.4 m, quartz, and mafic minerals. the residuum is dark-yellowish-brown Pedogenic carbonate discontinuously clayey sand or loam (B soil horizon) coats clasts and is present as thin that grades downward to brown seams and soft masses at depths of clayey, pebbly grus. In some places 30–50 cm. In some areas, few or no the grus layer grades laterally to a stones are present in the upper 1–1.5 thin lag of pebbles and cobbles on m; the clayey silt and sand in that part sandy claystone bedrock. In other of the material may be eolian places, the residuum grades sediment and (or) sheetwash downward to sandy claystone and alluvium. The residuum contains little clayey sandstone and firm to poorly or no swelling clay. Latite caprock cemented feldspathic sandstone. crops out as cliffs at the margins of Within the areas mapped as xsa there the gently undulating surfaces of both are some hills (8–15 percent slopes), North and South Table Mountain. A especially in southern Arapahoe rockfall hazard is present below the County, and small buttes that are cliffs. Vertical joints in the caprock thinly covered by intermixed are filled with brown lithic sand and colluvium, sheetwash alluvium, and granules. Thin (0.1–0.2 m) deposits residuum. Scattered throughout the of brown sand and silt and angular mapped area are outcrops of poorly pebbles and granules of latite veneer cemented conglomerate bedrock, the smooth tops of the caprock. which produce colluvium composed Residuum generally is 0.1–2 m thick of abundant cobbles and small boulders of granite, bluish-gray HOLOCENE TO MIDDLE PLEISTOCENE quartzite, sandstone, gneiss, volcanic tuff, and petrified wood. Unit cac, jbc Bouldery crystalline-rock landslide arkosic colluvium and sheetwash deposit—Bouldery and rubbly alluvium on hilly land, is landslide deposits, including some compositionally similar to unit xsa, sand and silt, mainly in mountainous and areas of the colluvium are areas underlain by igneous and included in the map unit. A lag metamorphic rocks. Thickness 2–8 m deposit of granitic cobbles is present xsg Feldspathic quartz-sand disintegration in some places. Sandy alluvial fan residuum—Pale-brown, light- deposits and sheetwash alluvium are yellowish-brown, and brown, on toe slopes. Pieces of red and feldspathic quartz sand and pebbles; brown silicified wood, 0.1–1 m long, more gravelly and feldspar-rich in or near the mountains. Less than 10 percent granules and pebbles of

15 granite and arkose. The residuum places the loess contains significant mantles a gently undulating upland amounts of clay and silty clay. surface. The parent material is high- Typically, the loess is nonstratified, level pediment alluvium and fan friable, and weakly compact, and it alluvium (the Nussbaum alluvium of stands in nearly vertical faces in Gilbert, 1897), of probable late exposures several meters high; it has Tertiary to early Quaternary age. blocky or columnar structure and Grain size of the residuum varies: vertical joints. Vertical permeability typically very fine to very coarse is moderate, and lateral permeability sand, predominantly medium sand. is very low. Loess is hard when dry, The composition varies from nonsticky to slightly sticky when wet. feldspathic quartz to more than 95 Subject to wind erosion; includes percent quartz; in places the sand deflation (blowout) basins. Some includes minor limestone or feldspar. deflation basins are occupied by Near the mountains the residuum lakes, for example, Bowles Lake (not includes small granitic pebbles. On mapped), southwest of Denver. the plains generally it contains about Previous workers have documented a 25 percent granules of olive-colored relict soil on the loess. Malde (1955, shale. Contains pebble-size fragments p. 242) described a 2-m-thick soil of of fossil wood locally. Thickness Wisconsin age developed on an generally 2–4 m, locally 6 m eolian silt and sand unit (which we mapped as elb) 1 km northwest of LATE PLEISTOCENE Louisville. He interpreted the soil to be possibly equivalent to the mid- elb Loess (Peoria Formation)—Moderate- Wisconsin soil of Nebraska and yellowish-brown, pale-brown, and Kansas. A relict soil on the loess west olive-gray wind-blown sandy silt, and south of Littleton was described clayey silt, or clayey fine sand. by Scott (1962, p. L29) as a strong Particles are predominantly quartz, Brown soil of mid-Wisconsin age, potassium-feldspar, plagioclase, having distinct prismatic or columnar calcite, and dolomite. Silt, probably structure (a similar soil is developed derived from glaciofluvial sediment on Louviers Alluvium). Scott on floodplains of the ancestral South considered the loess to be of early Platte River and its tributaries, from Wisconsin age. Machette (1977) eolian deposits, and from deflation of interpreted that the loess was bedrock sources, was blown onto deposited during late Bull Lake age upland surfaces. Presumably, glacial and during the following warmer, meltwater in the South Platte River interglacial period prior to the valley contributed a voluminous silt Pinedale glaciation. Loess covers load to the Colorado Piedmont. The gently undulating uplands and many silt was derived from rock flour that old alluvial deposits (ags, agm), and was scoured from bedrock in the high it thins downwind (east and mountains by glaciers. Volcaniclastic southeast) from river valleys and silt and clay eroded from bedrock in eolian sand deposits (es). Where the the Colorado Piedmont may have deposit thinly covers the Denver and provided some of the loess. In Dawson Formations (bedrock), northeastern Colorado (between lat excavations may encounter expansive 39° and 41° N. and between long clayey materials (for example, along 105° and 102° W.), the mean particle Highway C–470 east of Chatfield size of about 100 grain-size analyses Reservoir, in northernmost Douglas is fine sand (0.20 mm) to silt (0.02 County). In many areas, loess grades mm) (Muhs and others, 1999). The laterally to eolian sand, sheetwash loess is 60 percent or more silt. It is alluvium, colluvium, and (or) slightly to strongly calcareous, and it residuum. Where minor streams have contains calcium carbonate spots, cut through loess, the map unit nodules, and veinlets to depths of 1– includes areas of unmapped 1.5 m; locally, it may contain lenses colluvium (units cac or caa). The of sand and small pebbles. In some

