YOSEMITE Sketch of Yosemite National Park and an Account of the Origin of the Yosemite and Hetch Hetchy Valleys

EFFECT OF GLACIATION.

The question may be raised at this point whether the foregoing general explanation also applies to the Yosemite and Hetch Hetchy Valleys. These two valleys, though in many respects resembling the glacial canyons of the High Sierra, nevertheless possess a distinctive, if not exceptional, character that places them in a category by themselves. They may be described as deep-hewn, clean-cut moats bordered by precipitous rock walls that sharply trench across the surface of the uplands and spring abruptly from broad, level, grassy floors. Waterfalls and cascades of great height and beauty leap down into both chasms from the hanging valleys of the upland, and the cliffs themselves are sculptured into a variety of bold and picturesque forms, such as are scarcely known elsewhere.

In length the Yosemite Valley measures about 7 miles; in height its walls range from 3,000 to 4,000 feet; its floor width averages 1 mile. The Hetch Hetchy Valley, which is essentially similar in its characteristics, is only 3 miles long, half a mile wide, and some 2,000 feet deep. Both valleys lie at fairly low levels, the elevation of the floor of the Yosemite above the sea being 3,960 feet, that of the Hetch Hetchy Valley 3,660 feet.

Better, perhaps, than any verbal description, the accompanying bird's-eye view (fig. 9) will enable one to gain an understanding of the general character of the Yosemite Valley and the location of its various scenic features. The appended table further will furnish some idea of their altitudes above sea level and their heights above the valley floor.

FIGURE 9.—Birds-eye view of Yosemite valley looking eastward to the crest of the Sierra Nevada. 1, Clouds Rest; 2 Half Dome; 3, Mount Watkins; 4, Basket Dome; 5, North Dome; 6, Washington Column; 7, Royal Arches; 8, Mirror Lake and mouth of Tenaya Canyon; 9, Yosemite Village; 10, Head of Yosemite Falls; 11, Eagle Peak (the Thee Brothers); 12, El Capitan; 13, Ribbon Fall; 14, Merced River; 15, El Capitan Bridge and Moraine; 16, Big Oak Flat Road; 17, Wawona Road; 18, Bridalveil Fall; 19, Cathedral Rocks; 20, Cathedral Spires; 21, Sentinel Rock; 22, Glacier Point; 23, Sentinel Dome; 24, Liberty Cap; 25, Mount Broderick; 26, Little Yosemite Valley. (click on image for an enlargement in a new window)

Altitude and height of land features.

Altitude and height of waterfalls.

So extraordinary is the appearance of both the Yosemite and Hetch Hetchy Valleys that one involuntarily asks himself whether ice erosion alone is really sufficient to account for all their peculiarities or whether one should appeal to other forces for an explanation, as some of the earlier investigators have done. The tendency at first was, not unnaturally, to look for causes of a violent, cataclysmal sort. Features so extraordinary seemed to demand an unusual explanation, but the adequacy of slow-working everyday erosional processes to produce the results seen has come to be more and more universally recognized. It may be of interest, nevertheless, to briefly review the older theories here before giving what is now generally accepted as the correct explanation.

Prof. J. D. Whitney, the first scientist to study the Sierra, thought the deeply incased character of the Yosemite Valley to be the result of the sinking of a local block of the earth's crust having the exact outlines of the valley. Glaciers, he stoutly asserted, had never so much as entered it.

Galen Clark believed the valley to have originated by the exploding of a number of close-set domes of molten rock, subsequent stream and ice erosion having smoothed out the chasm to its present form.

Prof. Silliman considered the Yosemite as a great rupture caused by subterranean forces—a rent later partly filled with rock débris.

Clarence King was the first to point out the prominent role which the ice of the glacial epochs must have played in the elaboration of the Yosemite Valley. John Muir goes further and holds that the Yosemite, like all the canyons and other features of the Sierra Nevada, was sculptured almost wholly by ancient glaciers.

