PRE-EXISTING FRACTURES AND THE FORMATION OF AN ICONIC LANDSCAPE: TUOLUMNE MEADOWS, YOSEMITE NATIONAL PARK, USA

Tuolumne Meadows – in Yosemite National Park (USA) – is a large sub-alpine meadow in the Sierra Nevada. Immediately adjacent to it are near-vertical rock faces known for their exceptional rock climbing; both the Meadows and the steep faces are eroded out of the same bedrock lithology: Cathedral Peak Granodiorite. While the presence of a broad meadow suggests bedrock erodibility, the vertical rock walls indicate bedrock durability. We propose that the Tuolumne Meadows’s landscape is the result of variable glacial erosion due to the presence (or absence) of pre-existing bedrock fractures. In particular, the meadows and valleys formed because of concentrated tabular fracture clusters - a distinctive and locally pervasive type of fracturing - that were particularly susceptible to glacial erosion. In contrast, the vertical rock walls consist of sparsely fractured bedrock that were originally bounded by zones of pervasive tabular fracture clusters. Glacial erosion preferentially removed the highly fractured rock, forming prominent ridges in the upland surrounding Tuolumne Meadows. The orientation and spacing of the tabular fracture clusters, relative to ice flow, has exerted a fundamental control on the area’s geomorphology. The erosional variability exhibited by a single lithology indicates that the degree of fracturing can be more important than the host lithology in controlling landscape evolution.