16 map unit also includes unmapped Gile and others (1966), are common small areas of intensely weathered in semiarid areas (Shroba and upland gravel, stony and clayey Birkeland, 1984). Granitic clasts in colluvium, and outcrops of shale and the B and C horizons of soil sandstone. Thickness of loess varies developed in till of Bull Lake age considerably in the quadrangle. commonly are altered to grus. Generally, it is less than 3 m thick; it Maximum early Pinedale glaciation is entirely eroded adjacent to some near Winter Park, in the Fraser River drainageways. In the Louisville area, valley in the northwest corner of the loess is 0.6–4 m thick; south of Denver 1° × 2° quadrangle, occurred Commerce City it is as thick as 6–7 m later than 30,480+2800/−4300 radiocarbon years before present (14C LATE AND MIDDLE PLEISTOCENE yr B.P.) (DIC–482) and later than 30,050±1200 (14C yr B.P.) (SI–2912) Bouldery till (till of the Pinedale and Bull (Millington, 1977; Nelson and others, Lake glaciations)—Generally a 1979; Madole, 1986). An early nonsorted, nonstratified mixture of Pinedale mountain glacier in the subangular and subrounded boulders, North Boulder Creek valley dammed cobbles, pebbles, and granules in a a lake in the Caribou Creek valley on sandy matrix. The relative abundance the north boundary of the Denver of pebbles, cobbles, or boulders quadrangle, west of Highway 72 and varies; however, boulders typically northwest of Nederland. The lake was are conspicuously present and dammed by a lateral moraine 2.3 km commonly are abundant. Some upvalley from the terminal moraine boulders are larger than 5 m in across the North Boulder Creek diameter. Locally, till is overlain by valley, and formation of the lake was unmapped, thin loess and the map approximately synchronous with units (tbg, tbj) include subordinate maximum early Pinedale glaciation. stratified glaciofluvial deposits near Basal sediments from Lake Devlin the lower altitude limits of glacial yielded ages of 22,400+1070/−1230 deposits. 14 C yr B.P. (DIC–870), Till of Pinedale age (the younger − 14 till) is characterized by well- 22,040+740/ 820 C yr B.P. (DIC– − 14 preserved end moraines and lateral 871), and 21,640+610/ 650 C yr moraines and is mantled by B.P. (DIC–869). Farther north, in the unweathered to slightly weathered North St. Vrain valley northeast of surface boulders. Weakly developed Allens Park, till of Bull Lake age soil profiles in the till are yielded a uranium-trend age of characterized by 10- to 40-cm thick B 130,000±80,000 yr, and colluvium horizons containing 1–7 percent more overlying till of pre-Bull Lake age clay than is present in unweathered yielded a uranium-trend age of till. Pedogenic calcium carbonate 220,000±70,000 yr (Madole and horizons 40–80 cm thick (stage I Shroba, 1979; Rosholt and others, carbonate morphology) are common 1985). The till probably is 1–5 m in semi-arid areas. Few clasts are thick in most places, but it may be 30 weathered to grus. m thick in lateral moraines and end Till of Bull Lake age generally moraines forms slightly subdued end and tbg Crystalline-clast bouldery till—Light- lateral moraines that have broadened gray to grayish-brown till having a and rounded crests. Surface boulders micaceous, sandy, and grus-rich are moderately weathered and pitted, matrix (sand generally 55–70 percent and soil profiles typically are of matrix). Abundant clasts are characterized by 35- to 75-cm-thick B subangular to subround and horizons that contain 5–13 percent composed chiefly of granite, more clay than is present in granitoid rock, gneiss, or migmatite unweathered till. Pedogenic carbonate tbj Mixed-lithology-clast bouldery till— horizons 60–100 cm thick, having Clasts include granitoid rocks, gneiss, stage II carbonate morphology of migmatite, and sedimentary rocks.

17 Locally, along the northeast side of ags Alluvial sand, silt, clay, and gravel the Mosquito Range (west edge of (Louviers and Slocum Alluviums, quadrangle), the till includes undivided; late middle sandstone, conglomerate, limestone, Pleistocene)—Louviers Alluvium dolomite, and scarce monzonite clasts near Parker is ~11 m of stratified, ggq Outwash sand and gravel (outwash of pebbly, coarse sand; in paleochannels Bull Lake, Pinedale, and post- it contains lenses of pebbles, cobbles, Pinedale ages)—Stratified sand and and boulders (Scott and Lindvall, gravel deposited in valleys by 1970). The upper 1–2 m is yellowish- flowing, glacial meltwater. Mapped brown and brown silty sand; much or downvalley from till (units tbg, tbj, most of the silt may be loess. Locally, and tbk) and in glaciated valleys. The the matrix is light-reddish-brown outwash deposits generally underlie clayey sand and sandy clay and is uniformly sloping (4–10 percent) streaked or mottled with limonite and terrace surfaces 3–10 m higher than manganese oxides. A relict soil adjacent stream channels cut in commonly is developed in the upper Holocene time. Extensively dredged meter of the alluvium. The soil has a for placer gold at Fairplay, along clay-enriched B soil horizon that may Tarryall Creek (northwest Park overlie a carbonate-enriched Cca County), and along Swan River (west horizon; in places no Cca horizon is edge of map area, south Summit present. The soil is interpreted to County). Includes some nonglacial have developed after the Bull Lake alluvium. Outwash may be present glaciation (Machette, 1977). The farther downstream than shown on alluvium contains pieces of opalized the map; the boundary between wood. Clasts are mostly quartz, and outwash and nonglacial alluvium is minor feldspar. Plane bedded and arbitrary in some valleys. Sandy torrentially crossbedded. Contains gravel to gravelly sand having fossil bones, a horn core of a giant rounded to subangular clasts that bison was excavated at a site. grade upvalley into glaciofluvial Between Turkey and Deer Creeks, in deposits and till. Grain size, south-central Jefferson County, and at thickness, and particle composition the mountain front the deposit is silty, vary widely among small areas. clayey sand and minor gravel, 2–15 These properties reflect the distance m thick. from source glaciers and the variable Slocum Alluvium (the older of the types of source rocks. Thus, it was two alluviums) is yellowish-red, well- impracticable to map compositional stratified gravel that contains subtypes of outwash in the granules, pebbles, cobbles, and quadrangle. For example, outwash in boulders, generally in lenses. one watershed at the northwest end of Generally more bouldery near the South Park is red and light-gray, mountains. Large deposits mapped 25 calcareous gravelly loam composed km southwest of downtown Denver, of granules, pebbles, and cobbles of on the east foothills of the Front arkosic sandstone, pale-green felsic Range, are moderate-reddish-brown porphyry, hypabyssal rocks, to brown, silty, clayey coarse sand undifferentiated volcanic rocks, and and granules. These deposits are as minor flaggy, laminated siltstone. thick as 13 m south of Deer Creek Outwash in an adjacent watershed at (Bryant and others, 1973). Clasts are the northwest end of South Park is predominantly granitic rocks; some chiefly light-gray, sandy, clast- are pegmatitic, metamorphic, supported felsic and granitic pebbles volcanic, and sedimentary rocks and cobbles, and scarce fine-grained, derived, in part, from older alluvium igneous rock, scarce vein quartz, (agm). Sand is mostly quartz, schist, and medium-gray feldspar, mica, and magnetite. A skull microcrystalline limestone. Thickness of Bison latifrons was excavated in 2–8 m Slocum Alluvium near Cañon City (60 km south of this quadrangle).

