About This Item
Share This Item
The classic study of the erosional history of Big Horn basin is by Mackin in 1937. In it he studied the terrace levels which ranged in age from Late Tertiary to late Pleistocene. He postulated that the terraces were the product of stream captures or intervals of interglacial stability alternating with glacial incision. More recent studies have revised Mackin's classically simple model.
Detailed studies have increased the number of terrace levels, changed the timing of their stability episode, and estimated their ages. The number of terrace levels has been increased to nine along the Greybull and Bighorn Rivers and to six along the Shoshoni River. Because some of the different levels occur along each river, the number of unique levels within the basin is 12. The occurrence of a 600,000 and a 100,000 year old ash on two terrace levels allows the ages of the terraces to be estimated. The estimated ages range from 3 m.y. for the Tatman to 49,000 years for the Himes, which is the lowest level along the Bighorn River. Both ashes were deposited during river stability intervals and indicate that the Bighorn River and its eastern tributaries were stable late in the interglac al episodes. In contrast, the glaciofluvial gravels along the Shoshoni River at Cody indicate a late glacial stability episode for the western tributaries. The terrace cycles along the Bighorn River and its western tributaries are therefore out-of-phase. This relationship fostered the numerous stream captures recognized by Mackin, as well as some unusual terrace geometries. Comparison of the estimated terrace ages to termination in the marine isotopic record indicates that not all of the Pleistocene climatic cycles are preserved in the Big Horn basin terrace chronology. The present chronology is currently more complete than that of Mackin's pioneering work of the 1930s, but it has not changed his basic story.
End_of_Article - Last_Page 1351------------