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QUATERNARY GEOLOGIC MAP OF THE SHELBY 1° × 2° QUADRANGLE, MONTANABy

David S. Fullerton, Roger B. Colton, and Charles A. Bush2013

U.S. Department of the InteriorU.S. Geological Survey

Prepared in cooperation with theMontana Bureau of Mines and Geology

OPEN-FILE REPORT 2012–1170

Geology compiled in 1976–86, 1990–2002, 2004–2010Edit and digital layout by Jeffery R. Hartley

Geology digitized by Charles A. BushDigital cartography by Charles A. Bush

Published by U.S.Geological Survey Science Publishing Network DenverManuscript approved for publication July 27, 2012

INDEX TO 1:250,000 TOPOGRAPHIC QUADRANGLES Showing location of the Shelby 1° x 2° quadrangle in red

SPRINGS

BILLINGS

I D A H O WYOMING

48°

WALLACE

SULPHUR

KALISPELL

LEWISTOWN47°

44°

SHELBY

CHOTEAU

44°

116°

FORSYTH

C A N A D A 49°

EKALAKA

JORDAN

WOLF POINT

106°

46°

114°

BUTTE

48°

118°

HAMILTON

112°104°

BOZEMAN

112°

MILES CITY

GLASGOW

DUBOIS

CUT BANK

DILLON

SANDPOINT

47°

SPOKANE

104°

45°

114°106°

M O N T A N A 49°

WHITE

ASHTON

ELK CITY

ROUNDUP

116°

118°

GLENDIVE

HARDIN

GREAT FALLS

45°

108°

46°

110° 108°

HAVRE

110°

CONTOUR INTERVAL 100 METERS

1955 MAGNETIC DECLINATION FROM TRUE NORTH VARIES FROM 19¾° EASTERLY FOR THE CENTER OF THE WEST EDGE TO 18½° EASTERLY FOR THE CENTER OF THE EAST

EDGE. ANNUAL CHANGE IS 0°40’ WESTERLY

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 simplified compilation of data from many sources, listed in SOURCES OF INFORMATION, at several map scales. Some of the map data are simplified and generalized. For example, small areas of pediment, alluvial-fan, and terrace deposits (map unit pgd) that were mapped separately at larger scales by others, here are combined as single polygons. Many surficial deposits that can be shown accurately as small polygons at 1:250,000 scale are omitted here because they are too small to be shown at 1:1,000,000 scale. Map units are surficial deposits and materials, not landforms. The 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.

wlaal

Holocene

latePleistocene

afj

?

?

lda

tlx

(s)

tlx

(e)

ldb

lsslca lu

?

earlyPleistocene

lse

jea

crrcad

middlePleistocene

? ?

Pliocene

tlx

QUATERNARY

TERTIARY

clycaa

pgd

gg

Pleistocene

kgke

GLACIALDEPOSITS

ICE-CONTACTDEPOSITS

GLACIOFLUVIALDEPOSITS

ALLUVIALDEPOSITS

LACUSTRINEDEPOSITS

LANDSLIDEDEPOSITS COLLUVIAL

DEPOSITSBEDROCK

LATE WISCONSIN

CORRELATION OF MAP UNITS

R

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 WISCONSINFLOOD-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 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 redeposited outwash, ice-contact deposits, and lake deposits. East and northeast of Big Sandy, alluvium overlies a thick fill of older fluvial, glaciofluvial, and glaciolacustrine deposits and till (see description of till deposits associated with “Buried bedrock valleys of the ancestral Marias and Missouri Rivers” in the Explanation of map symbols). 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 gravel. Cobbles and boulders are common to abundant in some areas. Generally crossbedded or has crude horizontal bedding or lenticular bedding; local festoon bedding. Locally fossiliferous (chiefly vertebrate remains)

ALLUVIAL-FAN DEPOSITS—Yellowish-brown, grayish-brown, yellowish-gray, brownish-gray, gray, or mottled clay, silt, sand, and gravel. Calcareous. Poorly to well stratified; poorly to well sorted. Poorly-sorted gravel and pebbly sand near fan apex, grading to pebbly silt and clay near toe. Clasts are subangular to well rounded, reworked cobbles, pebbles, and granules derived from pediment, alluvial-fan, and fluvial-terrace deposits (pgd), till, and local bedrock. Includes some sheetwash alluvium and flood-plain and channel alluvium. Mapped as an alluvial-fan complex east of the East Butte of the Sweet Grass Hills. Small alluvial-fan deposits are included in other map units. Thickness 0.5–10 m; maximum thickness >10 m

