GLACIAL HISTORY OF THE YOSEMITE VALLEY
EVIDENCES OF GLACIAL ACTION IN THE YOSEMITE REGION
In no part of the Sierra Nevada have the evidences of glacial action been studied in greater detail than in the Yosemite region. John Muir was the first to engage in this work. During the years of his residence in the Yosemite Valley he devoted a large part of his leisure time to tracing the pathways followed by the ancient glaciers, not only in the immediate vicinity of the valley but also in the High Sierra above. Thirty years later Henry W. Turner, of the United States Geological Survey, undertook to locate the larger glacial deposits on a small-scale map, but he did not finish the task. It was not until 1913 that a systematic and detailed glacial surveythe survey on which the present account is basedwas instituted and that the data obtained were assembled on a large-scale map. In subsequent years this survey was extended to the High Sierra as well as to the country below the Yosemite region, until at length the entire area that once lay under the dominion of the Yosemite Glacier, with its tributaries and neighbors, was covered. As a result the length, breadth, and depth attained by each of these glaciers at the time of their greatest extension are now definitely known. What is more, some insight has been gained into the history of their advances and recessions in response to climatic changes, so that it is now reasonably certain that the Yosemite Valley was invaded by a glacier three times. There is even some warrant for the presumption that the valley was invaded more than three times, the glacial history of the Sierra Nevada, like that of the northern parts of the continent, having consisted of several wintry sub-epochs, or "glacial stages," alternating with mild subepochs, or "interglacial stages."
The reader not familiar with the methods of glacial research will wonder, probably, upon what manner of data these conclusions rest. It seems appropriate, therefore, to explain at the outset the nature and significance of the evidences searched for and the procedure followed in the glacial survey.
KINDS OF EVIDENCE STUDIED
The evidences of glacial action in a mountain region are in general of three kindsgrooved and polished rock surfaces, characteristic topographic forms, and deposits of ice-borne rock débris. Of these three kinds of evidence the first would naturally suggest itself, because of its highly distinctive nature, as the easiest to recognize and therefore the most valuable for the identification of ancient glacier paths.
Glaciers, indeed, literally grind, tool, and polish their beds with the rock débris that is frozen in their bottom layers. The large angular blocks produce grooves a fraction of an inch in depth; the smaller fragments produce fine parallel "striae"; and the sand and mud scour and polish the rock until it fairly gleams. The resulting striated and glassy rock surfaces are familiar to all who have visited intensely glaciated mountain areas. In the upper Yosemite region and the adjoining parts of the High Sierra such surfaces are particularly plentiful and extensive; there one may walk on "glacier polish" for considerable distances.
However, these vivid evidences of glacial action are not as a rule long lived. The reason is obvious; the rock, being constantly exposed to the weather and the wearing action of running water, inevitably decomposes and disintegrates in the course of time; the polish flakes off, the surface becomes increasingly rough and irregular, and the evidence of glaciation disappears. Some types of rock, naturally, weather more rapidly and lose their polish sooner than others, and this is true also in the Sierra Nevada. To assume, therefore, that the ancient glaciers on that range extended no farther than the polished and striated rock surfaces seem to indicate would be clearly unjustifiable. Actually the glaciers extended much farther; they spread over a vast contiguous territory from which all vestiges of glacial abrasion have now vanished.
Neither can the testimony of the topographic forms be safely relied upon. Among the more conspicuous forms that are generally held to be characteristic products of glaciation are U-shaped, troughlike canyons having spurless, parallel walls and stairwise descending floors that commonly hold shallow lake basins on their treads; also, hanging side valleys from whose mouths the waters cascade abruptly down into the main canyons. Such forms the reader already knows to be abundantly represented in the Yosemite region, and it might therefore seem to him that it would be a simple matter to determine the limits reached by the ice by observing how far these telltale forms extend down the flank of the range. However, it is to be borne in mind that the development of characteristic glacial forms is conditioned in part by the nature of the rock. Glaciers do not work with equal facility in all kinds of rock. As will be shown more fully further on, they work to best advantage in soft rocks and closely jointed rocks. In massive or sparsely jointed granite they accomplish relatively small results and seldom produce characteristic glacial forms. Thus it is that in the Sierra Nevada, where granitic rocks of massive habit prevail over large areas, many canyons and valleys retain in large measure their preglacial V shapes in spite of repeated vigorous glaciation. A conspicuous example is the Grand Canyon of the Tuolumne River, which lacks for the most part the characteristic U form of glaciation, although it was the pathway of the greatest and most powerful ice stream in the Sierra Nevada. Again, some valleys and canyons, though considerably remodeled by the ice, lack steps and lake basins in their floors, as, for instance, the valley of the Dana Fork of the Tuolumne River.
