Qaa

Qes

Qba

Qa3

Qay

Kp

Qac

Qea Qes

Kp

Kp

Kp

Qea

Qac

Qac

Kp

Qai2

Qea

Kp

Qay

Qay Kp

Qay

Qv

Qea

Kp

Qes

Qes

Qay

Qa3

Qes

Qea

Qa3

Qaa

Qaa

Kp

Kp

Qba

Qac

Qay

Qba

Qai2

Qac

Qay

Qai1

Qai2

Qai1

Qai2

af

Qbs

Qbs

Qbs

Qbs

Qbs

Qbs

Qa2

Qa1

Qaa

Qaa

Qa1

Qa2

Qa1

Qa1

af

Qbs

Qes

Qa1

Qaa

Qa2

Qa2

af

Qa1

Qa3 Qa1

Qa1

Qes

Qes

Qes

Qes

Qes

Qes

Qes

Qa3

Qa3

Qa3

Qa2

Qa1

Qa2

Qbs

Qa3

Qa2

af

Qba

Qba

Qba

Qfc

Qay

af

af

Qsh

Qsh

Qa3

Qai2

Qai1

Qai2

Qa1

Qa1

Qba

af

Qv

Qv

Qai2

Qai2

Qai2

Qac

Qac Qea

Qac

af

Qac

Qai2

Qai2 Kp

Qai2

Qay

af

Kp Qay

Qes

Qes

af

af

af

af

af

af

af

Qai2

Qai2

Kp

Qbs

Qbs

Qai1 Qai2

Qes

Qes

AK

BC

af

Qes

af

af

af

af

af

af

af

af

Kp

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]

SURFICIAL DEPOSITS

Artificial fill 

Artificial fill (latest Holocene)—Includes abutments where roadways cross South 

Platte River, raised track of railroad, artificial levees and banks, small earthen 

dams forming holding ponds, and dam embankments at Bijou No. 2 Reservoir. 

Artificial fill is present along most canals and ditches but only mapped locally. 

Chiefly consists of gravel, sand, silt, and rock and concrete fragments

Shoreline deposits

Shoreline deposits (latest Holocene)—Mostly pale brown, brown, or yellowish 

brown, moderately to well sorted, fine to medium sand, reworked from eolian 

sand deposits by shoreline processes at Bijou No. 2 Reservoir. Deposits 

post-date reservoir construction. Periodically inundated when water level in 

reservoir is high. Estimated thickness less than 2 meters (m)

Alluvial deposits

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

post-Piney Creek alluvium of Scott (1963). Very pale brown, pale yellow, 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 are common locally. 

Along Bijou Creek, alluvium mostly stratified sand, silty sand, and clayey silt. 

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

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

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

Estimated thickness 3–5 m

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

of Scott (1963). Forms low terrace approximately 1.5 m above active floodplain. 

Surface occasionally flooded. 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 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 

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

Along Bijou Creek, alluvium mostly stratified sand, silty sand, and clayey silt. 

Unit contains minor amounts of lignite fragments. Estimated thickness 3–6 m

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. 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. Weakly to 

moderately stratified. Loose to slightly hard dry consistence. Estimated 

thickness 1–5 m

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

of Scott (1963). Forms low terrace approximately 3 m above active floodplain. 

Surface occasionally flooded in places and rarely flooded in others. 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 

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 [USDA] 

Natural Resources Conservation Service, 2009). Krotovinas are common locally. 

Along Bijou Creek, alluvium mostly stratified sand, silty sand, and clayey silt. 

Unit contains minor amounts of lignite fragments. Estimated thickness 3–6 m

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

Gardner (1967) and Scott (1978). Forms terrace and low-gradient fans 6–11 m 

above active floodplain; Gardner (1967) and McFaul and others (1994) 

considered this terrace correlative to Kuner terrace of Bryan and Ray (1940), 

whose type locality is about 45 kilometers (km) west of Weldona, Colo. Includes 

sheetwash alluvium and colluvium interbedded with river alluvium, most 

notably on north side of South Platte River where valley wall is cut into Pierre 

Shale bedrock. Based on where terrace surface is well defined, gradient of 

terrace surface is less than gradient of floodplain such that relative height of 

terrace is higher downstream from the Narrows (fig. 1) than it is upstream in 

Orchard and Masters quadrangles (Berry and others, 2015a, b).

