Geological Survey Professional Paper 160 Geologic History of the Yosemite Valley

THE YOSEMITE REGION

GENERAL CONFIGURATION

That part of the Sierra Nevada of which the Yosemite Valley is the central feature is commonly referred to as the Yosemite region. By some the term is applied to the entire Yosemite National Park, but for the purposes of this paper it may to advantage be restricted to that part of the drainage basin of the Merced River which extends from El Portal (por-tal'), the lower entrance to the Yosemite National Park, up to the heads of the Little Yosemite Valley and Tenaya Canyon, the two main branches of the Yosemite chasm, and laterally out to the limiting divides on the bordering uplands. (See pl. 2.) The Yosemite region, as thus defined, embraces an area 20 miles long, measured in the direction of the master stream, and 20 miles broad. Beyond its upper border lies the High Sierra; beyond its lower border, what, for lack of a more precise term, may be called the lower Sierra slope.

A short distance above El Portal the winding, V-shaped Merced Canyon contracts abruptly to a sheer-walled gorge that has few turns, the Merced Gorge. This pronounced change in the character of the canyon is accompanied by a general change in the modeling of the landscape on each side, the choppy, intricate ridge and gulch topography of the lower Sierra slope giving way to massive, billowy features. The reason is that the Yosemite region is carved in prevailingly massive, resistant granitic rocks, whereas the lower Sierra slope is made up largely of upturned slates and other thin-bedded sedimentary rocks.

Proceeding up the Merced Gorge for a distance of 8 mies, the traveler emerges suddenly into the spacious Yosemite Valley. (See pl. 7.) On both sides its great cliffs reach up to undulating forested uplands that extend, unbroken by side canyons, to the head of the valley. (See pl. 8, B.) There, however, they are gashed by three converging branch chasms—the Little Yosemite (pl. 4, A), which is 2,000 feet deep; the Illilouette Valley, on the south side, which is between 1,500 and 2,000 feet deep; and Tenaya Canyon (pl. 8, A), on the north side, which is nearly 4,000 feet deep. The floor of the Yosemite itself has an altitude of about 4,000 feet above sea level; the uplands rise from 7,000 feet at their lower margin to 8,000 feet at the valley head.

PLATE 8.—A (top), TENAYA CANYON FROM GLACIER POINT. A typically U-shaped glacial canyon. At the right is Half Dome, and just beyond is the summit of Clouds Rest. At the left is Basket Dome, and above it, in the far distance, is Mount Hoffman, white with snow. In the center, darkened by the shadow of a cloud, is Mount Watkins, and below, at the mouth of the canyon, is Mirror Lake. Photograph by A. C. Pillsbury. B (bottom), VIEW ACROSS YOSEMITE VALLEY FROM RIBBON FALL. The depth of the chasm and the abruptness with which it trenches the gently undulating upland are strikingly brought out in this view. Taft Point is indicated by letter T. At the lower right is the base of the Cathedral Rock. Photography by F. C. Calkins.

Between the three branch chasms the uplands continue, covered with dense forests of lodgepole pine and fir, but they are surmounted by isolated domes, peaks, and crests of bare granite. South of the Little Yosemite the upland is particularly well defined. (See pl. 11, B.) It there rises gradually to an altitude of about 9,000 feet, terminating at the base of the Clark Range, the first great bulwark of the High Sierra. The upland on the north side of the Yosemite Valley extends all the way to the Grand Canyon of the Tuolumne River, which is 10 miles distant; that on the south side extends for a similar distance to the canyon of the South Fork of the Merced River. At the brinks of those canyons the uplands break off just as sharply as they do at the brinks of the Yosemite chasm. They are, therefore, in the nature of elevated table-lands that stand essentially undissected in the broad spaces between the main canyons.

The two upland tracts, moreover, accord closely in general height. In a broad view, especially from a point so far back from the brink of the Yosemite that that chasm is not visible, their billowy surfaces seem continuous (pl. 9, A) and appear to be parts of one and the same westward-slanting high-level surface. This high-level surface has no local name, but as it is a topographic feature of prime importance, which has its analogs in other parts of the Sierra Nevada, it will here be designated, in so far as it lies within the limits of the Yosemite region, the Yosemite upland.

PLATE 9.—A (top), VIEW NORTHWARD OVER YOSEMITE UPLAND FROM HORSE RIDGE. The Yosemite Valley is deeply ensconced below the general level of the upland and in a view such as this is difficult to find. The summit of El Capitan is marked with a cross. Photograph by F. C. Calkins. B (bottom), HANGING VALLEY OF YOSEMITE CREEK AND YOSEMITE FALLS, FROM GLACIER POINT. The most striking example of a hanging valley with a waterfall leaping from its mouth to be found in the Yosemite region. At the left is the overhanging rock of Glacier Point. Photograph by A. C. Pillsbury.

