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Kp

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Qg

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Qg

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Qay

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Qai2

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DESCRIPTION OF MAP UNITS

[Calibrated radiocarbon ages are expressed as “cal ka B.P.,” which stands for calibrated thousand 

years before present (0 yr B.P. = 1950 A.D.). Uncertainties are given at the 95 percent (2σ) 

confidence level. Calibrated ages are reported as the midpoint of the calibrated range. In cases 

where calibration produced more than one age range with a probability of 5 percent or more, ages 

are based on the mean of the ranges weighted by their probabilities and are presented without 

uncertainties.

Soil-horizon designations and other descriptive soil terminology used in this report follow 

criteria outlined in Soil Survey Division Staff (1993), Birkeland (1999), and Schoeneberger and 

others (2012). Most colors are field dry colors and based on Munsell soil color charts (Munsell 

Color, 1975). The term “consistence” is the resistance to crushing of soil or surficial material in the 

hand, as described by Soil Survey Division Staff (1993). Textures are field estimates. In descriptions 

of clast lithology, the term “granite” refers to phaneritic igneous or meta-igneous rock types that 

are felsic to intermediate in composition.

Geochronology sites are named by letters in their field numbers (tables 1–3, sheet 2), and 

correspond to letters shown in red on figure 1]

SURFICIAL DEPOSITS

ARTIFICIAL FILL 

Artificial fill and river control structures (latest Holocene)—Includes abutments 

where roadway crosses South Platte River, raised track of railroad, interstate 

roadbed, artificial levees and banks, metal river-control structures, small earthen 

dams, and areas extensively modified by industrial activities. Artificial fill is 

present along most canals and ditches, but only mapped locally. Chiefly consists 

of gravel, sand, silt, rock and concrete fragments, and waste from sugar refinery

Active and reclaimed gravel pits (latest Holocene)—Chiefly consists of piles of 

gravel and sand in central part of map area. Pits reclaimed in places. Mapped as 

Verdos Alluvium by Gardner (1967), but alluvium so extensively mined since 

then that exposures of alluvium no longer exist and the landscape is almost 

entirely modified

ALLUVIAL DEPOSITS

Active channel and floodplain alluvium (late Holocene)—Equivalent in part to 

post-Piney Creek alluvium of Scott (1963). Mapped along the South Platte River 

and its larger tributaries. In tributary drainages locally includes alluvium making 

up low terraces. South Platte River alluvium very pale brown, light brownish 

gray, light yellowish brown, or light pinkish brown. Mostly fine to coarse sand, 

pebbly sand, silty sand, and sandy pebble and minor small cobble gravel, interstratified with thin (<1–5 centimeters [cm]) layers or lenses of very pale brown 

silt and dark grayish brown, organic-rich clayey silt and clay. Poorly to well 

sorted, and weakly to moderately stratified. Sand beds commonly cross-stratified. 

Clasts mostly granite, gneiss, pegmatite, quartz, feldspar, and sandstone; most 

clasts subrounded to rounded. Locally, grains stained with reddish-yellow iron 

oxide and black manganese oxide at stream level. Uppermost 25–80 cm of 

sediment on floodplain typically light grayish brown, crudely stratified, poorly 

to moderately sorted sandy silt, silty clay, silty sand, and granule- to fine-pebbly 

sand. Less active parts of floodplain have weakly developed surface soil (A/C or 

A/AC/C profile). Krotovinas (filled animal burrows) are common locally. Unit 

contains minor amounts of detrital lignite eroded from bedrock (possibly Upper 

Cretaceous Laramie Formation; see Gardner, 1967) exposed along tributaries 

south and southwest of map area (Braddock and Cole, 1978; Sharps, 1980). 

Along Wildcat Creek, alluvium mostly light yellowish brown or pale yellow, 

moderately sorted sand, silt, and silty clay interstratified with minor beds or 

lenses of pinkish gray, poorly sorted pebbly sand; pebble rock types indicate 

many are reworked from South Platte River deposits. Estimated thickness 

1–3 meters (m) along Wildcat Creek and 3–5 m along South Platte River

Young alluvium 1 (late Holocene)—Equivalent in part to post-Piney Creek alluvium 

of Scott (1963). Mapped along the South Platte River and its larger tributaries. 

Forms a low terrace that is occasionally flooded. Along South Platte River, 

terrace is generally 1.5 m above active floodplain. Parts of surface were within 

active floodplain in the 1960s (Gardner, 1967), but shifts in river course and 

addition of artificial levees have protected these areas from frequent flooding. 

Uppermost meter or more of sediment historical in age, at least locally, based on 

corroded metal debris embedded in stratified sands at one locale. Alluvium very 

pale brown, light brownish gray, light yellowish brown, or light pinkish brown, 

fine to coarse sand, pebbly sand, and sandy pebble and minor small-cobble 

gravel. Interstratified with thin (<1–5 cm) layers or lenses of very pale brown 

silt and dark grayish brown, organic-rich clayey silt and clay. Poorly to well 

sorted and weakly to moderately stratified. Sand beds commonly cross-stratified. 

Loose to slightly hard dry consistence. Locally, grains stained with reddish-yellow 

iron oxide and black manganese oxide. Some of the finer grained beds show 

evidence of soft sediment deformation. Clasts mostly granite, gneiss, quartz, 

feldspar, pegmatite, chert, and sandstone; most clasts subrounded to rounded. 

Uppermost sediment packet in alluvial sequence typically light grayish brown or 

pale brown, crudely stratified, poorly sorted sandy silt, silty sand, and granuleto fine-pebbly sand with lenses of silt and dark grayish brown clayey silt and 

clay. Unit contains minor amounts of lignite fragments. Surface soil typically 

poorly drained A/C or A/AC/C profile. Krotovinas are common locally. Along 

Wildcat Creek, alluvium mostly light yellowish brown or pale yellow, 

moderately sorted sand, silt, and silty clay interstratified with minor beds or 

lenses of pinkish gray, poorly sorted pebbly sand; pebble rock types indicate 

many are reworked from South Platte River deposits. Estimated thickness 1–3 m 

along Wildcat Creek and 3–6 m along South Platte River

Young sidestream alluvium, undivided (Holocene)—Mapped in tributary drainages 

and small draws north of the South Platte River; includes alluvium in channels, 

on floodplains, and forming low terraces 1–3 m above valley bottoms. Also 

includes reworked loess and eolian sand deposited by sheetwash processes to 

form flat-floored valleys between low-relief divides at the heads of small stream 

drainages; flat floor of valleys commonly incised up to 3–5 m by steep-walled 

gullies (arroyos). Unmapped Qay deposits also partly cover Qa3 terraces of 

Wildcat Creek but are not shown so that the underlying terrace is discernable. 

Alluvium commonly consists of pale yellow, pale brown, brown, light yellowish 

brown, or light brownish gray, sandy silt or silty sand with scattered granitic 

granules, pebbles, and lenses and stringers of poorly sorted pebbly silty sand. 

Pebble rock types indicate that most are reworked from South Platte River 

deposits. Weakly to moderately stratified. Loose to slightly hard dry consistence. 

Locally, alluvial sequence contains buried soils indicating episodic 

deposition. A radiocarbon (14C) age of 2.93±0.07 cal ka B.P. (Aeon–1580, 

table 2, sheet 2) for a buried A horizon at site WC provides a minimum-limiting 

age for alluvium in which the buried soil formed and a maximum-limiting age 

for the 80 cm of alluvium that overlies it; age indicates that deposition of Qay at 

site WC spanned at least the last 2.9 ka and probably longer. A bison tooth 

sampled at approximately 90 cm depth from a gully headwall at site TIP has a 

14C age of about 0.13 cal ka B.P. (Aeon–1582, table 2, sheet 2), indicating that 

the upper meter of Qay filling the valley at this site could have been deposited 

since the early 1820s, and subsequently followed by as much as 5 m of gully 

incision. A very young age for uppermost meter of sediment at site TIP is 

supported by a lack of surface soil development at that site. Estimated thickness 

1–5 m

Young alluvium 2 (late Holocene)—Equivalent in part to post-Piney Creek alluvium 

of Scott (1963). Mapped along the South Platte River and its larger tributaries. 

