The mapped area, in southeastern Idaho, includes part of the Bannock Range, on the west, and Cache Valley, on the east. It is centered about 13 miles northwest of Preston, Idaho, and measures 12 miles in the north-south direction and 8.5 miles in the east-west direction. The Bannock Range, in the western part of the mapped area, consists of Precambrian and Cambrian stratigraphic units. The Precambrian units are as follows: (1) lower Precambrian argillite, (2) Precambrian quartzite, and (3) upper Mutual Formation. The Brigham Formation, which overlies the Mutual, is probably of Cambrian age; however, the lower part may be Precambrian. The Langston, Ute, Blacksmith, Bloomington, and Nounan Formations, all of Cambrian age, crop out locally. The Wasatch and Salt Lake Formations bf Tertiary age overlap older rocks near the western margin of the area. The Salt Lake Formation overlaps older rocks in the foothills along the western side of Cache Valley. It is separated from Precambrian argillite, on the west, by a major north-south gravity fault. Arnphibolite plutons intrude the Precambrian argillite at three localities in the Oxford Peak area; basalt flows and volcanic breccia are also present in the Precambrian argillite. The amphibolite represents metamorphosed diabase. Both the metamorphism of the plutons and the presence of extrusive igneous rocks in the Precambrian argillite indicate that the plutons formed during Precambrian time. Two major thrust faults are present in the Bannock Range. The Oxford Peak thrust fault places the Brigham Formation of Cambrian age, as well as Precambrian quartzite and the Mutual Formation of Precambrian age, over Precambrian argillite. The Clifton thrust fault places carbonate formations of Cambrian age and also a lower Paleozoic undifferentiated unit on various older rocks. Near the northwestern corner of the mapped area, however, the Clifton thrust fault underlies the Brigham Formation. Both thrust faults dip westward except where locally folded; the direction of movement was presumably eastward. The thrust faulting is probably related to the Laramide orogeny which occurred during the Cretaceous Period and the early part of the Tertiary Period. A major gravity fault extends along the eastern side of the Bannock Range. It places Salt Lake Formation of Tertiary age, on the east, against Precambrian argillite, on the west. A relatively short gravity fault, also down on the east, offsets the two major thrust faults east of Oxford Peak. The relative collapse of Cache Valley, during the Tertiary Period, produced great relief and, as a consequence, a major landslide formed northeast of Oxford Peak. It transported Precambrian argillite and overlying Brigham Formation down over Precambrian argillite.