Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

At first glance the walls and domes of the Yosemite region appear to be formed of one and the same kind of rock, for they present no striking variations in color suggestive of different rock types. Gray tints prevail throughout, and it is difficult even for a discerning observer to tell at a distance in what measure the differences in shade are expressive of differences in rock composition and in what measure they are due merely to the varied distribution of the lichens that grow on the surface of the rocks. In reality, however, there are present in the Yosemite region about a dozen distinct types of rock, all granitic and ranging in color from nearly white through gray to nearly black; but so universal is the mottled effect produced by the lichens that the contrasts in rock color are greatly subdued, or even completely obscured, especially on the little-sunned northward-facing cliffs, on which the lichens form an almost continuous veneer.

In spite of the obscuring lichens, nevertheless, the walls of the Yosemite Valley afford an exceptionally instructive field for the student of igneous intrusion: In few places elsewhere can he behold in cross section a more remarkable complex of intrusive bodies. To the layman also the rock walls, when duly interpreted, present a display of unusual interest; they show him the astonishing details of a portion of the earth's crust that once lay miles below the surface, beneath the roots of a former mountain system, and that was disturbed by repeated upwellings of molten rock.

In the following pages it is proposed briefly to outline the nature and significance of this highly complex assemblage of rocks and to describe, without going too far into petrographic technicalities, the specific characters of the individual rock types, so as to afford a guide for their identification in the field. Technical terms can not be avoided altogether in such descriptions; still it is hoped that the reader will not be deterred thereby from perusing this chapter and thus gaining an acquaintance with the materials of which the walls and domes of the Yosemite Valley are built.