FIGURE 10.—Map of Yosemite National Park. (click on image for an enlargement in a new window)

In contrast to this view is that of H. W. Turner and several others, according to whom the Yosemite is nothing but a stream-cut valley which has suffered little if any modification at the hands of the ice, but which owes much of its peculiar shaping to the influence of the strong vertical joints displayed in its walls.

Willard D. Johnson, a close student of ice erosion, considers the Yosemite and Hetch Hetchy Valleys to be products of stream erosion, subsequently widened by the characteristic sapping action of the ice. Others, notably E. C. Andrews, of New South Wales, and Douglas W. Johnson, have followed, all uniting in attributing considerable importance to glacial erosion, but differing somewhat in their estimates of the amount of work they believe should be assigned to it.

The most probable explanation is that the Yosemite and Hetch Hetchy Valleys both, like the other canyons and valleys of the High Sierra, have been developed through stream erosion consequent to the uplift of the Sierra block, and have later been greatly deepened and enlarged by repeated ice invasions; further, that they owe their strangely clean-cut, moat-like forms and the diversified sculpturing of their cliffs to the structure of the country rock, which has locally controlled the action of the eroding agents.

There is every reason to believe that the Yosemite and Hetch Hetchy Valleys are, in the first place, of stream-cut origin. Their relations to the other parts of the Merced and Tuolumne Valley systems leave no room for doubt on this point.

FIGURE 11.—View of Hetch Hetchy Valley. The meadows show the extent of the filled lake basin.

That both valleys have been visited by glaciers descending from the Sierra crest is now also settled beyond dispute. No one familiar with the evidences of ice work could be misled in this regard. Indeed, it is difficult to see how so eminent a scientist as Whitney could have overlooked the unmistakable signs of glaciation in the Yosemite Valley and should have pronounced that valley to have been ice-free at all times while admitting the Hetch Hetchy Valley to have been glaciated.

There are many indubitable proofs of the former glaciation of the Yosemite and Hetch Hetchy Valleys, but one of the best is found in the fact that both have deeply eroded rock basins in their floors. Running water does not erode inclosed rock basins of any considerable size or depth. Glaciers are the only agents known to produce such features by erosion. As for the possible formation of either of the basins by the subsidence of a local block, as contended by Whitney, or by the damming action of a landslide or volcanic eruption, there is no evidence whatever pointing to an origin of that sort.

To the layman the basins in the Yosemite and Hetch Hetchy Valleys may not at once be apparent. The fact is that they have long been filled with river sediment, and the picturesque lakes which they once held have been transformed into level expanses of meadow land. To the trained eye, however, this does not render the basins any less easy to recognize. The lake in the Yosemite Valley must have measured some 6 miles in length; that in the Hetch Hetchy Valley about 3 miles. In the Hetch Hetchy Valley the heavy rock sill that closes the basin at its lower end is still plainly visible, but in the Yosemite Valley the rock sill is buried under a ridge of glacial débris or "moraine," and its existence is only to be surmised. There is, however, ample justification for the conjecture, as few of the moraines in the Yosemite region measure more than a hundred feet in height, whereas cross sections of the valley, based on careful measurements and drawn without vertical exaggeration, seem to indicate that the depth of the present sand filling in the valley may amount to at least 500 or 600 feet. (See fig. 12.)

FIGURE 12 —Cross section of Yosemite valley from Eagle Peak to Sentinel Rock, showing probable depth of filling.

A few additional words about this interesting moraine may not be out of place. Consisting of rock débris carried by the glacier and dropped at its lower end, that ridge marks the position which the ice front occupied during a halt in its final recession. Stretching across the entire width of the valley from the base of the Cathedral Rocks to El Capitan, the moraine, after the ice had melted away, acted as a dam, raising the level of the waters in the rock basin until they rose within a few feet of its crest. But the Merced River, pouring through a low saddle in the ridge, has since cut a sharp notch, the very gap now spanned by the El Capitan Bridge.