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. Clay, silty clay, silty clay loam, and clay 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 places. Fossiliferous (gastropod tests) locally. Fragments of lignite (bedrock) are common. Calcareous. Clast free, with scattered granules and small pebbles, or with stringers, pods, and lenses of granule or pebble gravel. Clayey alluvium is soft, sticky, and plastic where moist; tough, hard, and blocky where dry. Clay minerals are dominantly montmorillonite. Granules and pebbles are chiefly local clastic sedimentary rocks; minor redeposited erratic limestone, dolomite, and igneous and metamorphic rocks. Sheetwash alluvium commonly overlaps flood-plain alluvium. Included in other map units in many places in the quadrangle. Mapped only in the Missouri River valley in the southeast corner of the quadrangle and north of the Tiber Reservoir. Thickness 1–4 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 masses (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 hummocky surface topography. Thin and discontinuous in some areas. Overlapped by flood-plain alluvium adjacent to some streams. 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, sheetwash alluvium, alluvial-fan deposits, or flood-plain alluvium. Thickness 1–5 m, locally >15 m

LAKE SAND AND SILT—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 that were ice rafted or reworked from till. Present beneath lake plains and in small basins formerly occupied by glacial and postglacial lakes. Local relief 2–4 m. Includes some lake clay and silt, flood-plain and channel alluvium, and till. In many areas, overlies lake clay and silt. Thickness 1–8 m, locally >10 m

LAKE CLAY, SILT, SAND, AND GRAVEL—Undivided deposit of lake clay, silt, sand, and fine gravel. Overlain by eolian sand and silt in some places. Includes inset and overlapping flood-plain and channel alluvium. Descriptions of lake deposits in map units lca and lss apply. Thickness 1–3 m, locally >10 m

LAKE AND EOLIAN SAND AND SILT—Complex map unit of lake sand and silt and eolian sand and silt. Lake deposits in many places were reworked by wind and are overlain by eolian deposits

Eolian sand and silt—Yellow, yellowish-brown, olive-brown, grayish-brown, brown, brownish-gray, gray, or mottled windblown sand and silt in blanket-like deposits; local low dunes. Calcareous. Well stratified; poorly sorted to well sorted. Typically loose, homogeneous, massive, fine sand and silt with faint beds or cross-beds. Very friable; weakly compact; slightly hard; subangular blocky structure; not sticky or plastic when moist. Generally clast free. Contains discontinuous accumulations of secondary calcium carbonate in some places; stained by iron oxides locally. Typically oxidized throughout. In some places, contains root casts and weakly developed buried soils (humic horizons). Mostly stable, with grass cover; local blowouts. Undulating or rolling topography. Thickness 0.5–3 m; maximum 6 m

Lake sand and silt—Similar to deposits in map unit lss

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. In some places, lake deposits are overlain by eolian sand and silt. Locally, glaciofluvial gravel and sand deposited by meltwater streams underlies the lake and alluvial deposits. 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. Channel alluvium is inset into other deposits; flood-plain deposits overlap some lake deposits. Sheetwash alluvium locally overlaps other deposits on valley sides; in some places it mantles much of the fill on the valley floor. 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 map unit caa. Thickness 0.5–8 mSheetwash alluvium—Similar to map unit wla. Thickness 0.5–4 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; sheet-wash-eroded or stream-eroded till or bedrock dissected by gullies; extensive areas of bedrock outcrop

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 blocks, 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 drift, typically it is boulders, cobbles, pebbles, and granules of erratic 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 map unit wlaLandslide deposits—Similar to map unit jea

HOLOCENE, LATE PLEISTOCENE, AND MIDDLE PLEISTOCENELOAMY COLLUVIUM2—Ownish-orange, yellowish-brown, orange-brown,

olive-brown, grayish-brown, brown, brownish-yellow, yellowish-gray, olive-gray, brownish-gray, gray, or mottled loam and sandy loam in the Sweet Grass Hills. Calcareous or noncalcareous. Nonstratified or weakly stratified; nonsorted or poorly sorted. Generally pebbly or cobbly. In most places, clasts are angular and subangular local bedrock: chiefly intrusive igneous rocks (syenite, syenite porphyry, and minor trachyte porphyry, diorite porphyry, and monzonite porphyry) and sedimentary rocks (sandstone, mudstone, shale, and minor limestone). Glacial erratics from Canada are present in some places on the margins of the Sweet Grass Hills. Includes some landslide deposits, flood-plain and channel alluvium, pediment, alluvial-fan and alluvial-terrace deposits, and extensive bedrock outcrops. Thickness 0.5–>6 m; maximum >15 m

FROST-RUBBLE AND TALUS-RUBBLE DEPOSITS—Blocks, slabs, flagstones, channers, boulders, and cobbles produced primarily by frost riving of trachyte porphyry, syenite porphyry, and syenite, and mass-movement of frost rubble. Most of the rubble is interpreted to have been formed in a periglacial environment when the Sweet Grass Hills were surrounded by glacial ice. Rubble is nonstratified and nonsorted; little or no matrix. Includes landslide deposits, rock-fall and rock-slide deposits, talus-flow and debris-flow deposits, colluvium, and bedrock outcrops. Thickness generally >1 m; locally >10 m