As shown on pages 34-35, side valleys can be left hanging as a result of other than glacial processes, notably by the rapid trenching of a master stream whose course has been steepened, in consequence of tilting, more than the courses of its tributaries. The Sierra Nevada abounds in hanging valleys of just such an origin, the Merced and the other southwestward-flowing master streams having intrenched themselves rapidly in consequence of the tilting of the range, whereas many of the small tributaries trending at right angles to their courses have not been able to trench at the same rate. In the Sierra Nevada, therefore, hanging valleys can not be accepted as prima facie evidence of glacial action.
The most reliable record of glacial activity, on the whole, is that embodied in the deposits of rock waste left behind by the glaciers, particularly those ridge-shaped deposits termed "moraines." These unobtrusive features, wherever well preserved, accurately define the limits reached by the ancient glaciers. It was, therefore, with the moraines that the survey was mainly concerned. What now are the characteristics and the significance of a moraine?
Every glacier carries large quantities of rock waste, ranging from huge boulders down to the finest mud. Of this material part has fallen upon its surface from the frost-riven canyon walls; part has been torn from the floor and sides of the canyon by the glacier itself. The margins and bottom layers of a glacier naturally are most heavily loaded, and in consequence, as the ice melts in summer, some of the rock waste is released and dropped at the sides and the front. If the glacier remains essentially unchanged in size for a considerable period, the rock waste accumulates around it in the form of a more or less sharp-crested ridge or moraine. Commonly the ridges along the flanks of a long-drawn glacier are called the lateral moraines, and the ridge at its front the terminal or, more properly, the frontal moraine, but in their most perfect development these ridges really form together a continuous loop.
The height to which moraines may be piled depends naturally upon the size and eroding power of the glacier, the abundance of rock waste, and the length of the period of accumulation. In the Sierra Nevada few moraines are over 60 feet in height and most are only 10 to 30 feet, for the glaciers were but lightly loaded, the granitic rocks over which they passed being for the most part too massive to be readily disrupted by the glacial processes. By contrast, in the Rocky Mountains and the Cascade Range, where the rocks are closely fractured and readily disrupted, many of the moraines, even those built by small glaciers, attain heights of a hundred or several hundred feet.
When a glacier melts back, as a result of amelioration of climate, the moraine loop formed during its previous stationary period is left standing as a sort of mold that bears witness to the former proportions of the ice mass. (See fig. 17.) Although composed of unconsolidated materials, the moraine loop may persist in a fair state of preservation for thousands of yearsfor a long time after the glacier has ceased to exist. It may be recognizable even when the glacier polish has all flaked off from the rocks. Thus it is in the Sierra Nevadathe farthest limits reached by the glaciers of the ice age there are still marked by moraines, or at least by remnants of moraines, although the glacier polish has disappeared over large areas. In addition, many phases in the slow decline of the glaciers remain similarly outlined, for whenever, in obedience to climatic variations, the glaciers made a prolonged halt in their retreat, or a minor readvance, they each left a subsidiary, recessional moraine loop. As a consequence each valley contains as a rule a series of moraine loops lying one within another and growing progressively shorter toward the crest of the range. The frontal parts of these loops curve across the valley floors in the form of low embankments, commonly breached by the streams, and the lateral parts extend for miles along the valley sides, wherever these are not too steep, in the form of boulder-strewn ridges or terraces paralleling each other at different levels.