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. 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 

125 km southwest of the 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 B 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 near the Narrows (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; 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) and 11.95±0.24 

cal ka B.P. (Aeon–995, table 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). 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. (AA–11084A, 

uncalibrated age of 10.11±0.09 14C ka B.P.) 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 is 

constrained by a date from McFaul and others (1994) of 5.84±0.18 cal ka B.P. 

(Beta–42564, 5.12±0.08 14C ka B.P.) 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 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) 

for a buried A horizon developed in Qa3 terrace alluvium of Wildcat Creek in 

adjacent Fort Morgan 7.5 ́ quadrangle (site FMR; fig. 1). 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 sheetwash alluvium about 4.9 cal ka. Estimated thickness 

1.5–5 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, b). 

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 

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 

here in Weldona 7.5 ́ quadrangle, near the Bijou Creek confluence. Height of 

surface decreases progressively downstream from Bijou Creek confluence to 

approximately 15 m above the active floodplain at eastern edge of Fort Morgan 

7.5 ́ quadrangle. Deposits have been identified as far downstream as Prewitt 

Reservoir (roughly 45 km east) and Atwood, Colo. (roughly 60 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) attributed 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 

(Berry and others, 2015a) and Bijou Creek drainage. 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 B horizon with moderate prismatic structure and Bk horizon with 

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

sand in many places.

North of river, Qbs also includes a few remnants of sidestream terrace 

alluvium that grade to mainstream terrace alluvium (Qba) along the river. 

Granules and pebbles in the sidestream deposits are composed mostly of rock 

types associated with the South Platte River drainage, indicating that the clasts 

were likely reworked from older South Platte River deposits.

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). These age estimates are in good agreement 

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

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). 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 

18–24 m near the confluence of Bijou Creek and the South Platte River

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, mostly covered by eolian sand (northwest edge of 

quadrangle) or exposed downstream from the Narrows in isolated terrace 

remnants roughly 15–18 m above active floodplain; also locally exposed beneath 

Qa3 alluvium at site AK (fig. 1). 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 

granule-to-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 bluish gray 

quartzite), and sandstone. Largest clasts commonly 3–5 cm diameter, but cobbles 

and small boulders (up to 32 cm in diameter) present locally. Terrace remnants 

north of river also include subangular sandstone boulders of upper transition 

member Pierre Shale (Kp). Reddish yellow to yellowish red iron oxide and black 

manganese oxide common as grain coatings, masses in matrix, and 

accumulations along bedding planes. Surface soil profile typically has thin 

argillic B horizon with weak to moderate prismatic structure, and a thin Bk 

horizon with stage II carbonate morphology. 

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

70 cm of crumbly clay, pinkish silt, platy silty fine sand, and burrowed sandy silt 

with gray and yellowish green color banding above white silt with common shell 

fragments and small, articulated clam shells. 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; Berry and others, 2015a) was obtained 

for sediment collected at site AK (fig. 1, this 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 discussion above), 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; 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; 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; 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 50 km west of Weldona, 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; 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; 

thins to 1‒1.5 m north of South Platte River downstream from the Narrows 

Intermediate alluvium (middle Pleistocene)—In area covered by Weldona 7.5 ́ 

quadrangle deposits either previously unmapped (here mapped as Qai1) or 

mapped by Gardner (1967) and Scott (1978) as Verdos Alluvium (here mapped 

as Qai2). Thought to be younger than Verdos Alluvium based on height above 

river relative to other deposits of probable Verdos Alluvium in adjacent Fort 

Morgan 7.5 ́ quadrangle (Gardner, 1967; Scott, 1978, 1982). Deposits are made 

up of pebble and cobble gravel, pebbly sand, and silty sand in terrace remnants 

and pebble- and cobble-gravel lags. Alluvium light gray, very pale brown, light 

yellowish brown, or light reddish brown, locally cemented, and poorly to 

moderately sorted. Sand beds commonly cross-stratified. Pebbles and cobbles 

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

sandstone, and chert; subrounded to well rounded; diameter of largest clasts 

commonly 13‒16 cm but ranging up to 26 cm. Some granitic clasts are 

weathered (beginning to disaggregate). Also includes subangular to subrounded, 

sandstone and bioturbated claystone boulders up to 1 m in diameter of upper 

transition member Pierre Shale (Kp), probably derived locally. Thin coats (rinds) 

of calcium carbonate on clasts common. Where soil profile 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 of innermost calcium carbonate 