HOLOCENE AND PLEISTOCENESLUMP-BLOCK LANDSLIDE DEPOSITS AND ROCKSLIDE DEPOSITS—

Mapped only in the Sweet Grass Hills. Most of the individual landslide deposits elsewhere 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. Clasts in rockslide deposits are granules to slabs and blocks; some blocks are >3 m in diameter; some deposits are jumbled blocks. Most of the mapped deposits are stabilized by vegetation. Includes colluvium, frost-rubble and talus-rubble deposits, sheetwash alluvium, and bedrock outcrops. Thickness 3–5 m, locally >10m

LATE WISCONSINLOAMY TILL—Sediment deposited chiefly by ice of a Laurentide continental ice

sheet. Informal Fort Assiniboine till of Fullerton and Colton (1986). The till in the northwestern part of the quadrangle was deposited by the Shelby glacial lobe; the till in the northeastern part of the quadrangle was deposited by the Havre glacial lobe; the till in the southeastern part of the quadrangle was deposited by the Loma glacial sublobe (fig. 1). Regional ice flow patterns are recorded by ice-molded or ice-scoured landforms (shown by a symbol) and by the orientations of till ridges and meltwater channels (also shown by symbols). 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. Largest erratic boulders are >2 m in diameter. Clasts of older till and clasts 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 limestone and dolomite 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 from Tertiary and Quaternary gravel, pebbles are chiefly quartzite, argillite, chert, and chert conglomerate from the mountains to the west or volcanic rocks from the south. 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 erratic limestone and dolomite from Saskatchewan and Manitoba, and quartzite from the Rocky Mountains. Till everywhere is generally unstable; it moves by creep on hillslopes; slumps in exposures. Till is thin and discontinuous in many areas; 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 and 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 lake delta deposits, 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 small end moraine deposits. Maximum slope angles are generally less than 4°. Local relief is generally <3 m. Thickness 0.5–2 m; maximum thickness >6 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, long belts of hummocky or undulating ridges or (2) belts of narrow, discontinuous, sharply defined ridges that delineate the configurations of vanished ice margins. Large areas mapped between the Sweet Grass Hills are complexes of till ridges. Shallow closed depressions and lag cobbles and boulders are common on the surface in some areas. Local relief in some places is 4–>10 m. Till end-moraine deposits are locally replaced laterally by kame-moraine deposits that are included in the map unit. In some places, 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; in some places 40 m. In some places, deposits include 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 were redeposited by mass movement (flow and sliding) as a result of melting of buried ice long after initial deglaciation. In some places, boundaries between stagnation-moraine deposits and ground-moraine or end-moraine deposits are transitional and contacts are arbitrary. Thickness 4–20 m, locally 40 m

KAME-MORAINE DEPOSITS—Ice-contact gravel, sand, and minor silt similar to deposits in map unit kg. Description of map unit kg applies. Comprises segments of end moraines; gently to steeply sloping ridges or aligned hummocks and hills (kames) of sand and gravel. Relief is generally 4–10 m. Kame moraine deposits in some places are replaced laterally by till end-moraine deposits that are included in the map unit. Thickness 4–l5 m, locally >20 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. Gravel locally 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 ubiquitous 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 shallow 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; the ice-contact deposits are overlain by thin, discontinuous, lake clay, silt, sand, and gravel. Distinguished from kame-moraine deposits (map unit ke) arbitrarily in some areas. Includes some kame-moraine deposits, kame-delta deposits, and lake-delta deposits. Also 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; in some areas overlain by eolian silt and sand. Thickness 1–10 m, locally >15 m

OUTWASH SAND AND GRAVEL—Sediment deposited in subaerial 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 (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; local boulder beds; local blocks (clasts) of till or masses of flowtill. Clasts are subangular to very well rounded; diameter of largest clasts generally decreases in downstream direction in valley train deposits. 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 (limestone, dolomite, granite, pegmatite, gneiss, schist, diorite, and other igneous and metamorphic rocks). Fragments of lignite (bedrock) are ubiquitous in many areas. 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 1–5 m. Includes some till, ice-contact sand and gravel, lake-delta deposits, inset flood-plain and channel alluvium, and bedrock outcrops. In some areas, 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 >10 m

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

LAKE DENSITY-CURRENT UNDERFLOW-FAN DEPOSITS—Pale-yellow, grayish-yellow, yellowish-brown, brown, brownish-gray, olive-gray, gray, or mottled, calcareous silt, sand, and gravel. Well stratified; generally well sorted. Chiefly laminated silt and very fine sand interbedded with thin layers of clay or fine to coarse sand. Average grain size decreases from top to bottom. Foreset beds are absent; basal sediments commonly are lake clay and silt, but they are not delta bottomset beds. Silt and sand are crossbedded or horizontally bedded; ripple-drift lamination is common. Clasts are mixed rocks derived from surficial deposits and bedrock that were eroded, transported, and deposited by catastrophic-flood drainage into a glacial lake. Mapped at several localities east of Conrad. Thickness 3–>10 m