18 Lumps and layers of olive-gray shale amounts of stratified drift, especially eroded from the underlying bedrock at the lowest elevations of occurrence are common locally in the alluvium. of the till. Generally lacks Bedding is cross-stratified and planar, constructional morainal form. Surface and it contains cut-and-fill structures. boulders are deeply weathered and The lower part of the alluvium locally pitted. Many subsurface clasts are is thick, poorly sorted gravel overlain altered to grus. Soil profile commonly by 1–1.5 m of very pale brown, pale- has 1-m-thick B horizon containing brown, and mottled-white pebbly three or more times the clay content sand and silt that is slightly sticky and of unweathered till. In semiarid areas, plastic when wet and hard when dry. pedogenic carbonate horizons having The alluvium is slightly cemented by stage III or higher carbonate clay throughout (except in lower morphology (Gile and others, 1966) gravelly part) and by calcium are common. The till mapped may carbonate in the upper part where have been deposited during more than stones have thin, continuous to one glaciation discontinuous coatings of calcium gge Outwash sand and gravel (outwash of carbonate. In the upper 1–1.5 m of pre–Bull Lake glaciations)— the alluvium, a thick, reddish-brown Stratified sand and gravel deposited clayey B horizon and an underlying by glacial meltwater. Mapped white Cca (carbonate-rich) horizon downvalley from till (tbk). Generally accumulated during an extended poorly to moderately sorted, sandy period of weathering and deposition pebble and cobble gravel consisting of loess. In some places these of subrounded and rounded clasts. horizons are eroded. Deposit Texture is stony sandy loam and very commonly underlies two terrace stony fine sandy loam. The relative surfaces about 36 and 55 m above proportions of granules, pebbles, modern streams. Generally, unit is 3– cobbles, and boulders, the kinds of 7 m thick near the mountains (Scott clasts, and the thickness of the and Lindvall, 1970); 1–2 m thick outwash from place to place all vary more than about 15 km away from and reflect both the distance from the the mountains. Locally, the deposit is source glacier and the types of degraded to a lag that mantles an bedrock. Clasts are mostly quartz uneven, stream-dissected surface. The monzonite, gneiss, amphibolite, and lag deposit is grayish-red quartz and minor rhyolitic tuff in the upper feldspar sand and silty clay Fraser River Valley (northwest corner containing zones of bouldery gravel of map area). Many subsurface clasts (Maberry and Lindvall, 1972) are intensely weathered. Soil profile commonly has 1-m-thick B horizon MIDDLE AND EARLY PLEISTOCENE containing three or more times the clay content of unweathered outwash. tbk Bouldery till (till of pre–Bull Lake In semiarid areas, pedogenic glaciations)—Generally a nonsorted, carbonate horizons having stage III or nonstratified mixture of subangular higher carbonate morphology (Gile and subrounded boulders, cobbles, and others, 1966) are common. The pebbles, and granules in a loamy sand outwash locally contains Lava Creek to sandy loam matrix. In some places B volcanic ash (0.62 Ma; Izett and cobbles are the predominant clast Wilcox, 1982). Unmapped loamy size. Clasts in the upper Fraser River alluvium, colluvium, or loess deposits Valley (northwest corner of 0.5–2 m thick overlie the outwash in quadrangle) are quartz monzonite, places. The outwash forms terraces gneiss, amphibolite, and minor 80–400 m above modern stream rhyolitic tuff. This till generally is levels. Less than 1 m thick on eroded preserved in a few locations terrace surfaces; at least 3 m thick downvalley from the younger tills of elsewhere Bull Lake and Pinedale ages. A thin layer of unmapped loess commonly overlies the till. May include minor

19 agm Alluvial gravel and sand (Verdos and carbonate–enriched zone 0.6–1.2 m Rocky Flats Alluviums, undivided; thick (in places forming a laminar K early middle and early soil horizon). Abundant white Pleistocene)—Poorly sorted, calcium carbonate fills vertical joints intensely weathered, matrix- and is concentrated laterally along supported and clast-supported, contacts between gravel and sand subangular and subrounded cobble lenses (Malde, 1955); most clasts and pebble gravel containing a sand have carbonate rinds. Alluvium on and clayey sand matrix; bouldery in high, east-sloping surfaces at the east pediment and fan deposits near margin of the Front Range is mountains and in fluvial terrace attributed to pediment processes (see, deposits of the South Platte River. for example, Tator, 1952; Scott, Grain size decreases eastward from 1960). The alluvium includes debris- the mountains. The matrix is light- flow deposits. brown and light-red, oxidized clayey Rocky Flats Alluvium, which has sand, sandy clay, and a mix of sand, weathered longer than the Verdos silt, and clay in various proportions Alluvium, is generally redder in color that is marked by intense (reddish-brown, light-brown, and accumulation of calcium carbonate in light-red) and more clayey than the many places. Includes lenses and Verdos Alluvium. Various workers beds of pebble gravel, pebbly silty describe the prevalent grain size of sand, and gravelly clay. Pedogenic the Rocky Flats Alluvium as sandy clay thickly coats sand grains and gravel; slightly bouldery, cobble and clasts. Occurs as high-level fluvial pebble gravel that contains a clayey deposits on old erosion surfaces sand matrix; sandy pebble gravel, (pediments) at the eastern foot of the pebbly silty sand, and cobbly to Front Range, at two isolated locations pebbly sandy clay. Clasts are largest on the plains, and in Woodland Park near the mountains and decrease in (north Teller County). Clasts are size eastward. The alluvium forms a mostly granite, gneiss, sandstone, large fanlike deposit at Rocky Flats quartzite, and minor vein quartz, (lat 39°52′ N., long 105°15′ W.). This pegmatite, and schist; locally, alluvium is generally about 45–70 m claystone pebbles are present at the higher than modern streams, locally base. Clasts of granitoid plutonic as much as 90 m. At Rocky Flats, the rocks and gneiss crumble easily, are alluvium commonly is 1–10 m thick, coated with secondary