In the 1860's, when Professor Josiah D. Whitney (State Geologist of California, for whom Mount Whitney was named), sent the first scientific exploring parties into the Sierra Nevada, and in the 1870's, when John Muir and Professor Whitney engaged in their memorable controversy about the glaciation of the Yosemite Valley, the concept of the Ice Age, or glacial epoch, was still very new and ill defined. Sufficient evidence was at hand to show that the north-central and northeastern parts of the United States and the adjoining parts of Canada had once been covered by a vast ice sheet, but the precise limits which that ice sheet had reached were not definitely known. It was assumed to have formed part of an immense icecap that centered at the North Pole and to have mantled all the northern half of North America. Not unnaturally, this sheet was supposed to have also covered the great western mountain belt of the continent, may of whose ranges still bear glaciers at the present time. Not until later did it become clear that no icecap could have been at the North Pole of the Earth, because that pole was covered by an ocean of considerable depth. The ice sheet, it was found, originated on the North American continent itselfin Labrador and the region to the west of Hudson Bay, from which centers the ice flowed in all directions.
It was discovered, further, that this ice sheet did not overwhelm the western mountain ranges but, anomalously, stopped to the east of them, and that those ranges were independent centers of snow accumulation which generated large glaciers of their own. Those glaciers became confluent and filled the intermontane valleys with ice to depths of several thousand feet. A vast composite ice mass was thus formed that was not an icecap, strictly speaking, nor did it bury the higher peaks; yet it was continuous over the entire breadth of the mountain belt. This Cordilleran ice, as it is appropriately called, spilled eastward onto the plains and westward into the fords of the Alaskan coast and British Columbia. It lay almost wholly to the north of the Canadian boundary line but sent several broad lobes southward into Puget Sound, eastern Washington, and western Montana.
South of this composite ice mass, the higher mountain ranges also generated glaciers. The Sierra-Cascade chain is notable in that it bore glaciers throughout most of its great length. This chain extends for 1,000 miles over 14° of latitude from the 49th down to the 35th parallel. Thus the chain traverses regions of the utmost geographic and climatic diversity. It begins near the Canadian boundary, in a region of extremely wet, snowy climate, and terminates at the edge of the Mohave Desert, which is one of the driest and most torrid areas on the North American Continent.
The glacial covering of this chain was very extensive at the north end, where the great height and breadth of the Cascade Range, the enormous quantities of snow supplied by the westerly winds, and the prolonged winters together produced conditions favorable for glaciation. This part of the chain lay fairly smothered under snow and ice and sent forth glaciers 80 to 100 miles long.
South of Mount Rainier, however, the ice mantle contracted rapidly in breadth, mainly as a result of declining altitude. Throughout southern Washington and northern Oregon, where the crestline, not counting the isolated volcanic peaks, rises scarcely above 5,000 feet, the glaciers attained lengths of only a dozen or so miles. Still farther south, throughout the 200-mile stretch of which Crater Lake, Mount Shasta, and Lassen Peak are the dominant landmarks, there was no true ice mantle but only detached glaciers and snowfields that lay in sheltered canyons high up on the main peaks. This dearth of icethis state of semiariditywas due not to a further decline in altitude, for the range here again rises to 6,000 and in places even to 8,000 feet, but to deficient snowfall caused by the presence between the Cascade Range and the Pacific Ocean of a large complex of mountains which intercepted a considerable share of the moisture from the westerly winds.
Southward from the canyon of the Feather River, however, in the northern part of the Sierra Nevada, glaciation formed on an increasingly large scale, owing both to greater altitude and to greater snowfall; here the intercepting power of the Coast Ranges diminished with decline in height. In the vicinity of Lake Tahoe, where the Sierra Nevada attains altitudes of more than 9,000 feet, the icefields and ice streams were large enough to coalesce and produce trunk glaciers from 15 to 20 miles in length. And in the stretch from Lake Tahoe to Mount Lyell, in which the crest rises progressively to altitudes of 11,000, 12,000, and 13,000 feet, the snows were so abundant as to mantle the range continuously over a breadth of 20 to 30 miles and to create trunk glaciers 40 to 60 miles long. In short, another climax of glaciation was reached in this central part of the Sierra Nevadaa climax second only to that attained near the Canadian boundary, 800 miles to the north.
