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Because it is self evident that a moulin can not maintain itself for an indefinite period in actively moving ice it seems incredible that such large holes in the bed rock could be ground out in the time available. But it is to be noted that moulins develop primarily at the lower ends of very stagnant and inactive glaciers and, further, that the fall of the water may be of very great height. Thus Lubbock1 relates that the depth of a moulin on the Finster-Aar glacier was found to be 232 meters. This depth would undoubtedly lead to great force in the fall of the water. The mere impact of the water, however, has very little to do with starting the holes, according to Stone.15 It is chiefly the stones and sediment rolled about that erode. The falling water develops tremendous swirls at the base and these keep the rock tools in active motion. The moulin potholes of regions of continental glaciation are however thought by Upham1 to have been formed during an early stage of the glaciation, since during that stage the supply of tools would not be too great, as he says would be the case during the latter part of the glacial period.

Class A, Type 2:

Plunge pools are potholes, in general, of large size, occurring at the foot of a vertical or nearly vertical waterfall. At such sites the velocity of the falling water develops especially great energy in swirling stones at the foot of the plunge, and this results in the grinding out of potholes or plunge pools of exceptional diameter (depending on the volume of the water) and depth (depending apparently on the height of the fall). A large plunge pool occurs at the base of the Canadian side of Niagara Falls. According to Spencer," the depth of this pool is 72 feet. In most plunge pools the water is much deeper than it is in the stream channel on their downstream side. Smaller waterfalls develop plunge pools that quite closely resemble normal potholes. In fact there are practically all gradations from plunge pools to the normal type of pothole formed in existing stream channels. It must not be inferred from this, however, that all potholes in existing stream channels are initiated

14 Lubbock, Sir John, "The Scenery of Switzerland," Macmillan Co., 1898, pp. 92-93, 122.

15 Stone, G. H., "The Glacial Gravels of Maine and their Associated Deposits," U. S. G. S. Monograph, 34, pp. 324–326, 1899.

16 Upham, W., "Giants' Kettles Eroded by Moulin Torrents," Bull. Geol. Soc. of Am., Vol. 12, pp. 25–44, 1900.

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17 Spencer, Dr. J. W. W., Soundings in Niagara Gorge and under the Falls," Sci. American, Vol. 99, Aug. 1, 1908, pp. 76–77.

by waterfalls however small. This fact is illustrated in Fig. 2 which shows quite clearly that a pothole may become a plunge pool.

Class A, Type 3:

Normal Potholes.-These potholes occur in the beds of present day streams or recently abandoned stream courses, in places over which the water has flowed either constantly or during its periods of high volume. If still in the process of formation they must be located directly in the course of the water channel at least during occasional periods of flooding. One of the distinguishing characteristics of normal potholes is the presence of waterworn surfaces adjoining the hole. Normal potholes are described in greater detail below.

Class A, Type 4:

Cupholes have been described by Hudson1s as little potholes that have been cut, not by pebbles, but by sand and silt swirled about by water currents. They rarely exceed 12 centimeters in diameter and may be cut on very steep slopes of a rock surface. According to Hudson,19 cupholes are somewhat V-shaped, more strictly speaking,-parabolic in vertical sections. (See Figs. 3, 4, and 5.)

The same author also describes joint wells 20-another type of small depressions that occur along joints in rocks that have suffered glaciation. It is his opinion that joint wells (see Fig. 6) were cut by combined solution and silt erosion processes and that they are embryonic moulin-potholes in that they are the work of subglacial streams. Hudson, while describing the cupholes as "little potholes "21 caused by wave and undertow acting on the lake shore, is unwilling to have them classed as a form of incipient potholes on the ground that they may never become such. He regards their form as being so different from that of

18 Hudson, G. H., "Some Items Concerning a New and an Old Coast Line of Lake Champlain," N. Y. State Mus. Bull., No. 133, 5th Rept. of the Director, 1908, pp. 160–162, 1909.

19 Hudson, G. H., “Joint Caves of Valcour Island-Their Age and Origin," N. Y. State Mus Bull., No. 140, 6th Rept. of the Director, 1909, pp. 170-173.

20 Hudson, G. H., "Rill Channels and Their Cause," Report of the Vermont State Geologist, 1912, pp. 245-246.

21 Hudson, G. H., "Some Items Concerning a New and an Old Coast Line of Lake Champlain," N. Y. State Mus. Bull., No. 133, 5th Rept. of the Director, 1908, pp. 160–162, 1909.

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FIG. 1. INTERSECTING POTHOLES IN GORGE OF BUTTERMILK CREEK NEAR ITHACA, N. Y. Such a series of potholes indicates the primary importance of pothole erosion processes in the deepening of stream gorges.

VOL. V.-36

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Notice

FIG. 2. SERIES OF PLUNGE POOLS AND A WATERFALL IN LAVA IN HAWAII. how the fully developed pools below the falls have intersected because of enlargement below the water surface thus creating a series of natural bridges. When the pothole at the foot of the falls has developed sufficiently to intersect the base of the similar hollow in the stream bed above, the falls will again retreat and the upper pothole will become a plunge pool. This picture farther illustrates the important part that processes of pothole and plunge pool excavation play in gorge deepening.

[graphic]

Photo by G. H. Hudson, Plattsburg, N. Y.

FIG. 3. CUPHOLES CUT IN STEEP FACE OF ROCK ON VALCOUR ISLAND.

typical potholes as to entitle them to a name of their own and in a letter to the writer states that they are made by silt and sand carried by vortex motion and that in his opinion the cupholes tell of lake conditions or large bodies of water and not of river conditions.

If his interpretation of them is correct, it is possible that the cupholes are in origin akin to the so-called tide pools described in another paragraph and hence may be classed as minute forms of normal potholes. Furthermore, it also seems

[graphic]

FIG. 4.

BLOCK OF

Photo by G. H. Hudson, Plattsburg, N. Y.

PURE DOLOMITIC LIMESTONE CUT BY CONFLUENT CUPHOLES. Specimen taken from Valcour Island and is now in the New York State Museum.

probable that the joint wells and dentpits (a third type of similar small depression) are all very small forms of moulin potholes or solution potholes. Thus the cupholes are due primarily to erosive action of fine sediment whirled by the water; the dentpits to solution action; while the joint wells may be due to both solution and grinding.

Class B, Type 1:

Solution Potholes.-This type includes all the holes that are

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