rinds, subsampled from multiple clasts collected at site EIC (fig. 1, Masters 7.5 ́ 

quadrangle), produced reliable results and oldest 230Th/U ages that primarily 

cluster in two groups, one (table 3, Wp95–A1–2, Wp95–A2–1, Wp95–B1, and 

Wp95–B2) with an error-weighted-mean age of 334±9 ka and the other (table 3, 

Wp95–B3 and Wp95–E1) with an error-weighted-mean age of 382±16 ka (fig. 1–

7 in Paces, 2015). These data indicate with a high degree of confidence that the 

alluvial clasts at EIC were deposited prior to 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, which have a proposed age range of 390–320 ka (Kellogg 

and others, 2008)

Intermediate alluvium 1 (middle Pleistocene)—Remnant terrace gravels capping 

low ridges or forming thin lags 21‒25 m above active floodplain. Only identified 

downstream from the Narrows. Estimated thickness generally 1.5 m or less

Intermediate alluvium 2 (middle Pleistocene)—Forms broad terrace generally 

34‒40 m above active floodplain; locally preserved only as thin gravel lag. 

Mostly covered by eolian sand (Qes) or mixed eolian and alluvial deposits 

(Qea). More extensively preserved than Qai1. Estimated thickness 3–8 m in 

terrace deposits to less than 1 m in gravel lags

Verdos Alluvium (middle Pleistocene)—Made up of poorly sorted pebble and 

cobble gravel in inferred terrace remnants mostly 49–55 m above active 

floodplain. In places, preserved only as thin pebble- and cobble-gravel lag. 

Mostly covered by eolian sand (Qes) or mixed eolian and alluvial deposits 

(Qea). Two small remnants preserved at 44–45 m above the active floodplain 

may represent a lower (younger) terrace of Qv; two levels of Verdos Alluvium 

have been reported at sites near the mountain front (Scott, 1972; Kellogg and 

others, 2008). However, deposits in Weldona quadrangle are too poorly exposed 

to allow confirmation of two distinct levels. Qv deposits were previously 

mapped as Rocky Flats Alluvium by Gardner (1967) and Scott (1978), but are 

reinterpreted here as Verdos Alluvium based on height above river and 

correlation to deposits mapped as Verdos Alluvium by Gardner (1967) and Scott 

(1978) in adjacent Fort Morgan 7.5 ́ quadrangle. 

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

pinkish gray. Pebbles and cobbles mostly quartzite (including bluish gray 

quartzite), gneiss, granite, pegmatite, volcanic clasts, chert, sandstone, and 

petrified wood; subrounded to well rounded; diameter of largest clasts up to 

24 cm. Also includes subangular to subrounded, sandstone and bioturbated 

claystone boulders 0.5–1 m in diameter of upper transition member Pierre Shale 

(Kp), probably derived locally. Soil profile poorly exposed in Weldona 

quadrangle but includes variably cemented K horizon with thin to thick coats 

(rinds) of calcium carbonate on clasts. In adjacent Fort Morgan quadrangle 

where soil is better exposed, eroded profile includes a 1-m-thick K horizon with 

stage III–IV carbonate morphology and carbonate coats on clasts up to 1 cm thick.

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), which 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 Weldona 7.5 ́ quadrangle (Scott, 1978, 1982; Izett and 

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

quadrangle. Thickness of the alluvium is poorly constrained in Weldona 7.5 ́ 

quadrangle but could be as much as 6–8 m based on exposures in adjacent Fort 

Morgan 7.5 ́ quadrangle

Alluvial and colluvial deposits

Fan alluvium and hillslope colluvium (Holocene)—Mapped along south side of 

South Platte River, where hillslope processes are actively degrading river bluff 

made up of mainstream and sidestream Broadway Alluvium (Qba and Qbs). 

Locally includes individual and coalescing alluvial-fan deposits at mouths of 

headward-eroding gullies, colluvium mantles on slopes, and sheetwash alluvium 

at toes of slopes. Very pale brown, crudely stratified, poorly sorted pebbly sand 

and moderately sorted sand with silt lenses and medium to coarse sand stringers. 

Most clasts <3 cm diameter and subangular to subrounded. Many clasts 

composed of potassium feldspar. Surface soil weakly developed A/C profile. 