From Mount Lyell to Mount Whitney, over a stretch of fully 100 miles, the glacial mantle extended almost undiminished in breadth. It covered all those parts of the High Sierra which are drained by the San Joaquin, Kings, Kaweah, and Kern Rivers and which are crowned by the culminating peaks of the range. A short distance south of Mount Whitney, however, within the boundaries of Sequoia National Park, the 300-mile-long glacial mantle of the Sierra Nevada came abruptly to an end. The glacier system of the upper Kern Basin was the southernmost large system of its kind in the Sierra Nevada. Beyond this system there were only a few small detached ice bodies.
The southern limit of glaciation in the Sierra Nevada was imposed not primarily by latitudethat is, by the southward increase in warmth and consequent rise of the snowline during Pleistocene timesbut by the termination of the two lofty mountain ranges that bound the upper Kern Basin, for only these two ranges were high enough to reach above the snowline during glacial times.
The precise extent of the glacial covering of the Sierra Nevada was long a subject of conjecture and dispute, but as a result of the systematic survey of glacial deposits, the margin of the glacial mantle has been definitely mapped, and its position can now be determined tentatively within narrow limits. There is thus no further doubt that the glacial mantle was confined wholly to the upper parts of the Sierra and that at no point did it reach down to, or even near, the western base of the range. In the south-central part of the range, where the glacial mantle was broadest, its western margin descended to altitudes of somewhat less than 5,000 feet. The trunk glaciers, of course, descended to still lower levels, yet even these glaciers fell far short of reaching the foot of the range. The Tuolumne glacier, which was the longest ice stream north of the Yosemite region, attained a maximum length of 60 miles and projected about a dozen miles beyond the margin of the ice mantle that lay on the adjoining uplands. The Tuolumne glacier terminated, however, fully 30 miles from the foot of the range and at an altitude of about 2,000 feet. The Yosemite glacier at the time of maximum glaciation was 37 miles long and projected 7 miles beyond the margin of the ice mantle. The terminus of this glacier lay in the Merced Canyon about 50 miles from the foot of the range and about 2,000 feet above it, just below the site of El Portal. The San Joaquin glacier was nearly as long as the Tuolumne glacier, but it advanced only a few miles beyond the margin of the ice mantle on the flanking uplands. The San Joaquin glacier halted 45 miles from the mouth of its canyon, at an altitude of about 2,600 feet. The Kings glacier, most southerly of the great glaciers, despite the great altitude of the crest region which it drained, attained a length of only 44 miles (measured along its middle branch) and came to an end about 37 miles from the base of the range at an altitude of 2,500 feet.
The low levels reached by these trunk glaciers seem truly remarkable when it is considered that their lower parts lay wholly in the zone of wastage where, even in the shaded spots, summer heat was sufficient to remove the snows of winter. The Yosemite glacier, for instance, reached more than a mile below the level (somewhat above 8,000 ft) in which glaciers were formed in the Yosemite region. The Tuolumne glacier reached 6,000 feet below this level; the San Joaquin glacier, about 5,300 feet; the Kings glacier, about 6,000 feet. (The level of glacier generation rose gradually southward.)
The ability of these glaciers to reach such low levels in spite of the warmth that prevailed in the zone of wastage affords impressive testimony of the immense surplus of snow and ice that descended from the higher parts of the range. However, these low levels were also due in part to the protection from the sun's rays that was afforded to the glaciers by the high walls of the canyons; to the small surface areas, proportionate to bulk, that the glaciers, 3,000 to more than 4,000 feet in thickness, presented to the melting agencies; and to the relatively rapid movement of the ice, which in the thicker glaciers must have averaged several feet a day.
On its eastern flank also, the Sierra Nevada bore a great array of glaciers, there being a glacier in almost every canyon; but these glaciers were in general much shorter than those on the western flank, owing to the abruptness of the escarpment, the shortness of the canyons, and the small extent of glacier-generating territory at their heads. Most of these glaciers, nevertheless, reached down to the eastern foot of the range; not a few projected well out into the adjoining lowlands, especially in the north-central and south-central parts of the range, where these lowlands have altitudes of 5,000 to nearly 7,000 feet.
The basin of Mono Lake was invaded by no less than six ice tongues, each of which extended several miles out from the range. In the regions south of Mono Lake, the glaciers projected as a rule but little beyond the mouths of their canyons, and along the border of Owens Valley the glaciers were confined mostly to the upper parts of the canyons; still further south there were only scattered snowfields, and the array of ice bodies came to an end.
The courses of the ancient glaciers in Sequoia National Park were traced and mapped and their farthest limits were determined by the same method that had proved effective in the San Joaquin Basin and elsewhere, that is, interpretation of the testimony of glacial deposits rather than that of sculptural features; this method consists primarily of a systematic survey of the moraines that were built by the individual glaciers.