It is noteworthy that glacially excavated lake basins also occur on the treads of the great branch canyons of the Yosemite Valley. A lake 3 miles long, now filled with sediment, once occupied the entire extent of the Little Yosemite; and on the tread immediately above the Vernal Fall a small but none the less typical tarn still remains in the form of Emerald Pool. Tenaya Canyon once contained four lakes, situated at successively higher levels. Of these the three upper ones are filled, but the lowest and longest is still partly in existence. Mirror Lake is its last remnant, and though fast being extinguished by the ever-advancing delta of Tenaya Creek, it is likely for a long time to continue to please the sight-seer with its wonderful reflections.

Many other evidences of ice work in the Yosemite and Hetch Hetchy Valleys might be adduced, such as moraines, erratic bowlders, and striated rock surfaces, but it is desired only to invite attention to the testimony furnished by the great waterfalls. These are to be regarded as something more than mere spectacular features of the landscape; they afford a rough measure of the depth to which the main troughs have been overdeepened with respect to the "hanging" valleys on the upland. It is worth while in this connection to inspect the table of altitudes and heights of waterfalls on page 26. Many of the hanging valleys, it will be seen, debouch at heights ranging from 2,000 to 2,500 feet above the floor of the Yosemite Valley. It has been estimated from these figures, making due allowances for the lowering of the side valleys themselves and a number of other factors. that the Yosemite trough may have been excavated at least 2,000 feet. Whether all of this work is to be accredited to the ice, however or in part also to stream erosion is a question that can not yet be determined with certainty.

FIGURE 13.—Upper Yosemite Fall, 1,430 feet in height. The top is 2,565 feet above the valley floor. Figure representing Washington Monument is shown on the left.

INFLUENCE OF ROCK STRUCTURE.

Ice erosion alone does not explain all the characteristics of the Yosemite and Hetch Hetchy Valleys. It does not explain, for instance, why each of these valleys should so utterly differ in shape and general character from its own branch canyons. These also have been glaciated, but for some reason the ice in them appears to have modeled on essentially different lines. Thus, Tenaya Canyon, which enters the Yosemite at its head, though even more profound than that valley (4,500 feet deep opposite Clouds Rest) is relatively narrow in proportion to its depth. It is a constricted gash with sloping rock walls, a canyon or gorge rather than a valley. In antithesis to it stands the Little Yosemite, which is the path of the Merced. Only distantly analogous to the main Yosemite Valley, it is remarkable, above all, for its great width and relative shallowness. Further Tenaya Canyon debouches practically at the level of the main valley floor, whereas the Little Yosemite lies a full 2,000 feet above that floor. It has paradoxically the relations of a hanging valley, though really occupied by the master stream itself, which descends from it by a gigantic stairway, making two successive plunges, the Nevada Falls (594 feet) and the Vernal Falls (317 feet).

FIGURE 14.—Wapama Falls, Hetch Hetchy valley.

The Hetch Hetchy Valley is somewhat different in detail, but the problem involved is essentially similar. What most strikes one here is the difference between the open character of the Hetch Hetchy Valley, and the narrow, closed-in aspect of Tuolumne Canyon, the stupendous chasm debouching into it, which is about 15 miles long and fully a mile deep. So great is the disparity in the configuration of these two features, that at first it is difficult to understand how the two could have been produced by the same agent.

Further, there are a host of minor details that, on closer examination, seem anomalous and demand special explanation. Thus to confine our attention to the Yosemite Valley, that trough, as will be patent from the bird's-eye view, is strangely constricted near its middle by two great promontories that advance out into it from opposite sides, while immediately above and below this gateway the valley is wider than anywhere else. The hanging valley of Bridalveil Creek projects like a headland into the main trough, and the Bridalveil Fall itself anomalously leaps from the end of a cape. No less puzzling is the occurrence of the two ponderous granite bosses, Liberty Cap and Mount Broderick, that bar the Little Yosemite at its lower end. The valley walls themselves present an array of rock forms surprising both in size and in sculptured shape. Colossal undivided masses, such as El Capitan, thousands of feet high and broad, contrast with forms of utmost fragility, such as the finger-like minarets of the Cathedral Spires. Huge blank walls of mural straightness, as those under Glacier Point or the Yosemite Falls, alternate with sharp-cut reentrants and overhanging recessed arches, while upward the rock masses terminate for the most part in bare, rounded, helmet-like forms, such as North Dome and Sentinel Dome. Finally, at the head of the Yosemite Valley, dominating the entire landscape, stands that most enigmatic of all rock monuments of the High Sierra, the famous Half Dome.

