Little by little these ma.s.ses of rock worked their way down lower into the ice-bed. Sometime they must reach the bottom, yet, though they rest upon granite, and granite would cleave to granite, the irresistible pressure from above forces the ice and rock ma.s.ses forward. Thus the sharp-edged blocks of granite become the _blades_ in the tools that are to help cut out the contours of a world"s surface. In other words the ma.s.s of glacial ice is the grooving or smoothing _plane_, and the granite blocks, aided by the ice, become the many and diverse blades in this vast and irresistible tool. Some cut deep and square, others with flutings and bevelings, or curves, but each helps in the great work of planing off, in some way, the rocky ma.s.ses over which they move. Hence it will be seen that the grooving and marking, the fluting and beveling, the planing and smoothing processes of the ice are materially aided and abetted by the very hardness and weight of the granite and other rocks it carries with it.
Now let Joseph LeConte take up the theme and give us of the rich treasure-store of his knowledge and observation. In the _American Journal of Science and Arts_, Third Series, for 1875, he discussed the very field we are now interested in, and his fascinating and illuminating explanations render the subject perfectly clear. Said he:
Last summer I had again an opportunity of examining the pathways of some of the ancient glaciers of the Sierra. One of the grandest of these is what I call the _Lake Valley Glacier_.[1] Taking its rise in snow fountains among the high peaks in the neighborhood of Silver Mountain, this great glacier flowed northward down Lake Valley, and, gathering tributaries from the summit ridges on either side of the valley, but especially from the higher western summits, it filled the basin of Lake Tahoe, forming a great "mer de glace," 50 miles long, 15 miles wide, and at least 2000 feet deep, and finally escaped northeastward to the plains. The outlets of this great "mer de glace" are yet imperfectly known. A part of the ice certainly escaped by Truckee Canyon (the present outlet of the Lake); a part probably went over the northeastern margin of the basin. My studies during the summer were confined to some of the larger tributaries of this great glacier.
[Footnote 1: This is the name given by Dr. LeConte to the Basin in which Lake Tahoe rests and including the meadow lands above Tallac.]
[Ill.u.s.tration: Pyramid Peak and Lake of the Woods, near Lake Tahoe, Calif.]
[Ill.u.s.tration: Snow Bank, Desolation Valley, near Lake Tahoe]
[Ill.u.s.tration: Gra.s.s Lake, near Glen Alpine Springs]
_Truckee Canyon and Donner Lake Glaciers_. I have said that one of the outlets of the great "mer de glace" was by the Truckee River Canyon. The stage road to Lake Tahoe runs in this canyon for fifteen miles. In most parts of the canyon the rocks are volcanic and crumbling, and therefore ill adapted to retain glacial marks; yet in some places where the rock is harder these marks are unmistakable. On my way to and from Lake Tahoe, I observed that the Truckee Canyon glacier was joined at the town of Truckee by a short but powerful tributary, which, taking its rise in an immense rocky amphitheater surrounding the head of Donner Lake, flowed eastward. Donner Lake, which occupies the lower portion of this amphitheater, was evidently formed by the down-flowing of the ice from the steep slopes of the upper portion near the _summit_. The stage road from Truckee to the summit runs along the base of a _moraine_ close by the margin of the lake on one side, while on the other side, along the apparently almost perpendicular rocky face of the amphitheater, 1000 feet above the surface of the lake, the Central Pacific Railroad winds its fearful way to the same place. In the upper portion of this amphitheater large patches of snow still remain unmelted during the summer.
My examination of these two glaciers, however, was very cursory. I hasten on, therefore, to others which I traced more carefully.
Lake Tahoe lies countersunk on the very top of the Sierra.
This great range is here divided into two summit ridges, between which lies a trough 50 miles long, 20 miles wide, and 3000 to 3500 feet deep. This trough is Lake Valley. Its lower half is filled with the waters of Lake Tahoe.
The area of this Lake is about 250 square miles, its depth 1640 feet, and its alt.i.tude 6200 feet. It is certain that during the fullness of glacial times this trough was a great "mer de glace," receiving tributaries from all directions except the north. But as the Glacial Period waned--as the great "mer de glace" dwindled and melted away, and the lake basin became occupied by water instead, the tributaries still remained as separate glaciers flowing into the Lake. The tracks of these lingering small glaciers are far more easily traced and their records more easily read, than those of the greater but more ancient glacier of which they were once but the tributaries.
