BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 17, PP. 317-320, PLS. 40-42 JULY 27, 1906

MOULIN WORK UNDER GLACIERS*

BY G. K. GILBERT

(Read before the Cordilkran Section of the Society December SO, 1905)

CONTENTS Page Statement of the problem studied...... 317 Origin of the rock sculpture...... 317 The moulin, or glacial m ill...... 318 Explanation of plates...... 320

S tatem en t o r t h e P roblem studied

In glaciated regions I have several times encountered an aberrant and puzzling type of sculpture. Inclined surfaces, so situated that they can not have been subjected to postglacial stream scour, are sometimes carved in a succession of shallow, spoon-shaped hollows, and at the same time are highly polished. They resemble to a certain extent the surfaces some­ times wrought by glaciers on well-jointed rocks, where the hackly charac­ ter produced by the removal of angular blocks is modified by abrasion; but they are essentially different. Instead of having the salient elements well rounded and the reentrant angular, they have reentrants well rounded and salients more or less angular; and they are further distinguished by the absence of glacial striae. An example appears in the foreground of plate 40, representing the canyon of the South fork of the San Joaquin river, in the heart of the glaciated zone of the Sierra Nevada. The peculiarly sculptured spot is high above the river.

Or ig in of t h e R ock Sculpture

The key to the puzzle was found on a dome of granite standing at the southwest edge of Tuolumne meadow, Sierra Nevada, just north of the Tioga road. The dome is several hundred feet high and in general Is

•Published by permission of the Director of the U. S. Geological Survey. XXIX—B o l l . G eol. S oc. A m., Vol. 17, 1905. (317)

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smoothly curved, its form being due partly to exfoliation and partly to erosion by the deep Pleistocene ice-stream of the Tuolumne basin, which passed over it from east to west. On the southern face, where the general slope is about 20 degrees, occurs the peculiar flexuous sculpture, and in close association with it are several potholes. The pothole figured in plate 41 has its mouth about 40 feet above the visible base of the dome, and there are others somewhat higher. They are not to be explained by any conceivable river or creek, and I have no hesitation in ascribing them to moulin work at some stage of the last glaciation of the district. With their aid it is easy to recognize the associated shallow hollows as imper­ fectly developed potholes.

T h e Mo u lin , ok glacial M il l

A moulin, or glacial mill, is a stream of water plunging from top to base of a glacier through a well of its own maintenance. The water is derived from ablation, has a course on the surface of the glacier before reaching the well, and escapes from the bottom of the well by a channel beneath the glacier. The well originates in a crevasse, the crevasse re­ sults from a strain of the glacier, and the strain is related to some local deflection of the ice-stream. Initially the crevasse must extend from top to bottom of the glacier, so as to admit and transmit the water stream. Afterward it is closed below by the welding of its walls, except where the falling water maintains an opening. The opening thus acquires a cylin- dric form, and is completely adjusted to the water, permitting it to plunge downward, with little or no deflection, and strike the rock bed with great force. As the glacier moves forward the moulin is carried with it. After a time a new crevasse is opened at the same turn of the ice current; it intercepts the stream of water and a new moulin is made; and the earlier well, being deprived of its water, and therefore unable to resist the en­ croachment of the quasi-plastic ice, becomes sealed. The new moulin and others after it repeat the course and the history of the first. At the base of the ice the plunging water finds boulders and sand, and with these, its familiar tools, attacks the rock bed. Some detail of the configuration of the bed, the presence of a large boulder held by the ice, or some other local condition, permanent or temporary, guides the water in such wav as to determine scour at a particular spot, and a shallow hollow is made. As successive mouliris pass the spot the hollow itself serves as a condition to determine further scour at the same spot. At the same time the hol­ low serves to prevent scour in its immediate vicinity,, but when the moulin has moved beyond its influence another hollow may be initiated. As

