Excellent exposures in Snake River canyon in the Eagle Rock area near American Falls, Idaho, were supplemented by numerous drill holes in connection with the exploration for a damsite. The rocks are Lower Pliocene (?) Neeley lake beds and associated basaltic and siliceous sediments and tuffs possibly correlative with the Salt Lake formation to the east and the Payette formation to the west. Above the Neeley formation a welded rhyolite tuff (ignimbrite) contains lithophysae and spherulites and shows four distinct phases. Next higher is a siliceous tuffaceous shallow-water sediment, a thin-bedded sublacustrine basaltic tuff, with very thin diatomaceous interbeds, and finally thick basaltic tuffs, breccias, dikes, and lava flows of the Massacre volcanics. Agglutinated basaltic tuffs are also present. Unconformable on these rocks are the Raft lake beds, possibly correlative with the Lake Idaho deposits farther west, of late Pliocene age. A major erosional unconformity separates the Tertiary rocks from the late Pleistocene Cedar Butte basalt which displaced Snake River for 55 miles and produced a lake 40 miles long and 12 miles wide. The beds deposited by this lake constitute the American Falls formation. Abandoned spring coves cut in the downstream end of the Cedar Butte basalt testify to heavy leakage from this ancient lake. The region was warped into gentle anticlines and synclines near the close of the Tertiary and faulted into small blocks. Several erosional unconformities were found. The petrography is described.