In the glacial survey of the Yosemite region all identifiable moraines and remnants of moraines, even isolated glacial boulders, were duly located and mapped, and thus there is now at hand a detailed record of the successive advances and recessions of the ancient glaciers. This record, for the Yosemite Valley and the areas immediately contiguous to it, is embodied in the map of glacial and postglacial deposits forming Plate 29. Let us now, with the aid of this map, analyze the morainal system of the Yosemite Glacier and see what story it tells.
MORAINES IN THE YOSEMITE VALLEY
In the lower half of the Yosemite Valley six frontal moraines may be counted within the distance of 1 mile. The lowermost of the series is immediately above the Bridalveil Meadow. Being widely breached by the Merced River, it consists really of two short segments, one on the south side of the valley, the other on the north side. The southern segment is readily identified as a massive wooded ridge about 40 feet high, that extends in a northwesterly direction and breaks off abruptly at the bank of the river. The highway is cut through its southeast end, thus affording a glimpse of its internal composition. The northern segment consists of a narrower and sharper crested ridge that extends southwestward to the edge of the northern motor road, where it also breaks off abruptly.
To a casual observer the relations which these two ridges bear to one another are not likely to be manifest, owing to the obscuring forest growth; but a glance at their positions as platted on the map will at once convince him that the two ridges are indeed the severed remnants of a formerly continuous moraine loop. Just how much farther on each side this moraine loop originally extended can only be surmised, as its outer ends are now buried under masses of rock waste that have fallen from the cliffs in postglacial time; but it seems probable in any event that the moraine curved across the entire width of the valley floor, from cliff base to cliff base.
The materials of which this moraine is built may be readily inspected in the road cuts. Most conspicuous are the large, smoothly rounded boulders, but mixed with these are cobbles, pebbles, angular fragments of rock of different sizes, and an abundance of sand and mud. Many of the rounded boulders and cobbles, moreover, when washed clean and held to the sunlight, appear highly polished and in places scratched or scored, like the glaciated canyon floors of the High Sierra. This fact in itself proves that they have been brought down by a glacierthat they are glacier-worn as well as stream-wornfor, though their roundness of form doubtless is due largely to stream wear, their polish could have been imparted only by long-continued abrasion with "rock flour," that almost impalpable substance produced by the grinding action of glaciers, which is many times finer than the sand or mud in a stream bed.
To the trained eye of a geologist, moreover, it is readily apparent that among the boulders and cobbles, as well as among the angular fragments, there are some representing rock types foreign to the Yosemite region, though prevalent in the High Sierra. Among these are several types of quartzite, schist, and limestone, but the most readily identified is a buff or pinkish granite containing nearly rectangular crystals of white feldspar 1 to 2-1/2 inches in length and resembling pieces of domino sugar. Whoever has traveled observantly through the High Sierra will at once recognize this rock as the type of granite which is most abundant in the Merced and Tuolumne Basins and of which notably. Cathedral Peak and adjoining parts of the Cathedral Range are made. Its appearance is equally striking in the glacier-polished canyon floors, where the crystals lie as in a natural mosaic (pl. 52, B), and in the rough, weathered crags and blocks on the peaks, where the crystals project fantastically from the less resistant matrix. It is known, appropriately, as the Cathedral Peak granite. (See pl. 37, B.)
The presence of these materials of distant origin, both in rounded, polished boulders and in rough, irregular fragments, mixed with rocks of local derivation and with sand and mud, is characteristic of the make-up of a glacial moraine and, taken together with the forms and position of the ridges, leaves no doubt whatever as to their glacial origin.
The second and third moraines of the series are even more widely breached than the first; indeed, they are very largely demolished. Their remnants, moreover, are covered by thickets and therefore are not readily recognizable by their forms, yet they may be identified by their constituent materials, which project through the pine-needle litter here and there and are exposed in the sections at the banks of the river.
Of the fourth moraine, by contrast, the principal remnant stands out boldly in a place where it could scarcely be overlooked. It projects from the northwest base of the Cathedral Rocks in the form of a stony scantily timbered ridge about 30 feet high.