DELTA SAND AND GRAVEL—Brownish-yellow, yellowish-brown, olive-brown, grayish-brown, brown, olive, brownish-gray, olive-gray, gray, or mottled, calcareous clay, silt, sand, and gravel. Moderately or well stratified; moderately or well sorted. Bottomset beds generally are absent. Foreset beds are ripple-laminated or planar-bedded, coarse to fine sand and silt and well-sorted sand and gravel. Topset beds are (1) well-sorted coarse sand and gravel, (2) moderately-well sorted gravelly sand, or (3) planar-bedded, trough-crossbedded, or ripple-laminated fine or medium sand. Grain size of sediment in topset beds varies abruptly, laterally and vertically. Cobbles and boulders are common. Clasts are chiefly subangular; clast composition is chiefly local bedrock because the delta formed at the mouth of a lake spillway channel that was deeply incised into bedrock. Mapped only in the southwest corner of the quadrangle. Some small delta deposits are included in lake silt and sand (lss) and undivided lake deposits (lu). Thickness 3–10 m

PLEISTOCENE AND PLIOCENEPEDIMENT, ALLUVIAL-FAN, AND FLUVIAL-TERRACE DEPOSITS— White,

pink, reddish-yellow, yellow, yellowish-brown, reddish-brown, orange-brown, olive-brown, brown, brownish-yellow, yellowish-gray, olive-gray, brownish-gray, gray, or mottled gravel, sand, and silt. Calcareous or non-calcareous. Poorly to moderately well stratified; poorly to moderately well sorted. Boulders, cobbles, pebbles, and granules in a sand and silt matrix. Clasts in pediment and alluvial-fan deposits are chiefly subangular; clasts in fluvial-terrace deposits are subangular, subrounded, or rounded. Clast lithologies are chiefly syenite, syenite porphyry, trachyte porphyry, diorite porphyry, monzonite porphyry, sandstone, and shale. Fluvial deposits are present beneath multiple terrace surfaces on the Sweet Grass Hills; at least three surfaces are older than the late Wisconsin till. Till deposited during at least two pre-Wisconsin glaciations overlies buried pediment, alluvial fan, and terrace deposits in the subsurface; however, only the late Wisconsin surface till is mapped. Includes some colluvium, sheetwash alluvium, eolian silt and sand, and bedrock outcrops. Thickness generally 1–4 m, locally >10 m

PRE-QUATERNARYBEDROCK

1Sheetwash alluvium, for purposes of this map, is material that was transported and deposited by unconfined running water, chiefly sheet flow and rill wash.

2Colluvium, for purposes of this map, is material that was transported and deposited by slow mass-movement processes, chiefly creep.

SOURCES OF INFORMATIONAlden, W.C., 1924, Physiographic development of the northern Great Plains: Geological

Society of America Bulletin, v. 35, p. 385–423.Alden, W.C., 1932, Physiography and glacial geology of eastern Montana and adjacent areas:

U.S. Geological Survey Professional Paper 174, 133 p.Calhoun, F.H.H., 1906, The Montana lobe of the Keewatin ice sheet: U.S. Geological Survey

Professional Paper 50, 62 p.Christiansen, E.A., 1979, The Wisconsinan deglaciation of southern Saskatchewan and adjacent

areas: Canadian Journal of Earth Sciences, v. 16, p. 913–938.Clayton, Lee, and Moran, S.R., 1982, Chronology of late Wisconsinan glaciation in middle

North America: Quaternary Science Reviews, v. 1, p. 55–82.Colton, R.B., Lemke, R.W., and Lindvall, R.M., 1961, Glacial map of Montana east of the

Rocky Mountains: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-327, scale 1:500,000.

De Young, William, Youngs, F.O., and Glassey, T.W., 1932, Soil survey of the Milk River area, Montana: U.S. Department of Agriculture, Bureau of Chemistry and Soils, Series 1928, no. 22, 35 p.

Dyke, A.S., 2004, An outline of North American deglaciation with emphasis on central and northern Canada, in Ehlers, Juergen and Gibbard, P.L., eds., Quaternary glaciations—Extent and chronology, Part 2, North America: Amsterdam, Elsevier, p. 373–430.

Fox, P.P., 1946, Buried Pleistocene gorge of the Marias River, Montana [abs.]: Geological Society of America Bulletin, v. 57, no. 12, p. 1194.