Of the two summit ridges mentioned above the western is the higher. It bears the most snow _now_, and in glacial times gave origin to the grandest glaciers. Again: the peaks on both these summits rise higher and higher as we go toward the upper or southern end of the Lake. Hence the largest glaciers ran into the Lake at its _southwestern end_.
And, since the mountain slopes here are toward the northeast and therefore the shadiest and coolest, here also the glaciers have had the greatest vitality and lived the longest, and have, therefore, left the plainest records. Doubtless, careful examination would discover the pathways of glaciers running into the Lake from the eastern summit also; but I failed to detect any very clear traces of such, either on the eastern or on the northern portion of the western side of the Lake; while between the southwestern end and Sugar Pine Point, a distance of only eight or ten miles, I saw distinctly the pathways of five or six. North of Sugar Pine Point there are also several.
_They are all marked by moraine ridges running down from the summits and projecting as points into the Lake_.
The pathways of three of these glaciers I studied somewhat carefully, and after a few preliminary remarks, will describe in some detail.
Mountains are the culminating points of the scenic grandeur and beauty of the earth. They are so, because they are also the culminating points of all geological agencies--igneous agencies in mountain _formation_, aqueous agencies in mountain _sculpture_. Now, I have already said that the mountain peaks which stand above the Lake on every side are highest at the southwestern end, where they rise to the alt.i.tude of 3000 feet above the lake surface, or between 9000 and 10,000 feet above the sea. Here, therefore, ran in the greatest glaciers; here we find the profoundest glacial sculpturings; and here also are cl.u.s.tered all the finest beauties of this the most beautiful of mountain lakes.
I need only name Mount Tallac, Fallen Leaf Lake, Cascade Lake, and Emerald Bay, all within three or four miles of each other and of the Tallac House. These three exquisite little lakes (for Emerald Bay is also almost a lake), nestled closely against the loftiest peaks of the western summit ridge, are all perfect examples of glacial lakes.
South of Lake Tahoe, Lake Valley extends for fifteen miles as a plain, gently rising southward. At its lower end it is but a few feet above the lake surface, covered with glacial drift modified by water, and diversified, especially on its western side, by debris ridges, the moraines of glaciers which continued to flow into the valley or into the Lake long after the main glacier, of which they were once tributaries, had dried up. On approaching the south end of the Lake by steamer, I had observed these long ridges, divined their meaning, and determined on a closer acquaintance. While staying at the Tallac House I repeatedly visited them and explored the canyons down which their materials were brought. I proceed to describe them.
_Fallen Leaf Lake Glacier_. Fallen Leaf Lake lies on the plain of Lake Valley, about one and a half miles from Lake Tahoe, its surface but a few feet above the level of the latter Lake[2]; but its bottom far, probably several hundred feet, below that level. It is about three to three and one-half miles long and one and one-fourth miles wide. From its upper end runs a canyon bordered on either side by the highest peaks in this region. The rocky walls of this canyon terminate on the east side at the head of the lake, but on the west side, a little farther down.
The lake is bordered on each side by an admirably marked debris ridge (moraine) three hundred feet high, four miles long, and one and one-half to two miles apart. These moraines may be traced back to the termination of the rocky ridges which bound the canyon. On one side the moraine lies wholly on the plain; on the other side its upper part lies against the slope of Mount Tallac. Near the lower end of the lake a somewhat obscure branch ridge comes off from each main ridge, and curving around it forms an imperfect terminal moraine through which the outlet of the lake breaks its way.
[Footnote 2: Professor Price informs me there is a difference of eighty feet between the level of Lake Tahoe and Fallen Leaf Lake.]
On ascending the canyon the glaciation is very conspicuous, and becomes more and more beautiful at every step. From Glen Alpine Springs upward it is the most perfect I have ever seen.
In some places the white rocky bottom of the canyon, for many miles in extent, is smooth and polished and gently undulating, like the surface of a gla.s.sy but billowy sea. The glaciation is distinct also up the sides of the canyon 1000 feet above its floor.