In open country such a survey can readily be executed with sufficient accuracy for a reconnaissance map by locating the moraines by eye with respect to identifiable landmarks, of which the landscape of the Sierra Nevada affords a plenty; but in the forested tracts the larger moraines must be actually followed out and, in some places, located by traversea laborious and time-consuming process. Fortunately, in Sequoia National Park the forested areas, though of considerable extent, are so amply diversified by topographic and drainage features, as well as by occasional meadows, that a large share of the work could be done by following the moraines, or the swales between moraines, on horseback. In the rougher areas, of course, the mapping had to be done on foot.
In the morainal deposits of Sequoia National Park, as in the areas to the north previously mentioned, abundant and unmistakable evidence was found of two distinct stages of glaciationa later one, the Wisconsin, and an earlier one, El Portal, separated by a lengthy time interval; meager indications were also found of a third, very early stage, the Glacier Point (Matthes, 1929). Thus the observations on the morainal deposits of these different stages in Sequoia National Park bear out the correctness of the analysis that was made of the moraine systems of the other basins in the Sierra Nevada, and place on a firmer basis the author's interpretation of the succession of the events in the glacial history of the Sierra Nevada. Throughout the central and south-central Sierra Nevada, therefore, the moraine systems of the great trunk glaciers and their numerous branches spell out the same story of three distinct periods of extensive and long-continued glaciation during the Pleistocene Epoch.
The characteristics of the glacial deposits of the three stages found in Sequoia National Park and elsewhere on the west slope of the Sierra Nevada are set forth in the following sections.
The glaciers of the Wisconsin Stage are considered first, because they are most definitely known and were, for the most part, ice tongues confined to individual valleys. Having obtained a definite image of them, the reader can then visualize also the more extensive glaciers and ice fields of the earlier stages, which coalesced over divides and in part moved in disregard of them.
Throughout the Sierra Nevada the moraines of the Wisconsin Stage are, as a rule, well preserved and distinct. Many still retain, only slightly changed, the sharp-crested forms which the glaciers gave them (fig. 20). Much of the finer material has been washed from these moraines, but many boulders that form the crests remain in place, or substantially so. The frontal moraines are commonly breached by the streams, but some of the younger moraines still act as dams impounding lakes. On slopes of low or moderate declivity the lateral moraines are often splendidly developed, extending for long distances as regular embankments broken by only a few stream-cut notches. Though wholly absent on precipitous canyon walls, lateral moraines can usually still be traced along steep slopes by surviving patches of coarse debris or by single boulders.
Particularly useful as diagnostic materials are the boulders of different types of granitic rocks. Such rocks preponderate in the central and southern parts of the Sierra Nevada and make up the bulk of the moraines (figs. 21, 22). In the moraines of Wisconsin age, a large percentage of these granitic boulders are unweathered and unstained. They ring when struck with the hammer, and with fine resilience throw the hammer sharply back; this is true not only of the siliceous types of granite and granodiorite but also of the more basic rocksthe quartz diorite, diorite, and gabbro. Exception must be made, of course, for such boulders as were already weathered when picked up by the glacier.
The glacial deposits of Wisconsin age are associated with smoothed and polished floors, walls, roches moutonnées, and ledges of rock in place (figs. 23, 24, 25). These surfaces are most extensive in the areas of sparsely jointed siliceous granite, and surfaces that were glaciated during the later substages of the Wisconsin naturally are more perfectly preserved than those that were glaciated during the earlier substages. From the latter surfaces, usually, most of the polish has already flaked off, but the extent to which it has disappeared depends also in some measure on the character of the rocks, the acid types retaining the polish longer than the basic ones. Whether still polished or not, however, all rock surfaces planed down by the Wisconsin ice still exhibit today the smoothed forms that are well known to be characteristic products of glacial abrasion. The actual reduction effected by weathering ranges from virtually none to as much as 2 inches.
The signs of recency in the Wisconsin moraines, whether preservation of ridge forms or freshness of boulders, are likewise somewhat more accentuated in the later than in the earlier Wisconsin deposits. In the somewhat more subdued moraines of the earlier substages, although the acidic rocks are still generally sound and hard, an increasing percentage of the basic rocks are cracked or split along joint planes, and some of these rocks are so badly disintegrated that they crumble in the hand. However, there is no need in this study to differentiate between substages of the Wisconsin Stage. The Tahoe and Tioga glaciations, which have been distinguished on the east slope of the Sierra Nevada and regarded as stages of early and late Wisconsin age, respectively (Blackwelder, 1931), are now realized to be two distinct stages in the Sierra Nevada.
