The basin that held

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

Lake Musselshell is in the lower (north-flowing) reach of the river. It is underlain mostly by highly erodible

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

Colorado shale,

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

group

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility. Although its permeabil ...

and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

, and Hell Creek sandstone and shale. The bedrock is gently folded and affected by local faults and joints. There is a sequence of nine terraces and more than 100 glacial boulders. The terraces are older than the erratics as the erratics rest on the terraces.Extent and Timing of Laurentide Glacial Lake Musselshell, Central Montana; Nicole Kristina Davis; Montana State University; Bozeman, Montana; July 2004

Glacial stages

Advancing out of

Alberta Alberta is a Provinces and territories of Canada, province in Canada. It is a part of Western Canada and is one of the three Canadian Prairies, prairie provinces. Alberta is bordered by British Columbia to its west, Saskatchewan to its east, t ...

a lobe of the late Wisconsin Laurentide Ice Sheet advanced southeastward into east and central

and southwestern

Saskatchewan Saskatchewan is a Provinces and territories of Canada, province in Western Canada. It is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, to the northeast by Nunavut, and to the south by the ...

, leaving the Cypress Hills and Boundary Plateau unglaciated. As the Laurentide ice crossed the area of the present day Milk River valley in southern Alberta, it was divided into two lobes by the Sweetgrass Hills, which also remained unglaciated. The western lobe, the Shelby lobe, advanced southward to the Missouri River, in the vicinity of Great Falls, Montana. The lobe that extended east of the Sweetgrass Hills, the Havre lobe, moved in two directions. The Lorna sublobe, advanced across the Missouri River to north of the Highwood Mountains. The Malta sublobe expanded southeastward along the present-day Milk River, bounded to the north by the Boundary Plateau and to the south by the Little Rocky Mountains and the region of the Mussellshell River.Geologic Framework and Glaciation of the Central Area, 1-1-2006; Christopher L. Hill; Boise State University, Boise, Idaho; 2006 The Lake Great Falls provides an approximate date and southern boundary for the maximum advance of the glacial lobes into Montana, impounding the

Missouri River The Missouri River is a river in the Central United States, Central and Mountain states, Mountain West regions of the United States. The nation's longest, it rises in the eastern Centennial Mountains of the Bitterroot Range of the Rocky Moun ...

Yellowstone River The Yellowstone River is a tributary of the Missouri River, approximately long, in the Western United States. Considered the principal tributary of the upper Missouri, via its own tributaries it drains an area with headwaters across the mountain ...

, and Little Missouri River rivers, each valley forming a glacial lake; i.e., Lake Mussellshell, Lake Circle, Lake Jordan, Lake Lambet, Lake Glendive, and Lake Mikkelson.

References

{{reflist

Bibliography

*Alden, W. C., 1958, Physiography And Glacial Geology Of Eastern Montana And Adjacent Areas, U. S. Geological Survey Professional Paper 174. *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 Geological Investigations Map I-327. *Howard, A. D., 1958, Drainage Evolution In Northeastern Montana and Northwestern North Dakota, Bulletin of the Geological Society of America, v69, 575-588. *Thornbury, W. D., 1965, Regional Geomorphology Of The United States, John Wiley & Sons, Inc., New York.

Glacial stages

References

Bibliography

See also