Estimated thickness up to about 3 m

Mixed eolian and alluvial deposits (Holocene and late Pleistocene)—Mostly 

sheetwash deposits mapped north of the river, consisting of reworked eolian sand 

mixed with lesser amounts of terrace gravel; typically overlies middle 

Pleistocene terrace alluvium (Qai1, Qai2, and Qv); locally includes small areas 

of eolian sand and terrace alluvium that have not been reworked. Pale brown, 

brown, or light yellowish brown, poorly sorted silty sand, sandy silt, granules, 

and scattered rounded pebbles and small cobbles. Clasts mostly granite, gneiss, 

quartz, feldspar, pegmatite, chert, and sandstone. Estimated thickness 3‒6 m

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 soil 

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

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

includes 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 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 

northeastern part of quadrangle, unstippled parts also include Valent-Vona 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 are mainly mapped south of South Platte River 

where eolian sand covers late Pleistocene sidestream deposits of Broadway 

Alluvium (Qbs). County soil survey shows mostly Bijou, Bresser, 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 and near Weldona 7.5 ́ quadrangle: 0.64±0.11 cal ka B.P. 

(Beta–70542, 0.68±0.08 14C ka B.P.) near Bijou No. 2 Reservoir; 1.14±0.16 cal 

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

Orchard 7.5 ́ quadrangle; 0.88±0.21 cal ka B.P. (Beta–84821, 0.96±0.11 14C ka 

B.P.) at south edge of Orchard 7.5 ́ quadrangle near Bijou Creek; and 0.82±0.16 

cal ka B.P. (Beta–62192, 0.89±0.10 14C ka B.P.) 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. 

(CAMS–8234, 1.56±0.07 14C ka B.P., 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 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; 

Berry and others, 2015a, b). At Milliron Draw, in adjacent 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 about 6.33±0.16 cal ka B.P. 

(Beta–72203, 5.51±0.09 14C ka B.P.) and a minimum-limiting age of about 

1.14±0.16 cal ka B.P. (Beta–70543, 1.23±0.08 14C ka B.P.) 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 about 3.90±0.20 cal ka B.P. 

(CAMS–6378, 3.60±0.07 14C ka B.P., 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 therein). 

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

extensive in northern part of Weldona 7.5 ́ quadrangle as well as regionally 

(Muhs and others, 1996; Madole and others, 2005). In Masters 7.5 ́ quadrangle at 

site ER (fig. 1), 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; 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. (CAMS–11339, 27.30±0.17 14C ka 

B.P.), 31.36±0.48 cal ka B.P. (CAMS–16612, 27.42±0.30 14C ka B.P.), and 

30.58±0.51 cal ka B.P. (CAMS–16604, 26.41±0.24 14C ka B.P.). Eolian sand 

deposition during the late Pleistocene likely was intermittent, and may have 

resulted in late Pleistocene deposits of various ages within the region; some may 

be as young as 12 or 13 ka (Madole and others, 2005).

Eolian sands in the Weldona quadrangle 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 the 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 Weldona quadrangle is generally consistent with 

regional trends reported by Muhs (1985) and Muhs and others (1996) and 

indicates sand-transporting winds that formed the dunes primarily came from the 

northwest (Muhs and others, 1996; Madole and others, 2005). Currently, dunes 

are mostly stabilized by sandsage and short grass prairie vegetation (Chapman 

and others, 2006). Thickness up to 10 m in compound parabolic dunes south of 

river, but thins to less than half a meter locally

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 1 m 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 crests

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, Bijou No. 2 Reservoir, and 

channels of South Platte River wide enough to be mapped at 1:24,000 scale. 

Based on 2015 NAIP orthoimagery

Optically stimulated luminescence (OSL) sample location—With site name that 

links to sample field number (table 1)

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

field number (tables 1 and 2)

INTRODUCTION

The Weldona 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 Pleistocene 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 Weldona quadrangle. 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 deep 

paleochannels (Bjorklund and Brown, 1957) now covered by younger alluvium. The river shifted 

back to the north during the late Pleistocene. Kiowa and Bijou Creeks (fig. 1) 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 their confluences, 

forming a broad, low-gradient fan of sidestream alluvium that could have occasionally dammed the 

river for short periods of time (Scott, 1982). 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 Weldona 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 (USDA Farm Service Agency Aerial 

Photography Field Office, 2009, 2011, 2013, 2015); Quality-level 2 lidar data (1-m 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-m 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 2015. Digital mapping, completed in 2016 using ArcMap software, was done on 2015 NAIP 

orthoimagery; 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 central pivot irrigation. The 1-m resolution lidar data, 

available for roughly 95 percent of the quadrangle, were used to enhance recognition and facilitate 

mapping of valley margins, river and stream terraces, sand dune crests and blowouts, natural and 

artificial levees, and floodwater inundation zones. 

