REGIONAL GLACIATION OF
NORTHERN GREAT PLAINS
James S. Aber
|Digital elevation model for northern Great Plains, including parts of North
and South Dakota, Minnesota, and the southern edge of Canada. The Missouri
Coteau, Prairie Coteau and Turtle Mountains are prominent uplands that guided
ice-lobe movement along the intervening lowlands.
DEM derived from TOPO30 database; image processing by J.S. Aber.|
|Badlands exposure of Upper Cretaceous Bearpaw Shale containing large sandstone concretions, near Manyberries, southeastern Alberta. Upper Cretaceous shale and sandstone underlie vast areas of the northern Great Plains in Alberta, Saskatchewan, Montana and the Dakotas. Photo date 8/84, © J.S. Aber|
|Upland Tertiary gravel at Jones Peak, southern Saskatchewan. Tertiary gravel derived from the Rocky Mountains is preserved on top of many plateaus and hills of the Great Plains region. Photo date 5/93, © J.S. Aber|
|Northern margin of the Prairie Coteau, a prominent glacial upland in eastern South Dakota and southwestern Minnesota. View toward the northeast, near Summit, South Dakota. Morainic deposits in the foreground cap the Coteau, which rises 100s of meters above the Red/Minnesota lowland visible on the horizon. Photo date 6/96, © J.S. Aber|
Glacial landforms are especially well preserved and displayed in North and South
Dakota, as a result of relatively arid climate and grassland vegetation.
These glacial landforms are arrayed in patterns that reflect the lobate
pattern of glaciation, which was in turn determined by bedrock topography
and configuration of ice sheet--see Fig. 16-3. Major elements of
the landscape include:
|Narrow spillway channel cut through Sioux Quartzite bedrock at Palisades,
South Dakota. Photo date 7/96, © J.S. Aber.|
Kite aerial photographs of the Palisades--see KAP.
The Devils Lake vicinity in northeastern North Dakota displays a remarkable assemblage of glacial landforms--see Fig. 16-4. Devils Lake occupies several connected depressions that were formed by glacier pushing of bedrock and sediment. Sullys Hill, Crow Hill, and other hills immediately south of the lake are built of material thrust out of the Devils Lake depressions. The ice-scooped depressions and ice-shoved hills are among the largest and best-developed glacial landforms of this type in the United States (Aber, Bluemle et al. 1993).
|Satellite image of Devils Lake vicinity, North Dakota. Landsat MSS false-color composite of visible (bands 1 and 2) and near-infrared (band 4). Active vegetation appears red, pink and red-brown in this autumn scene. Sullys Hill is part of an ice-shoved ridge complex south of the ice-scooped basin of Devils Lake. Landsat multispectral scanner (MSS) data, 23 Sept. 88; image processing by J.S. Aber.|
Sullys Hill, immediately south of Devils Lake Main Bay, is a focal point for glacial geomorphology of the region. This prominent hill is comprised of brecciated shale and deformed glacial sediment scooped from Main Bay. Two series of ice-shoved hills and associated source depressions lead away from Sullys Hill, one to the southwest, and the other to the southeast. These two trends mark the margins of two ice lobes that converged at Sullys Hill. Spillway channels drain away from the hills to the south, and tunnel valleys lead into Main Bay from the north. Glacier thrusting of the hills was associated with large-volume release of water from beneath the ice and from the Spiritwood aquifer. Devils Heart Butte is a conical mound of sand and gravel deposited by a hydrodyanmic blowout during the glaciotectonic thrusting (Bluemle 1993).
|Crow Hill, south of Devils Lake, northeastern North Dakota. Crow Hill is composed of ice-shoved bedrock (shale) and sediment derived from the West Bay basin of Devils Lake. Such conspicuous hills are characteristic of ice-margin positions. Photo date 6/92, © J.S. Aber.|
|View from west with Sullys Hill on horizon to right (south) and part of Devils Lake visible to left (north). Sullys Hill rises about 200 m above the floor of Devils Lake. The forested hill is composed of bedrock (shale) and sediment thrust out of the lake basin. Photo date 7/91, © J.S. Aber.|
|View from top of Sullys Hill looking northward across Devils Lake (Main Bay). The basin and hill represent a very large and well formed hill-hole pair, which was created by ice thrusting over the Spiritwood aquifer that runs beneath the lake basin. Photo date 7/91, © J.S. Aber.|
|Big Coulee is the broad, shallow depression in the middle distance (crossed by road). This channel was eroded as a melt-water spillway draining away from the ice margin at Sullys Hill (on left horizon). Photo date 6/92, © J.S. Aber.|
|The conical hill in scene center is Devils Heart Butte. This hill of sand and gravel was deposited by a hydrodynamic outburst of water from under the ice margin and from the Spiritwood aquifer at the time of ice thrusting at Devils Lake. The hill stands about 50 m high and has a base 240 m across (Bluemle 1993). Photo date 6/92, © J.S. Aber.|
Exceptionally large ice-pushed ridges are developed along the Missouri Coteau of southern Saskatchewan and eastern Alberta--see Fig. 16-5. The Dirt Hills and Cactus Hills are among the best developed ice-pushed ridges in the world (Aber 1993a). They are located on the Missouri Coteau upland southwest of Regina in southern Saskatchewan. The highest elevations of the Dirt Hills exceed 2880 feet (880 m), more than 1000 feet (300 m) above the Regina Lake Plain immediately to the north, and 400 feet (120 m) above the Missouri Coteau upland to the south.
