h Clarence
King the theory of glacial origin began its long career. John Muir
carried this theory to its extreme.

Since the period of Muir's speculations, the tremendous facts concerning
the part played by erosion in the modification of the earth's surface
strata have been developed. Beginning with W.H. Turner, a group of
Yosemite students under the modern influence worked upon the theory of
the stream-cut valley modified by glaciers. The United States Geological
Survey then entered the field, and Matthes's minute investigations
followed; the manuscript of his monograph has helped me reconstruct the
dramatic past.

The fact is that the Yosemite Valley was cut from the solid granite
nearly to its present depth by the Merced River; before the glaciers
arrived, the river-cut valley was twenty-four hundred feet deep opposite
El Capitan, and three thousand feet deep opposite Eagle Peak. The valley
was then V-shaped, and the present waterfalls were cascades; those which
are now the Yosemite Falls were eighteen hundred feet deep, and those
of Sentinel Creek were two thousand feet deep. All this in pre-glacial
times.

Later on the glaciers of several successive epochs greatly widened the
valley, and measurably deepened it, making it U-shaped. The cascades
then became waterfalls.

But none will see the Yosemite Valley and its cavernous tributary
canyons without sympathizing a little with the early geologists. It is
difficult to imagine a gash so tremendous cut into solid granite by
anything short of force. One can think of it gouged by massive glaciers,
but to imagine it cut by water is at first inconceivable.

To comprehend it we must first consider two geological facts. The first
is that no dawdling modern Merced cut this chasm, but a torrent
considerably bigger; and that this roaring river swept at tremendous
speed down a sharply tilted bed, which it gouged deeper and deeper by
friction of the enormous masses of sand and granite fragments which it
carried down from the High Sierra. The second geological fact is that
the Merced and Tenaya torrents sand-papered the deepening beds of these
canyons day and night for several million years; which, when we remember
the mile-deep canyons which the Colorado River and its confluents cut
through a thousand or more miles of Utah and Arizona, is not beyond
human credence, if not conception.

But, objects the sceptical, the Merced couldn't keep always tilted; in
time it would cut