811 °

1 41 °

 AH VRE

CHO ET AU

 DUBOIS

45°

SHELBY

106°

LE I W STOWN

4 °6

45°

 B ZO EMAN

10 °4

WHITE

47°

H RA DIN

 I L ND L O

U P US L H R

1 ° 110° 1 6

116°

108°

GL SA GOW

114° 112°

44°

S NA DPOINT

110°

84 °

 PS R NI GS

EL CK ITY

SPOKANE

O N T A M N A

EK L A AKA

H MA ILTON

49°

104°

G E D VE L N I

ROUN UD P

GREAT FAL SL

MI EL S C TI Y

4 °8

WALLACE

4 °9

 I L N B L I GS

KALISPELL CUT BANK

44°

C A N A D A

JORDAN

1 81 ° 112°

47°

A HS TON

1 60 °

4 °6

W L OIN T O F P

FORSYTH

108°

U TB T E

QUATERNARY GEOLOGIC MAP OF THE GLASGOW 1° × 2° QUADRANGLE, MONTANA

By

David S. Fullerton, Roger B. Colton, and Charles A. Bush

2012

U.S. Department of the Interior

U.S. Geological Survey

Prepared in cooperation with the

Montana Bureau of Mines and Geology

OPEN-FILE REPORT 2012-1217

Geology compiled in 1976–86, 1990–2002, 2004–2008

Edit and digital layout by Jeffery R. Hartley

Geology digitized by Charles A. Bush

Digital cartography by Charles A. Bush

Published by U.S.Geological Survey Science Publishing Network Denver

Manuscript approved for publication September 27, 2012

Base hydrology from U.S. Geological Survey, 1974

Base topography from National Elevation Dataset (NED), U.S. Geological Survey 1977 (http://seamless.usgs.gov)

Base cities, roads, and railroads from Montana State Library 2011 (http://nris.mt.gov)

TRANSVERSE MERCATOR PROJECTION

1927 North American Datum

CONTOUR INTERVAL 100 METERS

1970 MAGNETIC DECLINATION FROM TRUE NORTH VARIES FROM 181⁄2° EASTERLY FOR 

THE CENTER OF THE WEST EDGE TO 171⁄2° EASTERLY FOR THE CENTER OF THE EAST EDGE

INDEX TO 1:250,000 TOPOGRAPHIC QUADRANGLES

Showing location of the Glasgow 1° x 2° quadrangle in red

NOTE:

 This map was prepared to serve as a database for compilation of a Quaternary geologic map of the 

Lethbridge 4° x 6° quadrangle, United States and Canada (scale 1:1,000,000). Letter symbols for the map 

units are those used for the same units in the Quaternary Geologic Atlas of the United States map series. 

This map is a compilation of data from many sources, listed in SOURCES OF INFORMATION, at several 

different map scales. Some of the map data are simplified and generalized. Many surficial deposits that 

can be shown accurately at 1:250,000 scale are omitted here because they are too small at 1:1,000,000 

scale.

 Map units are surficial deposits and materials, not landforms. Deposits that comprise some 

constructional landforms (e.g., ground-moraine deposits, end-moraine deposits, and stagnation-moraine 

deposits, all composed of till), are distinguished for purposes of reconstruction of glacial history. Deposits 

and materials are distinguished on the basis of genesis, lithology or composition, texture or particle size, 

and other physical, chemical, and engineering characteristics. It is not a map of soils that are recognized in 

pedology or agronomy. Rather, it is a generalized map of soils as recognized in engineering geology, or of 

substrata or parent materials in which pedologic or agronomic soils are formed

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IMPORTANT STRATIGRAPHIC SECTIONS

Stratigraphic units are listed from youngest to oldest. All of the named stratigraphic units are informal allostratigraphic units. Unit thicknesses in exposures are 

maximum thicknesses. Till nomenclature from Fullerton and Colton (1986)

 

1.2 m oxidized and unoxidized Loring till; Pleistocene, 0.6 m truncated paleosol; Illinoian, 1.7 m oxidized and unoxidized Markles Point till; cobble 

pavement; middle Pleistocene, 3.7 m oxidized upper unit of Perch Bay till, 0.5 m paleosol (humic clay and silt), >0.3 m oxidized lower unit of Perch Bay till. 

Measured in 1985

and pebble gravel (channel fill inset into older till); Illinoian, 1.2 m unoxidized Markles Point till; middle Pleistocene, 5.5 m oxidized Perch Bay till, 3.7 m 

unoxidized till of anomalous composition [glaciotectonic raft(?)], 1.8 m oxidized Perch Bay till. Measured in 1985

abandoned gravel pits and gullies. Pleistocene, 0.9 m eolian sand and silt; Illinoian, 0.9 m oxidized Markles Point till (basal contact sharply truncates ice 

wedge casts that penetrate the underlying till units); Pleistocene, 1.8 m cobble and pebble gravel, 0.5 m truncated paleosol; middle Pleistocene, 1.5 m 

oxidized Perch Bay till, 0.3 m silt and clay, 1.5 m oxidized Perch Bay till, 0.6 m sand and gravel, 0.4 m sand and silt, >2.8 m oxidized Perch Bay till; middle 

Pleistocene, early Pleistocene, and (or) late Pliocene, 1.5 m truncated soil pendants in indurated stony "calcrete" till or colluvium, 0.3 m loess. A cobble of 

pink granite from the Canadian Shield was extracted from near the base of the indurated till or colluvium; all other clasts in that unit were redeposited 

quartzite, argillite, chert, and chert conglomerate derived from Miocene gravel exposed nearby. Measured in 1984, 1985, 1986

•1 Loring section, NW1⁄4 sec. 28, T. 36 N., R. 30 E., Mont.—Stratotype for Loring till and reference section for Markles Point till and Perch Bay till. Late Wisconsin, 

•2 Anderson Ranch section, SW1⁄4 sec. 5, T. 36 N., R. 32 E., Mont.—Late Wisconsin, 2.8 m unoxidized Loring till; 3.l m covered interval; Pleistocene, 1.2 m cobble 

•3 Glasgow Air Force Base site, SW1⁄4 sec. 17, T. 31 N., R. 40 E., Mont.—Reference exposures for Markles Point till and Perch Bay till. Composite section from 

lca lss eu

QUATERNARY

TERTIARY

LATE WISCONSIN

ILLINOIAN

PRE-ILLINOIAN

ggi

LACUSTRINE

DEPOSITS

wla

LANDSLIDE

DEPOSITS

agu

COLLUVIAL

DEPOSITS

als

RESIDUUM

?

BEDROCK

?

al

?

xlq

?

xsh

EOLIAN

DEPOSITS

?

jea cad caa

? ?

?

cgx xcg

xly

R

tlx tlx

(e)

tlx

(s) kg gg

tks

eux

?

GLACIAL

DEPOSITS

ICE-CONTACT

DEPOSITS

GLACIOFLUVIAL

DEPOSITS

ALLUVIAL

DEPOSITS

CORRELATION OF MAP UNITS

Holocene

Pliocene

 late

Pleistocene

 middle

Pleistocene

 early

Pleistocene

Pleistocene

1

al

cad

tlx

ggi

R

tlx(e)

tlx(s)

kg

gg

tks

cgx

xcg

xlq

xly

xsh

agu

als

lca

lss

eu

wla

eux

jea

caa

afj

agu

lca

lss

eu

wla

eux

cgx

xly

tlx

tlx(e)

tlx(s)

