Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

EVOLUTION OF THE YOSEMITE VALLEY IN PREGLACIAL TIME

LANDSCAPE OF BROAD-VALLEY STAGE

Although the broad-valley stage is the most remote, antedating the mountain-valley stage by probably 6,000,000 years and the canyon stage by perhaps 7,000,000 years, it is not difficult to picture the Yosemite at that early time, for its longitudinal profile and the profiles of several of its tributary valleys at that stage are definitely known, and the shapes of the flanking hills and ridges remain to-day but slightly changed. There are thus plenty of data from which a bird's-eye view such as that shown in Figure 13 may be constructed. This bird's-eye view the reader is requested to compare with the views following and also with Plate 15, representing the Yosemite Valley in its present state. All the views are from the same assumed high point and are drawn to scale.

FIGURE 13.—Bird's-eye view of the Yosemite Valley as it probably was in the broad-valley stage, prior to the first great uptilting of the Sierra Nevada. The Merced River meandered sluggishly over a broad, level valley floor flanked by rolling hills between 500 and 1,000 feet in height. The crown of El Capitan stood about 900 feet high. There were no cliffs or waterfalls. The tributary streams all sloped gently down to the level of the Merced. The entire region was covered with dense rain-loving vegetation. The drawing is based on the broad-valley profile of the Merced and on the profiles of the upper set of hanging valleys. For key see Figure 14. (click on image for an enlargement in a new window)

The Yosemite in the broad-valley stage was a broadly open, level-floored valley flanked by rolling hills. Most of these hills stood between 500 and 1,000 feet above the valley. The crown of El Capitan, which then had much the same rounded contour as now, rose in gentle curves to a height of 900 feet. Sentinel Dome stood somewhat more than 1,000 feet high, and Half Dome, as yet a bulky, irregularly shaped mass that fell off on the northwest side in a steep, ravined slope, but not a cliff, reared its summit 1,500 feet above the valley.

The landscape as a whole was characterized by subdued, billowy forms and smooth, curving lines. Cliffs, pinnacles, and other angular rock forms were absent. The Cathedral Rocks were represented only by a massive, hummocky spur that sloped down gradually to the valley floor. The Leaning Tower did not yet lean but was a mere knob on another, somewhat narrower spur. The Cathedral Spires and Sentinel Rock were still to be carved from the sloping south side of the valley, and the high ridge on which Glacier Point is situated was not yet cut off by a precipice but continued northward in the form of a projecting spur. Another still longer spur sloped from the base of Half Dome down to the confluence of Tenaya Creek and the Merced River.

Neither were there any waterfalls or cascades in the landscape. The tributary streams, without exception, entered the main valley at the level of its floor and emptied into the placid master stream with scarcely a ripple. The giant stairway at the mouth of the Little Yosemite had not yet been hewn, the Merced winding lazily in serpentine curves down a gently sloping valley floor. Tenaya Creek occupied a valley commensurate in size with its small volume—a valley that lay somewhat higher than the path of the Merced and was flanked mostly by wooded slopes. The entire region was densely covered by rain-loving vegetation, which included such species as laurel, magnolia, maple, sycamore, and willow. It is probable, nevertheless, that Clouds Rest already had some cliffs of bare granite, and that Mount Starr King had much the same domed form that it has today, for massive granite produces cliffs and domes even in regions of humid climate, as is attested by Stone Mountain, in Georgia, and other mountains in the southern Appalachian region.

The peaks and crest of the High Sierra stood far above the general level of the country, just as they now stand above the Yosemite upland, but they were full-bodied and rounded in contour, instead of angular and hollow-sided, as they now are; they were simply wooded mountains, no more distinguished in appearance than many others of to-day. Their sides sloped down in gentle curves to the broad floors of the Merced and Tuolumne Basins, neither of which had yet been trenched below the shoulders and benches that now flank them.

LANDSCAPE OF MOUNTAIN-VALLEY STAGE

The mountain-valley stage of the Yosemite Valley is less easy to portray than the broad-valley stage, for the valley had become, much deeper and more complexly modeled. As may be seen in Figure 14 the Yosemite had developed into a broadly V-shaped and fairly rugged mountain valley about 1,600 feet in depth. On both sides it was flanked by uplands, a number of its tributary valleys not having been cut down to its level. No sharp rims defined the edges of the uplands, but the change in declivity from upland surface to valley side was fairly abrupt. El Capitan had a distinct brow separating its rounded summit from the new, steep slope below. The brow rose 1,200 feet above the floor of the valley, but the drop was distributed over a horizontal distance of about half a mile, and consequently the slope was not marked by precipitous cliffs. Probably it was, like the valley sides elsewhere, largely wooded, though, owing to the massive structure of the granite, it must have been studded with crags and pinnacles.