There can be no doubt, therefore, that a glacier once came down this canyon filling it 1000 feet deep, scooped out Fallen Leaf Lake just where it struck the plain and changed its angle of slope, and pushed its snout four miles out on the level plain, nearly to the present sh.o.r.es of Lake Tahoe, dropping its debris on either side and thus forming a bed for itself.
In its subsequent retreat it seems to have rested its snout some time at the lower end of Fallen Leaf Lake, and acc.u.mulated there an imperfect terminal moraine.
_Cascade Lake Glacier_. Cascade Lake, like Fallen Leaf Lake, is about one and one-half miles from Lake Tahoe, but, unlike Fallen Leaf Lake, its discharge creek has considerable fall, and the lake surface is, therefore, probably 100 feet above the level of the greater lake. On either side of this creek, from the very border of Lake Tahoe, runs a moraine ridge up to the lake, and thence along each side of the lake up to the rocky points which terminate the true mountain canyon above the head of the lake. I have never anywhere seen more perfectly defined moraines. I climbed over the larger western moraine and found that it is partly merged into the eastern moraine of Emerald Bay to form a medial at least 300 feet high, and of great breadth. From the surface of the little lake the curving branches of the main moraine, meeting below the lake to form a terminal moraine, are very distinct.
At the head of the lake there is a perpendicular cliff over which the river precipitates itself, forming a very pretty cascade of 100 feet or more. On ascending the canyon above the head of the lake, for several miles, I found, everywhere, over the lip of the precipice, over the whole floor of the canyon, and up the sides 1000 feet or more, the most perfect glaciation.
There cannot, therefore, be the slightest doubt that this also is the pathway of a glacier which once ran into Lake Tahoe. After coming down its steep rocky bed, this glacier precipitated itself over the cliff, scooped out the lake at its foot, and then ran on until it bathed its snout in the waters of Lake Tahoe, and probably formed icebergs there. In its subsequent retreat it seems to have dropped more debris in its path and formed a more perfect terminal moraine than did Fallen Leaf Glacier.
_Emerald Bay Glacier_. All that I have said of Fallen Leaf Lake and Cascade Lake apply, almost word for word, to Emerald Bay. This beautiful bay, almost a lake, has also been formed by a glacier. It also is bounded on either side by moraines, which run down to and even project into Lake Tahoe, and may be traced up to the rocky points which form the mouth of the canyon at the head of the bay. Its eastern moraine, as already stated, is partly merged into the western moraine of Cascade Lake, to form a huge medial moraine. Its western moraine lies partly against a rocky ridge which runs down to Lake Tahoe to form Rubicon Point. At the head of the bay, as at the head of Cascade Lake, there is a cliff about 100 feet high, over which the river precipitates itself and forms a beautiful cascade. Over the lip of this cliff, and in the bed of the canyon above, and up the sides of the cliff-like walls, 1000 feet or more, the most perfect glaciation is found. The only difference between this glacier and the two preceding is, that it ran more deeply into the main lake and the deposits dropped in its retreat did not rise high enough to cut off its little rock basin from that lake, but exists now only as a _shallow bar_ at the mouth of the bay. This bar consists of _true moraine matter_, i.e., intermingled bowlders and sand, which may be examined through the exquisitely transparent water almost as perfectly as if no water were present.
All that I have described separately and in detail, and much more, may be taken in at one view from the top of Mount Tallac. From this peak nearly the whole course of these three glaciers, their fountain amphitheaters, their canyon beds, and their lakes enclosed between their moraine arms, may be seen at once. The view from this peak is certainly one of the finest that I have ever seen. Less grand and diversified in mountain forms than many from peaks above the Yosemite, it has added beauty of extensive water surface, and the added interest of several glacial pathways in a limited s.p.a.ce. The observer sits on the very edge of the fountain amphitheaters still holding large ma.s.ses of snow; immediately below, almost at his feet, lie glistening, gem-like, in dark rocky setting, the three exquisite little lakes; on either side of these, embracing and protecting them, stretch out the moraine arms, reaching toward and directing the eye to the great Lake, which lies, map-like, with all its sinuous outlines perfectly distinct, even to its extreme northern end, twenty-five to thirty miles away. As the eye sweeps again up the canyon-beds, little lakes, glacier scooped rock basins, filled with ice-cold water, flash in the sunlight on every side. Twelve or fifteen of these may be seen.