xsh

kg

gg

tks

ggi

R

f

als

jea

caa

cad

xcg

xlq

IT

Whitewater

Whitewater

Creek

Creek

Pea Lake

Whitewater

Reservoir

Austin

Lake

Frenchman

Reservoir

Frenchman

Creek

Frenchman Creek

Frenchman Creek

Crow

Creek

East Frok Crow

Creek

Bluff Creek

Snake Creek

Crow

Creek

McEachern Creek

Rock

Creek

South Fork Rock Creek

South Creek

Roanwood Creek

West Fork

Poplar River

West Fork

Poplar River

Spring Creek

Spring Creek

Coal Creek

Middle Fork

Porcupine Creek

East Fork Willow

Creek

Willow

Creek

Rock

Creek

West Fork Bitter Creek

South Fork Bitter Creek

Bitter Creek

Willow

Creek

Rock Creek

East Fork Cash Creek

West Fork Cash Creek

Papoose Creek

Rock

Creek

Lime Creek

First Creek

Second Creek

Third Creek

Fourth Creek

Numerous small lakes

Numerous small lakes

Little Sevenmile Creek

Dix Creek

Tallow Creek

Box Elder Creek

Beaver Creek

Sevenmile Creek

Alkali Creek

Beaver Creek

Flat Creek

Spring Creek

West Alkali Creek

Big Warm Creek

Alkali Creek

Dodsor

Milk River

S outh

Canal

Lake Bowdoin

Fort Peck Lake

Dry Lake

Mahan and Hoyt

Reservoir

Beaver Creek

Milk

River

Nelson Reservoir

Assiniboine Creek

Garland Creek

Cottonwood

Creek

Milk River

Beaver Creek

Little Cottonwood

Creek

Numerous

small

lakes

Numerous small lakes

Numerous

small

lakes

Martin Lake

Larb

Creek

Larb Creek

Square Creek

North

Fork

Antelope Creek

South

Fork Antelope Creek

Hardscrabble Creek

Bush Fork Bear Creek

Bear Creek

Canyon Creek

Buggy

Creek

Cherry Creek

Dry

Fork

Creek

Middle Fork

Porcupine Creek

West Fork

Porcupine Creek

East Fork

Porcupine Creek

Porcupine Creek

Porcupine Creek

East Fork

Little Po rcupine Creek

West Fork Little Porcupine Creek

East Fork Little Porcupine Creek

Little Porcupine Creek

Spring

Creek

Tomato Can Creek

Little Porcupine Creek

West Fork Charley Creek

Kintyre Creek

Charley

Creek

MISSOURI

RIVER

Milk River

Milk

River

Bear Creek

Antelope Creek

Vandalia

Vandalia

South

South

Canal

Canal

Buggy

Creek

Milk

River

Spring Creek

West Fork Cherry Creek

Ea st Fork Cherry Creek

Cherry Creek

CherryCreek

Brazil Creek

Sargent Creek

Willow

Creek

Willow Cree k

Mud Cree k

North Fork Little Beaver Creek

Little Beaver Creek

South Fork Beaver Creek

Beaver Creek

Sage Hen Creek

Coyote Creek

Nelson South Canal

Whitewater Creek

White Creek

Cash Creek

5 0 5 10 15 MILES

5 0 5 10 15 20 KILOMETERS

LIST OF MAP UNITS

HOLOCENE AND LATE WISCONSIN

FLOOD-PLAIN AND CHANNEL ALLUVIUM

FLOOD-PLAIN AND CHANNEL ALLUVIUM

FLOOD-PLAIN AND CHANNEL ALLUVIUM, LAKE

 DEPOSITS, AND SHEETWASH ALLUVIUM

LAKE CLAY AND SILT

LAKE SILT AND SAND

EOLIAN SILT AND SAND

HOLOCENE, LATE PLEISTOCENE, AND MIDDLE PLEISTOCENE

SHEETWASH ALLUVIUM

EOLIAN SILT AND SAND

SLUMP-BLOCK LANDSLIDE DEPOSITS AND 

 EARTHFLOW AND MUDFLOW DEPOSITS

COLLUVIUM, SHEETWASH ALLUVIUM, AND LANDSLIDE 

 DEPOSITS

COLLUVIUM, SHEETWASH ALLUVIUM, LAKE DEPOSITS, AND 

 GLACIOFLUVIAL DEPOSITS

GRAVELLY AND SANDY COLLUVIUM

CLAYEY DISINTEGRATION RESIDUUM, COLLUVIUM, 

 SHEETWASH ALLUVIUM, AND TILL

HOLOCENE AND PLEISTOCENE

LOAMY DISINTEGRATION RESIDUUM, COLLUVIUM, AND

 SHEETWASH ALLUVIUM

QUATERNARY AND TERTIARY

LOAMY DISINTEGRATION RESIDUUM

SANDY TO LOAMY DISINTEGRATION RESIDUUM

LATE WISCONSIN

LOAMY TILL

Ground-moraine deposits

End-moraine deposits

End-moraine deposits

ICE-CONTACT SAND AND GRAVEL

OUTWASH SAND AND GRAVEL

ILLINOIAN

LOAMY OR SANDY TILL

OUTWASH SAND AND GRAVEL

PRE-QUATERNARY

BEDROCK

EXPLANATION OF MAP SYMBOLS

Known or suspected glaciotectonic deposits or glaciotectonic terrain—Bedrock and surficial

 deposits that were thrust, stacked, and deformed by glacial ice. Chiefly stacked or imbricated

 slices of bedrock and Quaternary (and Tertiary?) stratified sediments. Till overlies some of the

 glaciotectonic deposits. The Tiger Butte deposits southeast of Glasgow were described and

 mapped by Jensen (1951a) and Jensen and Varnes (1964). Unmapped glaciotectonic deposits

 likely are included in stagnation-moraine deposits in the northwest corner of the quadrangle. 

 Maximum thickness >30 m 

Man-made land—Fort Peck Reservoir earthen dam 

Lake or river

Contact

Shoreline

Limit of late Wisconsin glaciation—Dashed where inferred; ticks are on the side of the advance

Limit of Illinoian glaciation—Dashed where inferred; ticks are on the side of the advance

Limit of a glacial readvance, or position of a significant stillstand of an ice margin—Dashed

 where inferred; Ticks are on the side of the advance

Minor moraines—Mainly till. Most of the ridges delineate the configurations of vanished ice

 margins. Local relief of ridges of late Wisconsin age typically is 1–2 m. Gravelly moraines 

 of Illinoian age on the Boundary Plateau are 1–>9 m high. Includes "transverse ridges," "small

 moraines," "washboard moraines," "annual moraines," "ice-crack moraines," "crevasse fillings,"

 and "corrugated moraines." Wild (2003) proposed that landforms in the southwest corner of

 quadrangle were formed by subglacial meltwater floods, not by glacial ice

Southeast limit of residual erratic glacial boulders on dissected bedrock surfaces covered

 by disintegration residuum—Residual erratic boulders from Canada are dominantly in area

 mapped as xlq; locally, boulders are on or are pressed into gravel deposits of Tertiary age (map

 unit xsh). Most of the boulders are igneous and metamorphic rocks from the Canadian Shield.

 Most of the erratic clasts smaller than boulders were removed by erosion. The boulders

 apparently were transported to the region by ice during one or more late Pliocene or early

 Pleistocene continental glaciations (Fullerton and others, 2004b)

Ice-molded or ice-scoured landform—Linear drumlin, rock drumlin, or fluting. Long axis of

 symbol is parallel to the direction of ice movement. Some symbols represent more than one

 landform. Wild (2003) proposed that landforms in the southwest corner of the quadrangle were

 formed by subglacial meltwater floods, not by glacial ice

Esker—Subglacial or englacial stream deposit

Buried bedrock valleys of the ancestral Missouri and Musselshell Rivers—West of Nashua

 the Milk River occupies the ancestral valley of the Missouri River. The modern (glacially diverted)

 Missouri River enters and then leaves the quadrangle in the southeast corner of the quadrangle.

 The north-flowing ancestral Musselshell River was a major tributary of the ancestral Missouri River.

 Beaver Creek now occupies the ancestral valley

Location of important stratigraphic section (see IMPORTANT STRATIGRAPHIC SECTIONS)

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

by the U.S. Government.

This database, identified as OFR 2012–1217, 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.

Publishing support provided by: 

Denver Publishing Service Center

Manuscript approved for publication September 27, 2012

For more information concerning this publication, contact:

Center Director, USGS Geology and Environmental Change Science Center

Box 25046, Mail Stop 980

(303)236-5344

Or visit the Geology and Environmental Change Science Center Web site at:

http://esp.cr.usgs.gov/ 

For sale by U.S. Geological Survey, Information Services, Box 25286, Federal Center, Denver, CO 80225,

1–888–ASK–USGS

Digital files available at http://pubs.usgs.gov/of/2012/1217

Suggested Citation: Fullerton, D.S., Colton, R.B., and Bush, C.A., 2012, Quaternary geologic map of the

Glasgow 1 ̊ x 2 ̊ quadrangle, Montana: U.S. Geological Survey Open-File Report 2012–1217, scale 1:250,000 

[http://pubs.usgs.gov/of/2012/1217]. 

