Yosemite > Library > François Matthes > The Striped Rock Floor of the Little Yosemite Valley >
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About a stone’s throw from where the Clouds Rest Trail leaves the flat of the Little Yosemite Valley, there is a curious expanse of smooth, bare granite, an acre or more in extent. It is a part of the solid rock floor of the valley, which, buried under river gravel and glacial material elsewhere, is here exposed to view, cleared of all debris. Indeed, so scrupulously clean swept does it look, one might fancy some cyclopian broom had been at work on it—and a new one at that.
Round about, in all directions lie glacial boulders, some singly, some in clusters, some in heaps mixed with fine debris. Sparse pines and cedars rise from what few cracks the stone floor affords as a root-hold, giving the place a singularly genial, parklike aspect. But the cleared tract itself has not a tree on it—its surface stretches unbroken and continuous, unmarred by a single fissure.
As one approaches from the lower end and looks up the gentle slope—for the floor inclines appreciably—the eye is almost at once held by the peculiar “painted” appearance of the space. Irregular, blotchy white ribbons set off conspicuously against the prevailingly gray tint of the rock floor, sprawl over it here and there. Wholly unlike the dark water stains that stripe most of the Yosemite cliffs, they seem, even to one thoroughly familiar with the various markings common to the rock surfaces of the region, altogether novel and enigmatic. All trend downward with the slope, but beyond this there seems no discoverable law in their arrangement, nor anything else immediately suggestive of their mode of origin. The majority occur in loosely connected groups, but some lie off by themselves, like pale islands in a dark ocean. As the view on page 51 well shows, they generally commence abruptly and terminate abruptly, without definite relation to the unevenesses of the floor itself. Some divide, others merge downward; some gain in width, others taper down toward their lower ends.
In dimensions they are equally varied. While they average between four and five feet in length and from two to three inches in breadth, there are individuals among them but a fraction of a foot long and others exceeding twelve or even fifteen feet; and some are less than an inch across, while others—like those in the immediate foreground of the view—span six inches and over. Nor is the breadth always proportionate to the length: Some of the longest are very narrow, some of the shortest very broad.
On closer inspection they are seen to consist simply of narrow tracts from which the lichens that otherwise uniformly mottle the rock have been removed, and it becomes plain that it is merely the light color of the unweathered granite thus exposed that makes them prominent. These stripes, then, are not stains at all; rather, they owe their brilliancy to their very stainlessness—to the absence of coloring matter of any sort.
By what agent the lichens were cleared off, however, seems at first a mystery. That it was some substance that moved downhill under the influence of gravity is patent from the invariable downhill trend of all the stripes, but what the nature of that substance was, is not easily guessed. One feels tempted to believe it was some corrosive fluid that was poured out upon the rock and flowed down slowly, eating away the lichens as it went. There are places in the Yosemite Valley where such a thing has actually happened, so the theory is not so utterly absurd as at first blush it may seem. On the road to Mirror Lake, for instance, there is a great block of granite on the flat side of which some enterprising individual once painted him an advertisement in bold, glaring type. The true history of the affair may be better known to some of the readers of this journal than to the writer, but he gathers from a casual look that the “ad” was subsequently effaced by a zealous guardian. Whatever material the latter employed to remove the paint, removed the lichens too, running down in vertical, blotchy stripes remarkably similar to those on the Little Yosemite floor. Again, at the site of the “Old Blacksmith Shop,” near the foot of the Coulterville Road, a space has been cleared on a huge rock by means of some caustic, and the same streaky effect has been produced.
But the stripes in the Little Yosemite Valley clearly were not the work of marring man. Besides, the same sort of markings exist in many other places in the Yosemite region, in seldom frequented spots too, as a rule. It was on such a spot, in fact—on the north slope of Liberty Cap that the writer first found a clue to their mode of origin. A small rock fragment, derived from a disintegrating shell of the great rock hump, had evidently slid here several feet from its place of starting, and, extending from it, pointing up the slope, was a little white path cleared of lichens. Not far away were other fragments each likewise leaving a flaming trail. The width of the stripe produced corresponded in each case to the dimensions of the fragment. A tiny bit of granite, no larger than a thimble, lay at the end of a delicate white ribbon, and an uptorn tree stump had made a dozen markings, one with each of its dragging root tips and a broad swath with its heavy broken end. Surely, here was the key to the enigma! Here were the stripes in process of being made.
What, however, impelled the rocks and the tree stump downward? None of them appeared in motion, and none when dislodged, would slip or roll. The slope was not steep enough for that. Observation further showed that no stripes ever occur on very steep slopes—they are restricted to surfaces of moderate inclination such as the crowning portions of the domes, and wherever the declivity approaches the “angle of repose” the stripes invariably come to an end. A pretty and striking instance was seen on a small dome spur in the Little Yosemite Valley. Here the stripes, diverging downward like meridians on a globe, all terminated abruptly as by concert at the same level, the same parallel of latitude. Below that line, evidently, the debris had slid or rolled away. The inclination here, it should be noted, was too great to stand on safely, but farther up, among the stripes, one could walk even with hobnails by exercising a little care.