Of greatest interest are the fifth and sixth moraines, which together form a nearly straight dam across the valley just below the El Capitan Meadow. Only a small gap interrupts the continuity of this damthe gap cut by the river and now spanned by the El Capitan Bridge. North of the bridge the road is laid upon the sixth moraine, as upon a causeway raised a few feet above the level of the meadow, and it is flanked on the west by the crest of the fifth moraine, which attains a maximum height of 15 feet. Northward both moraines gradually disappear from view under the sloping mass of rock waste derived from the north wall of the valley. Their southern extremities are similarly buried under the coarse débris that has fallen from the sheer cliffs of the Cathedral Rocks.
The height of these moraines above the meadow is probably but a fraction of their total height above their base on the rock floor of the valley. Only their crests are now exposed, the major part of their bodies being buried, on the upstream side under stratified sand and gravel, on the downstream side under stream-washed glacial débris, in the manner shown in Figure 18. The glacial débris was doubtless deposited at the time when the Yosemite Glacier lay with its front directly against the fifth moraine. The waters flowing from the ice mass at different points then spread part of their load of débris in front of the moraine in the form of an "outwash apron." The sand and gravel on the upstream side, on the other hand, must have been laid down in a lake basin that was created by the moraine dam when the Yosemite Glacier finally melted away. The postglacial lake whose former presence these sediments attestancient Lake Yosemite it may be calledprobably reached at first to the very head of the valley and had a length of 5-1/2 miles, but it was filled in time by the gradual forward growth of the delta of the Merced River and thus was ultimately replaced by the present level valley floor.
That the two moraines should have been buried by the sediments so nearly up to their crests is explained by the inability of the river to breach the dam to any great depth. Inspection of the gap shows that the cutting action of the river was impeded by the presence of many large boulders. As a consequence the lake was maintained at a high level to the very last.
Though the full height of the moraines above the rock floor of the valley remains conjectural, it may reasonably be estimated at not more than 60 or 70 feet, for no other moraines in the Yosemite region are known to attain greater height. It may seem surprising, that a moraine dam of such moderate height should have sufficed to impound the water for a distance of 5-1/2 miles, but, as shown on page 94, there are good reasons for believing that the dam rests on a swell or sill in the rock floor of the valley and that a glacially excavated basin formerly extended up the valley from the sill.
As early as 1863 Clarence King recognized the existence of a terminal moraine in the lower part of the Yosemite Valley. He also interpreted the present level valley floor as having been created by the filling of a temporary lake impounded by the moraine. However, the passage in Whitney's report45 relating to King's observations is so loosely worded that there is no certainty as to which of the several moraines actually existing is meant. The description given seems best to fit the lowermost moraine, the one near the Bridalveil Meadow, but that moraine is fully three-quarters of a mile below the one that impounded Lake Yosemite.
King also noted that in the part of the valley above the terminal moraine observed by him débris is conspicuously scant and in places almost wholly absent at the bases of the cliffs, whereas in the part of the valley below the moraine the walls are cloaked with débris to a great height. He therefore concluded, not unnaturally, that this moraine marks the farthest limit reached by the ancient Yosemite Glacier, and that below it the valley has remained unglaciated. As a matter of fact, the moraine next to the Bridalveil Meadow marks the farthest limit reached by the glacier during the last stage of glaciation onlythe last chapter of glacial history, so to speak. In the earlier glacial stages the ice extended many miles beyondas far as the site of El Portal, as is demonstrated on page 67. The six frontal moraines between the Bridalveil Meadow and the El Capitan Meadow, therefore, are to be interpreted as recording the oscillations of the Yosemite Glacier's front during and immediately after the climax of the last glacial stage.
Above the El Capitan Bridge no further moraines are to be found in the Yosemite Valley for a stretch of about 5 milesthat is, to a point within a short distance of the head wall. There, however, is the largest and most conspicuous moraine of all. It is a hummocky ridge 50 to 60 feet high and half a mile long that extends from the head wall directly down the middle of the valley and declines gradually to the level of the valley floor at the Clark Bridge. The road that leads from the Happy Isles to Mirror Lake passes through a saddle in this ridge. From the cut have been taken many rounded and polished boulders of Cathedral Peak granite and other rock types from the High Sierra.