Correlation of the last stage of Pleistocene glaciation in the Sierra Nevada with the Wisconsin Stage of the Laurentide glaciation was based initially on the comparable degree of preservation of their respective morainal deposits and on the largely unweathered state of the boulders in them. But there is additional warrant for the correlation in the fact reported by Alden (1932) that in northern Montana, where the Keewatin drift of Wisconsin age overlaps the moraines of the last Cordilleran glaciation, both deposits have about the same degree of freshness. There can be no doubt, on the strength of this evidence, that the last Pleistocene glaciation of the Rockies and the last Pleistocene glaciation of the adjoining plains were contemporaneous. It is assumed, further, that the last Pleistocene glaciation of the Sierra Nevada occurred at about the same time as that of the Rockies; though incontrovertible proof of this synchronism is lacking, there is at present no known reason for thinking otherwise.
EL PORTAL STAGE
The ill-defined round-backed moraines of El Portal Stage stand in striking contrast with the distinct sharp-crested moraines of the Wisconsin Stage in the Sierra Nevada. Disintegration of the boulders in El Portal moraines has continued for so long a time as to destroy completely the original crests. It has resulted also in mantling these older moraines with considerable arkose sand that gives them a smoother and less bouldery appearance than the Wisconsin moraines usually have. Although far more bulky, El Portal moraines are relatively obscure features that may easily be overlooked by an untrained observer of the landscape.
To identify these obscure moraines beyond possible doubt, their constituent materials must usually be sought in gullies, roadcuts, trailcuts, or holes left by uprooted trees. Although gullies are ordinarily plentiful, the author was obliged more than once to rely wholly on the evidence furnished by uprooted trees. Fortunately, some of these trees held boulders and cobbles aloft in their exposed roots, as if for the convenience of the geologist. Almost invariably such boulders and cobbles are light buff or yellowish because of their limonite coatting. The entire deposit of which they form part is commonly of the same yellowish hue, in contrast to the morainal material of Wisconsin age, which is mostly gray. The coating on the boulders and cobbles generally masks their lithologic character and makes many of the granitic rocks indistinguishable. When broken open, the boulders and cobbles are usually found to be stained by ferric oxides to a depth of one-fourth to one-half inch.
The boulders composed of the more siliceous granites are as a rule still firm, but they have lost so much of their resilience that the hammer bounces back only feebly from them. Many break readily; some even crumble into discrete granules. The granodiorite and quartz diorite boulders usually have still less coherence, and the diorite and gabbro rocks are so weak that they are smashed to bits by a light blow. These boulders are often traversed by ramifying cracks; some are in a crumbling stage. These characteristics vary, of course, with the prevailing moisture conditions in the deposits. In well-drained locations, decomposition and disintegration make much slower progress than in poorly drained ones. On strongly isolated platforms of bare rock, which dry out quickly, scattered cobbles and boulders are apt to be surprisingly well preserved (figs. 26, 27). Boulders on top of the moraines, exposed to the heat of the sun and therefore drying quickly after a wetting, generally have some cohesive strength left, but those in the interior of the moraines are as a rule so weak that the picks of the road workers cut right through them. In some roadcuts such boulders, cut flush with the wall, appear outlined as rusty rings.
Terminal moraines of El Portal Stage are generally wanting in the main canyons of the west slope of the Sierra Nevada. Their absence can hardly be attributed to complete destruction by stream-and-weather erosion after El Portal time, for in some of the canyons the topography is decidedly favorable to the preservation of at least the wings of such moraine. A more probable explanation is that the trunk glaciers of El Portal Stage built either no terminal moraines at all or else only very small ones, because the ice fronts rested for no considerable length of time at any point during the maximal phases. Whatever the explanation, the farthest limits reached by the trunk glaciers of El Portal Stage cannot, as a rule, be determined with any great accuracy because of the lack of terminal moraines.
The lateral moraines left by the glaciers of El Portal Stage, on the other hand, are generally of massive proproportions that dwarf the corresponding laterals of the Wisconsin Stage. Though gashed or even transected by gullies, and though in some places almost destroyed, they are nevertheless not difficult to trace, once they have been identified. It is, indeed, by the systematic mapping of these old lateral moraines that the courses of the glaciers of El Portal time are most surely traced.