Fullerton, D.S., Christiansen, E.A., Schreiner, B.T., Colton, R.B., and Clayton, Lee, 2007, Quaternary geologic map of the Regina 4° × 6° quadrangle, United States and Canada: U.S. Geological Survey Miscellaneous Investigations Series Map I–1420 (NM–13), 37 p., map scale 1:1,000,000.

Fullerton, D.S., and Colton, R.B., 1986, Stratigraphy and correlation of the glacial deposits on the Montana Plains, in Richmond, G.M., and Fullerton, D.S., eds., Quaternary glaciations in the United States of America: Quaternary Science Reviews, v. 5, p. 69–82.

Fullerton, D.S., Colton, R.B., and Bush, C.A., 2004, Limits of mountain and continental glaciations east of the Continental Divide in northern Montana and north-western North Dakota, U.S.A., in Ehlers, Juergen, and Gibbard, P.L., eds., Quaternary glaciations—Extent and chronology; Part 2, North America: Amsterdam, Elsevier, p. 131–150.

Fullerton, D.S., Colton, B.B., and Bush, C.A., 2011, Quaternary geologic map of the Havre 1° x 2° quadrangle, Montana: U.S. Geological Survey Open-file Report 2011–1028, map scale 1:250,000.

Fullerton, D.S., Colton, R.B., Bush, C.A., and Straub, A.W., 2004, Map showing spatial and temporal relations of mountain and continental glaciations on the Northern Plains, primarily in Montana and northwestern North Dakota: U.S. Geological Survey Scientific Investigations Map SIM–2843, 36 p., map scale 1:1,000,000.

Giesecker, L.F., Morris, E.R., Strahorn, A.T., and Manifold, C.B., 1933, Soil survey (reconnaissance) of the Northern Plains of Montana: U.S. Department of Agriculture, Bureau of Chemistry and Soils, ser. 1929, no. 21, 74 p.

Hill, C.L., 2002, Late Quaternary stratigraphy and radiocarbon chronology from the lower Marias River Valley, northern Montana: American Quaternary Association (AMQUA), 17th biennial meeting, Anchorage, Alaska, Program and Abstracts, p. 68.

Horberg, Leland, and Robie, R.A., 1955, Postglacial volcanic ash in the Rocky Mountain piedmont, Montana and Alberta: Geological Society of America Bulletin, v. 66, p. 949–955.

Kemp, J.F., and Billingsley, Paul, 1921, Sweet Grass Hills, Montana: Geological Society of America Bulletin, v. 32, p. 437–478.

Lemke, R.W., Laird, W.M., Tipton, M.J., and Lindvall, R.M., 1965, Quaternary geology of the northern Great Plains in Wright, H.E., Jr., and Frey, D.G., eds., The Quaternary of the United States: Princeton, N.J., Princeton University Press, p. 15–27.

Lemke, R.W., Mudge, M.R., Wilcox, R.E., and Powers, H.A., 1975, Geologic setting of the Glacier Peak and Mazama ash-bed markers in west-central Montana: U.S. Geological Survey Bulletin 1395-H, p. H1–H31.

Lindvall, R.M., 1956a, Geology of the Kenilworth quadrangle, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map I–129, scale 1:62,500.

Lindvall, R.M., 1956b, Geology of the Big Sandy quadrangle, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map I–130, scale 1:62,500.

Lindvall, R.M., 196l, Geology of the Boxelder quadrangle, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map I–338, scale 1:62,500.

Lopez, D.A., 1994, Preliminary geologic map of the Shelby 30' x 60' quadrangle: Montana Bureau of Mines and Geology Open-file Report MBMG 315, scale 1:100,000.

Lopez, D.A., 1995, Geology of the Sweet Grass Hills, north-central Montana: Montana Bureau of Mines and Geology Memoir 68, 35 p.

Lopez, D.A., 2002a, Geologic map of the Sweet Grass Hills 30’ x 60’ quadrangle, north-central Montana: Montana Bureau of Mines and Geology Open-file Report MBMG 443, 17 p., map scale 1:100,000.

Lopez, D.A., 2000b, Geologic map of the Conrad 30’ x 60’ quadrangle, north-central Montana: Montana Bureau of Mines and Geology Open-file Report MBMG 444, 10 p., map scale 1:100,000.

Lopez, D.A., 2000c, Geologic map of the Chester 30’ x 60’ quadrangle, north-central Montana: Montana Bureau of Mines and Geology Open-file Report MBMG 445, 10 p., map scale 1:100,000.

Lopez, D.A., 2002, Geologic map of the Lonesome Lake 30’ x 60’ quadrangle, north-central Montana: Montana Bureau of Mines and Geology Open-file Report MBMG 446, 12 p., map scale 1:100,000.