Forms a low terrace that is occasionally flooded in places and rarely flooded in 

others. Along the South Platte River, terrace is generally 3 m above active 

floodplain. Alluvium very pale brown, light brownish gray, light yellowish 

brown, or light pinkish brown, fine to coarse sand, pebbly sand, and sandy 

pebble and minor small cobble gravel. Interstratified with thin (<1–5 cm) layers 

or lenses of very pale brown silt and dark grayish brown, organic-rich clayey silt 

and clay. Poorly to well sorted and weakly to moderately stratified. Sand beds 

commonly cross-stratified. Loose to slightly hard, dry consistence. Locally, 

grains stained with reddish-yellow iron oxide and black manganese oxide. Some 

beds show evidence of soft sediment deformation. Clasts mostly granite, gneiss, 

quartz, feldspar, pegmatite, chert, and sandstone; most clasts subrounded to 

rounded. Uppermost sediment packet in alluvial sequence typically light grayish 

brown or pale brown, crudely stratified, poorly sorted sandy silt, silty sand, and 

granule- to fine-pebbly sand with lenses of silt and dark grayish brown clayey 

silt and clay. Surface soil variable, ranging from poorly drained A/C profiles to 

profiles with weakly developed textural B horizons (U.S. Department of 

Agriculture Natural Resources Conservation Service [USDA], 2009). Krotovinas 

are common locally. Unit contains minor amounts of lignite fragments. Along 

Wildcat Creek, alluvium mostly light yellowish brown or pale yellow, moderately 

sorted sand, silt, and silty clay interstratified with minor beds or lenses of pinkish 

gray, poorly sorted pebbly sand; pebble rock types indicate many are reworked 

from South Platte River deposits. Estimated thickness 1–3 m along Wildcat 

Creek and 3–6 m along South Platte River

Young alluvium 3 (Holocene and latest Pleistocene?)—Piney Creek Alluvium of 

Gardner (1967) and Scott (1978). Forms terrace and low-gradient fans 8–12 m 

above active floodplain of South Platte River, and terrace 6–9 m above Wildcat 

Creek. Gardner (1967) and McFaul and others (1994) considered Qa3 terrace 

correlative to Kuner terrace of Bryan and Ray (1940), whose type locality is 

about 60 kilometers (km) west of Fort Morgan, Colorado. Includes sheetwash 

deposits and colluvium interbedded with terrace alluvium at valley sides. 

Locally, covered by unmapped deposits of Qay along Wildcat Creek. Alluvium 

pale yellow, light yellowish brown, light olive brown, or grayish brown. Poorly 

to well sorted, weakly to moderately stratified, fine sand, silt, clayey silt, pebbly 

sand, sandy pebble gravel, cobble gravel, and silt with coarse sand lenses and 

granule- to fine-pebble stringers; along Wildcat Creek alluvium mostly fined 

grained, thinly stratified sand, silt, and silty clay. Soft to slightly hard dry 

consistence. Clasts mostly granite, gneiss, pegmatite, quartzite, quartz, feldspar, 

chert, and sandstone; includes bluish-gray quartzite that probably originated 

from outcrops in Coal Creek Canyon (Lindsey and others, 2005), roughly 130 km 

southwest of map area. Most clasts subrounded to rounded. Locally, weakly 

developed buried soil separates deposits within alluvial sequence. Soil survey 

(USDA Natural Resources Conservation Service, 2009) shows surface soil as 

typically having thin argillic (Bt) horizon and weakly developed Bk horizon 

with stage I–II carbonate morphology.

Radiocarbon (14C) and optically stimulated luminescence (OSL) age 

estimates from a section exposed in a stream cut along a small unnamed 

tributary in adjacent Weldona quadrangle (site AK, near section AK of Gardner, 

1967; fig. 1) indicate that basal alluvium was deposited in latest Pleistocene or 

early Holocene (tables 1 and 2, sheet 2; Berry and others, 2013; 2015a). Probable 

Succinea snail shells collected at depths of approximately 4.7 m and approximately 3.6 m yielded 14C ages of 11.90±0.28 cal ka B.P. (Aeon–994, table 2, 

sheet 2) and 11.95±0.24 cal ka B.P. (Aeon–995, table 2, sheet 2), respectively. 

Sediment collected at a depth of approximately 3.5 m, from a site about 10 m 

upstream from where the snail shells were collected, gave an OSL age estimate 

of 9.1±0.9 ka (UNL–3503, 5 percent moisture, table 1, sheet 2). A latest 

Pleistocene to early Holocene age for basal alluvium is also supported by an age 

of 11.68±0.36 cal ka B.P. (uncalibrated age of 10.11±0.09 14C ka B.P., 

AA–11084A) obtained by Haynes and others (1998) for humic acids in charcoal 

from Kuner terrace alluvium (of Bryan and Ray, 1940) at the Bernhardt site, 

roughly 80 km west of site AK. 

Timing of uppermost alluvium deposition and surface stabilization along the 

South Platte River is constrained by a date from McFaul and others (1994) of 

5.84±0.18 cal ka B.P. (5.12±0.08 14C ka B.P., Beta–42564) for soil humate from a 

buried A horizon developed in Qa3 terrace alluvium at a site in the Masters 7.5 ́ 

quadrangle. Degree of development of the buried soil indicates that the Qa3 

surface may have been stabilized for a few thousand years prior to its burial at 

that site by eolian sediment about 5.8 cal ka (McFaul and others, 1994). 

Stabilization of the Qa3 surface by middle Holocene time is supported by degree 

of development of soils mapped on the South Platte Qa3 terrace surface by 

USDA Natural Resources Conservation Service (2009), and by a 14C age of 

4.92±0.07 cal ka B.P. (Aeon–2101, table 2, sheet 2) for a buried A horizon 

developed in Qa3 terrace alluvium of Wildcat Creek (site FMR). Degree of 

development of the buried soil profile, which has a weakly developed Bt horizon 

roughly 35 cm thick and filamentous stage I carbonate accumulation, is consistent with several thousand years of soil development on the terrace surface prior 

to its burial at that site by episodic accumulation of sheetwash deposits 

(unmapped Qay) about 4.9 cal ka. Estimated thickness 2–6 m

Sidestream deposits of Broadway Alluvium (late Pleistocene)—Mapped as 

informal Bijou Flats tongue of Broadway Alluvium by Gardner (1967) and upper 

member of Broadway Alluvium by Scott (1978). Overlies and inferred to 

interfinger with mainstream deposits of Broadway Alluvium (Qba) south of 

South Platte River. Consists mainly of sheetflood deposits interpreted to have 

been deposited primarily by large-magnitude floods along Bijou and Kiowa 

Creeks (fig. 1; Gardner, 1967; Scott, 1978, 1982; Berry and others, 2015a, 2018). 

Deposits form a low-gradient fan that slopes roughly 3 m/km toward the South 

Platte River. Distribution of deposits suggests that influx of large amounts of 

sidestream alluvium during flood events deflected the South Platte River to the 

north side of its valley and may have episodically dammed the river for short 

periods of time (Scott, 1982). Accumulation of the sidestream alluvium built up 

the surface to a level that now stands approximately 21 m above the active 

floodplain in Masters 7.5 ́ quadrangle (Berry and others, 2015b), 21‒24 m above 

the active floodplain in Orchard 7.5 ́ quadrangle (Berry and others, 2015a), and 

reaches a maximum height of approximately 27 m above the active floodplain in 

adjacent Weldona 7.5 ́ quadrangle, near the Bijou Creek confluence (Berry and 

others, 2018). Here in Fort Morgan 7.5 ́ quadrangle, height of surface decreases 

from approximately 22 m near Bijou Creek to 15 m at the east edge of the 

quadrangle. Deposits have been identified as far downstream as Prewitt 

Reservoir (roughly 35 km east) and Atwood, Colorado (roughly 50 km east) by 

Gardner (1967) and Scott (1978), respectively. 

Alluvium light yellowish brown, light olive brown, pale yellow, or light 

gray, poorly to well sorted, moderately stratified, coarse to very fine sand and 

silty sand in beds generally 20–60 cm thick. Sand beds commonly separated by 

thin (1–5 millimeters [mm]) mats of organic debris and clay, or packets (5–10 cm 

thick) of thinly laminated dark-grayish brown or black organic-rich clay interbedded with layers of pale yellow or light gray silt and fine sand. Contains 

abundant detrital lignite that Gardner (1967) and Scott (1978) attribute to Upper 

Cretaceous Laramie Formation (see text below). Graded beds that fine upward 

from poorly sorted coarse sand and granules to mostly fine sand are common, as 

well as sand beds that are cross-stratified, finely laminated, crudely stratified, or 

massive. Sand grains and granules mostly subangular to subrounded, and are 

mostly composed of quartz and feldspar. Few scattered small pebbles of tuff, 

shale, and granitic rocks. Soft to hard dry consistence. Iron oxide nodules, 

masses, or pore linings present locally, particularly in sandy clay layers. 

Typically forms vertical exposures, especially along South Platte River bluff and 

Bijou Creek drainage (Berry and others, 2015a; 2018). Contact with mainstream 

alluvium typically sharp, most notably marked by difference in color and dry 

consistence attributable to differences in provenance. Soil profile characterized 

by thin argillic (Bt) horizon with moderate prismatic structure, and Bk horizon 

with stage II filamentous carbonate morphology. Soil profile buried by younger 

eolian sand in many places. 

OSL and 14C age estimates indicate a late Pleistocene age for Qbs (Berry 

and others, 2015a). A dated section at the mouth of Kiowa Creek (site KC, in 

Orchard 7.5 ́ quadrangle; fig. 1) yielded OSL age estimates (5 percent moisture) 

of 12.0±1.1 ka (UNL–3462) at approximately 1.7 m depth, 16.8±1.7 ka 

(UNL–3466) at approximately 2.6 m depth, and 15.2±1.5 ka (UNL–3463) at 

approximately 3.6 m depth (table 1, sheet 2). These age estimates are in good 

agreement with a 14C age of 14.53±0.56 cal ka B.P. (Aeon–1064, table 2, sheet 2) 

obtained for probable Succinea snail shells collected at a depth of approximately 

3.7 m from a section of Qbs exposed in the South Platte River bluff about 4.8 

km downstream from Kiowa Creek (site H–R, in Orchard 7.5 ́ quadrangle, at 

section H of Gardner, 1967; fig. 1). The ages also are in good agreement with 

OSL age estimates (5 percent moisture) for a section exposed in the north 

cutbank of Bijou Creek (site BC, in Weldona 7.5 ́ quadrangle about 13.5 km 

upstream from the river confluence; fig. 1): 12.4±1.1 ka (UNL–3498) sampled at 

approximately 1.1 m depth; and 14.6±1.2 ka (UNL–3499) sampled at approximately 1.9 m depth (table 1, sheet 2).