From appropriate positions on the surface of Lake Tahoe, also, all the moraine ridges are beautifully seen at once, but the glacial lakes and the canyon-beds, of course, cannot be seen.
There are several questions of a general nature suggested by my examination of these three glacial pathways, which I have thought best to consider separately.
_a. Evidences of the existence of the Great Lake Valley Glacier_. On the south sh.o.r.e of Lake Tahoe, and especially at the northern or lower end of Fallen Leaf Lake, I found many pebbles and some large bowlders of a beautiful striped agate-like slate. The stripes consisted of alternate bands of black and translucent white, the latter weathering into milk-white, or yellowish, or reddish. It was perfectly evident that these fragments were brought down from the canyon above Fallen Leaf Lake. On ascending this canyon I easily found the parent rock of these pebbles and bowlders.
the It is a powerful outcropping ledge of beautifully striped siliceous slate, full of fissures and joints, and easily broken into blocks of all sizes, crossing the canyon about a half mile above the lake. This rock is so peculiar and so easily identified that its fragments become an admirable index of the extent of the glacial transportation. I have, myself, traced these pebbles only a little way along the western sh.o.r.es of the great Lake, as my observations were princ.i.p.ally confined to this part; but I learn from my brother, Professor John LeConte, and from Mr. John Muir, both of whom have examined the pebbles I have brought home, that precisely similar fragments are found in great abundance all along the western sh.o.r.e from Sugar Pine Point northward, and especially on the extreme northwestern sh.o.r.e nearly thirty miles from their source. I have visited the eastern sh.o.r.e of the Lake somewhat more extensively than the western, and nowhere did I see similar pebbles. Mr. Muir, who has walked around the Lake, tells me that they do not occur on the eastern sh.o.r.e. We have, then, in the distribution of these pebbles, demonstrative evidence of the fact that Fallen Leaf Lake glacier was once a tributary of a much greater glacier which filled Lake Tahoe.
The only other agency to which we could attribute this transportation is that of sh.o.r.e ice and icebergs, which probably did once exist on Lake Tahoe; but the limitation of the pebbles to the western, and especially the northwestern sh.o.r.es, is in exact accordance with the laws of glacial transportation, but contrary to those of floating ice transportation--for lake ice is carried only by winds, and would, therefore, deposit equally on all sh.o.r.es.
Again: I think I find additional evidence of a Lake Tahoe "mer de glace" in the contrasted character of the northern and southern sh.o.r.es of this Lake.
All the little glacial lakes described above are deep at the upper end and shallow at the lower end. Further, all of them have a sand beach and a sand flat at the upper end, and great bowlders thickly scattered in the shallow water, and along the sh.o.r.e at the lower end. These facts are easily explained, if we remember that while the glacial _scooping_ was princ.i.p.ally at the upper end, the glacial _droppings_ were princ.i.p.ally at the lower end. And further: that while the _glacial_ deposit was princ.i.p.ally at the lower end, the _river_ deposit, since the glacial epoch, has been wholly at the upper end.
Now the great Lake, also, has a similar structure. It also has a beautiful sand and gravel beach all along its upper sh.o.r.e, and a sand flat extending above it; while at its lower, or northern end, thickly strewed in the shallow water, and along the sh.o.r.e line, and some distance above the sh.o.r.e line, are found in great abundance _bowlders of enormous size_.
May we not conclude that similar effects have been produced by similar causes--that these huge bowlders were dropped by the great glacier at its lower end? Similar bowlders are also found along the northern portion of the eastern sh.o.r.e, because the princ.i.p.al flow of the ice-current was from the southwest, and in the fulness of glacial times the princ.i.p.al exit was over the northeastern lip of the basin.