It may appropriately be added that for polished and striated rock surfaces dating from El Portal Stage one need not search. The glistening floors, walls, ledges, and roches moutonnées that are so abundant and so impressive in some parts of the Sierra Nevada, as already noted, all date from the Wisconsin Stagemostly from its later substages. The rocks surfaces that were planed by El Portal ice have long since been destroyed and changed beyond recognition by the granular disintegration, scaling, or spalling of the rocks (figs. 28, 29). In many places the granitic rocks appear to have been stripped of disintegration products to depths of 10 to 12 feet. Approximate measurements of such stripping are afforded by residual rock pedestals supporting perched glacial boulders and by dikes of slow-weathering aplite that remain standing in high relief (Matthes, 1930, p. 70-74; 1950a, fig. 51; 1950b, p. 101-102; 1960, fig. 32). Several other types of criteria suggest themselves, notably the depth of stream channeling accomplished since El Portal and Wisconsin climaxes, respectively, and the erosional changes produced in valleys and other landforms; but these criteria generally furnish no more definite measures for comparison than the pedestals and dikes.
This marked difference in the depth of disintegration and stripping of the granite since El Portal Stage and since the Wisconsin Stage affords a good index of the relative antiquity of those two stages of glaciation. If the time since the climax of the Wisconsin Stage is to be measured in tens of thousands of years, then surely the time since the climax of El Portal Stage is to be reckoned in hundreds of thousands of years. Mainly for that reason the author holds that El Portal and its probable correlative on the east side of the Sierra Nevada, the Sherwin Stage (Blackwater, 1931), are not younger than the Illinoian Stage of the continental glaciation and may include deposits of two or more unseparated stages, perhaps both the Illinoian and Kansan (Matthes, 1933, p. 33).
GLACIER POINT STAGE
The earliest glaciation that has been recognized on the west slope of the Sierra Nevada, the Glacier Point Stage, which presumably corresponds to Blackwelder's McGee Stage on the east flank (Blackwelder, 1931), is indicated by morainal deposits in only a few localities in the range. Two circumstances account for the scarcity of these deposits: (1) Their obliteration over large areas by the extensive and voluminous deposits of El Portal Stage, and (2) their vestigial character, their constituent materials having in large part disintegrated and been carried away.
The few small deposits which the author has shown on his maps as probably belonging to the Glacier Point Stage lie, in most places, at levels 200 to 300 feet above the upper limit of the massive El Portal moraines, where they clearly have escaped obliteration. Their positions at those high levels, however, do not necessarily indicate that the ice streams of the Glacier Point Stage attained greater depth in the canyons than did the ice streams of El Portal Stage; for it stands to reason that during Glacier Point time the canyons were not yet cut to the depth which they later attained in El Portal time. A given quantity of ice would have filled them to a higher level in Glacier Point time than it would have in El Portal time.
Some of the deposits of the Glacier Point Stage lie far out on the uplands flanking the canyons, and thus show that the ice of that stage overflowed the canyon rims, whereas the moraines of El Portal Stage show that the glaciers of that stage remained largely confined within the canyons. But that fact, too, is very probably explained by the circumstance that in Glacier Point time the canyons had smaller cross-sectional areas, and therefore less capacity for holding ice, than they had later in El Portal time.
In contrast to the massive El Portal moraines, the deposits of the Glacier Point Stage have but meager volume and a decidedly depleted aspect. They are reduced for the most part to skeletonlike rows of erratic boulders composed of resistant quartzite and highly siliceous granite, the rest of the constituent materials having vanished. In some places the deposits are entirely destroyed. The weaker rocks, such as the diorites, are represented as a rule only by a chance fragment here or there. Many of the siliceous boulders, even, have lost their glacial contours as the result of spalling, exfoliation, or granular disintegration; some have been reduced to strangely cavernous or basined forms. The fact that the morainal materials have been transplanted can be established only on lithologic grounds and by a knowledge of their provenance. Needless to add, it requires a trained eye to identify such scanty, vestigial deposits; and to know where to look for them, one must have some conception of the extent of the earlier ice in the Sierra Nevada and also a knowledge of the habits of glaciers and the manner in which they adjust their movements to different types of topography.
That the moraines of the Glacier Point Stage are much older than the moraines of El Portal Stage is readily evident from the foregoing. Even if it be granted that the relative scantiness of the Glacier Point deposits may be due to the shorter duration of the Glacier Point Stage as compared with El Portal, it is manifest from the more advanced state of disintegration of the boulders in the Glacier Point moraines that the Glacier Point Stage preceded El Portal Stage by a considerable length of time. The Glacier Point Stage clearly is to be assigned to the early Pleistocene, and some argument may even be found for correlation with the Nebraskan, for the Glacier Point is the earliest stage of glaciation of which any recognizable deposits remain in the Sierra Nevada (Matthes, 1933, p. 33).
Last Updated: 03-Aug-2009