YOSEMITE Vegetational Changes in Yosemite Valley NPS Occasional Paper No. 5

Pleistocene glaciers cut Yosemite Valley into its present pattern of cliffs and hanging valleys and quarried away several hundred meters of granite from the valley floor to form ancient Lake Yosemite (Wahrhaftig 1962). At least three glaciers occupied the valley, as indicated by moraines in the vicinity of Bridalveil Falls and by three layers of lake fill which have a total thickness of about 600 m (1,980 feet) (Gutenberg et al. 1956). Ancient Lake Yosemite became completely filled with sediment. Later, the Merced River cut the moraine, reworked the lake fill, and produced the present flood plain about 5 m (16.5 feet) below the older lake terraces. Thus, lake fill has been exposed in a few places (Fig. 1). Most lake deposits are covered with recent colluvial and alluvial materials from the canyon sides. Rocks deposited in the valley are predominantly siliceous igneous intrusive, and the sands principally quartz, alkali feldspars, and biotite.

Fig. 1. Cross section of Yosemite Valley showing relationships of several colluvial and alluvial deposits. (click on image for a PDF version)

Frequent winter and spring floods sometimes cover as much as 50% of the valley floor leaving layers of fine sand and silt. Abandoned river channels and low areas gradually are filled. Most efforts to confine the river to its channel and to prevent cutting of banks have failed. Flooding and deposition, as well as a low water table during the dry summer, affect soil development and vegetation. Less marshland occurs now than when the valley was discovered because the Merced River channel has deepened.