_b. Origin of Lake Tahoe_. That Lake Tahoe was once wholly occupied by ice, I think, is certain; but that it was scooped out by the Lake Valley glacier is perhaps more doubtful. All other Sierra lakes which I have seen certainly owe their origin to glacial agency. Neither do I think we should be staggered by the size or enormous depth of this Lake. Yet, from its position, it may be a plication-hollow, or a trough produced by the formation of two parallel mountain ridges, and afterward modified by glacial agency, instead of a pure glacial-scooped rock-basin. In other words, Lake Valley, with its two summit ridges, _may be regarded as a phenomenon belonging to the order of mountain-formation and not to the order of mountain sculpture_. I believe an examination of the rocks of the two summit ridges would probably settle this. In the absence of more light than I now have, I will not hazard an opinion.[3]
[Footnote 3: This question practically has been settled by Mr.
Lindgren, and his conclusions are given in an earlier chapter.]
_c. Pa.s.sage of slate into granite_. From the commencement of the rocky canyon at the head of Fallen Leaf Lake, and up for about two miles, the canyon walls and bed are composed of _slate_. The slate, however, becomes more and more metamorphic as we go up, until it pa.s.ses into what much resembles _trap_. In some places it looks like _diorite_ and in others like _porphyry_. I saw no evidence, however, of any outburst. This latter rock pa.s.ses somewhat more rapidly into _granite_ at Glen Alpine Springs. From this point the canyon bed and lower walls are granite, but the highest peaks are still a dark, splintery, metamorphic slate. The glacial erosion has here cut through the slate and bitten deep into the underlying granite. The pa.s.sage from slate through porphyritic diorite into granite may, I think, be best explained by the increasing degree of metamorphism, and at the same time a change of the original sediments at this point; granite being the last term of metamorphism of pure clays, or clayey sandstones, while bedded diorites are similarly formed from ferruginous and calcareous slates. Just at the junction of the harder and tougher granite with the softer and more jointed slates, occur, as might be expected, cascades in the river. It is probable that the cascades at the head of Cascade Lake and Emerald Bay mark, also, the junction of the granite with the slate--only the junction here is covered with debris. Just at the same junction, in Fallen Leaf Lake Canyon (Glen Alpine Basin), burst out the waters of Glen Alpine Springs, highly charged with bicarbonates of iron and soda.
_d. Glacial Deltas_. I have stated that the moraines of Cascade Lake and Emerald Bay glaciers run down to the margin of Lake Tahoe. An examination of this portion of the Lake sh.o.r.e shows that _they run far into the Lake_--that the Lake has been filled in, two or three miles, by glacial debris. On the eastern margin of Lake Tahoe, the water, close along the sh.o.r.e, is comparatively shallow, the sh.o.r.e rocky, and along the sh.o.r.e-line, above and below the water, are scattered great bowlders, probably dropped by the main glacier. But on the west margin of the Lake the sh.o.r.eline is composed wholly of moraine matter, the water very deep close to sh.o.r.e, and the bottom composed of precisely similar moraine matter. In rowing along the sh.o.r.e, I found that the exquisite ultramarine blue of the deep water extends to within 100 to 150 feet of the sh.o.r.e-line. At this distance, the bottom could barely be seen. Judging from the experiments of my brother, Professor John Le Conte, according to which a white object could be seen at a depth of 115 feet, I suppose the depth along the line of junction of the ultramarine blue and the emerald green water is at least 100 feet. The slope of the bottom is, therefore, nearly, or quite, 45 degrees. It seems, in fact, a direct continuation beneath the water of the moraine slope. The materials, also, which may be examined with ease through the wonderfully transparent water, are exactly the same as that composing the moraine, viz: earth, pebbles, and bowlders of all sizes, some of them of enormous dimensions. It seems almost certain that _the margin of the great Lake Valley glacier, and of the Lake itself when this glacier had melted and the tributaries first began to run into the Lake, was the series of rocky points at the head of the three little lakes, about three or four miles back from the present margin of the main Lake; and that all lakeward from these points has been filled in and made land by the action of the three glaciers described_. At that time Rubicon Point was a rocky promontory, projecting far into the Lake, beyond which was another wide bay, which has been similarly filled in by debris brought down by glaciers north of this point. The long moraines of these glaciers are plainly visible from the Lake surface; but I have not examined them. Thus, all the land, for three or four miles back from the Lake-margin, both north and south of Rubicon Point, is composed of _confluent glacial deltas_, and on these deltas the moraine ridges are the _natural levees_ of these ice-streams.
