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Tenaya Lake lies, like a sapphire gem, ensconced in the bosom of the Tenaya Basin, near the headwaters of the stream that dashes down rugged Tenaya Canyon. The lake is situated at an altitude of 8141 feet, somewhat more than 4000 feet above the level of the Yosemite Valley, and is surrounded by peaks of massive and imposing sculpture that rise to elevations between 10,000 and 11,000 feet. To the visitor who comes up from the valley by the Tenaya Lake Trail or the Tioga Road, the Tenaya Basin is the first bit of real High Sierra, fascinating both by reason of its scenic aspect and because of the extraordinarily vivid evidences of glacial action that appear on every hand.
On the west and northwest sides the basin is enclosed by Mount Hoffmann (10,836) and Tuolumne Peak (10,875) which form really a continuous range. On the northeast side stands sharp-spired Cathedral Peak (10,933); overlooking the lake from the east is blunt sheerwalled Tenaya Peak (10,300), and beyond it loom the clustered pinnacles of the Echo Peaks (11,000).
Directly from the north shore of the lake rises a smaller, roundish mountain of massive and almost wholly bare granite, which is known as Polly Dome (9786). It deserves more attention than is usually bestowed upon it, for its smoothed and in part polished sides and crown proclaim the fact that the glaciers of the Ice Age have repeatedly overwhelmed it. There are even glacial lakelets perched high up on its slopes. Polly Dome stands 1645 feet above Tenaya Lake, and so it is evident that the ice here attained a depth of more than 1600 feet.
To one not familiar with the details of glacial action in the High Sierra it might seem well nigh incredible that the ice could have attained so great a depth in the Tenaya Basin, yet Polly Dome does not even afford the full measure. Tenaya Peak, which rises 2160 feet above the lake, also was overswept, as is clearly proved by its ice-smoothed summit and the “erratic” boulders that are scattered upon it. To find the highest level attained by the glaciers one must go to the base of the pinnacles of the Echo Peaks or the neighboring crest of Columbia Finger. The frail, splintered forms of those pinnacles show that they have never been overridden by the ice, while the smoothly rounded mountains which they surmount have clearly been abraded. The ice line lies fully 2460 feet above Tenaya Lake.
Where did these vast quantities of ice originate, you may ask? The great bulk of it came from the upper Tuolumne Basin, of which the Tuolumne Meadows are now the central feature. That basin was formerly an immense ice reservoir, the largest of its kind in the High Sierra. Into it flowed dozens of sluggish ice streams from the amphitheater-like hollows or “cirques” on the sides of Mount Lyell, Mount Maclure, Kuna Crest, Mount Dana, and the other peaks round about. From it issued the mighty Tuolumne Glacier, which reached a length of 60 miles and was the largest ice stream in the Sierra Nevada. This great trunk glacier, nevertheless, was unable to carry off the ice as fast as it accumulated, and as a consequence the level of the ice sea rose until at last overflow took place in various directions through gaps and saddles between the peaks. By far the largest amount spilled over the hummocky divide that separates the Tuolumne Basin from the Tenaya Basin. It was a flow four miles wide, between Cathedral Peak and Tuolumne Peak. This ice flow invaded the Tenaya Basin, where its volume was augmented by local ice streams that issued from cirques on Cathedral and Tenaya peaks, Sunrise Mountain, and Mt. Hoffmann.
So abundant was the ice in the basin that the outflowing Tenaya Glacier, although 2500 feet thick, was unable to draw off all of it, and as a consequence a broad ice sheet spread over the rugged upland north of Tenaya Canyon, as far west as Porcupine Flat. Even Mount Watkins, which as viewed from Mirror Lake seems like another El Capitan, was completely overwhelmed. This fact is attested by several huge ice-borne boulders that lie on its bald, rounded summit (which, by the way, is easily accessible and well worthy of a visit).