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

and uranium-series (230Th/U) age determinations (tables 1–3; 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 (CAM) 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; 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 our study (table 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 using procedures outlined in “USGS–DRIL–01, R0 

Uranium-Thorium Disequilibrium Studies” (accessible at 

http://esp.cr.usgs.gov/projects/uth/USGS_DRIL_01cR0_U_series.pdf). Details about the samples 

analyzed and results obtained are provided in Paces (2015). The 230Th/U analyses provide 

constraints on the minimum ages of deposits from which samples were collected (table 3; 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 map. 

Pebble to cobble gravel, sand, and finer grained alluvium filling paleovalley located south of the 

modern South Platte River; buried by younger alluvium but inferred from subsurface lithologic data 

from test holes and water wells in Weldona and adjacent quadrangles (Bjorklund and Brown, 1957; 

Colorado Division of Water Resources, 2013). Inferred to be Louviers Alluvium by Gardner (1967) 

and Scott (1978). Some of the deeper paleochannels locally may contain deposits of older alluvium. 

Surface exposures previously mapped as Louviers Alluvium by Gardner (1967) are reinterpreted 

here, mostly as mainstream deposits of Broadway Alluvium (Qba). Gardner (1967) similarly 

recognized these deposits as late Pleistocene in age, but mapped them as Louviers Alluvium 

because he inferred the age of Louviers Alluvium to be only about 50–10.5 ka.

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

Madole, 1991). 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, 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 30–45 m thick in Weldona quadrangle.

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 William R. Page (USGS) provided thorough reviews that greatly improved this 

scientific investigations map.

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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) inch (in.) 0.3937

late 0–4.2 ka

middle 4.2–8.2 ka

early 8.2 ka–11.7 ka

Qv

Qfc

Qea

Qac

Kp

Qai1

Qai2

Qba af

Qsh

Qaa

Qa1

Qa2

Qay

Qa3

Qbs

Qes

Qes

Qes

BC

AK

Qa3

Kp

Exposure of weathered upper transition member 

Pierre Shale (Kp) capped by thin veneer of 

young alluvium 3 (Qa3) terrace gravels 

southeast of the Narrows. Shovel is 68 cm long. 

Photograph by M.E. Berry, June 2015. 

Kp

Qai1

Exposure of weathered upper transition member Pierre Shale (Kp) cropping 

out in a gully southeast of the Narrows. Qai1 gravels cap surface at skyline. 

Shovel is 68 cm long. Photographs by M.E. Berry, September 2015.

Qa3

Qa3

Qa3

View to the northwest of young alluvium 3 (Qa3) terrace southeast of the 

Narrows. The terrace surface in this area is dissected by small gullies, as 

seen in middle ground. The South Platte River, visible to the left, has 

overflowed its banks and floodwater is covering the active floodplain. 

Photograph by M.E. Berry, June 2015.

Composite view to the southeast (3 photos) of active channel and floodplain alluvium (Qaa) exposed in north cut bank of the South Platte River downstream of the Narrows. High potassium feldspar content 

gives pebbly sands a distinctive pinkish hue. Grayish brown overbank sediment overlies the pebbly sands. Shovel is 68 cm long. Photographs by M.E. Berry, August 2015.

Greeley

Sterling

Fort Morgan

Weldona

Quadrangle

Fort Morgan

104 ̊15'00“ 104 ̊00'00“ 103 ̊45'00“

40 ̊22'30“

40 ̊15'00“

South Platte River

Fort Morgan

Riverside

Reservoir

Empire

Reservoir

Jackson

Reservoir

Wildcat Creek

Bijou Creek

Kiowa

Creek

0 10 KILOMETERS 2468

02468 10 MILES

Figure 2. 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

K io w

a

C re e k

Bi jou

C re e k

WELD COUNTY

MORGAN COUNTY

ER

KC

TK–R

H–R

FMR

EIC

EF

BC

AK

Fort Morgan

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

S o u t h P la tt e R iv e r

34 39

52

144

76

76

Wi l dc a 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 and Masters quadrangles published as Scientific Investigations Map 3331 

and Scientific Investigations Map 3344, respectively (Berry and others, 2015a, b). 