SCALE 1: 250 000

SHELBY LOBE

W

 LOBE

WHITE TERA

LOMA SUBLOBE

 LOBE

YELLOWSTONE

MISSOURI V

 LOBE 

 GLASGOW OR

ALLEY

Pincher

Brocket

Fort Macleod

MonarchKipp

Lethbridge

aberT

Drywood

Waterton Park

iewVMountain 

Cardston

Kimball

Raley Spring Coulee

Magrath Raymond

Del Bonita win RiverT

Milk River

Bow Island

Robinson

Foremost Etzikom Pakowki

Pinhorn

Comrey

Kincorth Maple Creek

Battle Creek

Willow Creek

Rangeview

Divide

Eastend

Ravenscrag

Climax

Shaunavon

Neville

Kincaid

Val Marie

Orkney

Rosefield

Reliance Summercove

Lafleche

Ettington

Killdeer

eyburnW

Ratcliffe

Ardill Galilee

Crane alleyV

Dummer Yellow Grass

Vwin T alley

iceroyV

Bengough

Hardy Radville

Coronach Big Beaver

Big Muddy

Lake Alma

Osage

Stoughton

orquayT Outram Estevan

Frobisher

Crosby

Fortuna

Bowbells

Alamo

Lostwood

White Earth

Ray

Williston

Grenora

Flaxville Plentywood

Scobey Peerless

Medicine Lake

Brockton Culbertson

Poplar

Frazer Wolf Point

Whitewater

Hinsdale

Bowdoin

Malta

Glasgow

Fort Peck

Turner

Chinook

Havre Harlem

Dodson

Box Elder

Rocky Boy

Lodge Pole

Sunburst

Shelby Chester

Conrad Big Sandy

Babb

Cut Bank Browning

West Glacier

East Glacier Park

alierV

Pendroy

Hall Coulee

Marias Pass

Bynum

Choteau

Augusta

Fort Benton

Power

Portage

Great Falls

Cascade

Eagleton

Landusky

Iliad

Denton Roy alentineV

Lewistown

Sun Prairie

Jordan

Sidney

Richey

Circle

Intake

Glendive

Keene

Cartwright atford CityW

Grassy Butte Dunn Center

alleyVGolden 

Opheim

Figure 1. Regional ice flow during maximum

 late Wisconsin glaciation.

MOUNTAIN VALLEY AND PIEDMONT GLACIATION

Pinedale till

LAURENTIDE CONTINENTAL GLACIATION

Late Wisconsin till

Illinoian till

Isolated and scattered exposures of

 pre-Wisconsin till and glacial erratic boulders

Pre-Illinoian till on north margin 

 of Bearpaw Mountains

Scattered glacial erratic boulders, no till

Glacial lakes Great Falls and Musselshell. 

 Full extent of lakes not shown

Not glaciated

PT 

W L

IL

PW

PI

GB

LAURENTIDE CONTINENTAL GLACIATION

Late Wisconsin

Limit of late Wisconsin glaciation (LW),

 defined by stratigraphy and (or) surface 

 morphology 

Limit of "C2" glacial readvance in southwestern Alberta

Limit of a regional glacial readvance caused 

 by reorganization of glacial dispersal centers; 

 defined by stratigraphy and surface morphology.

 Hachures on the "up-glacier" side of limit

Illinoian

Limit of Illinoian glaciation (IL), defined by stratigraphy 

 and (or) surface morphology 

Pre-Wisconsin

Limit of pre-Wisconsin till and glacial 

 erratic boulders (limit of PW). Till is not present

 as far south as the limit of boulders

IL

d h i r

u t R e

J i v

W L

IL

IL

M s r v s u i r i o i R e

IL 

WL

Teton Riv er

PW

M i i r r s R e a a v

Reservoir

Fresno

PI

i i r M k R e l v

PT

PT

e e e P l t s u e a s a P r l

o a v P

l i r

p r R e

PT

o t i

o

y M n

R

s k u a n c 

WL

r k

C n C t a e

u B k e

EXPLANATION

Frances

Lake

Tiber Reservoir

Fort

Reservoir

Peck

Glacier

Park

National

Highwood

Mountains

Mountains

Bearpaw

Little Rocky Mts

Cypress

l siH l 

Willo n w u h B c Lake

M e s L u t v o t i e i i r s i r l R

W b ny re u 

v u e h l i M r s s l R e s l e

102°

a i r S n M

y R e i r v a t

Mountains

Killdeer

Plateau

Boundary

O

m i r d n R e l a v

104° 106°

t i r

a r n R e W e o v t

108° 110° 112°

d i r n R e

l a O

v 

m

Pre-Illinoian Pleistocene

Southern limit of known surface exposures of 

 pre-Illinoian Pleistocene till (limit of PI) on

 north flank of Bearpaw Mountains

Limit of glacial erratic boulders on bedrock

 erosion surfaces (limit of GB)

Location of Glasgow quadrangle

Generalized direction of ice flow

River, stream, and modern lake shoreline

114°

v i i r M k R e l

50°

49°

48° Williston

 

47°

Shelby

Glasgow

Cut Bank

l d eG n v e i

Choteau

Two Medicine

 lobe

Wood u a M n i o t n 

Lewistown

(792 m; 2,600 ft) confluent

glacial Lake Musselshell

Late Wisconsin phase of

with glacial Lake Jordan

Jordan

Late Wisconsin

(792 m; 2,600 ft)

Lake Jordan

phase of glacial

F e s r o o m t

atford CityW

Great F l al s

Lethbridge

of glacial Lake Great Falls

Late Wisconsin phase

(1142 m; 3,715 ft)

C2

Cypress Lake

A E AB TL R 

a l w t r s i

S

s e G s H l e

olf PointW

CANADA

T E N A A H A S K C W S

C2

Havre

UNITED STATES

 Upland

Wood Mountain

IL

W L

N N O A M T A

U

M S

R

 O A

S

U C 

E

 I O

 I T

IL

WL

H A T N T D O R K A O

N K TAT A OR D O H

IL

IL S K C W A A H A S T E N

IL

LW

O A M T AN N

GB

w o e r o t e i Y e l s n R v l

B TL R A E A

IL 

u v i o i i r

M s r R e s

 Plain

Flaxville 

IL

WL

IL 

i R

M s r v r s u i e i o

WL

25 0 25 50 75 100 KILOMETERS

25 0 25 50 75 100 MILES

Lake Sakakawea

TLMA A SUBLOBE

AH VRE LOBE

Waterton Lakes

National Park

Hills Porcupine

v e y i r B l R e l

tlx xsh

tlx

tlx(s)

xlq

tlx(e)

tlx

xlq tlx(e)

agu

tlx

caa

tks

jea

tks tks

tks

tks

tks

agu

agu

jea

xly

caa wla

R

tks

tlx(s)

tks

tlx(s)

tks

wla

tlx(s)

tks

tlx(e)

tks tks

caa

xsh

xlq

tks

tlx(e)

xsh

tks

xsh

xsh xly

tks

tks

tks

wla

tks

tlx(s)

tks tks tks

tlx(e)

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49°00'

108°00'

49°00'

106°00' 107°30' 107° 106°30'

106°00'

48°00'

108°00'

48°00'

48°30'

LARB HILLS

Tiger Butte

BOUNDARY

PLATEAU

PEERLESS

PLATEAU

Milk River

DESCRIPTION OF MAP UNITS

[Map unit thickness is typical range; in some areas the map units may be thicker 

or thinner than the given range]

HOLOCENE AND LATE WISCONSIN

FLOOD-PLAIN AND CHANNEL ALLUVIUM—Yellowish-brown, 

olive-brown, grayish-brown, brown, olive, yellowish-gray, brownish-gray, 

olive-gray, gray, black, or mottled clay, silt, sand, and gravel. Calcareous or 

noncalcareous; partly or completely oxidized. Clasts are angular to well 

rounded. Clast composition reflects composition of other surficial materials 

and bedrock in the drainage basin. Fossiliferous (gastropod, pelecypod, and 

vertebrate remains; wood fragments and plant detritus) in some places. 

Included in other map units in many areas. Includes some sheetwash 

alluvium, alluvial-fan deposits, alluvium beneath low terraces, outwash and 

ice-contact sand and gravel, lake deposits, and bedrock outcrops. Alluvium 

in some areas is dominantly reworked outwash, ice-contact deposits, and lake 

deposits. Thickness 1–6 m, locally >15 m

 Flood-plain alluvium—Chiefly poorly-sorted to well-sorted clay, silty clay, silty 

clay loam, clayey silt, and silt with scattered granules and pebbles; locally, 

loam, sandy clay loam, sandy loam, or fine sand. Moderately to well 

stratified. Commonly has either obscure bedding or pronounced horizontal 

bedding; weakly laminated in some places. Clay, silt, sand, or gravel lenses 

or small channel fills of sand and gravel are common. Dense and compact in 

most places. Clayey alluvium is moderately plastic, soft, and sticky where 

moist and hard where dry. Textures may vary abruptly, laterally and 

vertically. Local thin buried soils (humic horizons). Concentrations of 

detrital lignite (bedrock) fragments as large as 1 cm are common in 

crossbedded sand. Where flood-plain alluvium directly overlies lake clay, 

silt, and sand or sheetwash alluvium, in some places it is difficult to 

distinguish the deposits

 Channel alluvium—Chiefly loose, stratified, poorly-sorted to well-sorted, 

pebbly, fine to medium sand and (or) coarse sand and gravel; locally very 

poorly sorted sandy shale-fragment gravel. Cobbles and boulders are 

common to abundant in some areas. Generally crossbedded or has crude 

horizontal bedding or lenticular bedding; local festoon bedding. 

