Rates and Patterns of Postglacial Cliff Erosion in Yosemite Valley

Abstract:

The formerly glaciated granite cliffs of Yosemite Valley are now primarily modified by rockfalls, and vast accumulations of talus beneath the cliffs indicate the long-term importance of this process. Formation and weathering of exfoliation fractures provides a mechanism for relatively steady cliff erosion over thousand-year timescales, but how important are larger events? Yosemite Valley presents an ideal setting for investigating postglacial cliff erosion because (1) Last Glacial Maximum deglaciation left a wide, flat valley floor that has experienced only a few meters of subsequent aggradation, (2) deglaciation is known to have occurred ~15 ka, (3) lidar-based digital elevation models reveal postglacial talus accumulations in three dimensions, (4) talus deposits have been precisely dated by Be-10, and (5) historical rockfalls are well documented. Since deglaciation, ~180 million m³ of rock has been shed from 53 million m² of cliffs, representing a postglacial cliff retreat rate of ~0.2 mm/yr – among the faster landscape erosion rates measured in the Sierra Nevada. The average annual talus accumulation rate over the past 15 kyr is about 12,000 m³/yr, but evidence suggests that accumulation rates were punctuated by infrequent, extremely large events. Ten extremely large rockfalls, dated by Be-10, occurred between 13.2 and 1.0 ka and were likely seismically triggered. Volumetrically, these events represent about one-fifth of the total postglacial talus volume. Historical talus accumulations are similarly punctuated. A database of historical rockfalls documents ~1.55 million m³ of rock that has fallen from the valley walls since 1857, yielding a historical cliff retreat rate of ~0.2 mm/yr and an average annual accumulation rate of about 10,000 m³/yr, similar to rates averaged over 15 kyr. However, here again talus accumulation is dominated by a few extremely large (≥200,000 m³) events; a single 600,000 m³ rockfall in 1987 accounts for nearly one-third of the total historical volume. Thus, both long- and short-term rockfall records in Yosemite Valley indicate that, although formation and degradation of exfoliation fractures may allow for relatively steady cliff erosion, volumetrically this process is equaled or even exceeded by infrequent, extremely large rockfalls.