clay and and locally it is > 30 m thick (Shroba calcium carbonate, and are not and Carrara, 1996). It includes thick suitable for concrete aggregate but are channel-fill deposits of gravel used locally for road material. Pebble (Madole, 1991). Near the mountain counts on Rocky Flats (25 km front, it contains boulders as large as northwest of the State Capitol) 6 m across. Locally present are beds indicate that 60–77 percent of clasts and lenses of cobbly and silty clay of are very hard, subangular probable debris-flow origin. At the metaquartzite clasts derived from the east limit of Rocky Flats the average Coal Creek Canyon area (Boos and pebble diameter is 1 in. (Malde, Boos, 1934; Shroba and Carrara, 1955). Elsewhere in the quadrangle, 1996); these clasts make excellent the agm deposit is estimated to be 3– aggregate. The Verdos Alluvium, the 6 m thick younger part of the agm unit, contains the 0.62-Ma Lava Creek B volcanic PLEISTOCENE ash in some places (Izett and Wilcox, 1982). A relict soil is present in the pga Pediment gravel—Gray, bouldery alluvium top 2 m of the agm unit owing to composed mostly of andesite clasts in protracted weathering of alluvium. pediment remnants on flanks of The soil contains a reddish, clay- mountains. Poorly sorted and crudely enriched B soil horizon, which stratified. Map unit includes minor overlies a pedogenic calcium areas of reddish-brown, fairly well stratified and sorted sand and pebbles,

20 cobbles, and boulders composed of iron-rich accessory minerals (for sandstone, limestone, and locally, example, the Fountain Formation) gneiss and granodiorite. An intensely tends to produce light-reddish-brown developed calcium carbonate– to grayish-red, blocky, sandy loam enriched soil is present in the upper colluvium. More quartz-rich part of the alluvium. The alluvium sandstone commonly produces a forms a broad terrace or gently humus-rich, grayish-brown and light- sloping alluvial surface (several reddish-brown, blocky, sandy loam kilometers wide and long) about 25 m colluvium. Shale or mudstone in low, above the valley bottom in South gently sloping areas produce grayish- Park (southwest part of quadrangle). brown, clayey or silty colluvium, Thickness is about 6 m patchy alluvium, and small areas of grayish-brown clayey residuum. The QUATERNARY residuum is discontinuously covered by residual gravel of granite and cra Hogback and rangefront colluvium, gneiss. Gentle slopes are veneered by alluvium, and rock outcrop alluvial aprons of pebbly sand, very complex—Two or three rocky ridges fine sand, and silty sandy clay, which (hogbacks) and intervening valleys, locally contain shale and limestone eroded from steeply dipping chips. sedimentary rocks of variable Pediment gravel remnants at the composition and grain size along the base of the abrupt east face of the steep east front of the Front Range. Front Range are included in unit cra. Moderately steep to very steep (15– The granitic gravel covers gently 100 percent slopes), wooded bedrock east-sloping, planar, erosion surfaces ridges and associated colluvial and (pediments) on many divides between alluvial deposits extend north-south modern streams; the surfaces in compositionally different strips, generally are 30–100 m above the too narrow to differentiate on this streams. The deposits are similar to map. The deposits and rock outcrop those described as units agm and complex are mapped as a single unit. ags, but they are too small to map at Thick intervals of resistant arkosic this scale. They are 2–6 m thick and sandstone, quartzose sandstone, and 10–200 m in width. Floods and debris minor tuffaceous sandstone, flows deposited this material in early conglomerate, and thin intervals of and middle Quaternary time. limestone (and an exceptional mafic Piney Creek Alluvium (Holocene), latite ridge, the Ralston Dike, north of generally 2–8 m thick, is in channels Golden) underlie and crop out along and beneath floodplains of modern the ridges. The ridges are mantled by streams that follow the strike of blocky and gravelly sandstone inclined shale and mudstone bedrock colluvium, 1–8 m thick, that varies in units. Clasts in the alluvium are texture (sand, silty gravelly sand, silty chiefly granite and sandstone. Grain clayey sand, and other grain-size size is variable and includes gravel, mixtures) (Miller and Bryant, 1976). clayey fine to coarse sand, silt, and The colluvium contains fragments of clay. The alluvium commonly has a bedrock as large as several meters in dark-grayish-brown humic A soil diameter; and it locally includes horizon, 0.3–0.5 m thick, in its upper slump deposits and talus. Valleys part. It grades laterally to colluvium, between hogbacks are eroded into shaley debris-apron deposits, and softer rocks, mostly shale and talus mudstone, that locally are interbedded with thin sandstone beds EARLY PLEISTOCENE AND or layers of concretions. Colors and the mineral composition of the PLIOCENE? deposits vary, determined mainly by QTdd Diamicton—Till-like, gravelly deposits the varied colors and composition of along the South Boulder Creek– the source rocks. Feldspar-rich Boulder Creek divide near the north sedimentary bedrock that contains

21 boundary of the Denver quadrangle, Geologic Quadrangle Map GQ–1345, scale between Pinecliff and Nederland 1:24,000. (Boulder County). Deposits are Bates, R.L., and Jackson, J.A., eds., 1980, Glossary present above 2,500 m (8,200 ft) of geology (2d ed.): Falls Church, Va., American elevation. They are similar to deposits Geological Institute, 751 p. on Niwot Ridge, a 2-km-long Blair, R.W., Jr., 1975, Weathering and landform in alpine tundra about 10 geomorphology of the Pikes Peak granite in the km north of the quadrangle. Madole southern Rampart Range, El Paso County, (1982) concluded that no single Colorado: Golden, Colorado School of Mines, geologic process could convincingly Ph.D. dissertation, 115 p. account for the deposits (thus, the Boos, M.F., and Boos, C.M., 1934, Granites of the nongenetic name “diamicton”), but Front Range—The Longs Peak−St. Vrain that they may be mixtures of mostly batholith: Geological Society of America alluvium and colluvium. Previous Bulletin, v. 45, p. 303–332. workers have interpreted such Bryant, Bruce, 1976, Reconnaissance geologic map deposits as till. Deeply weathered, of the Bailey quadrangle, Jefferson and Park roughly round to angular boulders are Counties, Colorado: U.S. Geological Survey scattered on the surface; most Miscellaneous Field Studies Map MF–816, scale boulders are 0.25–0.5 m in diameter. 