Organic material collected from a thin (2–5 mm) interbed of organic debris 

and clay at approximately 2.5 m depth, just above OSL sediment sample 

UNL–3466 (site KC), yielded a 14C age estimate of 43.90±5.69 cal ka B.P. 

(Aeon–950, table 2, sheet 2). This is likely an infinite age, meaning the sample is 

probably too old to date by the 14C method; the age is at the practical upper limit 

of the method, where trace amounts of contamination by young carbon have a 

large effect. Gardner (1967) also dated organic material from Qbs that yielded 

an anomalously old date. Pollen analysis of his dated sample indicated a plant 

assemblage of Late Cretaceous age, leading Gardner (1967) to interpret the 

sample as lignite from the Laramie Formation, which crops out in Kiowa and 

Bijou Creek drainage basins (Braddock and Cole, 1978; Sharps, 1980). The 

dated sample (Aeon–950) also is suspected to contain lignite. Probable lignite 

debris is common to abundant in other exposures of Qbs along Bijou Creek and 

the South Platte River bluff, as also noted by Scott (1978). Estimated thickness 

18–20 m thinning to 6–9 m near east edge of quadrangle

Mainstream deposits of Broadway Alluvium (late Pleistocene)—Mapped as 

Broadway Alluvium by Gardner (1967) and lower member of Broadway 

Alluvium by Scott (1978). Underlies and inferred to interfinger with sidestream 

Broadway Alluvium (Qbs) from Kiowa and Bijou Creeks (fig. 1) south of South 

Platte River. North of river, preserved only in a few isolated gravel lags roughly 

15–18 m above active floodplain. Unit poorly exposed due to cohesionless 

nature of the sediment when dry. High potassium feldspar content gives unit a 

distinctive pinkish hue. 

Alluvium is pink, pinkish gray, or light reddish brown, moderately well- to 

poorly sorted, fine to coarse sand, pebbly sand, silty sand, and sandy granuleto-pebble gravel. Commonly cross-stratified. Locally has interbeds of pinkish 

gray to very pale brown silt or laminations of dark grayish brown clay. Sand 

grains mostly subrounded. Gravel typically rounded to subrounded clasts of 

granite, gneiss, pegmatite, feldspar, quartz, chert, quartzite (including bluishgray quartzite), and sandstone. Largest clasts commonly 3–5 cm in diameter, but 

cobbles and rare small boulders (up to 32 cm in diameter for sandstone boulders 

of upper transition member Pierre Shale) present locally. Reddish-yellow to 

yellowish-red iron oxide or black manganese oxide common as grain coatings, 

masses in matrix, and accumulations along bedding planes. Surface soil profile 

typically has thin argillic (Bt) horizon with weak to moderate prismatic structure, 

and a thin Bk horizon with stage II carbonate morphology. 

At a site in the Masters 7.5 ́ quadrangle (Berry and others, 2015b), unit 

includes an approximately 70-cm-thick section of (from top down) interbedded 

crumbly clay, pinkish silt, platy silty fine sand, and burrowed sandy silt with 

gray and yellowish green color banding and a few shell fragments, over thin 

beds of white silt with small articulated clam shells, common shell fragments, 

and a hard dry consistence. Interpreted as quiet-water deposits that may have 

been deposited at a time when the river was temporarily dammed by sidestream 

flood deposits (Qbs). Underlying sands brightly stained reddish yellow, 

yellowish red, or dark red due to iron oxide accumulation.

An OSL age estimate for mainstream Broadway Alluvium (8.0±0.7 ka, 

UNL–3502, 5 percent moisture, table 1, sheet 2; Berry and others, 2015a) was 

obtained for sediment collected at site AK (fig. 1, Weldona quadrangle; near 

section AK of Gardner, 1967) at a depth of approximately 5.1 m, just below its 

contact with young alluvium 3 (Qa3). This age estimate is anomalously younger 

than and stratigraphically inconsistent with basal ages obtained for the overlying 

Qa3 deposit (see previous discussion), and therefore not considered a realistic 

estimate for age of the sediment. Another OSL age estimate of 9.4±1.0 ka 

(UNL–3504, 5 percent moisture, table 1, sheet 2; Berry and others, 2015a) was 

obtained for mainstream Broadway Alluvium where it underlies approximately 

7 m of Qbs at a locality about 0.8 km upstream from Kiowa Creek confluence 

(site TK–R, fig. 1, Orchard 7.5 ́ quadrangle). This age estimate is anomalously 

younger than and stratigraphically inconsistent with ages obtained from samples 

analyzed for overlying Qbs (see discussion above), and therefore also not 

considered a realistic estimate for age of the sediment. Reasons for the anomalously young OSL age estimates are unknown, but could be due to a number of 

factors associated with sampling and dating, or exposure of sediment to sunlight 

by burrowing animals. Results of a dose recovery test performed on UNL–3504, 

however, rule out most systematic problems with luminescence behavior of 

these dated sediments. 

Uranium-series age analysis of a bone fragment (possibly bison-horn core) 

encased in fine-grained sediment from the uppermost meter of mainstream 

Broadway Alluvium at site EF (fig. 1, Masters 7.5 ́ quadrangle) indicates that 

burial of the bone occurred sometime between 15 and 11 ka (table 3, sheet 2; 

Berry and others, 2015b). The oldest ages are for two interior-most subsamples 

that produced apparent closed-system 230Th/U ages within analytical uncertainty 

of one another (Wp67–5 and Wp67–7, table 3, sheet 2; see fig. 1-1 in Paces, 

2015). The error-weighted average value of these two oldest ages, 15.24±0.06 ka, 

may most closely estimate the minimum age of the deposit that contained the bone. 

However, because data for the specimen indicate a complex history of uranium 

uptake and subsequent leaching (see discussion in Paces, 2015), data for additional 

specimens are needed to better constrain the minimum age of the deposit. 

Broadway Alluvium is considered coeval with the Pinedale glaciation (Bryan 

and Ray, 1940; Hunt, 1954; Scott, 1960, 1975; Madole, 1991), which spanned 

from >31 ka to about 15–13 ka (Nelson and others, 1979; Madole, 1986; 

Schildgen and others, 2002; Benson and others, 2004, 2007; Licciardi and Pierce, 

2008; Madole and others, 2010; Young and others, 2011; Schweinsberg and 

others, 2016). Fluvial sediment load may have been greatest during and shortly 

after deglaciation (Church and Ryder, 1972; Madole, 1991; Schildgen and others, 

2002; Lindsey and others, 2005), a process that started either about 17 ka 

(Licciardi and others, 2004; Benson and others, 2005; Schaefer and others, 2006) 

or about 16–15 ka (Young and others, 2011), and largely was completed between 

about 15 and 13 ka (Benson and others, 2007; Young and others, 2011; and 

references therein). Correspondingly, Clovis artifacts present in upper part of 

terrace alluvium near Kersey, Colo. (roughly 66 km west of Fort Morgan, Colo.), 

combined with other archaeological data, indicate that aggradation of Broadway 

Alluvium was still in progress between about 13.3 and 12.9 cal ka (11.5–11 14C ka) 

but completed and the surface stabilized by about 11.5 cal ka (10 14C ka) at the 

latest (Holliday, 1987). A 230Th/U age of 11.0±1.1 ka (Wp67–1, table 3, sheet 2; 

Berry and others, 2015b) for pedogenic carbonate coating the outer edge of bone 

fragment encased in uppermost Broadway Alluvium at site EF (fig. 1, Masters 

7.5 ́ quadrangle) further supports surface stabilization and the onset of soil 

formation sometime prior to 11 ka. Estimated thickness 12 m to as much as 30 m 

south of South Platte River; 1‒1.5 m in gravel lags north of river

Louviers Alluvium (middle Pleistocene)—Subsurface unit not shown on map. See 

Subsurface Alluvial Deposits section for description 

Intermediate alluvium (middle Pleistocene)—Poorly preserved deposits of pebble 

and cobble gravel, pebbly sand, and silty sand in terrace remnants and gravel lags 

mapped along the north side of the South Platte River and in sidestreams west of 

Cris Lee Draw; either extensively eroded (westernmost part of quadrangle) or 

mostly buried by eolian deposits (near Cris Lee Draw). Mapped as Slocum 

Alluvium by Gardner (1967), but here recognized at two discrete levels and 

mapped as Qai1 and Qai2 (as in adjacent Weldona 7.5 ́ quadrangle downstream 

from the Narrows; fig. 1; Berry and others, 2018).

Alluvium very pale brown, light yellowish brown, or light reddish brown. 

Pebbles and cobbles mostly quartzite (including bluish-gray quartzite), gneiss, 

granite, pegmatite, vein quartz, sandstone, volcanic clasts, and chert; subrounded 

to well rounded; diameter of largest clasts commonly 13–16 cm but ranges up to 

36 cm for sandstone boulders of upper transition member Pierre Shale (Kp). 

