Abstract

A middle or late Pliocene open-valley stage in the Ruby-East Humboldt Range is recognized and described. The anomalous position of the drainage divide in places east of the range crest is explained in terms of the geomorphic evolution of the range. Pediments and terraces on the range flanks and in the adjoining basins are described, and those on the west side formed under a régime of through-flowing drainage are compared with those on the east side formed on the borders of hydrographically closed basins.

Seven surfaces, the two highest being pediments and the others partial pediments and terraces, are recognized on the west flank of the range. The dissection of these surfaces is shown to be related to successive rejuvenations of the Humboldt River drainage. The exposure and dissection of surfaces on the east flank of the range are attributed chiefly to relative uplift of the range by faulting. These pediments and terraces have been formed between the middle Pleistocene and Recent.

The origin of pediments under contrasting conditions is considered, and the conclusion is reached that different geologic, climatic, and topographic conditions impose a difference in the efficacy of processes of pedimentation. In this region lateral planation is dominant in areas of permanent streams and soft rocks; and rill wash, rain wash, and weathering are dominant in areas of hard rocks, ephemeral streams, and low mountain masses.

Lateral erosion by streams accounts for approximately 40 per cent of the retreat of the mountain front, and approximately 60 per cent is due to weathering and various types of wash.

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