One characteristic of the Yosemite upland merits especial attention: Not one of its numerous vales and valleys that drain into the Yosemite slopes down to the bottom of that chasm; on the contrary, they continue with gentle gradients out to its lofty brinks and there terminate abruptly—some of them as abruptly as if cut off with a cyclopean knife. From the mouths of these "hanging valleys" leap most of the waterfalls and cascades for which the Yosemite region is renowned. (See pl. 9, B.)

Looking down into the Yosemite from the lip of one of these hanging valleys (pl. 8, B), at a height of 2,500 or 3,000 feet, one readily gains the impression that the chasm is an abnormal abyss that lies sunk deep below the general level of the land. Indeed, the abrupt termination of the upland valleys at its brinks, together with the prevailing sheerness of its walls, strongly suggests that the Yosemite was produced by a rupturing of the earth's crust or by the caving in of a portion of it.

HANGING VALLEYS

A typical example of a hanging valley is that of Yosemite Creek (pl. 9, B), the largest upland valley on the north side. Extending with moderate gradient to the brink, it is there cut off sharply at a height of 2,565 feet above the floor of the main chasm. From its lip pour the Yosemite Falls, the most famous of the waterfalls of the Yosemite region. The valley is about 10 miles long and traverses the entire breadth of the northern upland; one of its branches, in fact, heads close to the brink of the Grand Canyon of the Tuolumne. Much shorter is the hanging vale of Ribbon Creek, west of Yosemite Creek. (See pl. 7.) This vale is cut off at a height of 3,050 feet, and from its lip descends the Ribbon Fall, the highest of the Yosemite's waterfalls. On the south side, a little farther west, is the upland vale of Meadow Brook, which hangs at a height of 2,585 feet; and several miles to the east is the somewhat longer valley of Sentinel Creek, which is the highest of all, opening 3,340 feet above the Yosemite floor.

The hanging valley of Bridalvell Creek, which drains the major part of the southern upland, does not terminate abruptly at the brink but opens into a steep V-shaped gulch that projects, flanked by rocky spurs, a mile and a half into the Yosemite Valley. (See pl. 3.) Down through this gulch, as through an immense smooth chute, the stream glides with amazing speed, finally leaping over the 600-foot precipice at the bottom in the Bridalvell Fall. Even the upland valley is not typical throughout—broad and gently sloping in its middle course, it contracts, about 2 miles from the brink, to a rock-walled gorge having a somewhat steeper gradient. Bridalveil Creek, accordingly, may be said to traverse first a normal upland valley, next a somewhat steeper gorge, and then a precipitous chute, before it makes its final leap.

The hanging valleys of Indian Creek and its East Fork, which are east of Yosemite Creek, differ from any of the valleys above described in that their mouths are deeply gashed by V-shaped gulches. Through these gulches the streamlets cascade without making any waterfalls of note, joining each other midway in their descent. The gulch of the main stream is fully a mile long and is known as Indian Canyon because, being unobstructed by cliffs, it served the Yosemite Indians, before the white man built his trails, as a route of egress from the valley.

Illilouette Valley (ill-i-lou-ette') is cut to much greater depth below the general level of the Yosemite upland than most of the preceding valleys, yet it also is a true hanging valley, for it terminates abruptly at a height of 1,850 feet above the Yosemite floor. Its relatively great depth—nearly 2,000 feet—can hardly be accounted for by the volume of Illilouette Creek. That stream, it is true, is the largest of the Merced's tributaries in the Yosemite region and drains the most extensive territory of them all; but the depth of the valley nevertheless seems out of proportion to the size of the stream. The same impression is created by the hanging valleys of Tamarack and Grouse Creeks, in the lower part of the Yosemite region, which likewise are intermediate in height between the true upland valleys and the main chasm, yet which contain relatively small streams.

If the Illilouette Valley thus seems anomalously deep, Tenaya Canyon seems vastly more so, for it is cut down within 100 feet of the level of the Yosemite Valley and is next to that valley the greatest chasm in the Yosemite region, yet it is traversed by a streamlet comparable in volume to Yosemite Creek. (See pl. 8, A.) Moreover, its floor lies fully 2,000 feet below that of the Little Yosemite, which is the pathway of the Merced River. However, the Little Yosemite itself seems anomalous because of its lack of depth, considering that it forms part of the canyon of the master stream. It is cut off abruptly at a height of 2,000 feet above the main Yosemite and has the aspect of a hanging valley. From its mouth the Merced descends by successive leaps, forming the Nevada and Vernal Falls and a chain of turbulent cascades. (See pl. 10.) It will be readily understood, then, why Tenaya Canyon rather than the Little Yosemite has been by some observers regarded as the real upward continuation of the Yosemite Valley. The paradoxical relations between these two chasms constitute one of the most puzzling features of the Yosemite region.