1:24,000. Composed of boulders of exotic rock Bryant, Bruce, McGrew, L.W., and Wobus, R.A., in a poorly sorted matrix, chiefly 1981, Geologic map of the Denver 1° × 2° coarse to very coarse sand, on quadrangle, north-central Colorado: U.S. monzonite and other igneous rock Geological Survey Miscellaneous Investigations types on rounded interfluvial ridges Series Map I–1163, scale 1:250,000. and cols. The diamicton deposit is Bryant, Bruce, Miller, R.C., and Scott, G.R., 1973, older than local tills and similar to Geologic map of the Indian Hills quadrangle, them texturally, but it lacks morainal Jefferson County, Colorado: U.S. Geological form. Thickness unknown; base Survey Geologic Quadrangle Map GQ–1073, 7 concealed. Estimated thickness of 3– p., map scale 1:24,000. 35 m interpreted from seismic Colman, S.M., 1985, Map showing tectonic features methods for the deposit on Niwot of late Cenozoic origin in Colorado: U.S. Ridge (Madole, 1982). At edges, Geological Survey Miscellaneous Investigations deposit thins to 1–2 m, imperceptibly Series Map I–1566, scale 1:1,000,000. grading into talus and colluvium Colton, R.B., Holligan, J.A., and Anderson, L.W., 1975, Preliminary map of landslide deposits, PRE-QUATERNARY Denver 1° × 2° quadrangle, Colorado: U.S. Geological Survey Miscellaneous Field Studies R Bedrock—Various types of bedrock having Map MF–705, scale 1:250,000. no cover of detritus except for minor Crosby, E.J., 1978, Landslides in the Front Range gravel and thin soil in scattered urban corridor: U.S. Geological Survey pockets and crevices. Includes areas Miscellaneous Field Studies Map MF–1042, 3 of alpine tundra too small to map. sheets, scale 1:100,000. Mostly precipitous peaks, cliffs, Curry, R.R., 1966, Observation of alpine mudflows knife-edge divides (aretes), cols, in the Tenmile Range, central Colorado: cirque walls that include some talus, Geological Society of America Bulletin, v. 77, p. and steep canyon walls. Rockfall 771–776. hazard exists in canyons Ellefsen, K.J., Lucius, J.E., and Fitterman, D.V., 1998, An evaluation of several geophysical SELECTED REFERENCES methods for characterization of sand and gravel deposits: U.S. Geological Survey Open-File Report 98–221, 26 p. Baker, V.R., 1973, Paleosol development in Epis, R.C., Scott, G.R., Taylor, R.B., and Sharp, Quaternary alluvium near Golden, Colorado: W.N., 1976, Petrologic, tectonic, and Mountain Geologist, v. 10, p. 127–133. geomorphic features of central Colorado, in Epis, Barker, Fred, and Wyant, D.G., 1976, Geologic map R.C., and Weimer, R.J., eds., Studies in of the Jefferson quadrangle, Park and Summit Colorado field geology: Colorado School of Counties, Colorado: U.S. Geological Survey

22 Mines Professional Contributions no. 8, p. 301– ash beds (Pearlette family ash beds) of Pliocene 338. and Pleistocene age in the western United States Forman, S.L., Oglesby, Robert, Markgraf, Vera, and and southern Canada: U.S. Geological Survey Stafford, Thomas, 1995, Paleoclimatic Miscellaneous Investigations Series Map I–1325, significance of late Quaternary eolian deposition scale 1:4,000,000. on the Piedmont and High Plains, central United Kirkham, R.M., 1977, Quaternary movements on the States: Global and Planetary Change v. 11, p. Golden fault, Colorado: Geology, v. 5, p. 689– 35–55. 692. Fullerton, D.S., Bluemle, J.P., Clayton, Lee, Steece, Kirkham, R.M., and Rogers, W.P., 1981, Earthquake F.V., Tipton, M.J., Bretz, Richard, and Goebel, potential in Colorado, a preliminary evaluation: J.E., 1995, Quaternary geologic map of the Colorado Geological Survey Bulletin 43, 171 p. Dakotas 4º × 6º quadrangle, United States: U.S. Langer, W.H., Green, G.N., Knepper, D.H., Lindsey, Geological Survey Miscellaneous Investigations D.A., Moore, D.W., Nealey, L.D., and Reed, Series Map I–1420 (NL–14), 9 p., map scale J.C., Jr., 1997, Distribution and quality of 1:1,000,000. potential sources of aggregate, Infrastructure Fullerton, D.S., Moore, D.W., and Richmond, G.M., Resources Project area, Colorado-Wyoming: 1996, Maps of surficial deposits and materials of U.S. Geological Survey Open-File Report 97– the Nation—The USGS Quaternary Geologic 477, scale 1:500,000. Atlas of the Conterminous United States: Larsen, L.S., 1980, Soil survey of Elbert County, Geological Society of America Abstracts with Colorado, western part: U.S. Department of Programs, v. 28, no. 7, p. A–302. Agriculture, Soil Conservation Service, 135 p. Gardner, M.E., Hart, S.S., and Simpson, H.E., 1971, ———1981, Soil survey of El Paso County area, Preliminary engineering geologic map of the Colorado: U.S. Department of Agriculture, Soil Golden quadrangle, Jefferson County, Colorado: Conservation Service, 212 p. U.S. Geological Survey Miscellaneous Field Larsen, L.S., and Brown, J.B., 1971, Soil survey of Studies Map MF–308, scale 1:24,000. Arapahoe County, Colorado: U.S. Department of Gilbert, G.K., 1897, Pueblo, Colorado: U.S. Agriculture, Soil Conservation Service [in Geological Survey Geologic Atlas of the United cooperation with Colorado Agricultural States Folio 36, 9 p., 5 maps. Experiment Station], 78 p. Gile, L.H., Peterson, F.F., and Grossman, R.B., 1966, Lee, F.T., and Mystkowski, Walter, 1979, Loveland Morphological and genetic sequences of Basin slide, Colorado, U.S.A., Chapter 12 in carbonate accumulation in desert soils: Soil Voight, B., ed., Rockslides and avalanches, Science, v. 101, p. 347–360. volume 2, Engineering sites: Amsterdam, Goddard, E.N., Trask, P.D., De Ford, R.K., Rove, Elsevier Science Publications, p. 473–514. O.N., Singlewald, J.T., Jr., and Overbeck, R.M., Lindvall, R.M., 1978, Geologic map of the Fort 1970, Rock-color chart: Boulder, Colo., Logan quadrangle, Jefferson, Denver, and Geological Society of America. Arapahoe Counties, Colorado: U.S. Geological Hansen, W.R., 1973, Effects of the May 5–6, 1973, Survey Geologic Quadrangle Map GQ–1427, storm in the greater Denver area, Colorado: U.S. scale 1:24,000. Geological Survey Circular 689, 