To understand how these peculiar features have come into existence it is necessary to have some insight into the character of the rocks of the Yosemite region and also into the manner by which their structure has affected the sculpturing action of ice and water.

FIGURE 15.—Regularly jointed granite, an easily eroded rock. Kuna Crest.

The Yosemite and Hetch Hetchy Valleys lie hewn in granite, or, more strictly, in granitic rocks of various kinds. The larger part of the Sierra block consists of such rocks, all of them of igneous origin—that is to say, of rocks solidified from a molten state deep under ground. Stripped of their former rock cover by the long-continued denudation that took place in times preceding the Sierra uplift, they now appear at the surface. A few scattered remnants of the sedimentary rocks that once overlay them may still be found along the Sierra crest, mostly in the form of slates and quartzites.

The most marked peculiarity of the Sierra granites is the exceedingly irregular distribution of the so-called joints or natural partings in them. Most rocks in disturbed mountain regions are traversed by such partings, and these ordinarily occur in several sets crossing one another, the fissures of each set being parallel and spaced at more or less regular intervals. The entire rock mass thus is divided into fairly regular, flat-sided, smooth-edged blocks, called joint blocks.

Throughout the Sierra Nevada the jointing of the granites is full of vagaries, the partings being but a few feet, or even only a few inches, apart in places, while elsewhere they appear to die out, leaving the rock entirely massive and undivided. Figure 15 shows a typical example of jointed granite; figure 16 one of essentially massive rock. It is easy to see how the fissured rock must readily yield to erosive forces of any kind. It affords many avenues for percolating water and other weathering agents, and naturally tends to be converted into an aggregate of loose blocks and fragments, ready for removal. That it is especially susceptible to erosion by glaciers will also be clear; under the powerful drag of a heavy, overriding ice mass it is literally "plucked" or "quarried" away block by block.

The massive rock, on the other hand, because of the sparseness of its fissures, stands much better chance to resist the elements. Only its outer surface is vulnerable to the attacks of weathering agents, and consequently it is much slower to decay. As for erosion by glaciers, its individual joint blocks are so enormous in size and weight that no ice mass has the power to dislodge them. The only process by which the ice can reduce materials so massive is by abrading and grinding them with the rock débris held in its grip. But this process of abrasion is exceedingly slow and its results are not comparable with those of the quarrying method. Its characteristic products, the so-called glacier polish and striae, so often seen on ice-eroded valley floors, seem so impressive that one is apt to overestimate the amount of erosional work they represent. One would do better to regard them as evidences of retardation of glacial excavation due to the great resistance offered by the massive rock.

FIGURE 16.—Massive granite, not susceptible to glacial plucking, abraded and polished by ice.

It will be clear, then, that in a region of highly varied rock structure, such as the Sierra Nevada, erosion must necessarily proceed at locally varying rates. Where the rocks are most densely jointed they will succumb most rapidly and be worn into deep hollows, while the undivided massive rocks will resist the longest and in time remain standing out in high relief. All the knobs and domes that are so characteristic of the Sierra landscape have developed in this way; they are composed of sparsely fissured, essentially massive granite, and have survived the denudation that has swept away the weaker rocks that once surrounded them. As one studies more minutely the cirques and canyons along the Sierra crest he will see that they have largely been hollowed out in fissured material and that their individual peculiarities and minor details have all been determined by local inequalities in the structure of the rock. The High Sierra, one thus gradually comes to realize, owes its peculiar aspect not to ice erosion simply, but to ice erosion guided by the structures in the country rock.

