USGS1981
The Centennial Mountains include parts of Beaverhead County, Montana, and Clark and Fremont Counties, Idaho; the mountains trend east, and the Continental Divide follows their crest (fig. 1). They form a massive, almost impassable barrier between Centennial Valley on the north and the broad expanse of the Snake River Plain on the south. The proposed Wilderness Study Area encompasses the central more rugged part of the mountains and includes parts of both the Targhee and Beaverhead National Forests. Four contiguous parcels of land, totaling about 96,176 acres (38,922 hectares), make up the Wilderness Study Area (fig. 2). The first of these, encompassing about 46,126 acres (18,667 ha), is wholly in Montana (north of the Continental Divide) and consists of the U.S. Bureau of Land Mapagement's Centennial Mountains Instant Study Area. The second area, known as Management Unit 1 of the Centennial Planning Unit of the Targhee National Forest, consists of some 38,750 acres (15,682 ha) within Idaho (south of the Continental Divide). The third and fourth parcels include about 11,300 acres (4,573 ha) of National Forest land which cover the Mt. Jefferson and Sawtell Peak areas in the eastern part of the Centennial Mountains. Both are Forest Service Further Planning roadless areas (Unit 1-962 of the Beaverhead National Forest in Montana—the Mt. Jefferson area, and Unit 4-962 of the Targhee National Forest in Idaho—the Mt. Jefferson West area). Details of the geography and geology of the area are incorporated in a separate map prepared as an integral part of the mineral resource evaluation (Witkind, 1981). Only the more salient details of the geology described in that report are repeated here.