The Cretaceous Bear River and Aspen section in Willow Creek, 6 miles southeast of the junction of the Hoback and Snake rivers, Teton County, Wyoming, is described in detail, including the position and nature of its faunas. The Bear River is 539 feet thick, and the Aspen 1307 feet thick. On lithologic and paleontologic grounds the Bear River-Aspen contact is placed lower than in earlier reports. The Aspen section contains 63 feet of porcellanite, which suggests prolonged volcanic activity in the region.

Fourteen units of the Bear River are fossiliferous, but only 2 of these, in the upper 166 feet of the formation, contain invertebrates. One assemblage is similar to the typical fresh-water Bear River fauna; the other is brackish water. The Aspen has 25 fossiliferous units, of which only 9 contain invertebrates. Seven of these are fresh water, and 2 are brackish water or marine. In no case were fresh-water and marine or brackish-water elements mixed; each fauna is distinct and separated from the others by many feet of section. Conflicting age assignments for the Bear River and Aspen are discussed.

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