20 p. ———1979, Geologic map of the Arvada Hansen, W.R., and Crosby, E.J., 1982, quadrangle, Adams, Denver, and Jefferson Environmental geology of the Front Range urban Counties, Colorado: U.S. Geological Survey corridor and vicinity, Colorado: U.S. Geological Geologic Quadrangle Map GQ–1453, scale Survey Professional Paper 1230, 99 p. 1:24,000. Hart, S.S., 1974, Potential swelling soil and rock in ———1980, Geologic map of the Commerce City the Front Range Urban Corridor, Colorado: quadrangle, Adams and Denver Counties, Colorado Geological Survey Environmental Colorado: U.S. Geological Survey Geologic Geology no. 7, 23 p., 4 maps, scale 1:100,000. Quadrangle Map GQ–1541, scale 1:24,000. Hunt, C.B., 1954, Pleistocene and recent deposits in ———1983, Geologic map of the Sable quadrangle, the Denver area, Colorado: U.S. Geological Adams and Denver Counties, Colorado: U.S. Survey Bulletin 996–C, p. 91–140. Geological Survey Geologic Quadrangle Map Ives, J.D., Mears, A.I., Carrara, P.E., and Bovis, M.J., GQ–1567, scale 1:24,000. 1976, Natural hazards in mountain Colorado: Lozano, Efraim, 1965, Geology of the southwestern Association of American Geographers, Annals, Garo area, South Park, Park County, Colorado: v. 66, p. 129–144. Golden, Colorado School of Mines, M.S. thesis, Izett, G.A., and Wilcox, R.E., 1982, Map showing 115 p. localities and inferred distributions of the Maberry, J.O., and Lindvall, R.M., 1972, Geologic Huckleberry Ridge, Mesa Falls, and Lava Creek map of the Parker quadrangle, Arapahoe and

23 Douglas Counties, Colorado: U.S. Geological River drainage basin, Colorado: U.S. Geological Survey Miscellaneous Investigations Series Map Survey Geologic Investigations Series I–2644, I–770–A, scale 1:24,000. scale 1 in. = 5 mi. ———1977, Geologic map of the Highlands Ranch Malde, H.E., 1955, Surficial geology of the quadrangle, Arapahoe and Douglas Counties, Louisville quadrangle, Colorado: U.S. Colorado: U.S. Geological Survey Geologic Geological Survey Bulletin 996–E, p. 217–259. Quadrangle Map GQ–1413, scale: 1:24,000. McCain, J.F., and Hotchkiss, W.R., 1975, Map Machette, M.N., 1975, The Quaternary geology of showing flood-prone areas, greater Denver area, the Lafayette quadrangle, Colorado: Boulder, Front Range urban corridor, Colorado: U.S. University of Colorado, M.S. thesis, 105 p. Geological Survey Miscellaneous Investigations ———1977, Geologic map of the Lafayette Series Map I–856–D, scale 1:100,000. quadrangle, Adams, Boulder, and Jefferson Miller, R.D., and Bryant, Bruce, 1976, Engineering Counties, Colorado: U.S. Geological Survey geologic map of the Indian Hills quadrangle, Geologic Quadrangle Map GQ–1392, scale Jefferson County, Colorado: U.S. Geological 1:24,000. Survey Miscellaneous Investigations Series Map Madole, R.F., 1972, Neoglacial facies in the I–980, scale 1:24,000. Colorado Front Range: Arctic and Alpine Millington, A.C., 1977, Late Quaternary Research, v. 4, p. 119–130. paleoenvironmental history of the Mary Jane ———1976, Glacial geology of the Front Range, Valley, Grand County, Colorado: Boulder, Colorado, in Mahaney, W.C., ed., Quaternary University of Colorado, M.S. thesis, 214 p. stratigraphy of North America: Stroudsburg, Moore, Randy, 1992, Soil survey of Pike National Penn., Dowden, Hutchinson, and Ross, p. 297– Forest, eastern part, Colorado, parts of Douglas, 318. El Paso, Jefferson, and Teller Counties: U.S. ———1982, Possible origins of till-like deposits Department of Agriculture, Forest Service and near the summit of the Front Range in north- Soil Conservation Service, 106 p. central Colorado: U.S. Geological Survey Moreland, D.C., and Moreland, R.E., 1975, Soil Professional Paper 1243, 31 p. survey of Boulder County area, Colorado: U.S. ———1986, Lake Devlin and Pinedale glacial Department of Agriculture, Soil Conservation history, Front Range, Colorado: Quaternary Service, 86 p. Research, v. 25, p. 43–54. Muhs, D.R., Aleinikoff, J.N., Stafford, T.W., Jr., ———1991, Colorado Piedmont Section in Chapter Kihl, Rolf, Been, Josh, Mahan, S.A., and 15 of Morrison, R.B., ed., Quaternary nonglacial Cowherd, S.D., 1999, Late Quaternary loess in geologyConterminous U.S.: Geological northeastern Colorado, I—Age and paleoclimatic Society of America, The geology of North significance: Geological Society of America America, v. K–2, p. 456–462. Bulletin, v. 111, p. 1861–1875. ———1994, Stratigraphic evidence of desertification Muhs, D.R., Stafford, T.W., Jr., Cowherd, S.D., in the west-central Great Plains within the past Mahan, S.A., Kihl, Rolf, Maat, P.B., Bush, C.A., 1000 yr: Geology, v. 22, p. 483–486. and Nehring, Jennifer, 1996, Origin of the late ———1995, Spatial and temporal patterns of late Quaternary dune fields of northeastern Colorado: Quaternary eolian deposition, eastern Colorado, Geomorphology, v. 17, p. 129–149. U.S.A.: Quaternary Science Reviews, v. 14, p. Muhs, D.R., Stafford, T.W., Jr., Swinehart, J.B., 155–177. Cowherd, S.D., Mahan, S.A., Bush, C.A., ———1996, Late Pleistocene and Holocene alluvial Madole, R.F., and Maat, P.B., 1997, Late stratigraphy in glaciated valleys of the southern Holocene eolian activity in the mineralogically Rocky Mountains: American Quaternary mature Nebraska Sand Hills: Quaternary Association, 14th biennial meeting, Flagstaff, Research, v. 48, p. 162–176. Ariz., Program and Abstracts, p. 175. Nelson, A.R., Millington, A.C., and Andrews, J.T., Madole, R.F., and Shroba, R.R., 1979, Till sequence 1979, Radiocarbon-dated upper Pleistocene and soil development in the North St. Vrain glacial sequences, Fraser Valley, Colorado Front drainage basin, east slope, Front Range, Range: Geology, v. 7, p. 410–414. Colorado, in Ethredge, F.G., ed., Field guide, Noe, D.C., 1997, Heaving-bedrock hazards, northern Front Range and northwest Denver mitigation, and land-use policy—Front Range Basin, Colorado: Fort Collins, Colorado State piedmont, Colorado: Environmental University, Department of Earth Resources, p. Geosciences, v. 4, no. 2, p. 48–57. 123–178. Noe, D.C., and Dodson, M.D., 1997, An Madole, R.F., VanSistine, D.P., and Michael, J.A., investigation and evaluation of heaving bedrock 1998, Pleistocene glaciation in the upper Platte hazards in Douglas County, Colorado: Colorado Geological Survey Special Publication 42, 80 p.