Fossiliferous (chiefly Holocene vertebrate remains)

 In several areas on the Milk River valley floor in the vicinity of Malta, low 

terrace remnants are 2.4–3.0 m higher than the modern flood-plain surface. 

The terrace remnants are underlain by clay and silt, similar to lake deposits 

(map unit lca) elsewhere in the quadrangle. They are interpreted to be 

remnants of glacial lake deposits that are older than the flood-plain alluvium 

(Greg Snell, U.S. Department of Agriculture, Natural Resources 

Conservation Service, written and oral commun., 2005, 2006). The genesis 

of the clay and silt in the terrace deposits is uncertain; they are included in 

map unit al

FLOOD-PLAIN AND CHANNEL ALLUVIUM—Deposits similar to map unit 

al, preserved as terrace remnants north of Fort Peck Dam. The sand and 

gravel is younger than the deglacial lake deposits (map units lca, lss) and 

older than the flood-plain and channel alluvium of the modern Missouri 

River (map unit al). Thickness 1–6 m

FLOOD-PLAIN AND CHANNEL ALLUVIUM, LAKE DEPOSITS, AND 

SHEETWASH ALLUVIUM—Complex map unit of flood-plain and channel 

alluvium (map unit al), lake clay, silt, and sand (map units lca, lss), and 

sheetwash alluvium (map unit wla). Descriptions of those map units apply. 

Thickness 2–20 m

LAKE CLAY AND SILT—Pale-yellow, grayish-yellow, bluish-green, 

yellowish-brown, olive-brown, grayish-brown, brown, olive, yellowish-gray, 

brownish-gray, olive-gray, bluish-gray, gray, olive-black, bluish-black, black, 

or mottled, calcareous clay and silt. Generally stratified; well sorted; 

massive or weakly bedded. Commonly laminated, locally varved, clay and 

silty clay in lower part and massive or horizontally-bedded silty clay and silt 

in upper part. Upper part generally is clast free; in some places, 

iceberg-rafted granules, pebbles, cobbles, and boulders are common in lower 

part. Clasts generally are absent on the surface. Contains local thin, 

discontinuous beds, lenses, or stringers of sand or fine gravel; pockets or 

pods of iceberg-rafted granules and pebbles, or isolated rafted erratic cobbles 

or boulders. Interbedded with sand, fine gravel, till, or flowtill in some 

places; some rare ice-rafted clasts of till are 1–2 m in diameter. Graded beds, 

silt blebs and nodules, or rip-up clasts of till or lake silt and clay are present 

locally. Strongly contorted, with slump structures, load structures, and folds, 

in some places. Generally not faulted. Dessication cracks are common in 

some places. Commonly oxidized in upper 1–2 m; local iron-oxide stains. 

Soft to very firm; cohesive and compact. Very plastic and sticky when moist; 

hard when dry. Locally gritty. Dry clay and silt commonly breaks into chips 

or blocks. Clay minerals are dominantly montmorillonite. Selenite 

(gypsum) crystals <6 mm in length are present locally in fractures and are 

abundant in saline lake sediment. Secondary calcium carbonate concretions 

locally are present in upper part. Gullies are common in some places. 

Deposit is susceptible to landslide processes (chiefly slump) in exposures 

(see map unit jea). Most deposits are off-shore deposits of former lakes. 

Present beneath flat plains and in small basins formerly occupied by glacial 

and postglacial lakes and ponds. Flat to rolling or hummocky surface 

topography. Thin and discontinuous in some areas. Overlapped by 

flood-plain alluvium adjacent to some streams; overlapped by sheetwash 

alluvium adjacent to some bedrock outcrops. Includes lake silt and sand 

where lake clay, silt, and sand have not been distinguished. Includes some till 

with a thin veneer of silt and clay. Includes some ice-contact and outwash 

sand and gravel, small lake delta deposits, and bedrock outcrops. In some 

places, lake clay and silt are overlain by eolian silt and sand, alluvial-fan 

deposits, or floodplain alluvium. Thickness 1–5 m, locally >15 m

LAKE SILT AND SAND—Grayish-yellow, yellowish-brown, grayish-brown, 

brown, yellowish-gray, olive-gray, brownish-gray, gray, or mottled, 

calcareous silt and sand mixed with minor clay and gravel. Generally well 

stratified and well sorted. Typically horizontally bedded or massive, fine or 

medium sand containing scattered granules or small pebbles. Commonly 

interbedded or intercalated with massive, bedded or laminated, silt containing 

dropstones; in some areas, chiefly laminated silt and fine sand. Commonly 

slumped and contorted on hillslopes. Detrital fragments of lignite (bedrock) 

are abundant in many areas, concentrated in sand crossbeds. Clasts are 

chiefly subrounded and rounded erratic limestone, dolomite, and igneous and 

metamorphic rocks and local sandstone and shale that were ice rafted or 

reworked from till. Chiefly nearshore or shallow-water deposits. Present 

beneath lake plains and in small basins formerly occupied by glacial and 

postglacial lakes. Local relief 1–4 m. Includes some lake clay and silt, 

flood-plain and channel alluvium, and till. In many places, overlies lake clay 

and silt. Thickness 1–8 m, locally >10 m

EOLIAN SILT AND SAND—Yellowish-brown, olive-brown, grayish-brown, 

brownish-gray, gray, or mottled windblown silt and sand. Calcareous. Well 

stratified; well sorted. Faint horizontal beds or cross-beds. Very friable; 

slightly hard; subangular blocky structure. Not sticky or plastic when moist. 

Chiefly well-rounded quartz grains; less abundant feldspar grains. Quartz 

grains frosted or clear. Undulating surface topography. Typically oxidized to 

depths >1.5m. Thickness 1–3 m; maximum >4 m

HOLOCENE, LATE PLEISTOCENE, AND MIDDLE PLEISTOCENE

LOAMY AND CLAYEY SHEETWASH ALLUVIUM1

—Pale-yellow, 

yellowish-brown, olive-brown, brown, olive, yellowish-gray, olive-gray, 

brownish-gray, gray, or mottled alluvium transported and deposited by 

unconfined overland flow and rill wash. Typically (1) clay, silty clay, silty 

clay loam, and clay loam; (2) clay loam, silt loam, and sandy loam; or (3) 

loamy sand and sandy loam. Nonstratified to moderately well stratified; 

poorly or moderately well sorted. Massive, thinly laminated, or with weak 

horizontal bedding. Disseminated organic matter is abundant in some areas; 

local thin buried soils (humic horizons). Fossiliferous (gastropod tests) 

locally. Fragments of lignite (bedrock) are common in many areas. 

Generally calcareous and alkaline; saline in some places. Clast free or with 

scattered granules and small pebbles, or with stringers, pods, and lenses of 

granule or pebble gravel. Commonly interbedded with well-sorted, pebbly, 

coarse silt and very fine sand. Mixed, intercalated, or interbedded with 

eolian sand and silt or flood- plain and channel alluvium in some places. 

Clayey alluvium is soft, sticky, and plastic where moist; tough, hard, and 

blocky where dry. Clay minerals in most areas are dominantly 

montmorillonite. Granules and pebbles are chiefly local clastic sedimentary 

rocks of Tertiary and Cretaceous age; minor redeposited erratic quartzite, 

limestone, dolomite, and igneous and metamorphic rocks in glaciated areas. 

