Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

THE CUTTING OF THE MERCED CANYON

STREAM-CUT ORIGIN OF THE CANYON

Whatever doubt there may have been once as to the stream-cut origin of the Merced Canyon and the other great canyons that furrow the western slope of the Sierra Nevada, surely there is none to-day. That these canyons have been cut primarily by the streams that flow through them was recognized as far back as 1863 by Whitney and King and has since been verified by every geologist who has had occasion to study the range. The belief that still lingers in some quarters that they have been excavated wholly, or largely, by the glaciers of the ice age is untenable in face of the facts that are now known. To be sure, the middle and upper portions of the canyons bear unmistakable signs of remodeling by the glaciers, but it is entirely clear that they were already cut by the streams to depths of 1,500 to 2,000 feet before they were invaded by the ice. Besides, it is certain that the lower courses of all the Sierra canyons have remained wholly untouched by the ice and are products of stream cutting solely. The Merced Canyon, for instance, appears never to have been glaciated beyond a point just below El Portal, which lies about 50 miles from the foothills. The Tuolumne Canyon, similarly, appears to have remained unglaciated for a distance of 30 miles up from the foothills and the San Joaquin Canyon for a distance of 45 miles.

A little reflection will show that canyon cutting by the rivers must have been an inevitable consequence of the uplifts of the Sierra region and the final tilting of the Sierra block. Whenever through any cause the slope of a stream is appreciably steepened, its flow will be greatly accelerated and the wear on its bed correspondingly intensified. Precisely this happened whenever the slopes of the Merced and the other Sierra rivers were steepened by an uplift of the range. With each renewed tilting these streams were accelerated, and they vigorously attacked their beds, intrenching themselves in narrow, steep-sided gorges that eventually became the profound canyons, thousands of feet in depth, in which they flow at the present time.

EVIDENCES OF THREE STAGES OF CUTTING

In the unglaciated lower portions of the Sierra canyons there are many features that still indicate plainly the successive stages in the cutting, and from these indications it is possible with a fair degree of certainty to build up the general history of the development of the canyons. In the broad view of the lower Merced Canyon that may be had from the summit of El Capitan there appear, beginning at the sky line, long, fairly even-topped ridges that advance from right and left and that decline gently toward one another so as to outline the broadly flaring profile of a very ancient and rather shallow valley. (See pl. 26, A.) There is now but little left of that valley, for the ridges mentioned are mere "skeleton ridges" separated by broad spaces, and the level floor of the valley itself is entirely cut away.

Next below this ancient valley appear the much better preserved features of a somewhat narrower and steeper-sided valley, evidently of less remote origin. This valley was fully 1,500 feet deep, yet it also had a broad and fairly level floor, as is shown by the numerous flats and gently undulating surfaces that still remain. The Big Meadow flat, which is a few miles northeast of El Portal and is traversed by the Coulterville road, is the principal remnant of this old valley floor in the Yosemite region.

Sharply incised in the floor of this old valley is the narrow V-shaped inner gorge through which the Merced now flows. This gorge is 1,500 to 2,000 feet deep, but is so narrow that in the distant perspective it seems like a mere slotlike winding trench, an obscure feature in the landscape.

What is commonly and loosely termed the Merced Canyon, then, consists really of a steep-walled inner gorge cut into the floor of a spacious older valley, which in turn lies within a very broad valley of still greater antiquity. The lower canyons of the Tuolumne and San Joaquin Rivers have similar 3-story profiles; indeed, sculptural elements expressive of three stages of valley and canyon cutting may be discerned more or less clearly in the unglaciated lower course of every great Sierra canyon whose development has not been interfered with by volcanic flows. By the experienced eye they may be recognized even in the glaciated upper courses of some of the canyons, for the remodeling action of the ice has not everywhere been so thoroughgoing as to wipe out of existence these preglacial features.