24 Noe, D.C., Jochim, C.L., and Rogers, W.P., 1997, A Sampson, J.J., and Baber, T.G., 1974, Soil survey of guide to swelling soils for Colorado homebuyers Adams County, Colorado: U.S. Department of and homeowners: Colorado Geological Survey Agriculture, Soil Conservation Service, 72 p. Special Publication SP–43, 76 p. Sawatzky, D.L., 1967, Tectonic style along Elkhorn Pearl, R.H., 1968, Quaternary geology of the Thrust, eastern South Park and western Front Morrison quadrangle, Colorado: Mountain Range, Colorado: Golden, Colorado School of Geologist, v. 5, p. 197–206. Mines, D.Sc. thesis, 206 p., folded map in Pierce, K.L., and Schmidt, P.W., 1975a, pocket. Reconnaissance map showing relative amounts Schmidt, P.W., 1976a, Reconnaissance map showing of soil and bedrock in the mountainous part of relative amounts of soil and bedrock in the Eldorado Springs quadrangle, Boulder and mountainous part of the Morrison–Evergreen Jefferson Counties, Colorado: U.S. Geological quadrangles and adjoining areas to the west in Survey Miscellaneous Field Studies Map MF– Jefferson County, Colorado: U.S. Geological 695, scale 1:24,000. Survey Miscellaneous Field Studies Map MF– ———1975b, Reconnaissance map showing relative 740, scale 1:24,000. amounts of soil and bedrock in the mountainous ———1976b, Reconnaissance map showing relative part of the Ralston Buttes quadrangle and amounts of soil and bedrock in mountainous part adjoining areas to the east and west in Jefferson of the Platte Canyon quadrangle, Jefferson County, Colorado: U.S. Geological Survey County, Colorado: U.S. Geological Survey Miscellaneous Field Studies Map MF–689, scale Miscellaneous Field Studies Map MF–803, scale 1:24,000. 1:24,000. Price, A.B., and Amen, A.E., 1980, Soil survey of the ———1976c, Reconnaissance map showing relative Golden area, Colorado, parts of Denver, amounts of soil and bedrock in mountainous part Douglas, Jefferson, and Park Counties: U.S. of the Pine quadrangle, Jefferson County, Department of Agriculture, Soil Conservation Colorado: U.S. Geological Survey Miscellaneous Service, 405 p. Field Studies Map MF–804, scale 1:24,000. Reed, J.C., Jr., Sheridan, D.M., and Bryant, Bruce, ———1978, Reconnaissance map showing relative 1973, Map showing faults, joints, foliation, and amounts of soil and bedrock in mountainous part surficial deposits in the Evergreen quadrangle, of the Kassler quadrangle, Jefferson and Douglas Jefferson County, Colorado: U.S. Geological Counties, Colorado: U.S. Geological Survey Survey Map I–786–F, scale 1:24,000. Miscellaneous Field Studies Map MF–787, scale Reheis, M.C., 1980, Loess sources and loessial soil 1:24,000. changes on a downwind transect, Boulder– Schmidt, P.W., and Pierce, K.L., 1976, Mapping of Lafayette area, Colorado: The Mountain mountain soils west of Denver, Colorado, for Geologist, v. 17, p. 7–12. land use planning, in Coates, D.R., ed., Richmond, G. M., 1986, Stratigraphy and correlation Geomorphology and engineering: Binghamton of glacial deposits of the Rocky Mountains, the State University of New York Publications in Colorado Plateau, and the ranges of the Great Geomorphology, p. 43–54. Basin, in Richmond, G.M., and Fullerton, D.S., Schwochow, S.D., 1972, Surficial geology of the eds., Quaternary glaciations in the United States Eastlake quadrangle, Adams County, Colorado: of America: Quaternary Science Reviews, v. 5, Golden, Colorado School of Mines, M.S. thesis, p. 99–127. 152 p. Richmond, G.M., and Fullerton, D.S., 1986, Scott, G.R., 1960, Subdivision of the Quaternary Introduction to Quaternary glaciations in the alluvium east of the Front Range near Denver, United States of America, in Richmond, G.M., Colorado: Geological Society of America and Fullerton, D.S., eds., Quaternary glaciations Bulletin, v. 71, p. 1541–1543. in the United States of America: Quaternary ———1962, Geology of the Littleton quadrangle, Science Reviews, v. 5, p. 3–10. Jefferson, Douglas, and Arapahoe Counties, Robinson, C.S., Gallant, W.A., and Cochran, D.M., Colorado: U.S. Geological Survey Bulletin 1976, Land use and engineering geology of the 1121–L, p. L1–L53, map scale 1:24,000. Front Range, Colorado, in Studies in Colorado ———1963, Quaternary geology and geomorphic field geology: Professional Contributions of history of the Kassler quadrangle, Colorado: Colorado School of Mines, no. 8, p. 486–504. U.S. Geological Survey Professional Paper 421– Rosholt, J.N., Bush, C.A., Shroba, R.R., Pierce, K.L., A, 124 p. and Richmond, G.M., 1985, Uranium-trend ———1970, Quaternary faulting and potential dating and calibrations for Quaternary earthquakes in east-central Colorado: U.S. sediments: U.S. Geological Survey Open-File Geological Survey Professional Paper 700–C, p. Report 85–299, 48 p. C11–C18.