And this, as a matter of fact, is also the explanation of the extraordinary character of the Yosemite and Hetch Hetchy Valleys. These two chasms may differ from the other valleys and canyons of the High Sierra, but the difference is not one of kind but of degree; they are characteristic of the Sierra landscape, but they represent its most extreme phase.

It is in the neighborhood of the Yosemite and Hetch Hetchy Valleys that the structural peculiarities of the Sierra granites reach their climax, for here the contrasts between fissured and unfissured rock are most extreme and abrupt. Zones of finely sheared granite, consisting wholly of thin plates and slivers in some places pass through bodies of more or less coarsely and regularly jointed rock; in other places they lie contiguous to undivided masses of enormous extent. Perhaps the best mental image one can form of the structure of the rock of these localities is that of many huge, solid, massive lumps, hundreds and even thousands of feet in diameter, embedded in a matrix of jointed material, the whole mass being traversed in sundry directions by narrow zones of shearing and occasional single master joints.

To one who has gained a thorough appreciation of the effect which such exceedingly diverse structures must have on the action of the eroding agents, the mystery of the apparently anomalous character of the Yosemite and Hetch Hetchy troughs vanishes at once. The pronounced differences noted in the shape and aspect of these two troughs and their respective branch canyons are seen to correspond to the differences in the structure of the rock masses which these chasms respectively traverse. The Yosemite Valley evidently was carved from prevailingly fissured materials in which the ice was able to quarry to great depth and width. Tenaya Canyon, on the other hand, was laid along a rather narrow zone of fissuring, flanked by close-set solid masses; and the glacier that flowed through it, while permitted to carve deeply—more deeply even than the mightier Yosemite glacier—was impeded in its lateral excavating and has been able to produce only a narrow, gorge-like trough. The Little Yosemite, again, afforded conditions of an essentially opposite nature. Here the ice was allowed to quarry sidewise, but its downward cutting below a certain level was arrested by the huge horizontal beds of massive granite that form the valley's basement. All the ice could do was to abrade and polish these, as one may see in many places where the solid rock floor is exposed to view, notably at the head of the Nevada Falls.

But even in the Yosemite Valley itself the ice did not erode with equal facility at every point; some of the rock materials were more readily quarried away than others, and various inequalities in the trough form have resulted. The great width of the valley near the Cathedral Spires, as one might expect, is due to the particularly close jointing that is characteristic of the rock in that locality, which greatly facilitated the glacial quarrying. The narrowest place, on the other hand, occurs, it goes almost without saying, between two rock masses of exceptional solidity and strength—El Capitan and the Cathedral Rocks. Scarcely susceptible to glacial plucking, they have stubbornly resisted reduction by the passing ice, which was thus forced to squeeze through a narrow strait. Immediately below the constriction the valley walls flare out again, widening in proportion as the rocks become more fissile, and the trough regains its former width. The anomalous projecting character of the hanging valley of Bridalveil Creek is thus explained by the protection afforded it by the bulwarks of the Cathedral Rocks, though these have nevertheless seriously suffered at the hands of the ice; for the rocks as they now appear are merely the remnants of what was once a strong, continuous ridge. Its east half has been pared away by the ice, almost up to the crest line.

The constriction at the mouth of the Little Yosemite Valley is similarly explained. It was the essentially massive, unquarriable nature of the rock composing Liberty Cap and Mount Broderick that enabled these two bosses to hold out against the ice. The glacier coming down the Little Yosemite not only impinged against them, but at times even entirely overrode them, as is attested by the glaciated bowlders still lying on their summits. It was able, however, to reduce them but relatively little, though it was successful in stripping away the surrounding weaker rocks as well as the material from the fissured zone between them, now hollowed out into a profound, gaping cleft.

FIGURE 17.—El Capitan from the east.

DETAIL SCULPTURE.