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) link to sample field numbers (tables 1–3). The Narrows, formerly the site of a railroad 

post office (Elliott and Elliott, 1999), is near the start of 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).

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

Riverside

Reservoir

Empire Reservoir

Jackson

Reservoir

CORRELATION OF MAP UNITS

SURFICIAL DEPOSITS

af

QUATERNARY

Pleistocene

Holocene 

BEDROCK UNIT

Kp CRETACEOUS Upper

Cretaceous

Shoreline 

deposits 

Qsh

Eolian 

deposits 

Qes

Alluvial and colluvial 

deposits 

Qfc

Alluvial deposits

Qaa

Qa1

Qlv

Qai1

Qv

Qa3

Qa2

Artificial

fill

Qay

Qai2

Qea

Qac

?

[The Louviers Alluvium (Qlv) does not crop out in the study area but is present in the subsurface; 

for this reason, the unit is blank and not described but maintained for correlation purposes]

Qbs Qba

?

OSL–1–KC UNL–3462 Orchard 577915 4464174 1.7 5.3 20.0 3.8 67.6±3.3 22/30 2.9 5.77±0.32 11.7±1.0 5.0 5.64±0.35 12.0±1.1

OSL–4–KC UNL–3466 Orchard 577915 4464174 2.6 3.3 16.3 4.3 87.8±5.4 24/37 2.1 5.41±0.30 16.2±1.5 5.0 5.23±0.34 16.8±1.7

OSL–5–KC UNL–3463 Orchard 577915 4464174 3.6 3.7 15.8 4.3 80.6±4.8 22/32 2.4 5.45±0.31 14.8±1.4 5.0 5.29±0.35 15.2±1.5

OSL–6–ER UNL–3467 Masters 568185 4455813 0.7 2.9 11.4 3.4 109.7±5.6 20/37 5.0 4.15±0.27 26.4±2.5 5.0 4.15±0.27 26.4±2.5

OSL–7–ER UNL–3468 Masters 568198 4455836 1.0 2.4 8.1 3.7 31.4±2.2 29/37 1.9 4.22±0.25 7.4±0.7 5.0 4.07±0.28 7.7±0.8

OSL–BC–14 UNL–3498 Weldona 585123 4459026 1.1 4.8 19.5 3.7 67.0±2.8 31/48 7.7 5.23±0.39 12.8±1.2 5.0 5.39±0.34 12.4±1.1

OSL–BC–13 UNL–3499 Weldona 585123 4459026 1.9 4.3 17.6 4.1 79.1±2.9 36/54 6.7 5.33±0.38 14.8±1.3 5.0 5.43±0.35 14.6±1.2

UNL lab9 UNL lab9

OSL–AK–10 UNL–3502 Weldona 592468 4463758 5.1 5.7 23.3 2.7 40.2±1.4 43/54 7.0 4.92±0.35 8.2±0.7 5.0 5.03±0.31 8.0±0.7

OSL–AK–11 UNL–3503 Weldona 592478 4463772 3.5 3.5 9.9 2.6 32.5±1.8 27/35 5.9 3.52±0.24 9.2±0.9 5.0 3.56±0.23 9.1±0.9

OSL–TK–R–12 UNL–3504 Orchard 577216 4463702 8.0 1.5 6.7 3.9 35.8±2.0 48/54 3.3 3.88±0.26 9.2±0.9 5.0 3.81±0.28 9.4±1.0

Field

number

UNL lab

number

UTM

Easting2

UTM

Northing2

Approx.

depth

(m)

U

(ppm)

Th

(ppm)

Dose

rate

(Gy/ka)6

K2O

(wt.%)

CAM De

(Gy)

±1 Std. err.3

Aliquots

(n)4

In situ

H2

O field

(%)5

OSL

age

ka±1σ6

In situ

H2

0 est.

(%)5

Dose

rate

(Gy/ka)7

OSL

age

ka±1σ7

7.5 ́

quad

Table 1. Optically stimulated luminescence (OSL) age estimates with equivalent dose (De) and dose rate data (modified from Berry and others, 2015a).

