The Ice Age in Central Montana
By Karen Porter
Throughout the Pleistocene Epoch (2,600,000 to around 12,000 years ago), North America experienced four major glacial advances of the Laurentide Ice Sheet and the intervening interglacial periods. At its greatest extent, late in the fourth (Wisconsin) advance (85,000-11,000 years ago), the ice sheet periodically extended across the Missouri River and reached at least as far south as northern Fergus, Petroleum and Garfield Counties (Montana Bureau of Mines and Geology, 2007). In front of that fluctuating ice margin, the north-flowing Musselshell River was periodically blocked, backing up vast volumes of water and forming Lake Musselshell. This proglacial lake is one of six ice-dammed lakes recognized across central Montana by geologists in the field (Colton and others, 1961). The other lakes, from west to east, are lakes Cut Bank, Great Falls, Jordan, Circle, and Glendive. All are variously documented as forming in front (to the south or west) of the fluctuating Laurentide Ice Sheet’s southern margin in response to damming of north- or east-flowing streams. It was during this late Wisconsin glacial advance that the Missouri River was diverted from its original northern course across Montana southward and then eastward along its present course. The Missouri’s original northern course is the broad valley now occupied by the "underfit" Milk River, incongruously small for the size of its valley.
Lake Musselshell lay generally between Winnett and Sand Springs, Montana, and from the Missouri River south to the Melstone-Musselshell, Montana area, between 20,000 and 11,500 years ago (Davis, 2004). Thus, its presently recognized southwest margin lay only about ten miles east of the Milton Ranch.
Field evidence for Glacial Lake Musselshell is primarily the occurrence of "glacial erratics", boulders of various sizes composed of granitic and metamorphic rock types matching outcrops far to the north in Canada (Colton and Fullerton, 1986; Davis and others, 2006). These glacier-transported boulders, arriving at the front of the glacier, were iceberg-rafted out into the lake and randomly dropped as their icebergs melted. These boulders are now found scattered, sometimes partially buried, across the Late Cretaceous shale bedrock and grassy knolls of the Musselshell drainage.
Absent from the field data are typical glacial lake features such as varves (rhythmic, seasonally controlled deposits), delta deposits, and shorelines. Absence of these features suggests that Lake Musselshell was not a stable lake, but rather filled to various levels and emptied numerous times as the ice margin fluctuated (Davis, 2004; Davis and others, 2006). The lake probably drained to the north into the Missouri River, either under the ice margin or by breaching its shoreline and carving temporary new channels across or along the ice front. There is some evidence that the lake may have drained southeastward, through breached low hills such as the Larb Hills, into recognized low areas (Davis and others, 2006). Because this lake developed on erodable shales and sandstones, many features may have been, and continue to be, lost.
Although the great continental Laurentide Ice Sheet dominated the north-central Montana plains, all of central Montana experienced extensive mountain glaciation in the island ranges. The deeply scoured bowls (cirques) and ridges (arêtes) in the Big Snowy Mountains and Crazy Mountains are good examples of the shaping of mountain topography by the scouring, grinding, and transport processes of glacial ice. As these mountain glaciers melted, vast volumes of eroded debris were left behind as glacial till (in lateral, ground, and terminal moraines), or washed out onto the mountain flanks and high plains as glacial outwash deposits. This debris
apparently has been subsequently removed by erosion; if moraine deposits remain on cirque floors in the Big Snowies, they lie beneath the extensive modern landslide deposits.
Colton, R.B., and Fullerton, D.S., 1986, Proglacial lakes along the Laurentide ice sheet margin in Montana: Geological Society of America Abstracts with Programs, v. 18, n. 5, p. 347.
Colton, R.B., Lemke, R.W., and Lindvall, R.M., 1961, Glacial Map of Montana East of the Rocky Mountains: US Geological Survey Miscellaneous Investigations Series Map I-327, scale 1:500,000, 1 sheet.
Davis, N.K., 2004, Extent and timing of Laurentide Glacial Lake Musselshell, central Montana, M.S. Thesis, Montana State University, Bozeman, MT, 202 p.
Davis, N.K., Locke, III, W.W., Pierce, K.L., and Finkel, R.C., 2006, Glacial Lake Musselshell: Late Wisconsin slackwater on the Laurentide ice margin in central Montana, USA: Geomorphology, v. 75, p. 330-345.
Johnson, W.R., and Smith, H.R.,1964, Geology of the Winnett-Mosby area, Petroleum, Garfield, Rosebud, and Fergus Counties, Montana: U.S. Geological Survey Bulletin 1149, 91 p.
Montana Bureau of Mines and Geology, 2007, Geologic Map of Montana: Geologic Map GM 62, map scale 1:500,000).