_e. Parallel Moraines_. The moraines described above are peculiar and almost unique. Nowhere, except about Lake Tahoe and near Lake Mono, have I seen moraines in the form of _parallel ridges_ lying on a level plain and terminating abruptly _without any signs of transverse connection (terminal moraine) at the lower end_. Nor have I been able to find any description of similar moraines in other countries. They are not terminal moraines, for the glacial pathway is open below. They are not lateral moraines, for these are borne on the glacier itself, or else stranded on the deep canyon sides. Neither do I think moraines of this kind would be formed by a glacier emerging from a steep narrow canyon and running out on a level plain; for in such cases, as soon as the confinement of the bounding walls is removed, the ice stream spreads out into an _ice lake_.
It does so as naturally and necessarily as does water under similar circ.u.mstances. The deposit would be nearly transverse to the direction of the motion, and, therefore, more or less crescentic. There must be something peculiar in the conditions under which these parallel ridges were formed. I believe the conditions were as described below.
We have already given reason to think that the original margin of the Lake, in glacial times, was three or four miles back from the present margin, along the series of rocky points against which the ridges abut; and that all the flat plain thence to the present margin is made land. If so, then it is evident that at that time the three glaciers described ran far out into the Lake, until reaching deep water, where they formed icebergs. Under these conditions, it is plain that the pressure on this, the subaqueous portion of the glacial bed, would be small, and become less and less until it becomes nothing at the point where the icebergs float away. The pressure on the bed being small, not enough to overcome the cohesion of ice, there would be no spreading. _A glacier running down a steep narrow canyon and out into the deep water, and forming icebergs at its point, would maintain its slender, tongue-like form, and drop its debris on each side, forming parallel ridges, and would not form a terminal moraine because the materials not dropped previously would be carried off by icebergs_. In the subsequent retreat of such a glacier, imperfect terminal moraines might be formed higher up, where the water is not deep enough to form icebergs. It is probable, too, that since the melting of the great "mer de glace" and the formation of the Lake, the level of the water has gone down considerably, by the deepening of the Truckee Canyon outlet by means of erosion. Thus not only did the glaciers retreat from the Lake, but also the Lake from the glaciers.
As already stated, similar parallel moraine ridges are formed by the glaciers which ran down the steep eastern slope of the Sierras, and out on the level plains of Mono. By far the most remarkable are those formed by b.l.o.o.d.y most Canyon Glacier, described by me in a former paper. These moraines are six or seven miles long, 300 to 400 feet high, and the parallel crests not more than a mile asunder. There, also, as at Lake Tahoe, we find them terminating abruptly in the plain without any sign of terminal moraine. But higher up there are small, imperfect, transverse moraines, made during the subsequent retreat, behind which water has collected, forming lakes and marshes. But observe: these moraines are also _in the vicinity of a great lake_; and we have abundant evidence, in very distinct terraces described by Whitney[4] and observed by myself, that in glacial times the _water stood at least six hundred feet above the present level_. In fact, there can be no doubt that at that time the waters of Mono Lake (or a much greater body of water of which Mono is the remnant) washed against the bold rocky points from which the debris ridges start. _The glaciers in this vicinity, therefore, must have_ run out into the water six or seven miles, and doubtless formed icebergs at their point, and, therefore, formed there no terminal moraine.
[Footnote 4: _Geological Survey of California_, Vol. I, 451.]
That the glaciers described about Lake Tahoe and Lake Mono ran out far into the water and formed icebergs I think is quite certain, and that parallel moraines open below are characteristic signs of such conditions I also think nearly certain.
_f. Glacial Erosion_. My observations on glacial pathways in the High Sierra, and especially about Lake Tahoe, have greatly modified my views as to the nature of glacial erosion.
Writers on this subject seem to regard glacial erosion as mostly, if not wholly, a _grinding_ and _scoring_; the debris of this erosion as rock-meal; the great bowlders, which are found in such immense quant.i.ties in the terminal deposit, as derived wholly from the crumbling cliffs above the glacial surface; the _rounded_ bowlders, which are often the most numerous, as derived in precisely the same way, only they have been engulfed by creva.s.ses, or between the sides of the glacier and the bounding wall, and thus carried between the moving ice and its rocky bed, as between the upper and nether millstone. In a word, all bowlders, whether angular or rounded, are supposed to owe their _origin_ or _separation_ and _shaping_ to glacial agency.