PLATE 10.—THE GIANT STAIRWAY, FROM GLACIER POINT. In the center is the Nevada Fall, which leaps from the upper step, flanked on the left by Liberty Cap. Below is the Vernal Fall, which leaps from the lower step. On the far side of the Little Yosemite, which is behind Liberty Cap, are the water-streaked Cascade Cliffs, and beyond are the peaks of the High Sierra mantled with snow. At the left is Mount Florence; at the right is Mount Clark. Photograph by A. C. Pillsbury.

Both Tenaya Canyon and the Little Yosemite have hanging side valleys of the true upland type. The largest, on the north side of Tenaya Canyon, is the valley of Snow Creek, which heads on the south slope of Mount Hoffmann. It terminates at a height of 2,500 feet above the canyon floor. On the south side of the Little Yosemite is the broadly open upland vale in which lie the Starr King Meadows. The streamlet that issues from those meadows makes a precipitous descent of 1,700 feet.

STAIRLIKE STEPS IN CANYON FLOORS

The successive falls which the Merced River makes in its descent from the Little Yosemite leap from the steps of what may be characterized as a giant stairway hewn in granite. From Glacier Point, the far-famed scenic viewpoint on the south side of the Yosemite Valley, this natural stairway may be beheld in its entirety, and an excellent conception may be gained of the clean-cut forms of its steps and of the nature of the canyon in which it is cut. (See pl. 10.) The uppermost step, from which the Nevada Fall drops, is 600 feet high; the next lower step, which gives rise to the relatively insignificant Diamond Cascade, is about 50 feet high; the lowermost step, from which the Vernal Fall drops, is more than 300 feet high. The total descent is 1,200 feet in a distance of slightly more than half a mile.

Viewed from the tourist trail that leads directly up over the giant stairway to the Little Yosemite, the steps are seen to have well-defined edges, sheer fronts, or risers, and approximately level treads. (See pls. 24 and 25.) The lowermost step is the most perfectly modeled; its edge is neatly squared, and its front is essentially vertical and straight. The tread, however, is hollowed out, like a wooden stair tread that has suffered many years of wear. In the shallow basin lies picturesque Emerald Pool. All the steps, moreover, are much broader than the stream bed. They are canyon steps, not channel steps. The river, in fact, occupies but a small part of their width. In the distant view from Glacier Point it has the appearance of a mere trickle finding its way down a rugged pathway that is much too broad for its small volume. There is, nevertheless, a marked narrowing of the canyon at the edge of each step, followed by a widening on the tread below. At the Vernal Fall the constriction is produced mainly by a low spur on the south side of the canyon—the spur over which the horse trail leads in laborious zig zags. At the Nevada Fall the constriction is due to two great bosses of granite, Liberty Cap and Mount Broderick (pls. 10 and 44, B), which project from the north side. Immediately behind these obstructing masses the walls again flare out abruptly to the full width of the Little Yosemite.

Stairlike steps of this type accompanied by portal-like constrictions are present also in Tenaya Canyon. There is a long series of them, extending over a distance of 7 miles and making a total rise of 4,000 feet; but, with one exception, these steps are not so perfectly modeled nor are the portals so accentuated as those of the giant stairway below the Little Yosemite. Tenaya Canyon opens scarcely a hundred feet above the Yosemite Valley and is almost level-floored for a distance of a mile and a half. Then it rises steeply about 500 feet, but so narrow and gorgelike is its bottom here that the effect of a step is almost lost. Yet above the rise is a relatively level, broad-floored stretch nearly a mile in length that is truly suggestive of a tread.

The second step is much better defined than the first, its front rising abruptly in the form of a cross cliff about 700 feet in height. Tenaya Creek, nevertheless, does not leap over this cliff but issues from a narrow gorge which it has cut into the step. The south half of the step, which is the better preserved of the two, still has a smooth, tabular top and a definite, sharp edge. The tread was originally 1-1/2 miles long, but only its upper half remains untrenched.

The third and fourth steps are low and have short treads, but the fifth step, which is at the head of Tenaya Canyon, is remarkable both for its height and its long tread. It is one of the best-developed canyon steps in the Yosemite region. The front or riser is a sheer cliff 600 feet in height that connects without break with the canyon walls on each side. Down its marvelously smooth face the waters of Tenaya Creek glide rather than drop in the form of a continuous ribbon cascade. Above this great cliff begins a relatively shallow upper canyon carved throughout in bare granite. Its floor extends with gentle gradient for the distance of about a mile, then by a flight of minor rock steps it rises to the level of what may be termed the Tenaya Basin, whose central feature is Tenaya Lake.