Some granitic clasts are weathered (partly disintegrated). Thin coats (rinds) of 

calcium carbonate on clasts common. Where soil profile partially preserved 

includes carbonate horizons with stage III morphology. 

Probably correlative at least in part to intermediate alluvium (Qai) in 

Masters 7.5 ́ quadrangle, which has been dated using the uranium-series method 

(Paces, 2015). Uranium-series age analysis was done on innermost pedogenic 

calcium carbonate rinds, subsampled from multiple clasts collected at approximately 100–120 cm depth from Qai alluvium exposed beneath eolian sand in a 

ditch at site EIC (fig. 1, Masters 7.5 ́ quadrangle). The analysis produced reliable 

results and oldest 230Th/U ages that primarily cluster in two groups, one 

(Wp95–A1–2, Wp95–A2–1, Wp95–B1, and Wp95–B2, table 3, sheet 2) with an 

error-weighted-mean age of 334±9 ka and the other (Wp95–B3 and Wp95–E1, 

table 3, sheet 2) with an error-weighted-mean age of 382±16 ka (see fig. 1–7 in 

Paces, 2015). Age difference between clusters probably reflects difference in 

timing of calcium carbonate accumulation within the soil. Because method dates 

a product of soil development, oldest 230Th/U ages are considered closest 

temporally to minimum age of deposit in which the soil formed. These data 

indicate with a high degree of confidence that the alluvial clasts at EIC were 

deposited prior to about 334 ka; based on the two oldest ages, the best estimate 

for the minimum age of the deposit is somewhat older than 382 ka. These results 

support correlating Qai at site EIC with older deposits of Slocum Alluvium of 

Kellogg and others (2008), who recognized two levels locally along the Front 

Range and proposed an age range of 390–320 ka for the higher (older) deposits 

based on correlation with marine oxygen isotope stages and nonlinear rates of 

incision. However, more data are needed to substantiate the age of Slocum 

Alluvium and to establish its relation to Qai deposits along this part of the South 

Platte River corridor. More data also are needed to determine how Qai deposits 

at site EIC correspond to Qai1 and Qai2 deposits in Fort Morgan and adjacent 

Weldona quadrangles. Alluvium at site EIC is inferred to be part of a terrace 

remnant roughly 24 m above the modern floodplain, but the terrace is everywhere buried or not preserved in Masters and Orchard quadrangles (fig. 1), so it 

cannot be traced downstream. A height of 24 m would suggest correspondence 

with Qai1 terrace deposits, but some data indicate that at times in the past the 

gradient of the ancestral South Platte River was more gentle than that of the 

modern river through the reach between Masters and Fort Morgan (Berry and 

others, 2018). Therefore, it is conceivable that alluvium at site EIC corresponds 

to Qai2 terrace deposits instead

Intermediate alluvium 1 (middle Pleistocene)—Remnant gravel deposits roughly 

21–25 m above South Platte River floodplain. More extensively preserved than 

Qai2 (in contrast to relation in adjacent Weldona 7.5 ́ quadrangle; Berry and 

others, 2018). Includes sidestream terrace alluvium that grades to mainstream 

terrace alluvium; rock types in sidestream alluvium indicate clasts are mostly 

reworked from South Platte River deposits. Thickness commonly less than 

1–1.5 m but could be as much as 4–12 m based on borehole data (Colorado 

Division of Water Resources, 2013)

Intermediate alluvium 2 (middle Pleistocene)—Small, poorly preserved remnant 

gravel deposits roughly 32–40 m above South Platte River floodplain. Estimated 

thickness generally 1.5 m or less

Verdos Alluvium (middle Pleistocene)—Also mapped as Verdos Alluvium by 

Gardner (1967) and Scott (1978). Forms terrace remnants roughly 49–55 m 

above the South Platte River floodplain. Extensively mined for gravel and sand; 

some areas previously mapped as Verdos Alluvium by Gardner (1967) and Scott 

(1978) are now reclaimed gravel pits (gp). Alluvium largely covered by eolian 

sand (Qes) but locally exposed along canals and well-exposed in gravel pit in 

adjacent Brush West 7.5 ́ quadrangle (sec. 1, T. 4 N., R. 57 W.; photo 1, sheet 2). 

Alluvium very pale brown, yellowish brown, light reddish brown, or pinkish 

gray, poorly to moderately well sorted, pebble and cobble gravel, pebbly sand, 

sand, and silt. Sand and gravel commonly cross-bedded. Accumulations of 

yellowish-red and reddish-yellow iron oxide and black manganese oxide 

common in matrix, on grains, and along bedding planes. Pebbles and cobbles 

mostly quartzite (including bluish-gray quartzite), gneiss, granite, pegmatite, 

sandstone, chert, volcanic clasts, and petrified wood. Diameter of largest clasts 

commonly 14–20 cm. Most clasts subrounded to well rounded. Some clasts 

weathered (broken along foliation planes or beginning to disintegrate). Eroded 

soil profile includes 1-m-thick, variably cemented K horizon with stage III–IV 

carbonate morphology. Carbonate coats (rinds) on clasts up to 1 cm thick. May 

also contain some pedogenic silica. 

Age of Verdos Alluvium is constrained by its association with the Lava 

Creek B ash (Scott, 1960; Madole, 1991; Kellogg and others, 2008; and 

references therein), erupted from the Yellowstone Plateau volcanic field about 

640 ka (Lanphere and others, 2002). Beds of Lava Creek B ash in Verdos 

Alluvium have been documented at several sites along the South Platte River 

corridor northeast of Fort Morgan 7.5 ́ quadrangle (see Scott, 1978, 1982; Izett 

and Wilcox, 1982), although no ash deposits have been identified within the 

quadrangle. Estimated thickness of alluvium 6–10 m

Old alluvium (early Pleistocene)—Subsurface unit not shown on map. See Subsurface Alluvial Deposits section for description 

Nussbaum Alluvium (early Pleistocene? and Pliocene)—Old alluvial deposits 

mapped as Nussbaum Alluvium by Gardner (1967) and Scott (1978, 1982). 

Preserved in remnants roughly 122–137 m above active floodplain. Light reddish 

or yellowish brown, light brown, or pinkish gray, variably cemented, poorly to 

moderately sorted, pebble and cobble gravel, pebbly sand, sand, and sandy silt. 

Moderately to well stratified; sand and pebbly sand beds commonly crossstratified. Pebbles and cobbles mostly rounded to well rounded. Many cobbles 

13–18 cm in diameter. Clasts mostly granite, pegmatite, and quartzite, with lesser 

amounts of gneiss, volcanic porphyry, chert, and petrified wood. High potassium 

feldspar content of sand and granules imparts pinkish hue to deposits. Beds 

weakly to moderately, and locally strongly, cemented by calcium carbonate 

(photo 2, sheet 2). Basal 1–3 m of alluvium commonly cemented into an 

indurated conglomerate or conglomeratic sandstone that unconformably overlies 

Pierre Shale. Secondary silica may also be present based on the strength of 

cementation. Truncated soil with 15–20 cm of reddish brown Bt horizon over 

stage III+ carbonate horizon present locally. 

Scott (1982) considered Nussbaum Alluvium to be about 3 Ma (Pliocene) 

based on its geomorphic position, fossils remains, and inferred ages of Stegomastodon fossils collected from the alluvium at sites northeast of Fort Morgan 

(NE1⁄4SE1⁄4, sec. 12, T. 7 N., R. 55 W. and NW1⁄4NE1⁄4, sec. 2, T. 5 N., 

R. 56 W.). He used the fossil evidence and geomorphic position to correlate 

Nussbaum Alluvium with the Broadwater Formation of western Nebraska, and 

fossil evidence to show equivalence between the Broadwater Formation and the 

Blanco Formation of Texas (Scott, 1982; Madole, 1991), which has an age of late 

Pliocene and early Pleistocene based on dated ash beds contained within it (see 

Bell and others, 2004). Scott’s correlations, combined with the age range for 

Stegomastodon (Pliocene and early Pleistocene, see Bell and others, 2004), leave 

open the possibility that the age range of Nussbaum Alluvium likewise extends 

into the early Pleistocene. Thickness 7–12 m to as much as 21 m (Scott, 1982)

ALLUVIAL, COLUVIAL, AND GRAIN-FLOW DEPOSITS

Grain-flow deposits (late Holocene)—Mapped in northwest corner of quadrangle. 

Mostly made up of eolian sand that has flowed downslope by mass wasting 

processes. Demarcated by arcuate headscarp and lobate toe. Vegetated by 

shortgrass prairie (Chapman and others, 2006). Pale brown, brown, or yellowish 

brown, moderately to well sorted, mostly very fine to medium sand. Estimated 

thickness up to 3 m or more at toe

Alluvial and colluvial deposits (Holocene to middle? Pleistocene)—Mapped north 

of river mostly in areas where alluvial and colluvial deposits mantle hillslopes 

underlain by Pierre Shale (Kp). Largely deposited by overland flow and creep 

processes. Made up of reworked residuum from weathered bedrock mixed with 

sediments derived from river and eolian deposits higher on hillslopes. Locally 

includes areas where weathered residuum has not been reworked, and outcrops of 

Pierre Shale too small to map separately. Very pale brown, light yellowish brown, 

or light olive brown. Crudely stratified and poorly sorted. Mostly clayey silty 

sand or sandy clayey silt, with scattered, subrounded to rounded pebbles and 

cobbles of river alluvium and angular to subangular fragments of sandstone from 

the Pierre Shale. Deposits typically calcareous. Estimated thickness up to 3 m

EOLIAN DEPOSITS

Eolian sand (Holocene and late Pleistocene)—Forms simple and compound 

parabolic dunes and low-relief sand sheets. Blowouts common within dune 

fields. Pale brown, brown, or yellowish brown, moderately to well sorted, mostly 

fine to medium sand. Locally contains minor amounts of coarse to very coarse 

sand and a few scattered, subangular to rounded granules and small pebbles 

composed mostly of granitic and gneissic rock types. Loose to slightly hard dry 

consistence. Sand deposits of more than one age commonly separated by one or 

more buried soils. Locally includes sheetwash deposits of reworked eolian sand, 

and marsh or pond deposits in interdune and other low-lying wetland areas. 

