BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 51, PP. 337-372, A PUS.. 12 FIGS. MARCH 1, 1940 GE0M0RPH0L0GY OF THE RUBY-EAST HUMBOLDT RANGE, NEVADA BY ROBERT P. SHARP CONTENTS Page Abstract......................................................................................................................................... 338 Introduction................................................................................................................................. 338 Introductory statement..................................................................................................... 338 Location and physical features....................................................................................... 339 Geologic setting................................................................................................................... 341 Geomorphic features of the mountain block....................................................................... 342 General statement.............................................................................................................. 342 Open-valley stage............................................................................................................... 342 Relative position of range crest and drainage divide............................................... 343 Geomorphic features of the west flank and adjoining basins......................................... 346 General statement.............................................................................................................. 346 Description of surfaces..................................................................................................... 348 Identification and correlation of surfaces.................................................................... 353 Geomorphic features of the east flank.................................................................................. 353 General statement.............................................................................................................. 353 Description of surfaces..................................................................................................... 354 Gradients of pediments and terraces..................................................................................... 357 Dissection of surfaces................................................................................................................ 357 General statement.............................................................................................................. 357 Dissection of surfaces on west side of range............................................................... 357 Dissection of surfaces on east side of range................................................................ 359 Age of geomorphic features...................................................................................................... 359 Origin of pediments.................................................................................................................... 361 Summary....................................................................................................................................... 369 Works to which reference is made......................................................................................... 370 ILLUSTRATIONS Figure Page 1. Index map............................................................................................................................. 339 2. Geographic map.................................................................................................................. 340 3. Structure section................................................................................................................. 342 4. Open-valley remnants on Long Creek........................................................................... 343 5. Map of drainage divide and range crest....................................................................... 344 6. Map of surfaces in Boulder Creek area...................................................... 347 7. Map of surfaces in Thorpe Creek area....................................................... 348 8. Map of surfaces in South Fork area........................................................... 350 9. Map of surfaces in Smith Creek area......................................................... 351 10. Map of fault scarp and pediment................................................................................. 354 (337) Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/3/337/3415749/BUL51_3-0337.pdf by guest on 24 September 2021 338 E. P. SHARP----RTJBY-BAST HUMBOLDT RANGE Figure Page 11. Dissected pediment, east flank of Ruby Mountains................................................. 355 12. Plot of angles between stream courses and base of mountains............................... 367 Plate Pacing page 1. Interior Ruby Mountains................................................................................................. 344 2. Pediments and interior Ruby Mountains..................................................................... 345 3. Surfaces west of Ruby Mountains.................................................................................. 352 4. East side of East Humboldt Mountains....................................................................... 353 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 hydro- graphically 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 con­ clusion 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. INTRODUCTION INTRODUCTORY STATEMENT This paper is devoted chiefly to consideration of pediments and terraces flanking the Ruby-East Humboldt Range and in the adjoining basins. The geomorphic forms on the west side of the range, developed under a régime of exterior drainage, are compared with those on the east side, developed in an area of interior drainage. The geomorphological evolu­ tion of a progressively uplifted and tilted fault block, such as the Ruby- East Humboldt Range, would be of great interest, but, unfortunately, extended dissection of the mountain block makes this area unsuited for such studies. The glaciation of the range and some of the forms produced by block faulting have already been described (Sharp, 1938, 1939b) and will not be discussed further. A total of 5 months during the summers of 1936 and 1937 were devoted to field work in the Ruby-East Humboldt region with particular emphasis on its Cenozoic history. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/3/337/3415749/BUL51_3-0337.pdf by guest on 24 September 2021 INTRODUCTION 339 Sincere appreciation and acknowledgment are extended to Professors Kirk Bryan and Marland P. Billings, of Harvard University, for their interest, aid, and suggestions in the field and laboratory. Professor H. A. Meyerhoff, of Smith College, thoroughly criticised the manuscript and F ig u re 1.—Index map Ruby-East Humboldt Range shown in solid black. offered several excellent suggestions. Edward A. Schmitz aided in the preparation of illustrations. Profit has been derived from the interest and suggestions of Dr. Leland Horberg and Jean Todd Sharp. LOCATION AND PHYSICAL FEATURES The Ruby-East Humboldt Range is located in Elko County, north­ eastern Nevada, 200 miles west of Salt Lake City, Utah (Fig. 1). It lies well within the borders of the Great Basin section of the Basin-Range province. This range is a narrow, asymmetric fault block extending nearly 100 miles S. 20° W. from its northern end near Wells. Its high eastern face drops off steeply to the hydrographically closed, detritus-filled basins on Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/51/3/337/3415749/BUL51_3-0337.pdf by guest on 24 September 2021 340 E. P. SHARP----RUBY-EAST HUMBOLDT RANGE F igure 2.—Geographic map Showing drainage, larger topographic units, and location of detailed geomorphic maps of selected areas. the east. The western slope is longer and less steep,