In the central portion of the Tenaya Basin the eroding action of the ice was most vigorous, and there, consequently, only a sprinkling of boulders was left on the rock floors; but at the margins the rock debris was piled up in ridges or “moraines.” These are most numerous on the south side of the basin, where the Forsyth Trail comes down. They extend approximately parallel to one another, in concentric curves.
Naturally the Tenaya Basin exhibits in practically all its features the
effects of the intense erosive action of the ancient ice floods. Tenaya Lake, its
central feature, occupies a hollow that was literally gouged out in its rock
floor. The lake has a sounded depth of 114 feet. Not all this depth is to be
accredited to glacial excavation, however, for at its lower end the lake is
enclosed by glacial and stream-borne rock debris of unknown thickness. In that
Tenaya Lake, Yosemite. By Ansel Adams
[click to enlarge]
The filling is accomplished not by the settling of sediment in layers over the entire lake bottom, but by the forward growth of the deltas which the streamlets are building at their mouths. The extent to which these deltas already have encroached upon the lake is shown by the stretches of low sandy ground covered with willows and pines at the mouths of the streams. So extremely slow is the rate of advance of the delta that the waves stirred up by the daily southwest winds have no difficulty in keeping its front trimmed to a smooth curve.
At the Foot of Tenaya Lake is a feature of a relatively rare type that deserves a word of explanation. The shore there is raised to the form of a low, hummocky ridge crowned with glacial boulders. This ridge is only from one foot to three feet high above the level land immediately back of it, and is so unobtrusive that it might readily be overlooked. Similar hummocky ridges occur along the shores of a number of other lakes in the High Sierra—notably Ireland Lake—and also of many lakes in Wisconsin and Minnesota. For a long time the origin of such ridges was in doubt, but it is now known from actual observation that they are built of material that is literally shoved out of the lakes by the ice that forms on them in winter. These ridges are therefore appropriately known as “ice ramparts.”
An ice rampart is formed, in brief, as follows. The ice that freezes on a lake, although seemingly inert and unchanging, in reality contracts and expands appreciably with variations in the daily temperature. During a cold spell, with temperatures well below zero, the ice contracts sharply, and if it can not pull away from the shore becomes rent by open fissures. Water at once fills these fissures and, upon coming into contact with the cold air, freezes in them, thereby reuniting the sheet, which now fits snugly in the lake basin. With the next warm spell the ice again expands and as a consequence it pushes with great force against the shores. Either the ice breaks a short distance from shore and piles up in a pressure ridge, or its marginal portion is shoved bodily up on the land, carrying with it the rocks and mud that are frozen in it. The latter action may be repeated several times during a winter, and so in the course of time the lake bottom for some distance out from the shore is cleared and smoothed, and the material shoved up on the shore is piled together in a low, jumbled ridge. The tremendous force that is exerted by the ice in this apparently insignificant process is strikingly attested at Tenaya Lake by the large size of the boulders in and on the ice rampart.
To return to the evidences of glacial erosion in the Tenaya Basin: The walls, the slopes, and the floors of solid granite almost everywhere are abraded to smooth-flowing curves. Over considerable areas they still retain the polish, striae, and grooves that were imparted to them by the rock fragments held by the slow-moving ice mass. Indeed, next to the upper Merced Basin the Tenaya Basin offers the most impressive exhibit of glacially-worn rock that is readily accessible in the High Sierra. Some of the polished and gleaming granite is to be seen from the automobile road, especially along the shore of Tenaya Lake. Highly appropriate was the old Indian name for the lake—Py-we-ack, meaning the “water of shining rocks.” (The name Tenaya was given to it by the historic Mariposa Battalion, the first group of white men to set foot in the Yosemite Valley, in 1851, in memory of Chief Tenaya, of the Yosemite Indians, whom they captured at the lake.)
