Editing Hetch Hetchy (section)

===Geology===
The Hetch Hetchy Valley began as a V-shaped river canyon cut out by the ancestral Tuolumne River. About one million years ago, the extensive Sherwin glaciation widened, deepened and straightened river valleys along the western slope of the Sierra Nevada, including Hetch Hetchy, Yosemite Valley, and [[Kings Canyon National Park|Kings Canyon]] farther to the south.{{sfn|Huber|2007|p=80–83}} During the [[last glacial period]], the Tioga Glacier<ref name="USGS">{{cite book|url=http://geomaps.wr.usgs.gov/parks/yos/topobk.html|title=Geologic Story of Yosemite Valley|publisher=USGS|first=N. King|last=Huber|id=Bulletin 1595|access-date=2013-05-26|archive-url=https://web.archive.org/web/20100528095943/http://geomaps.wr.usgs.gov/parks/yos/topobk.html|archive-date=2010-05-28|url-status=dead}}</ref> formed from extensive icefields in the upper Tuolumne River watershed; between 110,000 and 10,000 years ago Hetch Hetchy Valley was sculpted into its present shape by repeated advance and retreat of the ice, which also removed extensive [[Scree|talus]] deposits that may have accumulated in the valley since the Sherwin period.{{sfn|Huber|2007|p=84}} At maximum extent, Tioga Glacier may have been {{convert|60|mi|km|abbr=on}} long and up to {{convert|4000|ft|m|abbr=on}} thick, filling Hetch Hetchy Valley to the brim and spilling over the sides, carving out the present rugged plateau country to the north and southwest.{{sfn|Matthes|1930|pp=87–90}} When the glacier retreated for the final time, sediment-laden meltwater deposited thick layers of [[silt]], forming the flat alluvial floodplain of the valley floor.{{sfn|Wohlforth|2004|p=419}}

Compared with Yosemite Valley, the walls of Hetch Hetchy are smoother and rounder because it was glaciated to a greater extent. This is because the Tuolumne catchment basin above Hetch Hetchy is almost three times as large as the catchment area of the [[Merced River]] above Yosemite, allowing a greater volume of ice to form.<ref name="USGS"/>