Deposits of Qes may be more extensive than mapped due to the difficulty in 

distinguishing thin eolian sand cover from sandy terrace alluvium in a region that 

has been farmed and cultivated for more than a century.

Unstippled parts of Qes are shown in county soil survey as having mostly 

Valent, Valentine-Dwyer, and Dwyer soils, characterized by weakly developed 

A/C or A/AC/C soil profiles (USDA Natural Resources Conservation Service, 

2009), typical of relatively young (late and middle Holocene) eolian sand 

deposits (Madole, 1995; Muhs and others, 1996; Madole and others, 2005). In 

the northern part of quadrangle, unstippled parts also include large areas of 

Vona-Dwyer soils that probably reflect both Holocene and late Pleistocene sand 

deposits in a distribution pattern too complex to map separately. 

Light stippled parts of Qes, located south of the South Platte River, are 

shown in county soil survey as having mostly Bijou and Truckton soils, characterized by argillic (Bt) horizon development in sandy, mostly alluvial soil parent 

material (USDA Natural Resources Conservation Service, 2009). Field investigation revealed argillic horizon to be part of a buried soil developed in the 

underlying stream and sheetflood alluvium and later covered by a veneer of 

younger eolian sand. Therefore, as in unstippled areas, eolian sand deposits in 

light stippled areas probably are Holocene in age, but generally thin (<1 m), such 

that underlying buried soil controls soil classification and series designation. 

Dark stippled parts of Qes, located north of the South Platte River, are 

shown in county soil survey as having mostly Vona and Ascalon soils, characterized by A/Bt/Bk (stage I–II carbonate morphology) soil profiles developed in 

eolian sand (USDA Natural Resources Conservation Service, 2009). These soil 

profiles are typical of late Pleistocene eolian sand deposits (Madole, 1995; Muhs 

and others, 1996; Madole and others, 2005). Locally includes sand deposits of 

Holocene age that are too small to map separately.

Widespread deposition of eolian sand during the late Holocene is indicated 

by radiocarbon ages reported by Madole (1994, 1995) and Madole and others 

(2005) for buried soils that provide maximum-limiting ages for uppermost sand 

deposits in adjacent or nearby quadrangles: 0.64±0.11 cal ka B.P. (0.68±0.08 14C 

ka B.P., Beta–70542) near Bijou No. 2 Reservoir in Weldona 7.5 ́ quadrangle; 

1.14±0.16 cal ka B.P. (1.23±0.08 14C ka B.P., Beta–70543) at Milliron Draw in 

Orchard 7.5 ́ quadrangle; 0.88±0.21 cal ka B.P. (0.96±0.11 14C ka B.P., 

Beta–84821) at south edge of Orchard 7.5 ́ quadrangle near Bijou Creek; and 

0.82±0.16 cal ka B.P. (0.89±0.10 14C ka B.P., Beta–62192) in Greasewood Lake 

7.5 ́ quadrangle. Radiocarbon ages for soil carbonate rhizoliths collected from a 

quarry exposure north of Orchard, Colo., are in good agreement, indicating a 

maximum-limiting age for uppermost eolian sand of about 1.44±0.13 cal ka B.P. 

(1.56±0.07 14C ka B.P., CAMS–8234, Muhs and others, 1996). Ages from 

elsewhere in the region also support widespread deposition of eolian sand during 

the late Holocene (Clarke and Rendell, 2003, and references cited therein). 

Eolian sand deposits of probable middle Holocene age are locally exposed, 

mostly underlying late Holocene sand deposits. At a site near Empire Reservoir 

(site ER, Masters 7.5 ́ quadrangle, fig. 1), approximately 1 m below the surface, 

a middle Holocene sand deposit covered by a thin veneer of late Holocene sand 

has an OSL age estimate of 7.7±0.8 ka (UNL–3468, 5 percent moisture, table 1, 

sheet 2; Berry and others, 2015a, b). At Milliron Draw, Orchard 7.5 ́ quadrangle, 

the age of a buried sand deposit is constrained by ages on buried soils that 

provide a maximum-limiting age of approximately 6.33±0.16 cal ka B.P. 

(5.51±0.09 14C ka B.P., Beta–72203) and a minimum-limiting age of approximately 1.14±0.16 cal ka B.P. (1.23±0.08 14C ka B.P., Beta–70543) for the deposit 

(Madole, 1995; Madole and others, 2005). Radiocarbon ages for soil carbonate 

rhizoliths collected from a quarry exposure north of Orchard, Colo., suggest a 

minimum-limiting age for buried eolian sand of approximately 3.90±0.20 cal ka 

B.P. (3.60±0.07 14C ka B.P., CAMS–6378, Muhs and others, 1996) at that site. 

Episodes of eolian sand deposition during the middle Holocene are also 

documented at other sites within the region (Clarke and Rendell, 2003, and 

references cited therein). 

Sand deposits of probable late Pleistocene age (marked by dark stipple) are 

extensive north of the South Platte River in the Fort Morgan 7.5 ́ quadrangle, as 

well as regionally (Muhs and others, 1996; Madole and others, 2005). At site ER 

(fig. 1, Masters 7.5 ́ quadrangle), a deposit of late Pleistocene sand has an OSL 

age estimate of 26.4±2.5 ka at 0.7 m depth (UNL–3467, 5 percent moisture, 

table 1, sheet 2; Berry and others, 2015a, b). This age is in good agreement with 

radiocarbon ages Muhs and others (1996) obtained for soil carbonate nodules 

from an underlying buried soil at a site about 2 km north of site ER that provide 

maximum-limiting ages for the overlying sand of 31.21±0.23 cal ka B.P. 

(27.30±0.17 14C ka B.P., CAMS–11339), 31.36±0.48 cal ka B.P. (27.42±0.30 14C 

ka B.P., CAMS–16612), and 30.58±0.51 cal ka B.P. (26.41±0.24 14C ka B.P., 

CAMS–16604). Eolian sand deposition during the late Pleistocene likely was 

intermittent, and may have resulted in late Pleistocene deposits of various ages in 

the region; some may be as young as 12 or 13 ka (Madole and others, 2005).

Eolian sands south of the South Platte River are part of a series of dune 

fields that make up the Fort Morgan dune field (fig. 2). Main sources of sand in 

the Fort Morgan dune field could include South Platte River sediments (Muhs 

and others, 1996; Aleinikoff and Muhs, 2010; Muhs, 2017) and the Tertiary 

Ogallala Formation (Muhs, 2017). North of the river, eolian sands are considered 

part of the Sterling dune field (Muhs and others, 1996). Sand source for the 

Sterling dune field has not been studied but could be similar to that for the 

adjacent Greeley dune field (fig. 2); likely sand sources for the latter are the 

Laramie Formation (Muhs and others, 1996; Aleinikoff and Muhs, 2010; Muhs, 

2017) and the Ogallala Formation (Muhs, 2017). Dune orientation in Fort 

Morgan quadrangle is generally consistent with regional trends reported by Muhs 

(1985) and Muhs and others (1996), and indicates sand-transporting winds 

primarily from the northwest (Muhs and others, 1996; Madole and others, 2005). 

Currently, dunes are mostly stabilized by short grass prairie vegetation (Chapman 

and others, 2006). Thickness up to 10 m; thins to less than half a meter locally

Loess (Holocene and late Pleistocene)—Mapped as Peorian and Bignell Loesses by 

Gardner (1967) and Peoria Loess by Scott (1978). Mostly wind-blown calcareous, very fine sandy coarse silt, coarse silty very fine sand, and silt. Locally 

reworked by sheetwash and creep processes. Very pale brown, pale brown, or 

light yellowish brown, moderately to well sorted, with a soft to slightly hard dry 

consistence. Deposits are mostly massive but locally contain stringers and thin 

lenses of medium and coarse sand. Surface soil variable; ranges from weakly 

developed A/C or A/Bk/C profile to A/Bt/Bk/C profile with stage I or II 

carbonate morphology (USDA Natural Resources Conservation Service, 2009). 

