| Summary: | Glacial Lake Musselshell is the middle link in a chain of lakes that formed along the Pleistocene Laurentide ice margin in central Montana. It was first recognized because scores of glacially-transported boulders from the Canadian Shield are found in the Musselshell River basin, yet there is no evidence that the Laurentide ice sheet advanced that far south. For a century, the ice-rafted boulders remained the only physical evidence associated with the lake. No other features typical of other large, ephemeral lakes - varved lacustrine sediment, inflow deltas, or lake shorelines - had been identified for Lake Musselshell. A sequence of nine river terraces and >100 previously located boulders provided the opportunity to place Lake Musselshell, and the corresponding Laurentide ice margin, in the context of regional and global chronologies. Terrace gradient and provenance, surface exposure ages of ice-rafted boulders, and identification of additional lake-related features were the most useful tools for establishing the extent and timing of Lake Musselshell. Lake Musselshell probably existed as one or more short-lived stage(s) that reached a maximum altitude of ~920 m. The absence of varves, deltas and shorelines suggests against one or more stable levels. Deposits of sheet-like silt and fine sand are interpreted as slackwater sediment from one or more (hydraulically dammed?) lakes. The lake(s) drained under or in front of the ice sheet, down the modern Missouri River channel. Strong evidence was found that Lake Musselshell existed during the Late Wisconsin stage. Twenty-seven 10Be surface exposure ages from ice-rafted boulders are all Late Wisconsin and younger (5.2-21.7 ka). Canadian Shield gravel occurs only in the lowest (probably Late Wisconsin) Pleistocene terrace. Additionally, upstream convergence of the Musselshell River terraces implies that displacement of the Missouri River by the Laurentide ice sheet occurred only recently (possibly Late Wisconsin). Pre-Late Wisconsin glacial advances into central ...
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