IT is known that since the beginning of time the surface of the earth has undergone various changes brought about by its cooling and shrinking and by internal eruptions and disturbances.
During one of these disturbances the region between the Pacific Ocean and the Rocky Mountains was affected. Here the surface of the earth was broken into great blocks and one of these, four hundred miles long and eighty miles wide, was pushed up at its eastern edge, separating it from the depressed region to the east, leaving a steep scarp, and pulled down at its western edge giving it a gentle slope to the sea.
The streams which flowed in diverse directions before the uplift of this block now were given a definite course flowing down the slope to the west and forming broad shallow valleys. One of these streams was the Merced River which now flows through the Yosemite Valley.
After a great period of time there occurred a second upward thrust which raised the eastern edge of the block to an elevation of several thousand feet making a distinct mountain range, later known as the Sierra Nevada. This second uplift gave to the western side a greater incline so that the Merced River was given enough velocity to enable it, through the millions of years elapsing before a third series of uplifts, to cut a narrower valley within its old broad valley.
The tributary streams which flowed parallel to the range and at right angles to the Merced River were not benefited by the tilting of the block, hence the deeper the main river cut the higher the side streams were left above it. With the broadening and leveling of its bed the Merced lost its cutting power and flowed lazily over the valley floor.
The third and last uplift, greater than any preceding it, which raised the crest of the range to an elevation of over seven thousand feet in the north and about fourteen thousand feet in the southern part, gave to the Merced River such rapidity and power that it cut in its old valley a deep narrow canyon. The deepening of the river bed again left all of the side streams high in their hanging valleys to cascade down steep V-shaped walls to join the master stream; except Tenaya Creek with its southwesterly course, and Bridal Veil
[paragraph continues] Creek, with its northwesterly course, both of which benefited by the tilting of the Sierra Block and also flowed over a less resistant bed of granite. These two tributaries were able to cut down to the level of the Merced River a depth two-thirds as deep as the present depth of the valley. The brink of the Bridal Veil Falls today marks the level at which the Merced River flowed just prior to the advent of the Ice Age.
The climate which, heretofore, had been of a semi-tropical nature gradually became frigid and snows piled up to a depth of thousands of feet in the higher mountains. The underlying snow became compact by the enormous weight into granular ice forming glaciers which moved clown the old stream beds of Tenaya and Illilouette Creeks and the Merced River. These three glaciers joined at the upper end of the valley into one mighty glacier several thousand feet thick, filling the canyon to the rim and extending down to about the present site of El Portal.
Through the hundreds of centuries that this glacier existed moving slowly forward with tremendous weight and irresistible force it deepened the canyon over a third and transformed it from a V-shaped canyon to a broad U-shaped valley, seven miles long, a mile in width and approximately thirty-five hundred feet deep. The cutting back of the V-shaped canyon walls to vertical cliffs caused the cascading tributary streams now to drop sheer from their hanging valleys and today produce Yosemite's beautiful waterfalls.
Gradually the climate became warmer causing the glacier to retreat back to the summit of the range and the Merced River resumed its old course through the valley for thousands of years, until the climate again grew colder causing the glacier to advance. This time the dominion of the ice was much shorter than before so that the glacier reached a depth of only a thousand feet and did little more than to add to the clift sculpturing started by the first glacier and to leave a moraine spanning the valley below El Capitan, where the ablation of summer balanced its forward movement. When the climate again became warmer causing the glacier to recede, this moraine acted as a dam, holding back the water from the melting ice forming a lake on the valley floor five and a half miles long.
During the twenty thousand years since the retreat of the ice the Merced River carrying sand and gravel has filled the, lake completely, a depth of about three hundred feet, bringing the valley floor to its present level. Remnants of this last ice field still exist at the base of some of the highest peaks in Yosemite National Park.
The granite in this region is made up of irregular blocks forming joints or cracks except those huge masses that stand out as Yosemite's
most prominent features, El Capitan, Cathedral Rocks and Half Dome. These, on account of their solidity, resisted the erosion of the glacier. The rounding of the domes is due to the expansion and contraction of the heat and cold which caused the rocks to shale off in layers like those of an onion.
On account of the compactness of the underlying rock in the Little Yosemite, upper Merced Canyon, the Merced Glacier was unable to quarry down to the depth of the Tenaya Canyon, which is two thousand feet lower, as the rock here, being closely jointed, facilitated the work of the glacier.
The floor of the Little Yosemite found in the upper Merced Canyon is the first of two giant steps made by the glacier in its descent and over which the river now flows forming Nevada and Vernal Falls.
Today great quantities of rock are to be found at the base of the cliffs forming talus slopes. This is caused by the weathering of the cliffs and also by intermittent earthquakes which have shaken down the rock from the valley walls. One of these earthquakes caused the rock to be thrown down from the cliff at the base of North Dome across Tenaya Canyon holding back the water of Tenaya Creek and forming Mirror Lake. The same fate is befalling Mirror Lake as that of Lake Yosemite, as Tenaya Creek increased in volume by the melting of the snow in the Spring heavily laden with sand, is extending its delta farther and farther into the lake. Eventually it will be filled and replaced by a meadow or a forest of pines.
Thus has God fashioned a Cathedral with the granite cliffs for the walls, the canopy of the heavens for the roof, and as the artist adds the last subtle touch to his masterpiece so has the Creator caused the forces of nature to deposit seeds of trees and beautiful plants to adorn the mighty cliffs and the valley floor.