Abstract

The geomorphic history of the Rocky Mountains of central western Montana is reconstructed as follows:

(1) Late Cretaceous-Paleocene: Laramide revolution produced a region of mountains and inter-montane basins.

(2) Eocene: lateritic weathering and erosion reduced the region to maturely dissected topography of moderate relief.

(3) Oligocene and Miocene: fluvial and aeolian volcanic ash deposition almost completely buried the pre-Oligocene landsurface; structural deformation dammed the outlet of the valley, forming a closed trough which preserved sedimentary fill from extensive erosion. At close of Miocene Rocky Mountains of central western Montana were an area of low relief in which low ridges of former mountains protruded as inselberge through the alluvial and aeolian volcanic ash cover.

(4) Pliocene: deformation tilted and folded Oligocene and Miocene sediments; subsequent erosion produced a surface of low relief on ash-filled basin and surrounding ranges forming accordant surfaces on or just below the present mountain summits (Pliocene subsummit peneplain).

(5) Late Pliocene: uplift initiated vigorous cycle of erosion with diversion of rivers, superposition of streams, exhumation of buried pre-Oligocene topography, and formation of extensive pedimentary surfaces or No. 1 benches. Exhumation followed dendritic drainage pattern established on pre-Oligocene landsurface except for minor adjustments, stream capture, and superposition.

(6) Early Pleistocene (Kansan?): Alpine glaciation deposited glacial drift in mountain valleys and on pedimentary surfaces (No. 1 benches); uplift and erosion produced No. 2 benches.

(7) Late Pleistocene (Wisconsin): Alpine glaciation carved mountain ranges and deposited moraines and outwash in valley heads and on No. 2 benches. Most trenching of No. 1 benches and development and trenching of No. 2 benches took place between these glacial stages. The No. 3 benches and minor lower stream terraces were formed by recurrent uplifting and lateral stream planation in late Wisconsin and Recent time.

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