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Abstract

Exotic Miocene and Pliocene river gravel lies on top of the Continental Divide along the Idaho-Montana border near Monida Pass. The gravel is interlayered with tuffs and basalt flows of the Heise volcanic field, which erupted from the site of the Yellowstone hotspot between 6.62 and 4.45 Ma. The gravel includes pebbles that may have been derived from bedrock outcrops in Nevada and Utah, implying a paleo-river with headwaters to the south of the modern Continental Divide and Snake River Plain. The river may have been a tributary of the pre–ice age Bell River of Canada.

The field trip examines evidence for the tectonic evolution of the Monida Pass area. The course of the Miocene river appears to have been diverted around growing mountain ranges, and then pinched off at Monida Pass on the northern shoulder of the Yellowstone hotspot track.

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