This ridge is commonly supposed to be a "medial moraine" that marks the line of junction between the Merced and Tenaya Glaciers, the two main branches of the Yosemite Glacier. The supposition would seem reasonable enough, for wherever two glaciers are confluent the débris carried on and in their coalescing margins is brought together in a narrow débris zone in the ice that extends down the middle of the trunk glacier and forms at its surface a hummocky dirt band known as a medial moraine. Doubtless the Merced and Tenaya Glaciers by their confluence gave rise to such a medial moraine on the Yosemite Glacier. The ridge at the head of the valley accordingly, is supposed to have been formed when the débris in this medial moraine and in the dirt zone in the ice underneath dropped to the ground as the glacier melted away.
The correctness of this interpretation, nevertheless, seems open to question, for although medial moraines are common features on glaciers, they are not known to give rise, upon the departure of the ice, to prominent ridges that extend longitudinally down the valleys. The fact is that a medial moraine consists only of débris in transit, and the quantity of material present in it at any given time would be far too small to produce a high, bulky moraine ridge if it were let down to the ground. Now the moraine at the head of the Yosemite Valley compares favorably in height and bulk with the largest frontal moraines left by the Yosemite Glacier. It must therefore be, like them, a product of long-continued deposition and can not have been formed by the dropping to the ground of the débris in a medial moraine. It is in all probability a frontal moraine, either of the Tenaya or of the Merced Glacier. Its position in the axis of the valley gives no clue as to which of these glaciers might have built it; neither do the boulders which it contains, for both glaciers carried about the same kinds of rock on their coalescing margins. However, the faint curvature of the ridge, concave toward the north, suggests that it is the remnant of a moraine loop of the Tenaya Glacier, and this suggestion finds support in the fact that there is a small remnant of a moraine also near the base of the Royal Arches, at a place where the opposite end of such a moraine loop might be looked for. It is not impossible, finally, that the ridge in question is compound, consisting of a frontal moraine of the Tenaya Glacier and a frontal moraine of the Merced Glacier that were pushed together by the opposing ice streams, but as to that there is no tangible evidence.
The complete absence of moraines in the 5-mile stretch between the El Capitan Bridge and the Clark Bridge can scarcely be explained by the great thickness of the sediments that fill the basin of ancient Lake Yosemite (the prominent ridge at the head of the valley doubtless lies in the shallow upper part of the basin). These sediments, it is true, may reach a thickness of more than 200 feet, as would appear from the cross sections of the valley in Figures 24 and 25, and might therefore readily cover up any moraine loops left by the Yosemite Glacier; but even if they did, it seems likely that the ends of the loops would remain visible at the sides of the valley. As no such ends of moraine loops exist, it may be concluded that the Yosemite Glacier did not deposit any frontal moraines above the El Capitan Bridge but melted back steadily or as a stagnant mass throughout the 5-mile stretch to the valley head. Observations on the moraine systems in a number of other valleys in the Sierra Nevada bear out the correctness of this conclusion: they show that each of the other glaciers, after depositing about half a dozen closely spaced frontal moraines, melted back for a considerable distance without leaving any distinct moraine loops.
MORAINES IN THE LITTLE YOSEMITE VALLEY
In the Little Yosemite Valley moraines are much more plentiful than in the main Yosemite. They form an orderly system rich in lateral and in frontal moraines and including several almost complete loops. The arrangement of its component members is strikingly brought out on the map of glacial and postglacial deposits (pl. 29), and its details record some interesting phases in the progressive decline of the Merced Glacier. For instance, it is evident from the positions of the moraines at the lower end of the Little Yosemite that at one stage the waning glacier, no longer able to overtop Mount Broderick (it did not overtop Liberty Cap), split into three lobes that cascaded down to the gorge of the Merced below, each through a separate gap. At a later stage, when the three lobes had become too short to cascade from the mouth of the Little Yosemite, the northernmost lay stagnant in the pocket between Mount Broderick and Half Dome. Doubtless it impounded there a small lake, which grew in size as the ice lobe melted back, until at last the rocky spur that projects eastward from Mount Broderick was entirely uncovered, when the water drained out. The present reed-grown pond known as Lost Lake, which is dammed at its east end by a small moraine left by the retreating glacier, is all that remains of that peculiar body of water.