FIGURE 14.—Bird's-eye view of the Yosemite Valley as it probably was in the mountain-valley stage, after it had been deepened about 700 feet by the Merced River in consequence of the first great uptilting of the Sierra Nevada. The valley was flanked by uplands, and Ribbon Creek, Yosemite Creek, Sentinel Creek, Bridalveil Creek, and Meadow Brook cascaded steeply from the months of hanging valleys. Tenaya Creek, Illilouette Creek, and Indian Creek, however, had cut their valleys down to the level of the Merced. The region was covered with mixed forests containing sequoias. The drawing is based on the mountain-valley profile of the Merced and on the profiles of the middle set of hanging valleys. (click on image for an enlargement in a new window) RC, Ribbon Creek. EC, El Capitan. EP, Eagle Peak. YC, Yosemite Creek. IC, Indian Creek. R, Royal Arches. W, Washington Column. TC, Tenaya Creek. ND, North Dome. BD, Basket Dome. MW, Mount Watkins. E, Echo Peak. C, Clouds Rest. SM, Sunrise Mountain. HD, Half Dome. M, Mount Maclure. L, Mount Lyell. F, Mount Florence. BP, Bunnell Point. CC, Cascade Cliffs. LY, Little Yosemite Valley. B, Mount Broderick. LC, Liberty Cap. SD, Sentinel Dome. SR, Sentinel Rock. SC, Sentinel Creek. CR, Cathedral Rocks. BV, Bridalveil Creek. DP, Dewey Point. MR, Merced River.

The Cathedral Rocks formed an asymmetric spur, much steeper on the east side than on the west and surmounted by two knobs corresponding to the two higher summits of the present group. The highest of these knobs stood about 900 feet above the floor of the valley. The third and lowest summit of the present group was still to be hewn from the rock below the level of the valley floor.

The Cathedral Spires and the other crags in their neighborhood were only dimly foreshadowed by rocky spurs separated by deep ravines. In the place of Sentinel Rock there was an irregularly shaped spur that projected far out into the valley and deflected the river to the north. Eagle Peak, on the other hand, less sharply attenuated than it is to-day, was the culminating summit of a massive asymmetric spur that deflected the river to the south. It stood fully 1,700 feet above the valley.

Half Dome was probably a long ridge, whose northwest side, furrowed by ravines and mostly wooded, carried as yet no suggestion of a sheer cliff with overhanging cornice. Its southeast side, on the other hand, in all likelihood was already in part smoothly curved and bare. The crest of the ridge rose at least 2,200 feet above the Little Yosemite and about the same height above the valley of Tenaya Creek. That stream joined the Merced, as before, opposite Glacier Point, but owing to the increased depth of their valleys the tapering spur between them had assumed much greater relative height. The granite from which the Washington Column and the cliff of the Royal Arches were later carved still lay beneath the level of the valley.

The most distinctive feature of the Yosemite landscape of that early epoch was the array of foaming cascades that descended from the mouths of the hanging upland valleys. Of leaping waterfalls such as adorn the sides of the Yosemite to-day there probably were none, but the descent of the streams was abrupt enough to cause their waters to break into cascades. Yosemite Creek tumbled from a hanging valley 600 feet high. As is shown by its profile, it had carved a gulch 1-3/4 miles in length, the head of which is now 1 mile back from the rim. Ribbon Creek had the highest hanging valley of all and cascaded down 1,400 feet in a horizontal distance of somewhat less than a mile. Meadow Brook made a mile-long cascade 1,200 feet high through a gulch that headed just back of the present upland rim. Bridalveil Creek descended about 900 feet through a gulch 2-1/2 miles long, and Sentinel Creek made a much steeper cascade of the same height through a gulch less than 1 mile long that headed not as far back as the present upland rim.

Only a few tributary streams had cut their valleys down to the level of the Merced and joined that river without cascades or falls. Among these were Indian Creek, Illilouette Creek, and Tenaya Creek. The basin of Indian Creek to-day still has the general character which it acquired during the mountain-valley stage, having been changed only slightly by glaciation. The Illilouette Basin has been modified more significantly by the glaciers, but for the most part only locally, and as a consequence it also retains in a broad way the configuration of the mountain-valley stage. Tenaya Canyon, on the other hand, offers in its present appearance no suggestion whatever of the mountain-valley which it replaces. The depth of that valley is, however, reliably indicated by the hanging valley of Snow Creek and by the upper Tenaya Basin. That basin, indeed, may be regarded as a large remnant of the mountain-valley landscape, severely though not profoundly remodeled by the ice.

