Northern Elkhorn Mountain was unmapped previous to this investigation. The mapped area is located north of Malad City, Idaho, in the Bannock Range. It is within the Basin and Range Province. The mapped area measures 5.4 mi. in the north-south direction and 8.9 mi. in the east-west direction. The oldest exposed stratigraphic unit, within the mapped area, consists of orthoquartzite and is of Early Cambrian age. Cambrian formations of the mapped area, in ascending order, are as follows: Camelback Mountain Quartzite, Gibson Jack Formation, Elkhead Formation, Bloomington Formation, Nounan Formation, and St. Charles Formation. Units of Ordovician age are the Garden City and Swan Peak Formations. The youngest unit of Paleozoic age, found within the mapped area, is the Fish Haven-Laketown Formation of Ordovician-­Silurian age. Rock types comprising the Paleozoic units are orthoquartzite, limestone, dolostone, and shale. Tertiary units present, within the area, are the Salt Lake Formation and volcanic rocks with the composition of andesite. These units occur only in isolated parts of the mapped area. Colluvial and alluvial deposits of Quaternary age are present in the valley west of Elkhorn Mountain and in the southeastern and northeastern parts of the mapped area. Numerous high-angle normal faults dominate the structure of the area. They trend generally north and northwest. A major high-angle normal fault extends along the western side of Elkhorn Mountain and is responsible for the present topographic relief. Several small asymmetrical anticlines and a low-angle thrust fault are also present. The structural features, within the area, resulted from two major periods of crustal deformation. The first event was the Laramide orogeny. Compressional forces, generated during this event, produced the anticlines and the thrust fault. Movement was eastward. The second event was Basin and Range faulting. It produced the high­-angle normal faults. Basin and Range faultinq has been active from Oligocene to Holocene. The marginal normal fault, west of Elkhorn· Mountain, is probably active at the present time.