Each major stream rising in the Bear-tooth Mountains in southern Montana and flowing from the east flank of the mountains to the adjacent portions of the Great Plains formed five distinct, well-developed terrace levels during the Pleistocene Epoch. The terraces are capped by coarse gravel derived from the crystalline rocks of the high-standing mountains; all deposits are fundamentally the same, regardless of elevation above the present stream. The gravel may represent valley fills deposited as the result of stream captures or as outwash during periods of glaciation in the mountains. The origin of any terrace gravel may, therefore, be partly fluvial and partly glaciofluvial.
Piracy of the major rivers by tributary streams heading in the plains would initiate aggradation because the coarse loads of the mountain-bred rivers cannot be transported on the low gradients of the capturing tributaries. Some of the terrace gravels can be traced into and partially through moraines of late Wisconsin age.
During deposition of the highest terrace gravels the rivers in the area drained to the northeast. Before deposition of the gravel capping the next lower terraces, each major river was diverted to a more northerly position. Terraces in any one valley are seldom matched, indicating that downcutting usually occurred in the nonresistant bedrock along the edge of the valley rather than through the coarse gravel deposits. Much of the downcutting may have been accomplished by headwardly eroding tributary streams which subsequently captured the main rivers.
Excavation of the Big Horn Basin began in the late Tertiary, ending a long period of basin filling. This downcutting trend has continued to the present except for intervals of valley filling represented by the terrace deposits.