25 ———1972a, Geologic map of the Morrison Survey Miscellaneous Investigations Series Map quadrangle, Jefferson County, Colorado: U.S. I–2526, scale 1:12,000. Geological Survey Miscellaneous Investigations Simpson, H.E., 1973a, Map showing landslides in the Series Map I–790–A, scale 1:24,000. Golden quadrangle, Jefferson County, Colorado: ———1972b, Map showing landslides and areas U.S. Geological Survey Miscellaneous susceptible to landsliding in the Morrison Investigations Series Map I–761–B, scale quadrangle, Jefferson County, Colorado: U.S. 1:24,000. Geological Survey Miscellaneous Investigations ———1973b, Map showing earth materials that may Series Map I–790–B, scale 1:24,000. compact and cause settlement in the Golden ———1972c, Map showing areas containing quadrangle, Jefferson County, Colorado: U.S. swelling clay in the Morrison quadrangle, Geological Survey Miscellaneous Investigations Jefferson County, Colorado: U.S. Geological Series Map I–761–D, scale 1:24,000. Survey Miscellaneous Investigations Series Map Sims, P.K., 1964, Geology of the Central City I–790–C, scale 1:24,000. quadrangle, Colorado: U.S. Geological Survey Scott, G.R., and Lindvall, R.M., 1970, Geology of Geologic Quadrangle Map GQ–267, scale new occurrences of Pleistocene bisons and 1:24,000. peccaries in Colorado: U.S. Geological Survey Soil Survey Staff, 1951, Soil Survey Manual: Professional Paper 700–B, p. B141–B149. Washington, D.C., U.S. Department of Sheridan, D.M., and Marsh, S.P., 1976, Geologic Agriculture, 503 p. map of the Squaw Pass quadrangle, Clear Creek, Sturchio, N.C., Pierce, K.L., Murrell, M.T., and Jefferson, and Gilpin Counties, Colorado: U.S. Sorey, M.L., 1994, Uranium-series ages of Geological Survey Geologic Quadrangle Map travertines and timing of the last glaciation in the GQ–1337, scale 1:24,000. northern Yellowstone area, Wyoming-Montana: Sheridan, D.M., Reed, J.C., Jr., and Bryant, Bruce, Quaternary Research, v. 41, p. 265–277. 1972, Geologic map of the Evergreen Tator, B.A., 1952, Piedmont interstream surfaces of quadrangle, Jefferson County, Colorado: U.S. the Colorado Springs region, Colorado: Geological Survey Miscellaneous Investigations Geological Society of America Bulletin, v. 63, p. Series Map I–786–A, scale 1:24,000. 255–274. Short, S.K., and Elias, S.A., 1987, New pollen and Taylor, R.B., 1975, Geologic map of the Bottle Pass beetle analyses at the Mary Jane site, Colorado quadrangle, Grand County, Colorado: U.S.  Evidence for late glacial tundra conditions: Geological Survey Geologic Quadrangle Map Geological Society of America Bulletin, v. 98, p. GQ–1224, scale 1:24,000. 540–548. Theobald, P.K., 1965, Preliminary geologic map of Shroba, R.R., 1980, Geologic map and physical the Berthoud Pass quadrangle, Clear Creek and properties of the surficial and bedrock units of Grand Counties, Colorado: U.S. Geological the Englewood quadrangle, Denver, Arapahoe, Survey Miscellaneous Geologic Investigations and Adams Counties, Colorado: U.S. Geological Map I–443, scale 1:24,000. Survey Geologic Quadrangle Map GQ–1524, Trimble, D.E., and Fitch, H.R., 1974, Map showing scale 1:24,000. potential gravel sources and crushed-rock ———1982, Physical properties and performance aggregate in the greater Denver area, Front characteristics of surficial deposits and rock units Range urban corridor, Colorado: U.S. in the greater Denver area, in Hansen, W.R., and Geological Survey Miscellaneous Investigations Crosby, E.J., Environmental geology of the Front Series Map I–856–A, scale 1:100,000. Range urban corridor and vicinity, Colorado: Trimble, D.E., and Machette, M.N., 1979, Geologic U.S. Geological Survey Professional Paper 1230, map of the greater Denver Area, Front Range p. 67–86. urban corridor, Colorado: U.S. Geological Shroba, R.R., and Birkeland, P.W., 1984, Trends in Survey Miscellaneous Investigations Series Map late-Quaternary soil development in the Rocky I–856–H, scale 1:100,000. Mountains and Sierra Nevada of the western Turner, A.K., 1974, Surficial geology investigations United States, in Porter, S.C., ed., Late- and evaluation of the sand and gravel deposits, Quaternary environments of the United States, Brighton quadrangle, Colorado: Colorado volume 1, the late Pleistocene: Minneapolis, School of Mines unpublished geologic report, 69 University of Minnesota Press. p. 145–156. p. Shroba, R.R., and Carrara, P.E., 1996, Surficial Van Horn, Richard, 1972, Surficial and bedrock geologic map of the Rocky Flats environmental geologic map of the Golden quadrangle, technology site and vicinity, Jefferson and Jefferson County, Colorado: U.S. Geological Boulder Counties, Colorado: U.S. Geological Survey Map I–761–A, scale 1:24,000.

26 ———1976, Geology of the Golden quadrangle, Wells, J.D., 1967, Geology of the Eldorado Springs Colorado: U.S. Geological Survey Professional quadrangle, Boulder and Jefferson Counties, Paper 872, 116 p. Colorado: U.S. Geological Survey Bulletin Varnes, D.J., 1978, Slope movement types and 1221–D, p. D1–D85. processes, in Schuster, R.L, and Krizek, R.J., Wyant, D.G., and Barker, Fred, 1976, Geologic map eds., Landslides, analysis, and control: National of the Milligan Lakes quadrangle, Park County, Academy of Sciences, Transportation Research Colorado: U.S. Geological Survey Geologic Board Special Report 176, p. 11–33. Quadrangle Map GQ–1343, scale 1:24,000.

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