Glacier polish is, even to the casual observer, an intensely fascinating thing, for it attests most vividly the grinding power of the ancient glaciers and by reason of its good state of preservation over large areas impresses one with the relative recency of the Ice Age. The fact is that glacier polish, because of its very smoothness, acts somewhat as a protective coating that retards the weathering of the rock. It permits water to run off more promptly than it would from a rough surface, and thereby lessens the proportionate amount that soaks into the rock. And this, again, retards the growth of mosses and lichens from whose decay are derived the acids that decompose the weaker minerals. It is a notable fact, that may be observed in many places in the High Sierra, that wherever the glacier polish finally scales off rock weathering is at once resumed at a relatively rapid rate (the normal rate).
The gleaming polish itself is produced, of course, by abrasion with the fine rock powder which the glacier carries in its lower layers. Far more impressive, as regards the pressure which the ice mass exerted on its bed, is the testimony of the grooves and striae. They show that the force was sufficient to cause angular blocks dragged by the glacier to furrow sound, unweathered granite to depths of a quarter or even a half inch, and to cause individual rock grains (of quartz, presumably) to leave tiny furrows (striae) of their own.
The pressure can be calculated from the known thickness attained by the
ice. Since a column of glacier ice 1000 feet high weighs about 30 tons per square
foot, the pressure on the rock floors about Tenaya Lake at the time of maximum
glaciation (when the ice was about 2400 feet thick) must have been about 74
tons per square foot. And to this must be added the forward thrust due to the
gravitation of the ice mass. Much of the grooving and scratching, however, must
have been done with less pressure than that stated, for many of the prominent
grooves are still rough and fresh looking, from which circumstance it is evident
Glacier polish and striae on aplite. Blocks disrupted by postglacial frost work.
By François Matthes
[click to enlarge]
One factor that accounts for the unusually fine display of polish, grooves,
and other glacial markings in the Tenaya Basin is that the granite there is very
massive—that is, it is broken by joint fractures only at long intervals. Normally
granite is jointed at intervals of a few feet. The partings run in parallel sets,
and there are three or more different sets that intersect one another at various
angles so as to divide the rock into angular blocks and slabs. Wherever the
rock is so divided glacier polish is apt to be scarce, for the glacier, instead of
merely grinding, quarries out the blocks and slabs. The Yosemite Valley was
quarried out in this fashion, and its walls in consequence are for the most part
hackled and faceted, and bear glacier polish only in a few spots. But in the
Tenaya Basin the joints are often scores and even hundreds of feet apart, so
that the glacier could not quarry but only grind and polish the surface of the
[click to enlarge]
Glacier polish, head of Tenaya Canyon. By David Brower
In the Tenaya Basin curving shells due to exfoliation are to be seen in many places, but they are not plentiful because the most of those that were loosened were quarried away by the glaciers, and sufficient time has not elapsed since the end of the Ice Age for the production of any new shells. In a general way, therefore, the smoothly curving, billowy rock forms of the Tenaya Basin have been fashioned in part by exfoliation and in part by glacial action.
Of the older rocks that formerly roofed over the granite a few remnants still exist in the vicinity of May Lake. The ridge that encloses the lake basin on the east side consists in part of brown quartzite and white marble, and the slope of Mount Hoffmann immediately above the lake is composed of similar materials. The quartzite is an ancient sandstone that was altered and made crystalline by the intense pressure and heat to which it was subjected when the strata were folded as the earlier mountain system arose. The marble was produced by the same process from limestone. The sandstone originated probably as sand on the shore of the continent, and the limestone was built up as a reef by corals and algae in a shallow sea.
It may seem surprising that these remnants of quartzite and marble occur at the base of Mount Hoffmann and not on its summit, which consists of pure granite. The explanation is that a large body of these metamorphic rocks hung down from the roof into the fluid granite, forming a dividing wall between two upwelling masses. Of that wall only the lower portions remain, the upper portions having long since been destroyed together with the roof. Many similar instances occur in different places in the Sierra Nevada. Among the most spectacular is a mass of contorted strata that hangs on the northwest slope of Tuolumne Peak.
See note on page 128.
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