Now, if such be the true view of glacial erosion, evidently its effect in mountain sculpture must be small indeed.
_Roches moutonnees_ are recognized by all as the most universal and characteristic sign of a glacial bed. Sometimes these beds are only imperfect _moutonnees_, i.e., they are composed of _broken angular surface with only the points and edges planed off_. Now, _moutonnees_ surfaces always, and especially angular surfaces with only points and edges beveled, show that the erosion by grinding has been only very superficial. They show that if the usual view of glacial erosion be correct, the great canyons, so far from being _formed_, were only very _slightly modified_ by glacial agency. But I am quite satisfied from my own observations, that this is not the only _nor the princ.i.p.al_ mode of glacial erosion. I am convinced that a glacier, by its enormous pressure and resistless onward movement, is _constantly breaking off large blocks_ from its bed and bounding walls. Its erosion is not only a grinding and scoring, but also a _crushing and breaking_. It makes by its erosion not only rock-meal, but also large _rock-chips_. Thus, a glacier is constantly breaking off blocks and making angular surfaces, and then grinding off the angles both of the fragments and the bed, and thus forming rounded bowlders and _moutonnees_ surfaces. Its erosion is a constant process of alternate _rough hewing and planing_. If the rock be full of fissures, and the glacier deep and heavy, the rough hewing so predominates that the plane has only time to touch the corners a little before the rock is again broken and new angles formed. This is the case high up on the _canyon walls_, at the head of Cascade Lake and Emerald Bay, but also in the _canyon beds wherever the slate is approached_. If, on the other hand, the rock is very hard and solid, and the glacier be not very deep and heavy, the planing will predominate over the rough hewing, and a smooth, gentle billowy surface is the result. This is the case in the hard granite forming the beds of all the canyons high up, but especially high up the canyon of Fallen Leaf Lake (Glen Alpine Basin), where the canyon spreads out and extensive but comparatively thin snow sheets have been at work. In some cases _on the cliffs_, subsequent disintegration of a glacier-polished surface may have given the appearance of angular surfaces with beveled corners; but, in other cases, in the _bed of the canyon_, and on elevated level places, where large loosened blocks could not be removed by water nor by gravity, I observed the same appearances, under conditions which forbid this explanation. Mr. Muir, also, in his _Studies in the Sierra_, gives many examples of undoubted rock-breaking by ancient glaciers.
_Angular_ blocks are mostly, therefore, the ruins of crumbling cliffs, borne on the surface of the glacier and deposited at its foot. Many _rounded_ bowlders also have a similar origin, having found their way to the bed of the glacier through creva.s.ses, or along the sides of the glacier.
But _most of the rounded bowlders_ in the terminal deposit of _great glaciers_ are fragments _torn off by the glacier itself_. The proportion of rounded bowlders--of upper or air-formed--to nether or glacier-formed fragments, depends on the depth and extent of the ice-current. In the case of the universal ice-sheet (ice-flood) there are, of course, no upper formed or angular blocks at all--there is nothing borne on the surface. The moraine, therefore, consists wholly of nether-formed and nether-borne severely triturated materials (_moraine profunde_). The bowlders are, of course, all rounded. This is one extreme. In the case of the thin moving ice-fields, the _glacierets_ which still linger among the highest peaks and shadiest hollows of the Sierra, on the other hand, the moraines are composed _wholly of angular blocks_. This is the character of the terminal moraine of Mount Lyell glacier. These glacierets are too thin and feeble and torpid to break off fragments--they can only _bear_ away what falls on them. This is the other extreme. But in the case of ordinary glaciers--ice-streams--the bowlders of the terminal deposit are mixed; the angular or upper-formed predominating in the small existing glaciers of temperate climates, but the rounded or nether-formed greatly predominating in the grand old glaciers of which we have been speaking. In the terminal deposits of these, especially in the materials pushed into the Lake, it is somewhat difficult to find a bowlder which has not been subjected to severe attrition.