[UNL Lab, University of Nebraska–Lincoln Luminescence Geochronology Laboratory; quad, quadrangle; UTM, Universal Transverse Mercator; Approx., approximate; m, meter; U, uranium; ppm, parts per million; 

Th, thorium; K2

O, potassium oxide; wt. %, weight percent; Gy, gray1; Std. err., standard error; n, number; H2O, water; ka, thousand years; ±1σ, represents estimates of uncertainty given at the 67 percent confidence interval.]

In situ8 In situ8

1

Gray is an SI unit of absorbed dose of ionizing radiation (Taylor and Thompson, 2008).

2

1927 North American Datum (NAD 27), zone 13.

3

Determined using the Central Age Model (CAM) of Galbraith and others (1999).

4

Accepted disks/all disks.

5

Assumes 100% error in estimated moisture content. Field, moisture content of sample, measured at UNL lab; Est., estimate of moisture content, which may better represent moisture history of sample. 6

Calculated using field moisture content of sample.

7

Calculated using 5% as estimated moisture content.

8

Measured in field with a portable gamma spectrometer.

9

Measured at UNL Lab using high-resolution gamma spectrometry (updated from Berry and others, 2015a, b).

Laboratory

number

7.5 ́

quad

UTM

Easting2

UTM

Northing2

Material

dated

Approx.

depth

(m) 

δ13C

(‰)3 P6

Aeon–2101 Fort

Morgan 602339 4469163 Humin fraction of 

buried A horizon FMR–3 1.8 -21.4 0.96 4.36±0.03 4.92±0.07

Field

number

14C age

(

14C ka B.P.)4

Aeon–1064 Orchard 581980 4466741

Probable Succinea

(four shells from

~15 cm thick bed)

H–R–3,4,5,9 3.7 -3.8 1.00 12.36±0.15 14.53±0.56

KC–245 Aeon–950 Orchard 577915 4464174 mostly lignite? 2.5 -23.2 1.00 39.80±3.10 43.90±5.69

Calibrated age

(cal ka B.P.)5

Table 2. Radiocarbon sample information, 14C ages1, and calibrated ages (modified from Berry and others, 2015a). 

[UTM, Universal Transverse Mercator; quad, quadrangle; δ13C, delta carbon-13; ‰, per mil; 14C ka B.P., carbon-14 thousand years before present; cal ka B.P., 

calibrated thousand years before present; P, probability; Approx., approximate; m, meter; cm, centimeter; ±1σ, represents estimates of uncertainty given at the 

67 percent confidence interval; ±2σ, represents estimates of uncertainty given at the 95 percent confidence interval.]

AK–1 Aeon–994 Weldona 592473 4463764 4.7 -6.8 0.98 10.22±0.07 11.90±0.28 Probable Succinea

(single shell)

AK–8 Aeon–995 Weldona 592473 4463764 3.6 -4.6 0.98 10.24±0.06 11.95±0.24 Probable Succinea

(single shell)

1

Radiocarbon (14C) activity measured by accelerator mass spectrometry (AMS). 2

UTM zone 13, 1927 North American Datum (NAD 27).

3

Relative difference between 13C/12C ratio of carbon extracted from subsample and that of Vienna Pee Dee Belemnite (VPDB) international standard. 4

Conventional radiocarbon age, normalized to -25‰, based on 5,568-year half-life; uncertainty ±1σ.

5

Calibrated age calculated using CALIB 7.0, IntCal13 dataset (Stuiver and Reimer, 1993; Reimer and others, 2013); 0 yr B.P. = 1950 A.D.; uncertainty ±2σ. 

 Calibrated age reported as midpoint of calibrated range. 

6

Probability of calibrated age falling within reported range as calculated by CALIB.

230Th/238U AR±2σ 234U/238U AR±2σ

Carbonate rind (subsample Wp67–1) and layers of bone (subsamples Wp67–2 

through Wp67–11). 