It is to be inferred from the above that a slow motion of the debris is essential for the production of the stripes. The explanation is here offered that the debris
These mysterious stripes on the floor of Little Yosemite
Valley are narrow paths from which slow-growing lichens
Snow and running water have slowly pushed this small
boulder across the gently sloping granite, wearing away
[click to enlarge]
The total progress thus effected may not exceed an inch or two per year, and this estimate, if it is at all correct, lends additional significance to the stripes: they indicate not merely the route traveled over by each fragment, they also embody a time record of the journey. Some of them represent a lapse of many years, the more impressive when it is reflected that during all that time no being, human or other, happened by in this solitude to interfere with the orderly continuance of the process.
The explanation above, however, accounts only for the movement of the debris. It does not yet make clear the production of the stripes themselves. That a heavy boulder might grind off the lichens from the bed it passes over seems quite natural, but that a bit of rock weighing an ounce or two should clear a path does not seem at all self-evident. The weight of the fragment, if it is a factor in the process, apparently plays but a minor role.
On picking up one of these traveling fragments, one finds it invariably imbedded in a small pad of loose rock grains that have collected under it. Now lichens cannot thrive under the thinnest veneer of sand or soil, as may be observed in a thousand places throughout the Sierra. Slanting rocks uncovered by the grading of a wagon road, for instance, show plainly by the boundary of their lichen growth where the surface of the ground used to be. Shallow basins in a rock floor or on large boulders that tend to accumulate sand, pine needles, and other litter, similarly remain white and bare of lichens. It is safe to say, therefore, that it is the sand pad under the fragments rather than the fragments themselves that clears the lichens from the stripes. And here, again, is substantiation of the view expressed regarding the slowness of the process. For, were the movement at all rapid, the lichens in any one spot would not remain covered for a sufficient length of time to utterly die and loose their hold. As a matter of fact there are places where they were not entirely stamped out and the stripes appear dim or interrupted. The debris must have advanced here with more than usual rapidity, owing to some local acceleration of the gradient, or perhaps through the pressure of an exceptionally heavy snow fall.
To come back now to the floor of the Little Yosemite Valley equipped with this insight into the stripe-producing process, let us look it over somewhat more closely. The feature that strikes us as most puzzling is the total absence of debris of any kind. Whatever material once traveled over the floor has in some manner disappeared. But not wholly, for here, near the east edge of the tract, lies a boulder weighing some twelve or fifteen pounds at the end of a long and glorious stripe. More than twenty feet it stretches, gradually fading in the distance like the smoke trail of a locomotive. A finer example would be difficult to find. The upward dimming of the stripe is in itself significant: so excessively slow has the progress of the boulder been that the lichens are already beginning to encroach again on the upper end, slow-growing plants though they be. When it is considered that rocks uncovered by road grading a score of years ago show scarcely any new lichens today, the great span of time represented by this stripe become doubly impressive. Its upper end, indeed, may date back to the time the Yosemite Valley was discovered.
But this stripe, after all, differs somewhat from the others on the floor. The rank and file are shorter and narrower; many split or fork irregularly, and all have ceased to grow in length through the removal of the debris that made them. Yet that material did not roll away, for the floor maintains about the same grade throughout and many stripes begin in the same latitude where others end. How, then, are these traits to be interpreted?
In the first place it seems certain that the material in question consisted of small, light fragments that were easily disturbed and thrown from their path by the feet of passing men or animals. That this must have happened more than once seems likely in view of the fact that the Little Yosemite is much frequented and in former days was inhabited by Indians, as the round holes in which they ground their acorns, near by, amply attest.
Again, the assumption seems legitimate that the fragments were in an advanced state of disintegration, and broke down and crumbled on the way. Much of the debris that litters the valley floor today is in just such a crumbly state. It has lain exposed so long to strong diurnal and seasonal temperature changes that the individual crystals in the granite, each expanding and contracting with a coefficient of expansion peculiar to the mineral composing it—feldspar, quartz, mica or hornblende have gradually worked loose and are ready to part company. Those readers who have mountaineered in the Sierra may have had the experience of picking up a rock that would break in the hand and run like sand through the fingers. This suggests an explanation for the forking of the stripes: A decomposing fragment, after having advanced some distance, would break in two. From then on there would be a double trail. Later, each of the pieces would divide, and the trail would split again. Some fragments broke down by degrees into an aggregate of half-loose crystals, and their trails widened out progressively. The end in each case came no doubt, when there was nothing left but a little heap of rock grains which the melting waters of springtime carried off with a rush.
Reprinted from Sierra Club Bulletin, January, 1911, pages 3-9. See also Matthes’ paper “Debris Tracks on the Domes of the Yosemite Region,” Science, n.s., Volume XXX, Number 758, pages 61-64, 1909.
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