Sheetwash alluvium is in fans and aprons on foot slopes, in glacial diversion 

channels and sluiceways, in swales and sloughs, and in depressions and other 

poorly drained areas. Included in other map units in many areas. Includes 

some inset or overlapping flood-plain and channel alluvium. Also includes 

some colluvium, till, and bedrock outcrops. Where sheetwash alluvium 

directly overlies flood-plain alluvium or lake deposits, commonly there is not 

a sharp contact between the deposits. Distinction between map units wla and 

cad is arbitrary in some areas. Thickness 1–4 m, locally >10 m

EOLIAN SILT AND SAND—Blanket-like wind-blown sediment. Pale-orange, 

pale-yellow, yellowish-brown, olive-brown, grayish-brown, brown, gray, or 

mottled, wind-blown silt (loess) and sheet sand. Calcareous where not 

leached; local white or pale-gray filaments, powdery interstitial fillings, or 

nodules of secondary calcium carbonate. Moderately alkaline. Nonsticky, 

nonplastic. Locally stained yellowish brown, reddish brown, or brown by 

iron oxides. Nonstratified or very poorly stratified; generally well sorted or 

very well sorted. Typically (1) loose, nonstratified, well-sorted, 

homogeneous, massive silt and very fine to medium sand or (2) silty fine 

sand containing laminae or thin lenses of silt. Local faint horizontal bedding 

or crossbedding; vague horizontal color banding is common. Friable; blocky 

structure common. Generally oxidized throughout. Chiefly subangular or 

subrounded quartz grains; less abundant feldspar grains. Quartz grains are 

frosted or clear. In some places eolian deposits contain weakly developed 

buried soils (humic horizons). Local small dunes. Modified by bioturbation, 

cryoturbation, solifluction, and creep. Mostly stable and covered by grass. 

Included in map unit xsh in many places. Thin, unmapped, eolian sand and 

silt discontinuously veneers Illinoian till in much of the area mapped as tks, 

and eolian sediment is a parent material in which residuum is developed (see 

map units xsh, xly). Thickness 0.3–2 m; maximum thickness 3 m

SLUMP-BLOCK LANDSLIDE DEPOSITS AND EARTHFLOW AND 

MUDFLOW DEPOSITS—Most of the individual landslide deposits in the 

quadrangle are too small to map at this scale. Slump-block landslide deposits 

are masses of bedrock and unconsolidated materials that rotated or slid 

downslope as a unit, with little or no flow. The physical properties of the 

transported materials are not greatly altered; original textures, stratification, 

bedding, and other sedimentary structures of slumped materials are disturbed 

but retained. Toes of some deposits form sub-parallel, arcuate, concentric, 

hummocky ridges, separated by elongate swales. Some of the mapped 

deposits are stabilized by vegetation. Includes areas of colluvium, sheetwash 

alluvium, and bedrock outcrops. Thickness 3–5 m, locally >10m

 Earthflow deposits—Heterogeneous, nonsorted mixtures of clay, silt, and sand 

containing scattered clasts, transported and deposited as a result of slow flow 

of wet, but not necessarily saturated, disintegrated bedrock and surficial 

materials. Most of the deposits are reworked slump-block deposits and 

debris derived from collapse of gully walls. Deposits typically are in gullies 

and in fans at mouths of gullies in dissected slump-block deposits, or on foot 

slopes. Thickness 1–5 m, locally >6 m

 Mudflow deposits—Heterogeneous mixtures of clay and silt containing scattered 

clasts, transported and deposited as a result of rapid flow of saturated, 

unconsolidated materials and disintegrated shale bedrock. Typically, deposits 

are debris washed from exposed shale surfaces or debris derived from 

collapse of gully walls. Deposits commonly form fans at mouths of gullies 

or on foot slopes. Thickness 1–5 m

COLLUVIUM,2

 SHEETWASH ALLUVIUM,1 AND LANDSLIDE DEPOSITS— 

Complex map unit on valley sides and eroded uplands. Includes areas of lag 

sand and gravel over till and (or) bedrock; alluvial-fan and 

sheetwash-alluvium fan or apron deposits at bases of hillslopes and at mouths 

of gullies; lag boulder and cobble accumulations; sheetwash-eroded or 

stream-eroded till or bedrock dissected by gullies; very thin disintegration 

residuum on bedrock; includes extensive areas of bedrock outcrops, 

including local badland terrain. 

 Colluvium—Yellowish-brown, olive-brown, brown, olive, yellowish-gray, 

brownish-gray, gray, or mottled debris transported and deposited by 

mass-movement processes. Calcareous or noncalcareous. Loose to compact. 

Nonstratified or faintly stratified; nonsorted or poorly sorted. Commonly 

massive. Where derived from local bedrock, colluvium typically is angular 

and subangular boulders, cobbles, pebbles, and granules of local bedrock in 

a clayey to sandy matrix. Where derived from sand and gravel, typically it is 

sand or pebbly sand. Where derived from glacial till, typically it is boulders, 

cobbles, pebbles, and granules of erratic quartzite, limestone, and dolomite, 

erratic igneous and metamorphic rocks, and local sandstone and shale in a 

clayey to sandy matrix. Three or more buried soils (humic horizons) locally 

are present in the colluvium; lignite (bedrock) fragments are ubiquitous in 

many areas. Thickness 0.5–5 m

 Sheetwash alluvium—Similar to deposits in map unit wla. Thickness 1–4 m,

 locally >10 m

 Landslide deposits—Similar to deposits in map unit jea. Thickness 3–5 m

COLLUVIUM,2

 SHEETWASH ALLUVIUM,1 LAKE DEPOSITS, AND 

GLACIOFLUVIAL DEPOSITS—Complex map unit in abandoned 

meltwater channels and glacial lake outlet sluiceways. Channel sides are 

sharply defined to indistinct. Some of the channels and sluiceways are 

shallow, broad flats. Valley sides are chiefly colluvium, sheetwash alluvium, 

till, lake deposits, and bedrock outcrops. Isolated large boulders or lag 

deposits of cobbles and boulders are present on valley sides in some places. 

Gullies locally are common on valley sides. Sheetwash alluvium derived 

from bedrock exposed on the valley sides locally overlaps other deposits on 

the channel floor; in some places it mantles much of the fill on the floor. 

Glaciofluvial deposits (outwash sand and gravel) possibly underlie alluvium 

on some channel floors. Distinction of map units cad and caa is arbitrary in 

some areas. Combined thickness of deposits is 1–10 m, locally >15 m

 Colluvium—Similar to deposits in map unit caa. Thickness 0.5–8 m

 Sheetwash alluvium—Similar to deposits in map unit wla. Thickness 0.5–4 m

GRAVELLY AND SANDY COLLUVIUM2

—Reddish-brown, yellowish-brown, 

grayish-brown, brownish-gray, and gray sandy gravel and pebbly sand with 

lenses of silt and clay, deposited on slopes by mass-movement processes. 

Derived chiefly from Tertiary sand and gravel that is exposed along eroded 

margins of till-covered terraces. Generally calcareous. Loose to compact. 

Nonstratified or weakly stratified; nonsorted or poorly sorted. Clasts are 

chiefly granules and pebbles; some cobbles; rare boulders. Pebbles and 

cobbles commonly are partly coated by calcium carbonate. Clasts are 

dominately quartzite, chert, and chert conglomerate from the Rocky 

Mountains; minor sandstone and shale derived from underlying bedrock. 

Reddish-brown, orange-brown, yellowish-brown, and brown iron oxide 

stains are common. Thickness of exposed gravel generally 6–10 m; thickness 

of colluvial apron 1–5 m

CLAYEY DISINTEGRATION RESIDUUM,3

 COLLUVIUM,2 SHEETWASH 

ALLUVIUM,1

 AND TILL—Complex map unit in the south-central part of 

the quadrangle, south of the Milk River. Includes small landslide deposits 

(map unit jea).

 Residuum—Yellowish-brown, olive-brown, grayish-brown, brown, olive, 

olive-gray, brownish-gray, gray, or mottled clay, silty clay, clay loam, or 

loam. Nonsorted, nonstratified; massive or has faint relict stratification. 

Loosely consolidated or compact. Generally is calcareous where not leached. 

Clay minerals are chiefly montmorillonite. Mildly to strongly alkaline. 

Plastic, sticky, and slippery where moist; hard and blocky where dry. 

Iron-oxide stains are common. Residual selenite (gypsum) crystals, 

fossiliferous calcium-carbonate and ironstone concretions, and redeposited 

pyrite, marcasite, or siderite crystals (from bedrock) are common locally. 

Shale fragments are common to abundant, particularly in lower part; locally 

the residuum contains thin layers of platy shale fragments. Very thin and 

discontinuous. Includes extensive areas of bedrock outcrops and badland 

terrain. Residual glacial erratic boulders of quartzite, limestone, dolomite, 

and igneous and metamorphic rocks are scattered on the land surface. 

Thickness 0.3–1 m

 Colluvium—Similar to colluvium in map unit caa. Dominantly angular and 

subangular fragments of micaceous and bentonitic shale in a clayey matrix. 