The Little Yosemite in the mountain-valley stage had been cut to a depth of about 1,200 feet. Liberty Cap, still a mere knob on a low spur that projected, uninterrupted by any gaps, southward from the base of Half Dome, stood only 600 feet above its floor. Above the Little Yosemite the mountain-valley cut by the Merced extended in the form of a narrow central trench all the way to the head of the Merced Basin. It was the antecedent of the present broad and deep glacial trough in which lie Merced Lake and Washburn Lake, but it decreased rapidly in depth toward its head, and as a consequence the side valleys hung at no great height above its floor.

LANDSCAPE OF CANYON STAGE

The great depth indicated by the profile C—C' in Plate 27, A, leaves no doubt that the preglacial Yosemite had the proportions of a canyon rather than a valley. Its depth was 2,400 feet, measured from the brow of El Capitan, and thence decreased gradually to 2,000 feet opposite Glacier Point. However, there is to be added the height of the hills on the upland, between which the original broad valley of Miocene time lay. The depth of the canyon thus measured averaged close to 3,000 feet. As the width from rim to rim was less than the present width, the ratio of depth to width was almost the same as it is to-day.

FIGURE 15.—Bird's-eye view of the Yosemite Valley as it probably was in the canyon stage, just before it was invaded by the glaciers of the ice age. The valley had been cut 1,300 feet deeper by the Merced River, in consequence of the second great uptilting of the Sierra Nevada, and had a V-shaped inner gorge and two sets of hanging side valleys. Illilouette Creek and Indian Creek now also made cascades, but Tenaya Creek and lower Bridalvell Creek had cut gulches down to the level of the Merced. The region was covered with coniferous forests adapted to a temperate climate with cold, snowy winters. The drawing is based on the canyon-stage profile of the Merced and on the profiles of the lower set of hanging valleys. For key see Figure 16. (click on image for an enlargement in a new window)

The preglacial Yosemite Canyon was not simply V-shaped in cross section, for the Merced had cut, in consequence of its acceleration by the last great Sierra uplift, a narrow inner gorge. This inner gorge, according to the testimony of the hanging valleys of the second or middle set, was 1,500 feet deep at El Portal. and 1,200 feet deep opposite Fireplace Bluff and thence extended with diminishing depth up through the whole length of the chasm. Opposite Glacier Point it forked; the southern and shorter prong came rather abruptly to a head just above the mouth of Illilouette Valley, and the northern and longer prong extended up the course of Tenaya Creek for 5 miles.

The narrowness of the preglacial canyon was accentuated by numerous craggy spurs that projected from both sides. These were the same spurs which existed in the mountain-valley stage, but they were more rugged and more angular of outline, owing to the increased depth of the chasm and the consequent development of sharp-cut ravines and gulches in its sides. The massive spur of the Cathedral Rocks now attained its greatest length—1-1/2 miles. It had acquired its third and lowest summit and, as its east side was gashed by deep ravines, all of its three summits began to stand out, severed from one another by gaps. The slender spur from which the Leaning Tower was to be carved also attained its greatest length, and between it and the Cathedral Rocks Bridalveil Creek was actively cutting its steep, V-shaped gulch all the way down to the river.

FIGURE 16.—Bird's-eye view of the Yosemite Valley as it probably was immediately after the ice age. The valley had been broadened and deepened to essentially its present proportions. The deepening accomplished by the ice ranged from 600 feet at the lower end to 1,500 feet at the upper end. A third set of hanging valleys had been added, and the Bridalveil Fair was produced. A lake 5-1/2 miles long occupied a basin gouged into the rock floor of the valley and dammed in addition by a glacial moraine. The vegetation consisted in the main of types now prevailing. The drawing is based primarily on a systematic survey of the moraines and the other glacial features of the valley. (click on image for an enlargement in a new window) RC, Ribbon Creek. R, Echo Peak. LC, Liberty Cap. EC, El Capitan. C, Clouds Rest. SD, Sentinel Dome. EP, Eagle Peak. SM, Sunrise Mountain. G, Glacier Point. YC, Yosemite Creek. HD, Half Dome. SR, Sentinel Rock. IC, Indian Creek. M, Mount Maclure. SC, Sentinel Creek. R, Royal Arches. L, Mount Lyell. CR, Cathedral Rocks. W, Washington Column. F, Mount Florence. BV, Bridalveil Creek. TC, Tenaya Creek. BP, Bunnell Point. LT, Leaning Tower. ND, North Dome. CC, Cascade Cliffs. DP, Dewey Point. BD, Basket Dome. LY, Little Yosemite Valley. MR, Merced River. MW, Mount Watkins. B, Mount Broderick.