Location of EF–Wp67: UTM 564654E., 4462502N.4

Wp67–1 22.5 1.64 5.7 0.117±0.011 1.221±0.004 11±1.1 1.228±0.004

Wp67–2 83.0 0.325 104 0.1338±0.0012 1.214±0.003 12.70±0.13 1.221±0.003

Wp67–3 107 0.044 1150 0.1520±0.0006 1.210±0.003 14.58±0.07 1.219±0.003

Wp67–5 116 0.025 2300 0.1588±0.0007 1.215±0.003 15.21±0.08 1.224±0.003

Wp67–7 115 0.028 2020 0.1600±0.0007 1.221±0.002 15.26±0.08 1.231±0.003

Wp67–9 116 0.032 1600 0.1458±0.0006 1.217±0.003 13.87±0.07 1.226±0.003

Wp67–11 104 0.124 322 0.1255±0.0006 1.217±0.003 11.82±0.06 1.225±0.003

Innermost carbonate rinds on alluvial clasts. Location of EIC–Wp95: 

UTM 567342E., 4458109N.4

Wp95–A1–1 5.92 0.280 66.9 1.040±0.008 1.125±0.007 248±9 1.252±0.010

Wp95–A1–2 7.36 0.309 76.7 1.061±0.007 1.084±0.003 337±15 1.217±0.009

Wp95–A2–1 10.2 0.274 121 1.065±0.009 1.085±0.003 341±20 1.222±0.012

Wp95–A3–1 7.12 0.549 41.3 1.048±0.009 1.094±0.004 294±13 1.216±0.008

Wp95–B1 7.90 0.901 28.2 1.057±0.01 1.077±0.004 345±24 1.205±0.012

Wp95–B2 7.60 0.509 47.6 1.048±0.007 1.079±0.003 324±15 1.197±0.008

Wp95–B3 9.58 0.672 46.3 1.069±0.007 1.072±0.003 397±27 1.221±0.014

Wp95–E1 9.91 0.461 67.7 1.037±0.006 1.054±0.003 372±21 1.153±0.008

Field

number 

Measured 230Th/232Th AR1

EF–Wp67

EIC–Wp95

Detritus-corrected activity ratios2

Lab sample name U concentration, 

in μg/g

Th concentration,

in μg/g 

230Th/U age

±2σ(ka)3

Initial 234U/238U AR±2σ3

1

Measured activity ratio (AR) corrected for mass fractionation, spike contributions, procedural blank and normalized relative to an atomic ratio value for National Institute of Standards and 

 Technology (NIST) Standard Reference Material (SRM) 4321B U-isotope standard of 234U/238U=0.0000529. 2

Ratios corrected for an assumed Th-bearing detrital component having an atomic Th/U of 4 with the following activity ratios and 2σ errors: 232Th/238U=1.276±0.64; 234U/238U=1.0±0.1; and 230Th/238U=1.0±0.25. 3 230Th/U ages, initial 234U/238U activity ratios (ARs), and associated errors are calculated using detritus-corrected activity ratio values. 4

UTM zone 13, 1927 North American Datum (NAD 27).

Table 3. U-Th concentrations, U-series isotope compositions, and calculated 230Th/U ages and initial 234U/238U activity ratios for subsamples of 

EF–Wp67 and EIC–Wp95 (taken from Berry and others, 2015b). 

[Abbreviated version of table 1-1 in Paces (2015). Note that ±2σ represents estimates of uncertainty given at the 95 percent confidence level; μg/g, micrograms per 

gram; σ, sigma; ka, thousand years.]

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 Weldona 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

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/sim3396

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.

This database, identified as SIM 3396, has been approved for release and 

publication by the U.S. Geological Survey (USGS). Although this database 

has been subjected to rigorous review and is substantially complete, the 

USGS reserves the right to revise the data pursuant to further analysis and 

review. Furthermore, it is released on condition that neither the USGS nor 

the U.S. Government may be held liable for any damages resulting from its 

authorized or unauthorized use.

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.

Base from U.S. Geological Survey Weldona 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

ISSN 2329-132X (online)

https://doi.org/10.3133/sim3396

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/

ScienceBase citation for the geologic map: 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 Weldona 7.5’ quadrangle, Morgan County, Colorado: U.S. Geological Survey data 

release, https://doi.org/10.5066/F7610Z63.

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 Weldona 7.5’ quadrangle, Morgan County, 

Colorado: U.S. Geological Survey Scientific Investigations Map 3396, 1 sheet, scale 

1:24,000, https://doi.org/10.3133/sim3396.

1

U.S. Geological Survey

2

University of Nebraska–Lincoln, School of Natural Resources

The geology was mapped by M.E. Berry from 2011 to 2016. 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.

U.S. Department of the Interior

U.S. Geological Survey

Scientific Investigations Map 3396

Publishing support provided by 

Denver Publishing Service Center

Edit and digital layout by J.A. Herrick

Manuscript approved for publication January 16, 2018