Mickelson, D.M., Clayton, Lee, Fullerton, D.S., and Borns, H.W., 1983, The late Wisconsin glacial record of the Laurentide ice sheet in the United States, in Porter, S.C., ed., Late-Quaternary environments of the United States, volume 1, the late Pleistocene: Minneapolis, University of Minnesota Press, p. 3–37.

Montagne, Clifford, Munn, L.C., Nielsen, G.A., Rogers, J.W., and Hunter, H.E., 1982, Soils of Montana: Montana Agricultural Experiment Station Bulletin 744, 95 p.

Simonson, G.H., Rogers, J.W., Brownfield, S.H., Richardson, R.E., and Nielsen, G.A., 1978, General soil map of Montana: Montana Agricultural Experiment Station Miscellaneous Publication 16, scale 1:1,000,000.

Smith, J.F., Jr., Witkind, I.J., and Trimble, D.E., 1959, Geology of the lower Marias River area, Choteau, Hill, and Liberty Counties, Montana: U.S. Geological Survey Bulletin 1071-E, p. 121–155.

Soller, D.R., 1993, Preliminary map showing the thickness and character of Quaternary deposits in the United States east of the Rocky Mountains: U.S. Geological Survey Open-File Report 93-543, scale 1:3,500,000.

Soller, D.R., 1994, Map showing the thickness and character of Quaternary sediments in the glaciated United States east of the Rocky Mountains-Northern Plains states (west of 102° West Longitude): U.S. Geological Survey Miscellaneous Investigations Series Map I-1970-D, scale 1:1,000,000.

Swenson, F.A., 1957, Geology and ground-water resources of the Lower Marias Irrigation Project, Montana [with a section on chemical quality of the ground water by H.A. Swenson]: U.S. Geological Survey Water-Supply Paper 1460-B, p. 41–98.

Tuck, L.K., 1993, Reconnaissance of geology and water resources along the north flank of the Sweet Grass Hills, north-central Montana: U.S. Geological Survey Water-Resources Investigation Report 93-4026, 68 p.

U.S. Department of Agriculture, unpublished soils maps of Toole, Liberty, Hill, Blaine, Pondera, Choteau, and Phillips Counties, Montana, provided by Robert Lund, Montana Geospatial Center, Bozeman, Montana.

Veseth, Roger, and Montagne, Clifford, 1980, Geologic parent materials of Montana soils: Montana Agricultural Experiment Station Bulletin 721, 117 p.

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SCALE 1: 250 000

5 0 5 10 15 MILES

5 0 5 10 15 20 KILOMETERS

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LIST OF MAP UNITS

HOLOCENE AND LATE WISCONSIN

EXPLANATION OF MAP SYMBOLS

FLOOD-PLAIN AND CHANNEL ALLUVIUM

ALLUVIAL-FAN DEPOSITS

LOAMY AND CLAYEY SHEETWASH ALLUVIUM

LAKE CLAY AND SILT

LAKE SILT AND SAND

LAKE CLAY, SILT, SAND, AND GRAVEL

LAKE AND EOLIAN SAND AND SILT

COLLUVIUM, SHEETWASH ALLUVIUM, LAKE DEPOSITS, AND GLACIOFLUVIAL-FILL DEPOSITSCOLLUVIUM, SHEETWASH ALLUVIUM, AND LANDSLIDE DEPOSITS

HOLOCENE, LATE PLEISTOCENE, AND MIDDLE PLEISTOCENELOAMY COLLUVIUM

FROST-RUBBLE AND TALUS-RUBBLE DEPOSITS

HOLOCENE AND PLEISTOCENESLUMP-BLOCK LANDSLIDE DEPOSITS AND ROCKSLIDE DEPOSITS

LATE WISCONSINLOAMY TILL

Ground-moraine deposits

End-moraine deposits

Stagnation-moraine deposits

KAME-MORAINE DEPOSITS

ICE-CONTACT SAND AND GRAVEL

OUTWASH SAND AND GRAVEL

LAKE DENSITY-CURRENT UNDERFLOW-FAN DEPOSITS

DELTA SAND AND GRAVEL

PLEISTOCENE AND PLIOCENEPEDIMENT, ALLUVIAL-FAN AND FLUVIAL-TERRACE DEPOSITS

PRE-QUATERNARYBEDROCK

Known or suspected glaciotectonic deposits or glaciotectonic terrain—Bedrock and surficial deposits that were thrust, stacked, and deformed by glacial ice. Chiefly (1) tilted bedrock blocks overlying till and stratified sediments; (2) stacked or imbricated slices of bedrock, till, and stratified sediments that form parallel or concentric ridges; or (3) deformed masses of bedrock and surficial deposits characterized by overturned folds. Some glaciotectonically transported materials are covered by till that was deposited by overriding ice. Thickness 5–>30 m