The story of the cutting of the Merced Canyon, as built up from these and other data, may then be briefly told as follows:

THE STORY OF THE CUTTING

When the Merced first established its course conformably to the southwestward slant of the Sierra region, it fashioned for itself a valley of moderate depth that sloped seaward with gentle gradient. For a long period the general character of this valley remained but little changed, though doubtless the river deepened the valley whenever the land rose and again built up its floor with sediments whenever the land returned to lower levels. Meanwhile the flanking hills were being worn lower at a very slow rate. On the whole but little is known of this long early chapter, which lasted, probably, through the first half of the Tertiary period, but it is clear that the outstanding characteristics of the valley of the Merced at that time were great breadth and shallowness. Throughout its lower course, probably, the river meandered sluggishly over a broad, level flood plain, and even in its middle course, in which the Yosemite was later developed, it swung leisurely from side to side. The valley was flanked by rolling hills and a few ridges of greater height, but the headwater region was relatively rugged and had mountains 1,000 to 3,000 feet high. This first stage in the cutting of the Merced Canyon, accordingly, may be termed the broad-valley stage. (See fig. 4, A.)

FIGURE 4.—Cross sections illustrating the successive stages in the cutting of the Merced Canyon. They are drawn to scale and show the general proportions of that part of the canyon which ultimately became the Yosemite Valley. A, The first or broad-valley stage, which dates back presumably to late Miocene time, when the Sierra region was still relatively low; B, the inner gorge cut by the Merced after the first strong tilting of the Sierra block; C, the second or mountain-valley stage, which dates back presumably to the end of the Pliocene epoch; D, the third or canyon stage, produced early in the Quaternary period by the rapid incision of the present inner gorge in consequence of the last great Sierra uplift.

The great uplifts that supervened toward the end of the Miocene epoch naturally steepened considerably the Merced's gradient and quickened its flow. Forthwith the stream abandoned its meandering habit and began vigorously to deepen its valley. Indeed, so rapidly did it erode its bed that in a relatively short period it cut a narrow, steep-sided inner gorge. (See fig. 4, B.)

Throughout most of the Pliocene epoch the river continued to cut downward, though at an ever-slackening rate, for as the gorge grew deeper the gradient became flatter and the waters lost their swiftness and eroding power. Meanwhile the sides of the gorge were being worn back to slopes of moderate inclination, and thus at length the inner gorge developed into a V-shaped inner valley of considerable depth and breadth. (See fig. 4, C.)

In the mostly thin-bedded and relatively unresistant sedimentary rocks of the lower slope of the Sierra Nevada the inner valley grew to be very broad—in places so broad as to obscure the older valley in which it was cut. But in the prevailingly massive and exceedingly resistant granitic rocks of the Yosemite region the inner valley remained relatively narrow and steep-sided. It maintained the aspect of a rugged mountain valley, and accordingly this second stage in the cutting of the Merced Canyon, produced in the Pliocene epoch, may be termed the mountain-valley stage. (See fig. 4, C.)

Such was the state of things when, at the beginning of the Quaternary period, came those stupendous dislocations and tilting movements that resulted in the elevation of the Sierra Nevada to its present height. The sources of the Merced on Mount Lyell were then raised not less than 6,000 feet above their previous altitude, the general slope of the river was greatly steepened, and its waters were accelerated to torrential speed. With more vigor than before they deepened their bed, scouring it with sand and gravel, pounding it with boulders, and plucking out slabs and blocks wherever the fractured state of the rock permitted, and so they again produced a steep-walled inner gorge. (See fig. 4, D.)

Throughout the Quaternary period, down to the present day, the Merced has continued to deepen this new gorge. So rapidly has the cutting proceeded that in the vicinity of El Portal the gorge now attains a depth of about 2,000 feet, or about twice the depth of the Pliocene mountain valley, yet the time involved has been probably less than a million years, or only one-seventh of the duration of the Pliocene epoch. That the cutting is still in progress is shown by the fact that even in its lower course, where it traverses the relatively unresistant sedimentary rocks, the river still flows in a rock channel and makes cascades and rapids over the more obdurate ledges.

It is the combined depth of the Quaternary gorge and of the Pliocene and Miocene valleys above it that gives the great trench worn by the Merced its present canyonlike character. It seems appropriate, accordingly, to refer to this last stage in the cutting of the Merced Canyon as the canyon stage.