Perhaps the most significant feature of the series of moraine loops left by the middle and southern lobes is their approximately regular spacing. Perhaps the spacing would appear even more regular on the map were all the moraines laid down by the glacier actually represented, but some of the moraines are now reduced, as a result of rain wash and other agencies, to only a few scattered boulders and therefore were not mapped. In any event, the spacing is so prevailingly regular as to justify the inference that the recurring fluctuations of the declining Merced Glacier were essentially periodic. The same inference may be drawn also from the spacing of the frontal moraines that curve across the entire width of the valley floor about a mile above Liberty Cap, though their intervals are considerably larger than those between the moraines at the lower end of the valley.
The moraines that span the entire width of the valley, above Liberty Cap, naturally are more voluminous than those which outline the individual lobes, yet they are for the most part inconspicuous in the landscape. The reason is that they are buried almost up to their crests in sand and gravelmaterials that were deposited by the Merced in a shallow basin that was excavated by the glacier in the rock floor of the Little Yosemite and was analogous to the much larger and deeper glacial basin in the main valley, which was occupied by the waters of ancient Lake Yosemite.
Particularly full and enlightening is the record of fluctuations of the Merced Glacier that is embodied in the lateral moraines. These moraines are best developed and most advantageously spread out for individual study on the north side of the valley, in the broad embayment southeast of Half Dome, where cliffs are lacking and slopes of moderate declivity prevail. No less than 30 distinct moraine ridges are situated there, one above another in a great, essentially parallel series. As seen on the map their arrangement in parallel curves and tangents and their coalescence by twos and threes at the east end strongly resemble the layout of the tracks in a large railroad yard.
The regular spacing of these moraines on the sloping valley side, again, is perhaps their most significant feature; it indicates much more fully and more convincingly than the spacing of the incomplete system of frontal moraines on the valley floor that the climatic pulsations by which the Merced Glacier was affected during its decline occurred with essentially rhythmic periodicity.
The Sunrise Trail zigzags up across the entire series of lateral moraines, thus affording many opportunities for their close inspection. Impressive even to the casual observer is the prevalence of large blocks in these moraines. Some of the ridges are made up almost wholly of large fragments and contain but little fine material. (See pl. 30, B.) Blocks 5 or 6 feet in length are common; not a few measure 10 or 15 feet to the side. Most of the blocks are imperfectly rounded, or at least subangular, their edges and corners having been blunted and worn smooth in the course of their long journey on and in the ice; but there are some sharply angular, clean-cut blocks that appear to have suffered almost no wear since the day when they were torn from the cliffs. The proportionate number of these sharp-edged blocks increases noticeably up the valley, toward the sources of the glacier.
The excellent state of preservation of many of the blocks is also worthy of comment. Though weathered boulders are plentiful in all these moraines, nevertheless the traveler on the Sunrise Trail is constantly impressed by the great number of fresh-looking, essentially unweathered blocks that are in view on every hand. These blocks ring when struck with a hammer and give every proof of being composed of sound, hard granite. This fact gains significance when it is considered that these blocks have lain exposed to the weather for certainly not less than 10,000 yearsthose in the highest moraines of the series more probably for about 40,000 years, that being the length of time which some geologists now estimate, on the strength of several lines of evidence, to have elapsed since the glaciers began to recede from their culminating stage.
One reason why so many of the moraines are composed largely of coarse material is that most of the finer particles have been washed from them by rain water. Granite sand inherently has poor binding qualities and in consequence is easily dislodged, grain by grain, by running water, even by the merest rain-water rills. The extent to which some moraines have been thus despoiled of their sandy constituents is particularly evident in those places where the grains washed down the slope have been arrested by the moraine next below and have accumulated so as to form a smooth sandy terrace. (See fig. 19.) Terraces of this kind afford good going, and the trail builder wisely has taken advantage of them for considerable stretches.