Contact

Shoreline

Limit of late Wisconsin glaciation on the Sweet Grass Hills

Limit of Illinoian glaciation on the Sweet Grass Hills

Limit of a late Wisconsin glacial readvance, or position of a significant stillstand of an ice margin—Ticks are on the side of the advance

Inferred limit of a glacial readvance marked by the distal margin of glaciotectonic deposits or glaciotectonic structures

Marginal end moraine—Single till ridge formed along the margin of an ice lobe

Interlobate moraine—Broad till ridge formed at the junction of two ice lobes south of East Butte of the Sweet Grass Hills

Minor moraines—Mostly till. Most of the ridges delineate the configurations of vanished ice margins. Local relief of ridges typically is 2–10 m. Includes "annual moraines," "washboard moraines," "small moraines," "corrugated moraines," and "transverse ridges"

Ice-margin alignment interpreted from ice-margin drainage channels and till ridges

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

Esker—Subglacial or englacial stream deposit

Buried bedrock valleys of the ancestral Marias and Missouri Rivers— A middle Pleistocene fill deposit is present in the subsurface in the buried valleys. It is not well exposed in the Shelby quadrangle, but it is well exposed in the adjacent Havre quadrangle in the vicinity of Havre (Fullerton and others, 2012). The alluvial and lacustrine fill deposit commonly includes, from younger to older: glacial lake sediment of Illinoian age; nonglacial overbank and channel alluvium and pond or lake deposits of pre-Illinoian age; and glacial lake deposits of pre-Illinoian age. The fill deposits are overlain by eolian deposits, channel and flood-plain alluvium, sheetwash alluvium, lake deposits, and till units of late Wisconsin and Illinoian age (informal Fort Assiniboine and Herron Park tills, respectively, of Fullerton and Colton, 1986). The fill deposits overlie till of pre-Illinoian age (upper unit of informal Havre till of Fullerton and Colton, 1986); lake and fluvial deposits; a paleosol in the lower unit of Havre till; glacial lake deposits; and nonglacial fluvial gravel from the mountains to the south and west. The basal nonglacial gravel beneath the fill deposits contains Irvingtonian vertebrate fossils; the fill contains rare vertebrate (ungulate) fossils

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GoldButte

SWEET GRASS HILLS

EastButte

WestButte

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsementby the U.S. Government.

Although this information product, for the most part, is in the public domain, it also contains copyrightedmaterials as noted in the text. Permission to reproduce copyrighted items must be secured from thecopyright owner.

This database, identified as OFR 2012–1170, has been approved for release and publication by the U.S.Geological Survey (USGS). Although this database has been subjected to rigorous review and issubstantially complete, the USGS reserves the right to revise the data pursuant to further analysis andreview. Furthermore, it is released on condition that neither the USGS nor the U.S. Government may beheld liable for any damages resulting from its authorized or unauthorized use.

Publishing support provided by: Denver Publishing Service CenterManuscript approved for publication July 27, 2012

For more information concerning this publication, contact:Center Director, USGS Geology and Environmental Change Science CenterBox 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/2012/1170

Suggested Citation: Fullerton, D.S., Colton, R.B., and Bush, C.A., 2013, Quaternary geologic map of theShelby 1˚ x 2˚ quadrangle, Montana: U.S. Geological Survey Open-File Report 2012–1170, scale 1:250,000 [http://pubs.usgs.gov/of/2012/1170].

Base hydrology from U.S. Geological Survey, 1955Base topography from National Elevation Dataset (NED), U.S. Geological Survey 1974 (http://seamless.usgs.gov)Base cities, roads, and railroads from Montana State Library 2011 (http://nris.mt.gov)