Aleinikoff and others (1999) and Muhs and others (1999a) identify sources 

for loess in northeastern Colorado as glaciogenic silt produced by valley 

glaciation in the mountains and dryland dust eroded from bedrock (primarily 

from Eocene–Oligocene White River Group with lesser amounts from Upper 

Cretaceous Pierre Shale). In the Fort Morgan 7.5 ́ quadrangle, Qel covers much 

of the uplands north of the South Platte River from Cris Lee Draw east, but in 

regional perspective these deposits constitute an isolated area of loess aligned in 

a NW–SE distribution pattern generally centered around a similarly oriented 

Wildcat Creek drainage (Scott, 1978). This distribution pattern lends support to 

dryland dust sources northwest of the quadrangle being particularly important 

here (for example, see Mason, 2001); the White River Group crops out in a broad 

band north of Fort Morgan quadrangle (see fig. 2 in Muhs and others, 1999a, and 

references therein), and the upper transition member of Pierre Shale crops out 

extensively within Wildcat Creek drainage upstream (NW) from the loess 

deposits (Scott, 1978). 

Age estimates obtained for loess elsewhere in northeastern Colorado 

(Forman and others, 1995; Muhs and others, 1999a; Mahan and others, 2009; 

Pigati and others, 2013) support a correlation to late Pleistocene Peoria Loess, 

deposited at a time corresponding to Pinedale glaciation in the headwaters and 

deposition of Broadway Alluvium downstream in the South Platte River valley 

(see description for Qba). Locally, uppermost parts of Qel probably are 

Holocene in age (Madole, 1991), and therefore probably correlative to Bignell 

Loess, which has been noted in adjacent Peace Valley School 7.5 ́ quadrangle 

(Gardner, 1967) and elsewhere in northeastern Colorado and Nebraska, where it 

is separated from Peoria Loess by the Brady Soil (see discussions in Muhs and 

others, 1999b; 2008; Bettis and others, 2003; Pigati and others, 2013). Deposition of Bignell Loess is thought to have started about 10.5–9 cal ka and continued episodically throughout the Holocene (Mason and others, 2003, 2008; Miao 

and others, 2007). No buried soils were observed within exposed sections of Qel

in the Fort Morgan quadrangle that would allow differentiation of late Pleistocene and Holocene loess units, however, exposures are few, distribution of 

Bignell Loess is known to be patchy (Muhs and others, 1999b; Bettis and others, 

2003), and locally its accumulation may have been small enough and slow 

enough to be incorporated into the surface soil rather than form a discrete deposit 

(see Mason, 2001). Thickness for Qel up to 6–9 m, but thins to less than half a 

meter locally

EOLIAN AND ALLUVIAL DEPOSITS

Eolian and alluvial deposits, undivided (middle Pleistocene)—Loess and eolian 

sand partly reworked by sheetwash processes, grading downward into gravelly 

alluvial deposits that are probably correlative in part to Qg. Corresponds to map 

unit Gardner (1967) described similarly but interpreted as Loveland Loess 

grading downward into gravelly deposits he equated to Slocum Alluvium. Only 

tentatively correlated to Loveland Loess by Scott (1978). Mapped in Wildcat 

Creek drainage and in the headwaters of Cris Lee Draw. Fills paleovalleys now 

cut by modern arroyos in northernmost part of quadrangle. 

Brown, light yellowish brown, or brownish yellow. Typically consists of an 

upper section of weakly stratified sandy silt with lenses, stringers or scattered 

grains of sand and small pebbles, massive beds of silty very fine sand, or massive 

beds of sandy silt; and a lower section of interstratified sand, pebbly sand, silt, 

and poorly sorted sandy pebble or cobble gravel with cobbles 10–20 cm in 

diameter common (photo 3, sheet 2). Clasts mostly quartzite, gneiss, granite, 

sandstone, volcanic clasts and chert; rock types indicate that many clasts are 

reworked from older South Platte River deposits (probably Nussbaum Alluvium). 

Sands rich in potassium feldspar. Massive beds have slightly hard to hard dry 

consistence. Eroded pedogenic carbonate horizon with stage III carbonate 

morphology developed in Qlg marks contact between it and the younger deposits 

that overlie it (Qel, Qes, or Qay). 

Timing of paleovalley cutting and filling is uncertain, but could have 

corresponded to when deep incision, followed by aggradation of Louviers 

Alluvium (coeval with Bull Lake glaciation in the mountains), occurred in the 

South Platte River valley; this sequence of events would likely have caused a 

similar response within tributary drainages. The gradational contact between 

loess and eolian sand and alluvial deposits in the unit further supports correlating 

the gravelly alluvial deposits in Qlg to Louviers Alluvium rather than Slocum 

Alluvium. Locally could contain older deposits of sheetwash, alluvium, and 

colluvium. Estimated thickness at least 4 m 

Gravel deposits (middle Pleistocene)—Pebble and cobble gravel in eroded and 

partially buried remnant terrace deposits and thin gravel lags along Wildcat 

Creek. Probably correlative in part with gravelly alluvial section of Qlg based on 

valley position, clast lithology, and weathering characteristics. Overlies Pierre 

Shale and largely covered by late Pleistocene and Holocene eolian deposits (Qel 

and Qes). Previously mapped as Slocum Alluvium by Gardner (1967) but here 

considered more likely correlative to Louviers Alluvium (see discussion of Qlg). 

Clasts mostly quartzite, gneiss, granite, sandstone, volcanic, and chert reworked 

from older South Platte River deposits (probably Nussbaum Alluvium). Cobbles 

10–20 cm in diameter common; locally includes a few boulders (up to 30 cm in 

diameter) of conglomerate probably derived from Nussbaum Alluvium. Locally, 

many clasts have thin, patchy coats of calcium carbonate. Estimated thickness 

1.5 m or less

BEDROCK UNIT

Pierre Shale (Upper Cretaceous)—Upper transition member of Pierre Shale 

composed chiefly of marine calcareous silty shale or claystone, shaly sandstone, 

and sandy shale (Gardner, 1967; Scott, 1978). Mostly covered by unmapped 

weathered residuum; locally includes colluvium and other surficial deposits not 

mapped separately. Outcrops are light gray, fine-grained sandstone and shaly 

sandstone weathering to yellowish brown or light olive brown, and dark gray 

silty shale weathering to olive gray. Selenite (gypsum) crystals are common. Also 

contains concretionary limestone layers up to one meter thick. In northeastern 

Colorado, upper transition member of Pierre Shale is reported to contain 

diagnostic ammonites Sphenodiscus (Coahuilites) sp. and Baculites clinolobatus 

(Scott, 1978). Thickness of Pierre Shale about 1,800 m in map area (Gardner, 

1967; Scott, 1978)

EXPLANATION OF MAP SYMBOLS

Contact—Dashed where approximately located

Dune crest

Blowout rim within dune field 

Light stipple—Areas where eolian sand (Qes) is likely Holocene in age (as in 

unstippled areas) but where sand deposits could be thin

Dark stipple—Areas where eolian sand (Qes) may be late Pleistocene in age

Intermittent wetlands and ponds—Low-lying areas prone to wetness and ponding 

within eolian sand deposits (Qes)

Water—Includes water in natural and artificial ponds, and river and stream channels 

wide enough to be mapped at 1:24,000 scale

Radiocarbon (14C) sample location—With site name that links to sample field 

number (table 2, sheet 2)

INTRODUCTION

The Fort Morgan 7.5 ́ quadrangle is located on the semiarid plains of northeastern Colorado, 

along the South Platte River corridor where the river has incised into Upper Cretaceous Pierre 

Shale. The Pierre Shale is largely covered by surficial deposits that formed from alluvial, eolian, 

and hillslope processes operating in concert with environmental changes from the late Pliocene to 

the present. The South Platte River, originating high in the Colorado Rocky Mountains, has played 

a major role in shaping surficial geology in the map area, which is several tens of kilometers 

downstream from where headwater tributaries join the river. Recurrent glaciation (and deglaciation) 

of basin headwaters has affected river discharge and sediment supply far downstream, influencing 

deposition of alluvium and river incision in the Fort Morgan quadrangle. Distribution and characteristics of the alluvial deposits indicate that during the Pleistocene the course of the river within 

the map area shifted progressively southward as it incised, and by late middle Pleistocene the river 

was south of its present position, cutting and filling a deep paleochannel near the south edge of the 

quadrangle (Bjorklund and Brown, 1957). The river shifted back to the north during the late 

Pleistocene. Kiowa and Bijou Creeks (figs. 1 and 2) are unglaciated tributaries originating in the 

Colorado Piedmont east of the Front Range that also have played a major role in shaping surficial 

geology of the map area. Periodically during the late Pleistocene, major flood events on these 

tributaries deposited large volumes of sediment at and near their confluences, forming a broad, 

low-gradient fan composed of sidestream alluvium that could have occasionally dammed the river 

for short periods of time (Scott, 1982). Wildcat Creek, also originating on the Colorado Piedmont, 

and the small drainage of Cris Lee Draw dissect the map area north of the river (fig. 2). Eolian sand 

deposits of the Sterling (north of river) and Fort Morgan (south of river) dune fields cover much of 

the quadrangle and record past episodes of sand mobilization during times of prolonged drought 

(fig. 2; Muhs and others, 1996). With the onset of irrigation and damming during historical times, 

the South Platte River has changed from a broad, shallow, and sandy braided river with highly 

variable seasonal discharge to a much narrower, deeper river with braided-meandering transition 

morphology and more uniform discharge (Nadler and Schumm, 1981; Harvey and others, 1985).