Sandstone and siltstone fragments and granules, derived from local bedrock, 

and pebbles of erratic quartzite, limestone, dolomite, and crystalline igneous 

and metamorphic rocks are present where colluvium is derived in part from 

eroded till. Thickness 0.3–1 m

 Sheetwash alluvium—Similar to map unit wla. Thickness 0.3–3 m, locally >9 m

 Till—See map unit tks

 HOLOCENE AND PLEISTOCENE

LOAMY DISINTEGRATION RESIDUUM,3

 COLLUVIUM,2 AND 

SHEETWASH ALLUVIUM1

—Brownish-yellow, yellowish-brown, 

olive-brown, grayish-brown, gray, or mottled loam, clay loam, silt loam, and 

silty clay loam; in some areas, sandy clay loam, sandy loam, or loamy sand. 

Noncalcareous or calcareous; fine seams and soft masses of secondary 

calcium carbonate are common where sediment is calcareous. Generally 

moderately alkaline. Nonsticky or slightly sticky and nonplastic or slightly 

plastic when moist; slightly hard to hard when dry and generally friable. 

Typically massive; prismatic structure or subangular blocky structure. Local 

reddish-brown, yellowish-brown, or brown iron-oxide stains. Residual 

concretions of calcium carbonate, limonite, or pyrite are common locally as 

clasts. In some places, contains fragments of lignite (bedrock). Clay 

minerals are dominantly montmorillonite in many areas. Modified by 

bioturbation, cryoturbation, and creep. Gullies are common on moderate and 

steep slopes. Includes small landslide deposits. Includes some redeposited 

sand and gravel containing quartzite clasts as large as 30 cm in diameter and 

patchy lag deposits of sand and gravel. Also includes dissected bedrock. 

Locally overlain by discontinuous eolian silt and sand. Residual glacial 

erratic cobbles and boulders of very resistant rocks from Canada are widely 

scattered on the land surface in the region mapped as unit xlq (Fullerton and 

others, 2004a, b) and in adjacent Saskatchewan (Klassen, 1992a, b). (See 

GB in fig. 1 and symbol for southeast limit of boulders on map). Limestone 

and dolomite clasts were removed by weathering processes. Some of the 

boulders and cobbles of coarse-grained igneous and metamorphic rocks that 

are preserved in colluvium are disintegrated (Klassen, 1992b, 2002). The 

residual erratic clasts are interpreted to record Pliocene or early Pleistocene 

continental glaciation (Fullerton and others, 2004a, b). Till deposited during 

that glaciation has not been identified in association with the residual cobbles 

and boulders.

 Residuum—Derived chiefly in place from mechanical weathering of bedrock or 

mixed windblown sediment and sheetwash alluvium overlying bedrock. 

Weathering profile typically extends into bedrock. Thickness <1 m

 Colluvium—Similar to colluvium in map unit caa. Where derived from local 

bedrock, clasts are dominantly soft sandstone, siltstone, and claystone. 

Where derived from sand and gravel, clasts are dominantly quartzite, 

argillite, chert, and chert conglomerate from the Rocky Mountains. Partly 

cemented by secondary calcium carbonate in some places. Thickness 1–4 m

 Sheetwash alluvium—Similar to map unit wla. Thickness 1–4 m

QUATERNARY AND TERTIARY

LOAMY DISINTEGRATION RESIDUUM3

—Yellowish-brown, grayish-brown, 

brownish-gray, or gray, calcareous or noncalcareous clay and silty clay. Soft, 

slightly sticky, and slightly plastic when moist; hard and flaky when dry and 

generally friable. Generally massive; sedimentary structures in parent 

material are not preserved; blocky structure common. Minor iron-oxide and 

manganese-oxide stains. Root casts are preserved locally. Residuum grades 

downward to parent material, very calcareous eolian silt and sand that 

contains lenses and interbeds of secondary calcium carbonate. Parent 

material in some places is "calcrete" cap rock. Parent material is the upper 

marl unit of the informal Flaxville formation of Collier and Thom (1918) (see 

unit xsh for discussion of informal designation of Flaxville), the informal 

Opheim formation of Whitaker (1980) and Patton (1987), and the "Ogallala 

Formation equivalent" of Colton and others (1989b). Map unit includes areas 

of other complex materials and deposits (map unit xsh) and also sheetwash 

alluvium (map unit wla). In some areas, residuum is overlain by wind-blown 

sand and silt (map unit eux). Thickness of residuum is 1–5 m. Thickness of 

parent material is generally 6–8 m, locally >10 m

 In the area interpreted to have been glaciated during Pliocene or early 

Pleistocene time (the southwestern limit of distribution of residual erratic 

boulders is shown by a symbol on the map; also see fig.1), glacial erratic 

cobbles and boulders of igneous and metamorphic rocks from the Canadian 

Shield, and also quartzite, limestone, and dolomite clasts from Canada, 

locally were pressed by ice into the calcareous residuum and the parent 

material of the residuum and later were buried, chiefly by eolian silt and 

sand. The very rare limestone and dolomite clasts were preserved because 

the enclosing sediment was very calcareous and because they were buried 

and protected from weathering by calcareous eolian sediments. Till 

deposited during a Pliocene or early Pliocene glaciation has not been 

identified in association with this map unit (see discussions in descriptions of 

map units xlq and tks)

SANDY AND LOAMY DISINTEGRATION RESIDUUM3

—Grayish-orange, 

pale-yellow, reddish-brown, yellowish-brown, olive-brown, grayish-brown, 

brown, yellowish-gray, brownish-gray, gray, or mottled loamy sand, sandy 

clay loam, sandy clay, loam, and clay loam. Parent material in most places is 

weathered wind-blown silt and sand that overlies fluvial sand and gravel of 

Tertiary (Miocene and Pliocene?) age. Residuum generally is calcareous; 

white masses or nodules of secondary calcium carbonate are common; 

cemented by calcium carbonate in some places; locally contains 

carbonate-coated root casts. Generally is strongly alkaline. Poorly stratified 

or nonstratified; poorly sorted or nonsorted. Commonly massive; 

sedimentary structures in parent materials are not preserved. Generally 

compact and moderately indurated. Moderately fissile. Angular or 

subangular blocky structure; minor clay films on ped surfaces. Granular, 

loose, and friable where very sandy. Silty or clayey residuum is slightly 

sticky to sticky and slightly plastic to very plastic when moist; hard when 

dry. Residuum typically is very sandy near base; grades downward to sand 

and gravel. Scattered, isolated granules and pebbles in upper part of 

residuum, chiefly well-rounded or rounded quartzite, argillite, chert, and 

chert conglomerate, are products of bioturbation. In some places, the long 

axes of pebbles and small cobbles in gravel are aligned vertically, a product 

of cryoturbation. In some places a buried soil is present between the fluvial 

deposits and the residuum. Multiple buried soils are locally present in the 

residuum; in some places a pedogenic composite soil more than 1 m thick is 

developed in the residuum. Modified by bioturbation, cryoturbation, 

solifluction, and creep. Gullies are common in residuum and the underlying 

sand and gravel in some areas. Map unit includes some colluvium; 

colluvium commonly margins exposed fluvial gravel deposits. The areas 

mapped in the northeast corner of the quadrangle, in the drainage basin of 

West Fork Poplar River beyond the limit of glacial till, are chiefly dissected 

remnants of pediments and terraces. In those areas, bedrock is overlain by 

redeposited Tertiary gravel and sand and the alluvium is overlain by eolian 

deposits. Residuum is developed in the alluvium and overlying eolian 

deposits. The thickness of the residuum in the latter region is 0.3–2 m; the 

thickness on higher plateau surfaces is 1–3 m, locally >8 m 

 The underlying sand and gravel on most of the Peerless Plateau is the Flaxville 

formation (Collier and Thom, 1918; Whitaker, 1980; and Patton, 1987) and 

the Wiota gravel (Jensen, 1951a, 1952; Jensen and Varnes, 1964; and Colton 

and others, 1989a, b). Those two stratigraphic units are distinguished solely 

on the basis of altitude, not on the basis of lithology or composition. 

Consequently they are here designated as informal allostratigraphic units, not 

formal lithostratigraphic units

LATE WISCONSIN

LOAMY TILL—Sediment deposited chiefly by ice of a Laurentide continental ice 

sheet; Fort Assiniboine till in the Havre glacial lobe, Loring till in the 

Whitewater glacial lobe, and Crazy Horse till in the Glasgow glacial lobe 

(Fullerton and Colton, 1986; also see fig. 1). Typical till is pale-yellow, 

grayish-yellow, yellowish-brown, olive-brown, grayish-brown, brown, 

brownish-olive, olive, yellowish-gray, brownish-gray, olive-gray, bluish-gray, 

gray, or mottled, calcareous clay loam and loam; in some areas it is clay, silty 

clay, silty clay loam, silt loam, sandy clay, or sandy loam. Very gravelly in 

some places; where till directly overlies shale bedrock, the matrix commonly 

is >90 percent shale fragments. In some places, till is interbedded with, 

intercalated with, or contains lenses, pods, and stringers of clay, silt, sand, or 

gravel. Generally nonstratified; nonsorted or very poorly sorted. Local weak 

horizontal layering where the ice margin fluctuated in a lake. Commonly 

massive; cohesive to friable. In some areas, gritty or mealy; crude fissility. 