TRANSVERSE MERCATOR PROJECTION1927 North American Datum

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Shelby

Virden Lake

Kevin

Sunburst

Trail

Cree

k

Sheep Cree

k

Hingham

Gildford

Rudyard

Box Elder

Big Sandy

Chester

Galata Lothair

Joplin

ReservoirTiber

Dunkirk

Dry Fork Willow Creek

Brady

Conrad

Willow Creek

Aloe Lake

Marias River

Dry F

ork M

aria

s River

Prichard Creek Bear Creek

Deer Cree

k

Strawberr

y Creek

Corral Creek

Governm

ent CreekHorse Creek

Eagle Creek

Laird Creek

Sage Creek

Little Sage Creek

Milk River

Frank Jurenka’sReservoir

Wild Horse Lake

Wild Horse Lake

Fresno Reservoir

Chain ofLakes

Howard Sailors Reservoir

LonesomeLake

Duck Creek

Four

teen

Mile

Cre

ek

Big Sandy C reek

MISSOURI RIVER

Skit Creek

Lees Lake

Mari as Ri ver

Cottonwood Creek

Eagle Creek

Little Sage Creek

SHELBY LOBE

WHITEWATER LOBE

LOMA SUBLOBE

YELLOWSTONE LOBE

GLASGOW ORMISSOURI VALLEY LOBE

PincherBrocket

Fort Macleod

MonarchKipp

LethbridgeTaber

Drywood

Waterton ParkMountain View

Cardston

Kimball

RaleySpring Coulee

MagrathRaymond

Del BonitaTwin River

Milk River

Bow Island

Robinson

Foremost Etzikom Pakowki

Pinhorn

Comrey

KincorthMaple Creek

Battle Creek

Willow Creek

RangeviewDivide

Eastend

Ravenscrag

Climax

Shaunavon

Neville

Kincaid

Val Marie

OrkneyRosefield

RelianceSummercove

Lafleche

Ettington

Killdeer

Weyburn

Ratcliffe

Ardill Galilee

Crane Valley

Dummer Yellow Grass

Twin Valley

ViceroyBengough

Hardy Radville

Coronach Big Beaver

Big Muddy

Lake Alma

Osage

Stoughton

Torquay Outram EstevanFrobisher

CrosbyFortuna

Bowbells

Alamo

LostwoodWhite Earth

Ray

Williston

Grenora

Flaxville PlentywoodScobeyPeerless

Medicine Lake

CulbertsonBrocktonPoplar

Wolf PointFrazer

Whitewater

HinsdaleBowdoin

Malta

Glasgow

Fort Peck

Turner

Chinook

Havre Harlem

DodsonBox Elder

Rocky Boy

Lodge Pole

Sunburst

Shelby Chester

Conrad Big Sandy

Babb

Cut BankBrowningWest Glacier

East Glacier Park

Valier

Pendroy

Hall Coulee

Marias Pass

Bynum

Choteau

Augusta

Fort Benton

Power

Portage

Great Falls

Cascade

EagletonLandusky

Iliad

Denton Roy Valentine

Lewistown

Sun Prairie

Jordan

Sidney

Richey

Circle

Intake

Glendive

Keene

Cartwright Watford City

Grassy Butte Dunn Center

Golden Valley

Opheim

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 bouldersPre-Illinoian till on north margin of Bearpaw MountainsScattered glacial erratic boulders, no till

Glacial lakes Great Falls and Musselshell. Full extent of lakes not shownNot glaciated

LAURENTIDE CONTINENTAL GLACIATIONLate 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-WisconsinLimit of pre-Wisconsin till and glacial erratic boulders (limit of PW). Till is not present as far south as the limit of boulders

Pre-Illinoian PleistoceneSouthern 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 Shelby quadrangle

Generalized direction of ice flow

River, stream, and modern lake shoreline

PT

LW

IL

PW

PI

GBIL

Judi

th R

iver

LW

IL

IL

Missouri

River

IL

LW

Teton River

PW

Marias River

FresnoReservoir

PI

Milk River

PT

PT

Peerless Plateau

Poplar River

PTRocky Mountains

LW

Cut Bank Creek

EXPLANATION

LakeFrances

Tiber Reservoir

Fort PeckReservoir

GlacierNationalPark

HighwoodMountains

BearpawMountains

Little Rocky Mts

Cypress Hills

Figure 1. Regional ice flow during maximum late Wisconsin glaciation.

Willow Bunch Lake

Little Missouri River

Weyburn

Musselshell River

102°

Saint

Mar

y Rive

r

KilldeerMountains

BoundaryPlateau

Oldman River

104°106°

Wate

rton River

108°110°112°

Oldman River

114°

Milk River

50°

49°

48° Williston

47°

ShelbyGlasgow

Cut Bank

GlendiveChoteau

Two Medicine lobe

Wood Mountain

Lewistown

Late Wisconsin phase ofglacial Lake Musselshell(792 m; 2,600 ft) confluentwith glacial Lake Jordan

Jordan

Late Wisconsinphase of glacialLake Jordan(792 m; 2,600 ft)

Foremost

Watford CityGreat Falls

Lethbridge

Late Wisconsin phaseof glacial Lake Great Falls(1142 m; 3,715 ft)

C2

Cypress Lake

ALBE

RTA

Sweet G rass Hills

Wolf Point

CANADASASK

ATCH

EWAN

C2

Havre

UNITED STATES

Wood Mountain Upland

IL

LW

MON

TAN

A

MISSOURI COTEAU

ILLW

NOR

TH D

AKOT

A

NORTH DAKOTA

IL

IL SASKATCHEWAN

IL

LW

GB

Yello

wsto

ne R

iver

ALBERTA

IL

Missouri River

Flaxville Plain

IL

LW

IL

Missouri River LW

25 25 50 75 100 KILOMETERS0

25 25 50 75 100 MILES0

Lake Sakakawea

MALTA SUBLOBE

HAVRE LOBE

Waterton LakesNational Park

Porc

upin

eHi

lls

Belly

Riv

er

MONTANA