METHODS

The geology of the Fort Morgan 7.5 ́ quadrangle was previously mapped by M.E. Gardner 

during the 1960s (Gardner, 1967). Gardner’s map (1967) provided the foundation for the digital 

geologic map presented here. This new geologic map was completed using a combination of 

methods, including field investigation, geochronologic research, and interpretation of the following: National Agriculture Imagery Program (NAIP) orthoimagery (U.S. Department of Agriculture 

[USDA] Farm Service Agency Aerial Photography Field Office, 2009, 2011, 2013, 2015); Qualitylevel 2 lidar data (1-meter resolution) from the 2013 South Platte River Flood Area 1 lidar data set 

(U.S. Geological Survey, 2015); 7.5 ́ topographic and 1/3 arc-second (10-meter resolution) digital 

elevation data; digital soil survey data (USDA Natural Resources Conservation Service, 2009); 

satellite imagery viewed with Google Earth; Federal Emergency Management Agency (1989) flood 

maps (1:19,200 approximate scale); stereoscopic pairs of National Aerial Photography Program 

(NAPP) color-infrared (1988, 1:40,000 scale) and historical black and white (1948, 1:39,230 scale; 

1949, 1:16,620 scale) aerial photographs; and subsurface lithologic data from test holes and water 

wells (Bjorklund and Brown, 1957; Colorado Division of Water Resources, 2013). Field mapping 

and research were carried out between 2011 and 2016. Digital mapping, completed in 2017 using 

ArcMap software, was done on NAIP orthoimagery taken in 2015; location and dimensions of the 

South Platte River and other water bodies shown on this map are based on the 2015 imagery. Much 

of the area has been extensively farmed and cultivated for more than a century, and in many places 

geomorphologic features are difficult to distinguish in orthoimagery or aerial photographs. 

However, they are commonly evident in the lidar images even within areas traversed by centralpivot irrigation equipment. The 1-meter resolution lidar data, available for roughly 75 percent of 

the quadrangle, were used to enhance recognition and facilitate mapping of floodplain and terrace 

deposits, as well as sand dune crests and blowouts, and natural and artificial levees. 

Geologic mapping was aided by optically stimulated luminescence (OSL), radiocarbon (14C), 

and uranium-series (230Th/U) age determinations (tables 1–3, sheet 2; Berry and others, 2015a, b). 

Sediment samples for OSL dating were processed and analyzed at the University of Nebraska–

Lincoln Luminescence Geochronology Laboratory using the single aliquot regenerative (SAR) 

method (Murray and Wintle, 2000). Equivalent dose (De

) was measured on a Risø DA–20 TL/OSL 

reader and values were calculated using the central age model of Galbraith and others (1999). Dose 

rates and age estimates were calculated using both field moisture of the sample and a fixed estimate 

of 5 percent moisture (table 1, sheet 2; Berry and others, 2015a). Moisture content of some of the 

samples (collected in November, 2011) was very low and unlikely to approximate an average 

condition; the fixed estimate of 5 percent (an intermediate value within the range of values for 

samples collected in November, 2011 and April–May, 2012) may better represent moisture history 

of the samples for the purpose of calculating estimated ages from the dose rates. Radiocarbon ages, 

including those obtained in this study (table 2, sheet 2, modified from Berry and others, 2015a) and 

previously published ages cited from the literature, were calibrated using the IntCal13 dataset and 

CALIB 7.0 (Stuiver and Reimer, 1993; Reimer and others, 2013) for better comparison to ages 

generated by other dating methods. Uranium-series age analyses were done at the U.S. Geological 

Survey Denver radiogenic isotope laboratory (DRIL) using standard operating procedure “USGS–

DRIL–01, R0 Uranium-Thorium Disequilibrium Studies” summarized in Paces (2015). Details 

about the samples analyzed and results obtained also are provided in Paces (2015). The 230Th/U 

analyses provide constraints on the minimum ages of deposits from which samples were collected 

(table 3, sheet 2; Berry and others, 2015b); the oldest 230Th/U ages are interpreted as being closest 

temporally to the minimum age of the depositional event. 

SUBSURFACE ALLUVIAL DEPOSITS

LOUVIERS ALLUVIUM (Qlv)

This middle Pleistocene unit is within the subsurface of the study area and not shown on this 

map. It is composed of pebble-to-cobble gravel, sand, and finer grained alluvium filling paleovalley 

located south of the modern South Platte River. Buried by younger alluvium (mostly Qba and 

Qbs) but inferred from subsurface lithologic data from test holes and water wells in Fort Morgan 

and adjacent quadrangles (Bjorklund and Brown, 1957; Colorado Division of Water Resources, 

2013), and considered to be Louviers Alluvium by Gardner (1967) and Scott (1978). Locally, unit 

may contain deposits of older alluvium. 

The Louviers Alluvium is considered coeval with the Bull Lake glaciation (Scott, 1975; 

Madole, 1991; see discussion of U-series ages for Louviers Alluvium in Szabo, 1980). Timing of 

Bull Lake glaciation is less well constrained than that of Pinedale, but ages that have been obtained 

for Bull Lake deposits suggest the glaciation spanned from about 190 to ≤130 ka (see discussions in 

Madole, 1991; Schildgen and others, 2002; Pierce, 2003; Sharp and others, 2003; Kellogg and 

others, 2008; Licciardi and Pierce, 2008; Schweinsberg and others, 2016). Fluvial sediment load 

may have been greatest during and shortly after deglaciation (Church and Ryder, 1972; Madole, 

1991; Schildgen and others, 2002; Lindsey and others, 2005); therefore, some of the youngest 

alluvium may post-date the Bull Lake glaciation by a few thousand years. Thickness is uncertain 

because unit is mostly buried by younger deposits along the South Platte River corridor between 

Greeley and the Colorado-Nebraska State Line (for example, see Colton, 1978; Scott, 1978), but 

subsurface lithologic data from test holes and water wells (Bjorklund and Brown, 1957; Colorado 

Division of Water Resources, 2013) indicate that in buried paleochannels, unit could be up to 

35–45 m thick in the Fort Morgan quadrangle.

OLD ALLUVIUM (Qao)

This early Pleistocene unit also is within the subsurface of the study area and not shown on 

map. Buried by eolian sand (Qes) but inferred from subsurface lithologic data from test holes and 

water wells (Bjorklund and Brown, 1957; Colorado Division of Water Resources, 2013) that 

indicate the presence of gravel deposits north of exposures of Verdos Alluvium and south of 

exposures of Nussbaum Alluvium. Uppermost gravels estimated to be roughly 60–70 m above the 

active floodplain, which supports a correlation with Rocky Flats Alluvium of Scott (1978, 1982). 

Gardner (1967) also recognized the buried alluvium and considered it Rocky Flats Alluvium. In the 

Masters 7.5 ́ quadrangle (Berry and others, 2015b), old alluvium exposed in a quarry and interpreted as Rocky Flats Alluvium (Gardner, 1967, p. 57) is light reddish or yellowish brown, moderately 

to well cemented, poorly to moderately sorted, sandy pebble and cobble gravel, pebbly sand, sand, 

and silty sand. Weakly to moderately stratified; sand beds commonly cross-stratified. Pebbles, 

cobbles, and minor small boulders (up to 30 cm in diameter) generally rounded to well rounded. 

Clasts mostly quartzite (including bluish-gray quartzite), gneiss, granite, pegmatite, and sandstone. 

Dense accumulations of red, yellowish-red, and reddish-yellow iron oxide and lesser amounts of 

black manganese oxide prevalent in matrix, on grains, and along bedding planes. Secondary silica 

may also be present based on the strength of cementation at that site. Calcium carbonate accumulations common along joints and as thin coats on clasts. 

Based on studies near the range front, Rocky Flats Alluvium has an estimated age of at least 

1.6–1.4 million years (Ma) (Birkeland and others, 1996) or about 1.5 Ma (Dethier and others, 2001) 

to about 2 Ma (Birkeland and others, 2003; Riihimaki and others, 2006; see discussion in Kellogg 

and others, 2008), although cosmogenic radionuclide studies indicate a complicated history for 

associated terrace surfaces, which may be much younger (Riihimaki and others, 2006; Dühnforth 

and others, 2012; Foster and others, 2015, 2016). More locally, a fossil tooth (Stegomastodon 

elegans) from a site downstream from Fort Morgan (NW1⁄4SW1⁄4, sec. 7, T. 5 N., R. 55 W.), 

collected from a deposit approximately 60 m above the active floodplain and mapped as Rocky 

Flats Alluvium by Scott (1978, 1982), was used by Scott (1982) to assign an early Pleistocene age 

to the deposit. Thickness is poorly constrained but locally could exceed 15–20 m.

ACKNOWLEDGMENTS

This work was part of the Greater Platte River Basins and Northern Plains Geologic Framework Studies project of the U.S. Geological Survey’s National Cooperative Geologic Mapping 

Program. The National Association of Geoscience Teachers (NAGT)—U.S. Geological Survey 

Cooperative Summer Field Training Program also provided support through a student summer 

intern. We thank the many landowners who graciously granted access to their property. We also 

thank the many other individuals who contributed to the project: Sarah R. Survis (NAGT Intern) 

assisted with field work; Shannon A. Mahan (USGS) provided a portable gamma spectrometer to 

measure OSL dose rates in the field; Jeffrey S. Pigati (USGS) provided helpful discussions 

regarding radiocarbon dating methods, samples, and interpretation, identified probable Succinea 

snail shells, and submitted our radiocarbon samples for analysis; and Ralph R. Shroba (USGS) and 

David J. Lidke (USGS) provided thorough reviews that greatly improved this Scientific 

Investigations Map.