Generally loosely compact or compact, but not hard. Sandy till is loose to 

firm; silty till is firm. Clayey till is soft, slightly sticky to sticky, and slightly 

plastic to plastic when moist; hard when dry. Dry till in some places breaks 

into small angular flakes and plates that have sharp edges; blocky structure in 

some other places. Parting typically is irregular or prismatic. Clay minerals 

are dominantly montmorillonite. Widely spaced, weakly-developed joints 

are common; in some places, joint surfaces are coated by powdery calcium 

carbonate or gypsum, or both. Joint or parting surfaces are locally weakly 

stained by iron or manganese oxides. Selenite (gypsum) crystals 1–3 mm 

long are present on joint surfaces in some places. Fragments of lignite 

(bedrock) are ubiquitous in till. Nearly pebble free to very pebbly; cobbles 

and boulders are rare to abundant. Boulders typically are more abundant in 

till of late Wisconsin age than in older tills; also, average diameter of 

boulders is larger than that in older tills. Largest erratic boulders are >2 m in 

diameter. Clasts of older till and clasts or rafts of lignite (bedrock) as large as 

2 m in diameter locally are included in the till. Glaciotectonic blocks or rafts 

of shale or sandstone tens of meters in length locally are included in the till. 

Pebbles in typical till are chiefly subangular to rounded erratic quartzite, 

limestone, and dolomite from Canada and erratic igneous and metamorphic 

rocks from the Canadian Shield; where till is derived primarily from local 

bedrock, pebbles are chiefly angular to subrounded shale, siltstone, and soft 

sandstone; where derived primarily from Tertiary and Quaternary gravel, 

pebbles are chiefly quartzite, argillite, chert, and chert conglomerate. 

Cobbles and boulders are chiefly subangular to well-rounded, erratic granite, 

pegmatite, gneiss, schist, diorite, and other igneous and metamorphic rocks 

from the Canadian Shield; less abundant limestone and dolomite from 

Saskatchewan and Manitoba, and quartzite. Till everywhere is unstable; it 

moves by creep on hillslopes and it slumps in exposures. Till is thin and 

discontinuous in many places; locally the map unit includes exhumed till of 

Illinoian and (or) pre-Illinoian age that is exposed at the surface. Includes 

some glaciotectonic deposits (symbol IT), flowtill, mud-flow deposits, 

debris-flow deposits, and landslide deposits. Includes areas of till that were 

modified by waves and currents in glacial lakes, with a surface litter of 

cobbles and boulders or a discontinuous cover of lake clay, silt, and sand. 

Includes some outwash and ice-contact sand and gravel, flood-plain and 

channel alluvium, and bedrock outcrops. Locally overlain by colluvium, 

sheetwash alluvium, lake clay, silt, and sand, or eolian sand and silt

 Ground-moraine deposits—Integrated drainage; flat, rolling, or undulating 

surface topography. In some places, includes some collapsed supraglacial 

sediment (stagnation moraine deposits) and minor moraines (see symbol). 

Maximum slope angles are generally less than 4°. Local relief is generally 

<3 m. Thickness 0.5–2 m; maximum thickness >15 m. Overlies older till in 

most areas; till thicknesses of >10 m generally include thicknesses of older 

subsurface tills 

End-moraine deposits—Till characterized by (1) broad belts of hummocky or 

undulating ridges or (2) belts of narrow, discontinuous, sharply defined 

ridges that delineate the configurations of vanished ice margins. Some 

mapped end- moraine deposits are suites or complexes of push-moraine 

deposits. Shallow, closed depressions and lag cobbles and boulders are 

common on the surface in many areas. Local relief in some places

 is 4–>20 m. Till end-moraine deposits are locally replaced laterally by 

kame-moraine deposits that are included in the map unit. In some areas, 

boundaries between end-moraine deposits and stagnation-moraine or 

ground-moraine deposits are transitional, and contacts are arbitrary. 

Thickness 4–15 m, locally >20 m

Stagnation-moraine deposits—Till characterized by hummocky or knob-and-kettle 

collapsed topography. Generally lacks distinct, continuous ridges. 

Nonintegrated or poorly integrated drainage; shallow ephemeral ponds, 

sloughs, and depressions are common. Ice-block depressions (kettles) are 

abundant in some areas. Local relief is generally >3 m; locally 40 m. In the 

northwest corner of the quadrangle, deposits likely include unmapped, 

glaciotectonically-folded and ice-thrust masses of bedrock (see symbol IT). 

Map unit includes collapsed glacial sediment (till and flowtill), collapsed 

supraglacial lake clay, silt, and sand, and outwash and ice-contact sand and 

gravel. Glacial, glaciolacustrine, glaciofluvial, and alluvial sediments likely 

were redeposited by mass movement (flow and sliding) as a result of melting 

of buried ice long after initial deglaciation. In some areas, boundaries 

between stagnation-moraine deposits and ground-moraine or end-moraine 

deposits are transitional and contacts are arbitrary. Thickness 4–20 m, locally 

50 m

ICE-CONTACT SAND AND GRAVEL—Deposited on or against glacial ice; 

sediment collapsed when ice subsequently melted. Pale-yellow, 

brownish-yellow, grayish-yellow, reddish-brown, yellowish-brown, 

olive-brown, grayish-brown, brown, olive, yellowish-gray, olive-gray, 

brownish-gray, gray, or mottled, calcareous sand and gravel and minor silt. 

Textures vary abruptly, laterally and vertically. Locally, map unit is 

dominantly boulder or cobble gravel; in some places, pebbly silt and fine 

sand. Sand and gravel commonly are interbedded with, or contain, lenses, 

inclusions, or clasts of clay, silt, till, or flowtill. Poorly to well stratified; 

poorly to moderately sorted. Irregularly bedded to well bedded; beds are 

discontinuous laterally. Faults, folds, and slump and collapse structures are 

common. In some places, gravel is cemented by iron oxides or secondary 

calcium carbonate in zones; intense iron-oxide stains on some clasts. Clasts 

are subrounded to well rounded; clast composition is similar to that of 

associated till. Typically, shale clasts are much less abundant than in 

associated till; resistant erratic clasts are more abundant. In some places, 

includes blocks (clasts) of redeposited till. Fragments of lignite (bedrock) are 

abundant in many places. Surfaces are flat to undulating or hummocky to 

knobby. In some areas deposits form isolated mounds or irregular hills. 

Commonly pitted with ice-block depressions (kettles). Boulder litters are 

present locally on the surface. Local relief is generally 3–15 m, locally >20 

m. In some places, deposits were modified by waves and currents in lakes; 

ice-contact deposits are overlain by thin, discontinuous lake clay, silt, sand, 

and gravel. Includes some outwash sand and gravel, lake clay, silt, and sand, 

inset flood-plain and channel alluvium, till, and bedrock outcrops. Locally 

overlain by till or flowtill. Thickness 1–10 m, locally >15 m

OUTWASH SAND AND GRAVEL—Sediment deposited in meltwater streams. 

Pale-yellow, brownish-yellow, grayish-yellow, reddish-brown, 

yellowish-brown, olive-brown, grayish-brown, brown, olive, yellowish-gray, 

brownish-gray, olive-gray, gray, or mottled, calcareous sand, pebbly sand, 

and gravel; locally pebbly sand and silt. Poorly to well stratified; poorly to 

well sorted. Stratification typically is (1) horizontal beds of well-sorted sand, 

(2) pebbly sand with ripple-drift, cut-and-fill, planar, or trough crossbeds, or 

(3) interbedded pebbly sand and pebble, cobble, or boulder gravel. Local 

lenses or beds of silt and clay. Clasts are subangular to very well rounded. 