On the rocky slopes of the promontory southeast of Sunrise Creek the higher moraines of the series gradually approach one another until at length they unite by twos or threes to form massive embankments from 10 to 50 feet in height. The topmost and largest of these embankments leads up over the very summit of the promontory, thence down nearly 100 feet to a saddle and again up in a grand spiral curve around the south side of Moraine Dome. It is, indeed, for this remarkable morainal embankment that the dome is named. The strip of forest which the moraine sup ports sets it off from the barren rock slopes above and below, thereby enhancing its prominence in the land scape. (See pls. 30, A, and 31, B.)
At a point about 150 feet below the summit of the dome the moraine is cut off by a precipice too steep to retain glacial débris; but a few hundred yards farther on, near the end of the ridge that extends northeastward from the dome, the moraine reappears, wrapped tightly around a knob that evidently stemmed the ice flood and caused it to divide. Pursued still farther, the moraine is found to plunge steeply down the north side of the ridge, dipping into the vale of Sunrise Creek, and to rise again on the other side to the base of the Clouds Rest massif. It describes, as is evident from the map, an arc outlining the front of a lobe that split off from the Merced Glacier. This lobe pushed through the saddle northeast of Moraine Dome but, curiously, advanced only a short distance, although favored by the steep downward slope. Behind the main moraine are several lesser ones disposed in concentric curves and marking successive phases in the recession of the lobe.
From the saddle just mentioned the moraines extend northeastward for a distance of 2-1/2 miles, as far as the steep rocky spur at the south end of Sunrise Mountain, forming another great series of parallel crests and terraces. It is here that the moraines attain their greatest volume and are best developed, many of them resembling artificial embankments, evenly graded and laid out in smooth curves. The topmost moraine is the largest and the most impressive. It bears on its crest a row of granite blocks which, shimmering white through the dark foliage of the trees, produces the effect in a distant view of a chalk line drawn across the mountain side to mark the highest level reached by the ice flood. (See pl. 31, A.)
On the south side of the Little Yosemite the lateral moraines are not so well displayed as on the north side, mainly because of the prevalence of smooth rock faces on which the glacial débris can find no resting place. The only moraines of the series that are strongly developed are the upper ones, which rest on a shoulder above the precipitous ice-carved cliffs. (See pl. 32, B.) This shoulder marks the lower margin of the relatively gentle upper slopes, which are remnants, but slightly modified, of the preglacial valley side. From the point where the trail leading from the Nevada Fall to Glacier Point reaches the shoulder the upper moraines extend eastward for a distance of 1 mile, the highest forming a mighty rock-crowned bulwark. They end at the ice-smoothed ledges on the salient north of Helen Lake, this being a place of glacial scouring and not of deposition; but farther east, in the deep recess between the salient mentioned and the Cascade Cliffs, the moraines reappear in force. They fill this recess so completely that their frontal slope is almost flush with the two flanking promontories. (See pl. 29.)
It is a significant fact that the topmost moraine, instead of curving into the recess and hugging the wall, extends across the opening in the form of an almost straight embankment, and that behind it there is a flat formed of granite sand that has been washed down from the slope above. What is more, in two places the low crest of a moraine projects slightly above this sandy flat, close to the slope. It is thus evident that before the Merced Glacier attained its highest stage and laid down, the straight morainal embankment it had attained a slightly lower stage during which it invaded the recess to its full depth and deposited a moraine against the slope. The question naturally arises: Why did not the glacier invade the recess also during its culminating stage and deposit the second moraine directly upon or against the first? There can be only one explanationthat the glacier, after invading the recess and depositing the first moraine against the slope, subsided with repeated fluctuations and deposited a series of moraines at successively lower levels, in the manner indicated in Figure 20, and that, having thus nearly filled the recess with débris, it rose to a second climax a little higher than the first. Being unable now to expand as broadly as before, it built its new moraine in the form of an embankment upon the old accumulation.