Not all the rock forms of the Yosemite and Hetch Hetchy Valleys, however, are of ice-hewn origin. The detailed carving of the cliffs has been mostly the work of the dismantling agents that took hold after the ice had left its task. Weathering, frost, and percolating water are the sculptors that have done the finer chiseling; the glaciers only modeled in the rough. Like the glaciers they have throughout followed the dictates of the structure, and being inherently more subtle in their action than the ice, with its coarse quarrying process, they have wrought far more minutely and with greater delicacy of touch. They have responded to every local change in fissuring and have brought out every structural incident in bas-relief.

Thus it is the exceptional solidity of its granite that has enabled El Capitan to maintain the rare boldness of its 3,000-foot front. So little débris has fallen from it since the days of the ice as to be scarcely noticeable at first glance. El Capitan, more than any other cliff, may be said to typify the "rock of ages." Yet immediately to the west of El Capitan is a deeply incised gulch, excavated along a zone of shattering, from which the crushed materials have fairly vanished before the onslaughts of the weather. (See bird's-eye view, fig. 9.)

FIGURE 18.—Cathedral Rocks and Spires. Notice the deep, narrow clefts etched out along zones of shattering.

Similar but narrower zones are being etched out into clefts gashing deeply into the otherwise massive Cathedral Rocks and threatening to sever them from each other. Hard by, again, in the region of the Cathedral Spires, wholesale ruin has overtaken considerable portions of the canyon wall, and the crumbling materials now rest in huge débris slopes from which only two lone, tottering shafts emerge. Farther up the valley the rock is firmer, but is traversed by many inclined joint planes, which appear to have guided the sculpturing agents and have given rise to strangely unsymmetrical, faceted shapes. The Three Brothers (fig. 19) afford the most impressive form of this type.

FIGURE 19.—The Three Brothers, sculptured along slanting joint planes. The highest point, known as Eagle Peak, is 3,183 feet above the valley.

Vertical joint planes have evidently determined the face of Sentinel Rock, and of the great blank walls under the Yosemite Fall and Glacier Point. Indeed, as the tourist studies the great cliff walls of the Yosemite Valley, he soon finds that each one coincides with a flat joint plane. It is worth while, in this connection, to take up the large-scale map of the Yosemite Valley,¹ and to note thereon how these cliffs accord in their respective trends. Many of them run in northeast-southwest directions; others at right angles to these—northwest-southeast. Evidently these are the directions of the two preponderant joint systems of the region, but there are also others, some running north-south, others east-west. The Glacier Point and Yosemite Falls cliffs belong to the east-west system.

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¹May be had for 10 cents on application to the Director of the U. S. Geological Survey.

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FIGURE 20.—Sentinel Rock, 3,000 feet high. Its face has been determined by a joint plane.

The peculiar orientation of the Cliffs of the Nevada and Vernal Falls was determined by the trend of certain joint planes. These cliffs are two of the great glacial steps cut out in the valley floor by the ice. Neither of them, however, extends squarely across the valley; both run obliquely to its axis. A glance at the map mentioned suffices to show that the upper one is controlled by a joint of the northeast-southwest system, the lower one by a northwest-southeast joint. However potent the glacier descending from the Little Yosemite may have been, clearly it was unable to carry on its excavating without regard to the structures of the locality, but was compelled to quarry in directions strictly according with their trends.

FIGURE 21.—A typical dome landscape. North Dome and Basket Dome in middle distance.

DOMES.

Not all the Yosemite cliffs, however, are of this rectilinear, clean cut sort a surprisingly large number are laid out on rounding curves. The dome-shaped eminences bordering the valley evidently belong to the same category of rock forms. Placed apparently at random in the landscape, these huge, bulging masses of bare granite give it in no small degree its unusual character. What is the explanation of their origin?

They represent the great, massive lumps in the Sierra granite that refused to fracture under the earth stresses which elsewhere produced the various systems of joints. Essentially solid and undivided, they are monoliths in the true sense of the word. Yet it is noteworthy that they tend to flake off, so to speak, and are invariably covered with thin, concentric shells, fitting one over the other like the layers of an onion. The real origin of this peculiar structure is not yet fully understood, but this much can be said with certainty, that it develops only at the surface and must be due to expansive stresses in the rock which have come into play since the domes were uncovered by denudation and became exposed to the weather.