Clast composition generally is similar to that in local till; locally dominantly 

shale or dominantly redeposited quartzite, argillite, chert, and chert 

conglomerate derived from older gravel. Boulders and large cobbles are 

chiefly subangular or subrounded, reworked, glacial erratics (quartzite, 

limestone, dolomite, granite, pegmatite, gneiss, schist, diorite, and other 

igneous and metamorphic rocks). Fragments of lignite (bedrock) are 

ubiquitous in many places. Clasts in some places are intensely stained by 

iron oxides or manganese oxides; gravel is locally cemented by secondary 

calcium carbonate. Surfaces are smooth, undulating, or gently rolling. Local 

relief is 0.5–2 m. In some areas, the deposits were modified by waves and 

currents in lakes, and outwash sand and gravel is overlain by lake clay, silt, 

and sand. Locally overlain by eolian silt and sand. Thickness 1–4 m, locally 

>5 m

 Some of the sand and gravel mapped as outwash deposits possibly is early 

postglacial alluvium (see map unit agu). Precipitation and runoff likely were 

greater than at present for several thousand years after initial deglaciation, 

and glacial, ice-contact, glaciofluvial, and glaciolacustrine sediments were 

eroded and redeposited as alluvial fills. The surface morphology, clast 

composition, and sedimentary structures of the deglacial outwash and early 

postglacial alluvium are similar, and the deposits have not been distinguished 

on published maps

ILLINOIAN

LOAMY OR SANDY LOAMY TILL—Sediment deposited by a Laurentide 

continental ice sheet. Markles Point till of Fullerton and Colton (1986). 

Pale-yellow, grayish-yellow, yellowish-brown, olive-brown, grayish-brown, 

brown, brownish-olive, olive, yellowish-gray, brownish-gray, olive-gray, 

gray, or mottled, calcareous clay loam, loam, or sandy loam; in some areas, 

matrix is silty clay loam, sandy clay, or sandy clay loam. Generally 

nonstratified; nonsorted or very poorly sorted. In some places, till is 

interbedded with, or contains lenses, pods, and stringers of, clay, silt, sand, or 

gravel. Commonly massive; cohesive and friable; crudely fissile. Generally 

compact or very compact; resists penetration by pick or shovel. Clayey till is 

slightly sticky to very sticky and slightly plastic to plastic when moist; hard 

and blocky when dry. Dry till in some places breaks into small, irregular 

pieces; commonly breaks through (rather than around) granules of sandstone, 

limestone, dolomite, granite, gneiss, and schist. Parting typically is crudely 

irregular or prismatic; platy structure locally. Clay minerals are dominantly 

montmorillonite. Widely spaced polygonal joints in some areas; joint 

surfaces commonly are coated by seams of secondary calcium carbonate or 

by powdery calcium carbonate or gypsum. Selenite (gypsum) crystals 1–4 

mm long, or clusters or crusts of crystals, are present on some joint surfaces. 

Stains, flecks, or scales of iron and manganese oxides are common on joint 

and parting surfaces. Fragments of lignite (bedrock) are common. Generally 

oxidized throughout where thin. Nearly pebble free to very pebbly; cobbles 

and boulders are rare to abundant. Clasts in some places are dominantly 

granules and pebbles. The surface is intensely deflated in many places, or is 

covered by a veneer of eolian sand and silt. Discontinuous lag deposits of 

granules and pebbles are common on washed or deflated till surfaces; 

isolated lag cobbles and boulders are widely scattered on till surfaces. In 

general, boulders are less abundant in till of Illinoian age than in till of late 

Wisconsin age. Large cobbles and boulders are dominantly subangular to 

well-rounded granite, pegmatite, gneiss, schist, diorite, and other igneous and 

metamorphic rocks from the Canadian Shield; erratic quartzite, limestone, 

and dolomite clasts from Canada are less abundant. Largest erratic boulders 

are >2 m in diameter. Erratic pebbles are dominantly subangular to rounded 

igneous and metamorphic rocks from the Canadian Shield and quartzite, 

limestone, and dolomite from Canada. Where till is thin and derived from 

local bedrock, pebbles are chiefly subangular to subrounded shale, siltstone, 

and soft sandstone. Where till is derived chiefly from older gravel, pebbles 

are dominantly quartzite, argillite, chert, and chert conglomerate. Drainage is 

integrated; flat, rolling, or undulating surface topography. Maximum slope 

angles in most places are <4°. Generally stable on hillslopes; till in natural 

exposures (such as stream cuts) and artificial excavations does not slump 

readily. Thin and discontinuous in some areas. Includes minor moraine 

deposits (see symbol for minor moraines). Map unit includes some 

flood-plain and channel alluvium, outwash sand and gravel, colluvium, 

sheetwash alluvium, and bedrock outcrops. Overlain by thin sheetwash 

alluvium or eolian silt and sand in many places. Thickness 0.3–3 m; 

maximum thickness 5m

 Till north of the Milk River and east of the Whitewater River, on the 

north-central part of the map, is mapped as directly overlying bedrock. 

However, the Flaxville formation and the younger Wiota gravel, both 

informal allostratigraphic units (see description of map unit xsh), underlie the 

glacial deposits on bedrock erosion surfaces in the northeast quarter of the 

quadrangle (Whitaker, 1980; Patton, 1987; Colton and others, 1989a, b). The 

gravel in many places is overlain by till deposited during two pre-Illinoian 

glaciations, and the pre-Illinoian till is overlain by till deposited during at 

least one Illinoian glaciation (see IMPORTANT STRATIGRAPHIC 

SECTIONS, #3). Where exposed, the pre-Illinoian till is included in map 

unit tks. Indurated "calcrete" till or colluvium beneath middle Pleistocene 

till (see IMPORTANT STRATIGRAPHIC SECTIONS, #3) is interpreted to 

be late Pliocene or early Pleistocene in age. A cobble of pink granite from 

the Canadian Shield was extracted from the base of the indurated till or 

colluvium. If present, glacial, glaciofluvial, and glaciolacustrine deposits of 

late Pliocene or early Pleistocene age are expected to be preserved only on 

uplands where they overlie Tertiary fluvial deposits and were not eroded. 

Surficial deposits in lowland areas were removed by erosion prior to middle 

Pleistocene glaciation.

 The upper part of the Wiota gravel on some low-altitude bedrock erosion 

surfaces north of the Milk River contains rare reworked granules, pebbles, 

and small cobbles of granite, pegmatite, gneiss, and other igneous and 

metamorphic rocks from the Canadian Shield (Jensen, 1951a; Jensen and 

Varnes, 1964). Reworked glacial erratics have not been observed in higher 

altitude deposits of Wiota gravel or in gravel of the older Flaxville formation. 

Till of pre-Illinoian age is not known to overlie the gravel that contains the 

erratic clasts. The glacial erratics apparently are products of middle 

Pleistocene glaciation, and the upper part of the Wiota gravel at those sites is 

reworked gravel. Most of the fluvial gravel that underlies map unit tks is 

nonglacial in origin and is much older than any of the tills that overlie the 

gravel

OUTWASH SAND AND GRAVEL—Sediment deposited in meltwater streams. 

Pale-yellow, brownish-yellow, reddish-brown, yellowish-brown, 

olive-brown, grayish-brown, brown, olive, or mottled, calcareous or 

noncalcareous, coarse sand, pebbly sand, and gravel. Poorly to well sorted; 

poorly to well stratified. Typically is interbedded pebbly sand and pebble, 

cobble, or boulder gravel. Lenses of silt and clay are present locally; local 

boulder beds. Clasts are subangular to very well rounded. Clast composition 

generally is similar to that of till in the same area; locally, dominantly 

redeposited quartzite, argillite, chert, and chert conglomerate derived from 

gravel of Tertiary to early Pleistocene age (see unit xsh). Boulders and large 

cobbles are dominantly subangular to well rounded, reworked glacial erratics 

(quartzite; limestone and dolomite; granite, pegmatite, gneiss, schist, diorite, 

and other igneous and metamorphic rocks). Clasts in some places are 

intensely stained by iron and manganese oxides. Mapped deposits are fills in 

melt-water channels. Distinction between outwash sand and gravel (map unit 

ggi) and colluvium, sheetwash alluvium, and glaciofluvial-fill deposits (map 

unit cad) is arbitrary in some areas because exposures on channel floors are 

lacking. Surfaces are smooth or undulating. Includes some till, inset 

flood-plain and channel alluvium, colluvium, and bedrock outcrops. 

Commonly overlain by sheetwash alluvium or eolian silt and sand. Thickness 

1–4 m, locally >6 m

PRE-QUATERNARY

BEDROCK-Includes areas of badland terrain

1

Sheetwash alluvium, for purposes of this map, is material that was transported and deposited 

by unconfined running water, chiefly sheet flow and rill wash.

2

Colluvium, for purposes of this map, is material that was transported and deposited by slow 

mass-movement processes, chiefly creep.

3

Disintegration residuum, for purposes of this map, is material that was derived primarily 

from in-place mechanical disaggregation of clastic bedrock or other materials, with no 

appreciable lateral transport. It is an earth material, not sediment or a deposit. Where 

residuum is thin, pedogenic soils may extend through the residuum to or into bedrock.

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