To learn about the USGS and its information 

products visit https://www.usgs.gov/

1-888-ASK-USGS

This report is available at

https://doi.org/10.3133/sim3408

CONVERSION FACTORS

Multiply By To obtain

late 11.7–126 ka

middle 126–781 ka

early3 781 ka–2.58 Ma

DIVISION OF QUATERNARY TIME USED IN THIS REPORT1

Holocene2

1

Ages for time divisions are from Walker, J.D. and others (2012), Walker, M.J.C and others (2012), and 

 Cohen and others (2013; updated). Ma, million years; ka, thousand years. 

2

Subdivisions of Holocene are informal divisions advocated by Walker, M.J.C and others (2012).

3

Calabrian and Gelasian Ages.

Quaternary

Period Epoch Age

Pleistocene

meter (m) foot (ft) 3.281

millimeter (mm) 0.03937 inch (in.)

kilometer (km) 0.6214 mile (mi)

centimeter (cm) 0.3937 inch (in.)

late 0–4.2 ka

middle 4.2–8.2 ka

early 8.2 ka–11.7 ka

Qv

Qao

QNn

Qba

af

gp

Qaa

Qay

Qa1

Qac

Qa2

Qai2

Qai1

Qgf

Qes

Qlg

Qg

Kp

Qel

Qes

Qes

TIP

Qlv

Qa3

Qbs

Cris Lee Draw

Greeley

Sterling

Fort Morgan

Fort Morgan

104 ̊15'00“ 104 ̊00'00“ 103 ̊45'00“

40 ̊22'30“

40 ̊15'00“

Riverside

Reservoir

Empire

Reservoir

Jackson

Reservoir

Wil

at Creek

Bijou Creek

Kiowa Creek

0 10 KILOMETERS 2 4 6 8

0246 8 10 MILES

Figure 2. Location map for the Fort Morgan quadrangle. Shaded relief was derived from U.S. Geological Survey National 

Elevation Dataset (NED) with 10-meter resolution elevation data (accessed June 17, 2014, at http://ned.usgs.gov/).

dune field

dune field

dune field

dune field Fort Morgan

Quadrangle

Fort Morgan

dc

Fort Morgan

South Platte RiverSouth Platte RiverSouth Platte River

Cris Lee Draw

K i o wa

Cr e e k

Bi jou

Cre e k

WELD COUNTY

MORGAN COUNTY

ER

KC

TK–R

H–R

FMR

EIC

EF

BC

AK

Orchard

Weldona

Narrows

POINT OF ROCKS GREASEWOOD 

LAKE

SUNKEN LAKE JUDSON HILLS PEACE VALLEY

SCHOOL

DEAD HORSE

SPRINGS

DEARFIELD MASTERS ORCHARD WELDONA

FORT MORGAN

BRUSH WEST

ROGGEN OMAR WIGGINS VALLERY LAMB ROUND TOP

34 39

52

144

76

76

Wi l dca t C r e e k

104 ̊07 ́30 ̋ 104 ̊

40 ̊22 ́30 ̋

40 ̊15 ́

104 ̊22 ́30 ̋ 104 ̊15 ́ 103 ̊52 ́30 ̋ 103 ̊45 ́ 103 ̊37 ́30 ̋

40 ̊07 ́30 ̋

40 ̊30 ́

Figure 1. Index map showing map area (in yellow) and nearby 7.5 ́ quadrangles. Geologic maps of Orchard, Masters, 

and Weldona quadrangles published as U.S. Geological Survey Scientific Investigations Maps 3331, 3344, and 3396, 

respectively (Berry and others, 2015a, b, 2018). Geochronology sample sites shown where green and red symbols 

represent the following: squares indicate 230Th/U sample sites, solid circles indicate optically stimulated luminescence 

sample sites, and red cross within circles indicate radiocarbon sample sites; site names (in red) correspond to sample 

field numbers (tables 1–3: sheet 2) as shortened versions of the field numbers. The Narrows, formerly the site of a 

railroad post office (Elliott and Elliott, 1999), is near a natural constriction of the South Platte River valley proposed as 

a dam site for Narrows Reservoir, which was never built (Gardner, 1967; Minges, 1983; Rogers, 2009).

Riverside

Reservoir

Empire Reservoir

Jackson

Reservoir

Fort Morgan

TIP WC

South Platte River

KC

FMR

EXPLANATION

230Th/U sample site and 

identifier

EIC

Optically stimulated 

luminescence sample 

site and identifier

Optically stimulated 

luminescence and

radiocarbon sample site 

and identifier

Radiocarbon sample site 

and identifier

ER

CORRELATION OF MAP UNITS

SURFICIAL DEPOSITS

af

BEDROCK UNIT

Kp

gp

Eolian deposits 

Qes

Alluvial, colluvial, 

and grain-flow 

deposits 

Qgf

Alluvial deposits

Qaa

Qa1

Qlv

Qai1

Qv

Qbs

?

Qa3

Qba

Qa2

QUATERNARY

Pleistocene

Holocene 

CRETACEOUS Upper

Cretaceous

Artificial fill

Qay

Qai2

Qac

Qao

QNn

Qel

Eolian and alluvial 

deposits 

?

Pliocene

?

[The Louviers Alluvium (Qlv) and old alluvium (Qao) do not crop out in the study are but are present in 

 the subsurface; for this reason, the units are blank but maintained for correlation purposes]

Qlg

Qg

NEOGENE

TIP

WC

FMR

Any use of trade, firm, or product names is for descriptive purposes only and does not imply 

endorsement by the U.S. Government.

Although this information product, for the most part, is in the public domain, it also contains 

copyrighted materials as noted in the text. Permission to reproduce copyrighted items must 

be secured from the copyright owner.

Although these data have been processed successfully on a computer system at the USGS, 

no warranty expressed or implied is made regarding the display or utility of the data on any 

other system or for general or scientific purposes, nor shall the act of distribution 

constitute any such warranty. The USGS or the U.S. Government shall not be held liable for 

improper or incorrect use of the data described and/or contained herein.

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3408

ScienceBase citation: Berry, M.E., Taylor, E.M., Slate, J.L., Paces, J.B., 

Hanson, P.R., and Brandt, T.R., 2018, Data release for the geologic map 

of the Fort Morgan 7.5’ quadrangle, Morgan County, Colorado: U.S. 

Geological Survey data release, https://doi.org/10.5066/F7QC02PQ.

ScienceBase citation for geochronology of the river corridor: Berry, 

M.E., Hanson, P.R., Paces, J.B., Taylor, E.M., and Slate, J.L., 2018, Data 

release of OSL, 14C, and U-series age data supporting geologic 

mapping along the South Platte River corridor in northeastern Colorado: 

U.S. Geological Survey data release, https://doi.org/10.5066/F7QN65M3.

Suggested citation: Berry, M.E., Taylor, E.M., Slate, J.L., Paces, J.B., 

Hanson, P.R., and Brandt, T.R., 2018, Geologic map of the Fort Morgan 

7.5’ quadrangle, Morgan County, Colorado: U.S. Geological Survey 

Scientific Investigations Map 3408, 2 sheets, scale 1:24,000, 

https://doi.org/10.3133/sim3408.

For more information concerning this publication, contact

Center Director, USGS Geosciences and Environmental Change Science Center

Box 25046, Mail Stop 980

Denver, CO 80225

(303) 236-5344

Or visit the Geosciences and Environmental Change Science Center Web site at

https://gec.cr.usgs.gov/

SCALE 1: 24 000

1 1/ 2 0 1 MILE

1000 1000 0 2000 3000 4000 5000 6000 7000 FEET

1 0 .5 1 KILOMETER

Geologic map of the Fort Morgan 7.5’ quadrangle, Morgan County, Colorado

By

Margaret E. Berry,1

 Emily M. Taylor,1 Janet L. Slate,1 James B. Paces,1 Paul R. Hanson,2 and Theodore R. Brandt1

2018

APPROXIMATE MEAN

DECLINATION, 2015

8 ̊

TRUE NORTH

MAGNETIC NORTH

Base from U.S. Geological Survey Fort Morgan Quadrangle, 1971; photorevised 1984.

Polyconic projection. 1927 North American Datum (NAD 27)

10,000-foot grid based on Colorado coordinate system, north zone

1,000-meter Universal Transverse Mercator grid ticks, zone 13

CONTOUR INTERVAL 10 FEET

NATIONAL GEODETIC VERTICAL DATUM OF 1929

COLORADO

QUADRANGLE LOCATION

1

U.S. Geological Survey

2

University of Nebraska–Lincoln, School of Natural Resources

The geology was mapped by M.E. Berry from 2011–2017. E.M. Taylor 

and J.L. Slate assisted with geochronology sample collection and 

stratigraphic interpretation. J.B. Paces processed and analyzed 

samples for U-series dating and interpreted results. P.R. Hanson 

processed and analyzed samples for OSL dating and interpreted 

results. T.R. Brandt prepared the digital topographic base, digital 

compilation, and GIS database of the geologic map.

Publishing support provided by 

Denver Publishing Service Center

Edit and digital layout by J.A. Herrick

Manuscript approved for publication May 24, 2018

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3408

Sheet 1 of 2