The remarkable roundness of the Sierra domes has by some been attributed to ice erosion, but this view is clearly based on a misconception. However much of the peculiar rock topography of the Sierra landscape the ice may have sculptured, it did not carve the domes. Indeed, the domes should rather be looked upon as masses which by virtue of their extreme solidity have escaped remodeling by the ice. It is true that many of them have been overridden by the glaciers, but, aside from tearing off the loosened shells, the ice can not be said to have either produced or accentuated their rotundity. Stone Mountain, in the State of Georgia, affords as fine an example of a smoothly rounded granite dome as any to be found in the High Sierra, yet it has not been touched by the ice of the glacial epochs, for it stands hundreds of miles south of the southernmost limit reached by the ice.

FIGURE 22.—Back view of Half Dome. (Note the deep water-worn grooves.)

The fact is that the domes have acquired their roundness largely by casting off successive shells. In the course of this process they have become progressively simpler and smoother in outline and more compact in form. It is highly probable that most of them did not possess smooth exteriors at the start, but that they were prevailingly irregular and angular in outline. As exfoliation went on, however, these initial irregularities were gradually subdued until at last they were entirely eliminated.

A few domes, neverthleless, even in their present outlines, still carry a suggestion of the strongly marked features which they formerly possessed. The most notable and interesting of these is Half Dome.

Cut down a sheer 2,000 feet on its northwest side, it at first impresses one as the remnant of a huge rock sphere, one-half of which has suddenly been engulfed. Examination of its cross profile and rear side from well-chosen points of view, however, soon reveals it to be a narrow, elongated rock mass, the steep front and rear sides of which are essentially parallel. (See figs. 22 and 23.) A look at the large-scale map of the Yosemite Valley further discloses the fact that the trend of these two faces accords with the northeast-southwest system of joints. There is every reason to believe, therefore, that originally the block was bounded on both sides by strongly jointed structures belonging to that system.

FIGURE 23—Telephoto view of Half Dome taken in line with its sheer northwest face.

The front and back of Half Dome, however, are by no means alike in aspect. The back seems normal in appearance in spite of its faint curvature, but the precipitous front, trenching abruptly across the otherwise flowing outlines, seems aberrant and demands special explanation.

In general, the back may be said to be the old side of the dome. It has evolved through normal shelling at an exceedingly slow rate. The length of time a single shell may remain clinging to its surface is strikingly attested by the deeply fluted aspect of the enormous shell that now envelopes the entire back of the dome. The loosened rock particles that for ages have been washed from the crown of the dome have worn deep furrows, which are visible at a distance of several miles.

The front of the dome, on the other hand, appears by contrast smooth and fresh. It has been formed rather recently through the rapid scaling off of successive thin plates or sheets cleft by close-set parallel partings of an accentuated fissure zone. A body of these plates still clings to the dome front at its northeast end, and it is there that one may observe the character of the fissure zone noted. Ice that formerly lodged at the foot of the great precipice no doubt has served to accelerate its recession. As for the remarkable overhang at the top of Half Dome, this is explained by new exfoliation, beginning on the exposed front of the monolith, by which the older shells on the summit are being undercut.

Thus every detail in the configuration of Half Dome is seen to be expressive of some structural attribute. The very hugeness and uniqueness of the rock monument are themselves the direct outcome of the exceptional character of the rock masses involved. And this characterization also holds for the Yosemite and Hetch Hetchy Valleys in their entirety; they are what they are inherently by virtue of the structural peculiarities of their rocks. In these materials streams and glaciers have modeled boldly and on unusual lines, while rain and wind and frost and sunshine, as well as the less obtrusive chemical processes of disintegration, have added the finer touches, bringing out the subtler structural phases of the country rock in the detail sculpture of the walls.