Lava eruptions west of Sheep Rock at least 140,000 years ago blocked the Bear River from draining into the Snake River system. Instead, the Bear was forced to drain into what was then Lakes Thatcher and Bonneville to the south. The Bear River's course forms a giant horseshoe shape between its source in the high Unita Mountains of eastern Utah and where it enters the Great Salt Lake northwest of Ogden.
This high, rocky, abrupt termination of the mountain, a prominent Oregon Trail landmark, is the northern terminus of the Bear River Range, which is continuous with the Wasatch Mountains. The Bear River flows around the base of the mountains before turning south.
Just west of Sheep Rock is Alexander Crater, a basaltic cinder cone less than 20,000 years old that was mentioned in several Oregon Trail diaries. John C. Fremont, in 1843, described Alexander Crater as "a very perfectly vertical, and disposed like masonry in a very regular manner..." In recent years the crater has been mined for gravel and landscaping materials.