|False-color Landsat MSS image of southern Saskatchewan, 21 May 78. The dark green region toward upper right is part of the Regina Lake lowland. Lighter yellow and pink region to lower left is the Missouri Coteau upland. Adapted from EROS Data Center.|
The Dirt Hills form a huge loop, stretching from Claybank to Galilee, comprised of multiple parallel ridges and intervening narrow valleys--see Figs. 16-6 and 16-7. The Cactus Hills form part of another great loop that includes hills to the west at Crestwynd. These prominent ridges are cored by upthrust or folded Upper Cretaceous bedrock. A mantle of glacial sediment covers most of the ridges, except in the southern Dirt Hills.
Remarkable agreement exists between orientations of deformed structures, trends of individual ridges, and the overall morphology of the Dirt Hills. Ice-shoved ridges on the Missouri Coteau are direct or first-order morphologic expressions of bedrock structures produced by ice pushing. Bedrock structures at most sites are related to a single direction of ice advance and to a single episode of deformation.
The northern and western Dirt Hills and Cactus Hills were overridden by active ice following glacier thrusting. However, the southern Dirt Hills apparently formed a nunatak between active ice to the north and older stagnant ice to the south--see Fig. 16-8. Maximum structural uplift of 200-250 m is developed in the nunatak area. Variations in bedrock competence clearly influenced structural development. Thrust faults are usually located within lignite or claystone beds; conversely, thicker sandstone units comprise the larger folds and fault blocks.
|View over ridged landscape in northern portion of Dirt Hills, Saskatachewan. Each ridge is supported by deformed Cretaceous bedrock, and small lakes occupy the swales between ridges. Photo date 6/86, © J.S. Aber.|
|Deformed bedrock within ice-shoved ridge of the southern Dirt Hills. The white mass to upper right is a block of Whitemud Fm. uplifted about 160 m, folded and thrust over the brown strata to right. The brown bedrock is also deformed into a partly overturned fold. Height of exposure about 40 m. Photo date 8/84, © J.S. Aber.|
|View of ice-thrust ridges in southern Dirt Hills, near Spring Valley, Saskatchewan. Ridges on horizon are cored by bedrock masses uplifted 200 m or more by ice pushing. The channel in the foreground leads into a spillway that crosses the ridges in the background where the road disappears. Photo date 6/86, © J.S. Aber.|
|Clay mine on crest of ridge in Cactus Hills, Saskatchewan. Whitemud Fm. (U. Cretaceous) megablock is folded into a gentle anticline and draped by a layer of lodgement till (brown cap). Several small faults offset the fold, as shown by lignite (black) layers. Photo date 7/84, © J.S. Aber.|
|Closeup view of small thrust fault on flank of anticline. Note repetition of lignite and smearing of lignite up along thrust fault. Brown till caps upper lignite/thrust zone. Scale pole marked in feet. Photo date 7/84, © J.S. Aber.|
Three local ice tongues of the Weyburn ice lobe caused thrusting of the Dirt Hills and Cactus Hills. The main thrusting of ridges occurred during a readvance of the Galilee and Spring Valley ice tongues. They overran the older ice-pushed ridges and thrust up new ridges to the south. The Avonlea ice tongue also readvanced at this time. Building of the Ardill end moraine system and cutting of associated spillways are thought to be related to the same ice advances that caused the main phase of thrusting.
The Lancer ice-thrust moraine is located on the Shackleton escarpment south of the South Saskatchewan River in western Saskatchewan--see Fig. 16-5. According to Kupsch (1962, p. 585), the Lancer area "... shows possibly the best developed sharp-crested ridges in western Canada ..." The ridges are composed primarily of till that contains deformed floes of lacustrine sediment. Organic peat and soil within the deformed ridges have yielded a radiocarbon date of 31,300±1400 years BP (Campbell, 1995, pers. com.). Uplifted sandstone bedrock of the Belly River Beds forms the cores of some ridges. Three topographic depressions north of the ice-shoved ridges mark source basins from whence material was thrust into the ridges.
|Overview of Lancer ice-thrust moraine, western Saskatchewan. Ridges in foreground are composed of deformed glacial sediments and uplifted sandstone bedrock. Source depressions are present in the Saskatchewan valley seen in the left background. Photo date 6/92, © J.S. Aber.|
|Exposure of layer of organic-rich peaty soil thrust into the Lancer moraine. The peat has yielded a radiocarbon date of 31,300±1400 years BP. This material was presumably derived from mid-Wisconsin interstadial deposits within the Saskatchewan valley to the north. Photo date 5/93, © J.S. Aber.|
The present relief of the Lancer moraine is not the original deformational morphology. The moraine was modified by deposition of stratified sand and gravel between ridges and by drapping of lacustrine sediment of Glacial Lake Stewart Valley. Postglacial erosion has removed some of the lacustrine cover and has exhumed the ridges by gully erosion in a trellis pattern. The Lancer moraine is part of a larger assemblage of ice-shoved ridges that form a loop in the South Saskatchewan valley. These ice-shoved hills and associated source depressions define the lateral and frontal positions of an ice lobe situated within the valley (Aber 1993b).
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ES 331/767 © J.S. Aber (2012).
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