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MOULIN WORK

San Joaquin Canyon, Sierra Nevada

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MOULIN WORK

Near Tuolumne Meadows, Sierra Nevada

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BULL. GEOL. SOC. AM

MOULIN WORK Mokelumne Canyon, Sierra Nevada THE GLACIAL MILL 3 1 9

moulin follows moulin and summer follows summer, the hollows are deepened and assume the character of potholes. As I understand it, a pothole is developed only where rock fragments are carried round and round by whirling water. Mere impact of the plunging water is not suffi­ cient if there is no inclosure or obstruction to determine a whirl. After a hollow has been made and the condition for a whirl thus permanently localized, the whirl may be maintained by violent motion of the water anywhere about its rim; so that the deepening of the pothole progresses whenever a moulin stream strikes near it. If a moulin stream of pure water strikes the divide between two potholes it may furnish power for the simultaneous drilling of both holes without eroding the partition between them. If the surface conditions of the glacier are such that successive moulins follow closely the same track, there may be a long row of potholes, and with changing conditions there may result either parallel rows or an irregular distribution. In Turner’s classic photograph of the potholes of Mokelumne canyon (plate 42) at least three parallel rows may be seen. The imperfect potholes of the peculiar flexuous surfaces may some­ times be imperfect in the sense that they are incipient, but the greater number are probably surviving parts of potholes that were once complete. Just as there are one-sided lake valleys and one-sided water channels where the complementary sides were of Pleistocene ice, so, I think, the com­ plementary parts of these imperfect potholes were of ice, more or less fortified by included rock débris.* After the water of the moulin has reached the rock bed it must escape along some course beneath the ice. In flowing away it may accomplish erosion of the ordinary type, and the sculpture resulting from stream erosion may therefore be associated with moulin sculpture.

»Since the writing of this paragraph I have learned that the same explanation was published by Von Post forty years ago. Brogger and Reusch cite him as follows : “The parts now wanting in what now appear unfinished kettles he believes were composed of ice ; so that in this case also water might have whirled stones and rubbish round tlio inside of a kettle consisting partly of rock and partly of ice.” Quart. Jour. (ieol. Soc. London, vol. xxx, p. 765, 1874.

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Explanation op Plates

Plate 40.—Moulin Work, San Joaquin Canyon, Sierra Nevada The view looks up the canyon of the South fork of San Joaquin river toward the mouth of Evolution creek. The rock of the foreground is slate, with vertical structure. The sculpture of a tract in the foreground, especially the spoon-shaped hollows with débris, a, 6, c, is ascribed to moulin action. Just beyond this tract the profile of the canyon wall, outlined against trees, is of the normal glacial type.

Plate 41.—Moulin Work, near Tuolumne Meadows, Sierra Nevada The view looks eastward. The general direction of Pleistocene ice movement was from distance to foreground. The pothole in the foreground is nearly filled by débris, among which are highly polished boulders. The pine at the left is rooted in another pothole, and the more distant pine probably occupies a third. At various points beyond the nearer pothole the general slope is interrupted by imperfect potholes. The gran­ ite is without joints and wifhout schistosity. The lines, sug­ gestive of structure, which descend the slope are surface stains.

Plate 42.—Moulin Work, Mokelumne Canyon, Sierra Nevada The locality is under the north wall of the canyon of the North fork of Mokelumne river, 5 miles northwest of Bloods (north boundary of Big Trees quadrangle of U. S. Geological Sur­ vey Atlas). The view looks upstream. The direction of ice movement may be assumed as parallel to the canyon wall. It will be observed that a considerable number of the potholes are arranged in rows parallel to the canyon wall. There is a row of four holes between A and B, one of seven holes between C and D, and one of four holes between E and F. The last mentioned row possibly begins at Q- and includes eight holes between G and F. The photograph is by H. W. Turner, who described the locality in the American Journal of Science in 1892 (third series, volume 44, pages 453-454). He states that the potholes are about 250 in number, that they are from 0 inches to 6 feet apart, that few or none coalesce, and that the lowest of them are 20 or 30 feet above the river.

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