GLACIAL SEDIMENTS OF THE LITTLE BLACKFOOT RIVER VALLEY, MONTANA: REVISITING THE PLEISTOCENE BOULDER MOUNTAINS ICE CAP

1:24,000-scale field mapping of Quaternary sediments in the Little Blackfoot River valley near Elliston, Montana, reveals extensive glaciogenic features including hummocky end moraines with erratic clasts, well developed lateral moraines, multiple generations of glaciofluvial terraces, and lateral meltwater channels incised into bedrock. Terrain analysis employing digital elevation models and GIS software provides additional insight into the relationships between these glaciogenic features. Erratic-bearing diamicts with hummocky surfaces occur at two levels in the valley: a lower surface as much as 30 m above present river level, and an upper surface as much as 150 m above present river level. The upper hummocky surface occurs on a broad terrace also underlain by Tertiary volcanics. These glaciogenic features were created by the northern terminus of the Pleistocene Boulder Mountains ice cap, first described by Ruppel in 1962. The results presented here represent the first 1:24,000-scale mapping of glacial deposits in the Little Blackfoot River valley, and place the maximum extent of ice as much as 4 km farther down valley than existing maps indicate. Present data cannot constrain the maximum extent of ice chronologically. The upper hummocky surface may represent a maximum Pinedale position with the lower hummocky surface representing a recessional position. An alternative hypothesis is that the upper hummocky surface represents an older Bull Lake or early Pinedale position with the lower hummocky surface representing a younger Pinedale position. The diamicts described here contain quartz-rich boulder lithologies which provide an ideal target for cosmogenic nuclide dating methods. Resolution of this chronologic ambiguity may help further constrain our knowledge of regional Pleistocene climate.