It is to be concluded, then, that there were really two ice floods separated by an interval during which the glacier melted down to a relatively low level or perhaps withdrew from the valley altogether. The interval, however, was not a long one, for the moraine of the first ice flood looks but little older than the embankment produced by the second, and the continuity of the embankment shows that the mass of débris left in the recess by the first ice flood had suffered but little from erosion when the second ice flood supervened. Similar evidence pointing to the occurrence of two ice floods separated by a brief interval is to be found farther west on the south side of the Little Yosemite Valley and at several places on the north side. There is, further, an abundance of confirmatory evidence in the moraine series in other valleys of the Sierra Nevada.
It is entirely probable, in view of all these facts, that the six frontal moraines which the main glacier left in the Yosemite Valley are to be divided into two series also. The four lower moraines, which are widely breached by the river, may record the first ice flood; the two better-preserved moraines which together form the dam at the El Capitan Bridge may record the second ice flood.
MORAINES IN THE TENAYA BASIN
Tenaya Canyon, in contrast to the Little Yosemite, contains scarcely any moraines. Muir46 sought to explain this fact by supposing that the Tenaya Glacier toward the end of the ice age "became torpid, withering simultaneously from end to end"; but a systematic survey of the Tenaya Basin above the head of the canyon and to the west of it reveals a fine series of regularly spaced lateral moraines analogous to those left by the Merced Glacier in the Little Yosemite. The Forsyth Trail leads down into the basin over the left laterals, and the Tenaya Lake Trail crosses the right laterals in the rugged country near Hidden Lake. From this neighborhood the right laterals swing northward to Snow Flat, around which they curve in concentric loops, outlining an ice lobe, and thence they extend northeastward to the gentle rock slopes under the bench of May Lake, where they spread out broadly.
To one who views these series of moraines comprehensively on a topographic map it is manifest that the Tenaya Glacier fluctuated repeatedly during its decline, in precisely the same way as the Merced Glacier. Doubtless the two ice streams were affected by the same periodic oscillations of the climate. Moreover, it is to be inferred from the general trend of the moraines in the Tenaya Basin and from the levels at which they lie that they would be prolonged for considerable distances along the sides of Tenaya Canyon if there were suitable slopes to which they might cling. The absence of lateral moraines in Tenaya Canyon, therefore, is seen to be due primarily to the excessive steepness and smoothness of its walls.
It may seem inconsistent with this explanation, perhaps, that the bottom of the canyon is not littered with large quantities of morainal material that slid from the walls, but the fact is that the Tenaya Glacier was very lightly loaded with débris. The moraines which it left in the Tenaya Basin are notably of very meager volume, individually, compared with those of the Merced Glacier. The reason is obvious: the Tenaya Basin is carved in hard and prevailingly massive granite that was, necessarily, very difficult for the glacier to erode.
The scarcity of distinct frontal moraines in Tenaya Canyon is explained by the exceeding ruggedness of its bottom, by the paucity of glacial débris, and also, doubtless, by the fact that the Tenaya Glacier, not having any great tributaries in its upper course, bore no strong medial moraines such as would have provided material for the building of frontal moraines.
A small amount of glacier-worn débris, a mere remnant of a frontal moraine, occurs near the lower end of Mirror Lake, opposite the first bay. Mirror Lake itself, however, does not owe its existence to a morainal dam. It is impounded by large masses of coarse, angular rock débris that evidently fell in avalanches from the cliffs back of the Washington Column and from the wall opposite at a time long after the glacial epoch. Clarence King47 believed these obstructing masses to be a moraine loop of the Tenaya Glacier, but the material of which they are composed, upon close examination, proves to be derived wholly from the adjacent canyon walls.
Were space available it would be in order, next, to describe the remarkably complete and well-preserved systems of moraines in the Illilouette Basin; likewise the moraines left by the glaciers that occupied the hanging valleys of Yosemite Creek, Bridalveil Creek, and the lesser upland streams. It seems preferable, however, in order not to weary the reader, to describe instead the glaciers themselves which are so clearly outlined by these moraines. Before doing so, however, it is important to invite attention to certain facts that tell, unmistakably, of earlier and more extensive ice invasions in the Yosemite region than the one that is recorded in the distinct moraines just described.
Last Updated: 28-Nov-2006