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### Geologic history of the Yosemite Valley

#### Professional Paper 160

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#### Links

- Document: **Report**                                
  (pdf)
- Plates:
- **Plate 2**                                
    (pdf)
- **Plate 7**                                
    (pdf)
- **Plate 29**                                
    (pdf)
- Download citation as:  RIS

#### Abstract

The problem of the origin of the Yosemite Valley inherently demands a 
solution in quantitative terms. Its essence is, To what extent is the valley a 
product of glacial action, to what extent a product of stream erosion? The 
principal result of the investigations upon which this report is based is the 
determination within narrow limits of the preglacial depth of the Yosemite 
Valley and of other facts concerning its preglacial development which permit 
fairly definite estimates of the proportionate shares of work performed by 
stream and by glacier. The investigations comprise a detailed survey of the 
glacial and geomorphologic features of the Yosemite region and an equally 
intensive study of its rock formations, supplemented by reconnaissance work of 
both kinds in adjoining parts of the Sierra Nevada. The petrologic studies were 
made by Frank C. Calkins; the glacial and geomorphologic survey by Francois E. 
Matthes.

Detailed mapping of the morainal system of the ancient Yosemite Glacier has 
served not only to determine the farthest limits reached by that glacier but to 
throw new light on the significance of the hanging valleys of the Yosemite 
region. It is reasonably certain that the glacier never extended more than about 
a mile beyond the site of El Portal. The hanging side valleys of the Merced 
Canyon below El Portal, therefore, hang not because of any glacial deepening 
suffered by that canyon. The explanation is offered that they hang because their 
streamlets have been unable to trench as rapidly as the Merced River since the 
rejuvenation of the Merced by the last uptilting of the Sierra Nevada. The 
streamlets were handicapped not only by their comparatively small volume but 
also by the fact that their courses trend northwestward and southeastward, 
substantially at right angles to the direction of the tilting, and therefore 
have remained essentially unsteepened, whereas the Merced's course trends 
southwestward, directly down the slope of the Sierra block, and therefore has 
been appreciably steepened.

Projection of the longitudinal profiles of these hanging valleys forward to 
the axis of the Merced Canyon shows that they are closely accordant in height. 
Their profiles indicate a series of points on a former profile of the Merced 
with respect to which the side streams had graded their courses prior to the 
last uplift. This old profile can be extended upward into the glaciated part of 
the Merced Canyon above El Portal and even into the profoundly glaciated 
Yosemite Valley, accordant points being furnished by a number of hanging side 
valleys (due allowance being made for glacial erosion suffered by those 
valleys). However, not all the hanging valleys of the Yosemite region are 
accordant with this set. Several of them, including the upland valley of 
Yosemite Creek, constitute a separate set indicating another old profile of the 
Merced at a level 600 to 1,000 feet higher than the first. Others, including the 
hanging gulch of lower Bridalveil Creek, point to an old profile of the Merced 
about 1,200 feet lower than the first. There are thus three distinct sets of 
hanging valleys produced in three cycles of stream erosion. The valleys of the 
upper set, like those of the middle set, were left hanging as a result of rapid 
trenching by the Merced induced by an uplift of the range, there having been two 
such uplifts. Only the valleys of the lower set hang because of glacial 
deepening and widening of the Yosemite Valley, the cycle in which they were cut 
having been interrupted by the advent of the Pleistocene glaciers. They 
consequently indicate the preglacial depth of the Yosemite Valley. That depth, 
measured from the brow of El Capitan, was about 2,400 feet; measured from the 
rim at Glacier Point it was about 2,000 feet.

#### Study Area



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