Cretaceous and Tertiary Planation in Northern Idaho

Abstract

Three stages of peneplanation, representing the erosional history of north Idaho mountains, are discussed in this article. These mountains belong to the maturely dissected plateau type and compose a maze of ridges attaining accordant summit levels. The oldest erosion surface or peneplain is recorded on the crest of the highest mountain ridges and was formed apparently near the close of Cretaceous time. A second peneplain is represented on the crests of ridges of lower elevation and was probably completed by the end of Eocene time. A third erosion surface on still lower ridge tops records the bottoms of old-age valleys carved in the earlier peneplains, most likely in early Miocene time. The third surface has been maturely dissected by the present streams, but no peneplain of younger age has been developed. This history differs from that recorded in the adjoining mountains of southern and central Idaho where the summit ridges in general represent a peneplain of post-Miocene or Pliocene age.