Stratigraphy and Quaternary Landscape Evolution in the Vicinity of the Marias River, North-Central Montana

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Stratigraphic exposures near the Marias River contain evidence of multiple glacial advance by lobes of Laurentide Ice Sheets (LIS) as well as a postglacial aggradational sequence containing two tephras and a series of buried soils. Field mapping was conducted in the region northwest of Fort Benton and southwest of Shelby in Liberty County, Montana, where the Marias River occupies a broad valley containing late-Mesozoic and Quaternary strata (Lopez 2002; Smith et al. 1959). Three Cretaceous units are exposed, including Colorado shale, the Telegraph Creek formation, and Eagle sandstone. Gravels that lie directly on top of the Colorado shale have been correlated with the Saskachewan or Wiota Gravels which contain Pleistocene fossils (Hill 2006; Jensen and Varnes 1964).

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