Tuolumne Meadows, Yosemite National Park, USA
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It’s Time!—Renew Your Membership & Save 15% NOVEMBER | VOL. 24, 2014 11 NO. A PUBLICATION OF THE GEOLOGICAL SOCIETY OF AMERICA® Preexisting fractures and the formation of an iconic American landscape: Tuolumne Meadows, Yosemite National Park, USA NOVEMBER 2014 | VOLUME 24, NUMBER 11 Featured Articles GSA TODAY (ISSN 1052-5173 USPS 0456-530) prints news and information for more than 26,000 GSA member readers and subscribing libraries, with 11 monthly issues (April/ May is a combined issue). GSA TODAY is published by The SCIENCE: Geological Society of America® Inc. (GSA) with offices at 3300 Penrose Place, Boulder, Colorado, USA, and a mail- 4 Preexisting fractures and the formation of an ing address of P.O. Box 9140, Boulder, CO 80301-9140, USA. iconic American landscape: Tuolumne GSA provides this and other forums for the presentation Meadows, Yosemite National Park, USA of diverse opinions and positions by scientists worldwide, regardless of race, citizenship, gender, sexual orientation, Richard A. Becker, Basil Tikoff, Paul R. Riley, religion, or political viewpoint. Opinions presented in this and Neal R. Iverson publication do not reflect official positions of the Society. © 2014 The Geological Society of America Inc. All rights Cover: Matthes Crest, just south of Tuolumne Meadows, is a reserved. Copyright not claimed on content prepared famous climbing locality. The ridge owes its prominence to the wholly by U.S. government employees within the scope of glacial erosion of tabular fracture clusters lying on either side of it. their employment. Individual scientists are hereby granted Photo courtesy of Frank Klein. 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Becker*, Basil Tikoff, Dept. of Geoscience, University commonly associated with bedrock erodibility (e.g., Augustinus, of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, 1995; Glasser and Ghiglione, 2009; Krabbendam and Glasser, USA; Paul R. Riley, ExxonMobil Production Co., 800 Bell St., Houston, 2011). In contrast, the nearby vertical rock walls—including Texas 77002, USA; and Neal R. Iverson, Dept. of Geological Cathedral Peak, Matthes Crest, and Lembert Dome—suggest and Atmospheric Sciences, Iowa State University, Ames, Iowa bedrock durability. Despite these geomorphic differences, the 50011, USA entire region is underlain by the same lithology, the Cathedral Peak Granodiorite (Bateman, 1992). In this paper, we present evidence that this anomalous land- ABSTRACT scape is the result of preferential glacial erosion of highly fractured Tuolumne Meadows, in Yosemite National Park (USA), is a bedrock. In particular, tabular fracture clusters (TFCs) are large sub-alpine meadow in the Sierra Nevada Mountains. common in the Cathedral Peak Granodiorite in the Tuolumne Immediately adjacent to Tuolumne Meadows—and underlain by Meadows area (Riley and Tikoff, 2010; Riley et al., 2011). TFCs are the same bedrock lithology (Cathedral Peak Granodiorite)—are dense networks of sub-parallel opening-mode fractures that are vertical rock faces that provide exceptional opportunities to clustered into discrete, tabular (book-like) zones. We conclude climbers. While the presence of a broad meadow suggests bedrock that Tuolumne Meadows resulted from ice flowing perpendicu- erodibility, the vertical rock walls indicate bedrock durability. We larly to high TFC concentrations. In contrast, ice flowing parallel propose that the Tuolumne Meadows’s landscape is the result of to variable TFC concentrations formed the vertical rock walls. variable glacial erosion due to the presence or absence of pre- Thus, the exceptional rock climbing around Tuolumne Meadows existing bedrock fractures. The meadows and valleys formed is a direct result of fracture-controlled variations in erodibility— because of concentrated tabular fracture clusters—a distinctive on the 10–100 m scale—within a single lithology. This finding and locally pervasive type of fracturing—that were particularly supports the contention that landscape evolution is strongly susceptible to glacial erosion. In contrast, the vertical rock walls controlled by bedrock fracturing (e.g., Matthes, 1930) and that consist of sparsely fractured bedrock that was originally bounded tectonic processes that result in fracturing may generally exert a by zones of pervasive tabular fracture clusters. Glacial erosion fundamental and underappreciated role in geomorphology preferentially removed the highly fractured rock, forming promi- (Molnar et al., 2007). nent ridges in the upland surrounding Tuolumne Meadows. The orientation and spacing of the tabular fracture clusters, relative to PREVIOUS WORK ON PREEXISTING FRACTURES AND ice flow, has exerted a fundamental control on the geomorphology GEOMORPHOLOGY IN YOSEMITE NATIONAL PARK of the area. The erosional variability exhibited by a single lithology Yosemite National Park is justifiably known as the landscape indicates that the degree of fracturing can be more important of John Muir. Yet, it is equally the landscape of François than the host lithology in controlling landscape evolution. Matthes, despite the fact that he is much less well known, even among geologists. Matthes studied Yosemite’s geomorphology INTRODUCTION for 25 years (Schaffer, 1997, p. 63–70) before publishing a Tuolumne Meadows in Yosemite National Park is an iconic benchmark paper (Matthes, 1930) in which he concluded that American landscape: It is a sub-alpine meadow surrounded by fracture concentrations were responsible for the size, shape, glacially sculpted granitic outcrops in the Sierra Nevada and location of the roches moutonnées as well as for the Mountains. Owing to its accessibility and aesthetic appeal, it has morphology of its stair-stepped valleys (see figures 33 and 34 been a focal point for both vacationers (up to ~4,200 people per in Matthes, 1930). He also observed that stair-stepped valleys day according to a 2014 National Park Service report [p. ES-19]) formed where fracture concentrations were oriented transverse and geological research in the Sierra Nevada (e.g., Coleman and to ice flow and speculated that deep, straight, and flat-floored Glazner, 1997; Loheide et al., 2009; Lowry et al., 2011). It also has valleys, such as Tenaya Canyon, formed where fracture concen- historical significance; the idea for a Yosemite National Park came 2014 trations were parallel to ice flow. Matthes (1930, p.