GLACIAL POTHOLES. 829 



direct impact of tlie water, until it is superseded hj the next crevasse, 

 which then repeats the process. But there must be an enlargement at the 

 base of the shaft, varying in size according to the size of the stream, the depth 

 of ice, and the amount of warmed water. This may in many cases permit 

 the water to scatter, so that it will not strike the rock in a round definite 

 stream. But be this as it may, the direct mechanical impact of the water 

 against the rock has very little, to do with eroding potholes except to start 

 the process It is chiefly the stones rolled round and round by the water 

 that do the Avork. Were the rock so soft that mechanical erosion by the 

 water exceeded the attrition of the whirling stones, we should have, not a 

 smooth-walled pothole, but a canyon of erosion with irregular surface. 

 When once a cavity is found or made which is deep enough to prevent the 

 stones swept along by the stream from being prematurely washed away, 

 the erosion by the rolling stones would, in case of hard rocks, so far exceed 

 the mechanical erosion of the water that the shape of the well would be 

 that due to the attrition of the stones, with hardly a trace of direct water 

 erosion. All that is needed is that the water in the pothole be kept whirl- 

 ing. When a nearly vertical cascade strikes the rock, the water must shoot 

 swiftly oiitward on all sides in nearly a horizontal plane. If a pothole 

 were within reach of these oiit-rushing Avaters, the water within it would be 

 kept whirling as well as if a vertical stream fell into it. Whether, then, the 

 water at a glacier mill falls directly into a pothole or anywhere near it, it 

 will continue to whirl the water in the hole. And the hole would be as 

 round as one formed by any other stream, unless the nature of the rock 

 permitted it to be easily eroded by the direct impact of the water. 

 Regarding glacial potholes, my conclusions are as follows: 



1. They may be formed by sub glacial streams. 



2. They may be formed at the foot of the waterfall where a superficial 

 stream pours down a crevasse. 



3. They form only where the stream carries but little sediment or is 

 swift enough to keep its channel clear of sediment, or nearly so. If a stream 

 begins to drop its sediment an incipient pothole would soon fill up and 

 ultimately would be covered by a mass of glacial gravel. 



4. The velocit}^ of subglacial streams is so great, since they are urged 

 by a great pressure from behind, that they might be able to form potholes 

 at a considerable depth beneath the sea or a lake into which they might 