The style of modeling of the other spurs in the Yosemite Canyon may be likewise inferred in a general way, as the structure of the rock, which controlled the modeling, is indicated in the present rock forms. The front of El Capitan, from the brow down, was carved probably in short, massive buttresses; the south side of the valley immediately opposite was dissected into a number of long, sharp-crested, forking spurs. The great spur that projected from Eagle Peak was doubtless asymmetric. It fell off abruptly and irregularly toward the east and sloped down smoothly and at a fairly constant angle toward the west. On the other hand, the spur which now ends in the sheer face of Sentinel Rock bore on its declining crest probably a series of pinnacles and crags. The spur under Glacier Point, again, was in all probability rounded and massive, though in some places castellated like the present cliffs east of Union Point.

The long spur that projected from the base of Half Dome westward into the head of the canyon is difficult to picture definitely, as nothing whatever remains of it to-day. Yet the mere fact that the glaciers were able so completely to destroy it throws light on the structure of the rock of which it was composed. That rock must have been thoroughly fractured, or jointed, and it may be inferred, therefore, that the spur sloped down rather evenly, undiversified by pinnacles, knobs, or buttresses.

Half Dome itself doubtless already possessed some features that foreshadowed its present unique configuration. The back probably was already in large part smoothly rounded and bare; perhaps the crown was bare also, but the great cliff face on the northwest side was yet to be hewn out, and in its place there was a craggy, splintered slope. Half Dome then stood about 3,800 feet high above the point of confluence of Tenaya Creek and the Merced and already was the highest and most remarkable eminence in the immediate vicinity of the chasm.

The Little Yosemite in this stage was not much deeper than in the preceding, for the Merced had carved its inner gorge but a short distance up beyond the mouth of Illilouette Valley, and from this point upstream it had accomplished but little cutting, owing largely to the exceedingly resistant nature of the rock. Doubtless the river at that time descended from the Little Yosemite, not by successive leaps, as it does to-day, but by a long chain of cascades. To judge by the canyon profile in Plate 27, A, the descent amounted to 1,000 feet in a distance of about 3 miles.

The valley of Tenaya Creek, on the other hand, was rapidly assuming the depth and aspect of a canyon, for though Tenaya Creek was inferior to the Merced in volume and cutting power, its cutting was greatly facilitated by a zone of closely fractured rock. Thus, it will be seen, even at this early epoch, prior to the coming of the glaciers, Tenaya Creek approached the Yosemite through a deeper valley than that of the Merced.

Tenaya Creek, however, was the only tributary stream that was able to cut to such great depth. All the other tributary streams had hanging valleys from whose mouths they cascaded steeply into the main canyon. Even Illilouette Creek and Indian Creek, which in the mountain-valley stage had cut their valleys down to the level of the master stream, now tumbled precipitously down from the brink of the inner gorge. Illilouette Creek, emptying near the head of the gorge, had a cascade only 600 feet in height, but Indian Creek had a chain of cascades 1,000 feet in height.

Yosemite Creek, Ribbon Creek, and the other streams of the upper set of hanging valleys had much higher cascades than in the preceding stage, owing to the added depth of the inner gorge. Some of them probably made two cascades in succession, an upper from the upland to the bottom of the old mountain valley, and a lower from the brink of the inner gorge to the river. Their total heights were as follows:

Spectacular as these lofty cascades doubtless were they did not add so much to the beauty of the landscape as the present leaping falls of the Yosemite Valley, for each of them descended through a narrow, sharply incised gulch flanked by craggy spurs. The gulch through which Bridalveil Creek now cascades down to the brink of its great fall is the only one of these preglacial gulches that remains well preserved. It is probably representative in a general way of the entire category, as it has suffered but slight change by glaciation, though there is reason to believe that it is more regularly V-shaped and more smooth-sided than most of the other gulches, its configuration being determined largely by inclined master joints in the granite, whereas the other gulches were cut in diversely structured rocks.