328 GLACIAL GEAVELS OF MAINE. 



northeast side of the ravine there is a narrow step or shelf situated about 

 halfway between the top and the bottom of the wall. Its position is shown 

 in the accompanying diagram. The hole is nearly round. It is 21 inches 

 in diameter, 16 inches deep on the lower side, and 2 feet on the upper. 

 The upper part of the interior is somewhat weathered and rough. The 

 lower part, which is generally filled with water, is very smoothly polished. 

 The bottom is almost hemispherical. The granite of the region weathers 

 rough. These facts prove it to be a pothole, not a freak of weathering. 

 Its situation halfway up the side of a cliif and within a few rods of the 

 highest part of the ridge, whence the water flows in several different direc- 

 tions, conclusiA^ely proves that it can not have been formed by anj' stream 

 of surface drainage. A glacier moving from the north would naturally be 

 broken by crevasses as it flowed over the cliff. This would be a favorable 

 place for a stream flowing on the surface of the ice to plunge 

 to the bottom and escape as a subglacial stream. I could 

 find no glacial gravel in the vicinity. A subglacial stream 

 flowing from this point southward would fall more than 200 

 feet within a mile, and might be expected to sweep its chan- 

 nel clear of sediment. If a subglacial stream from the north 



Fig. 26.— Section of 



cliff and pothole; flowcd up the loug hill and over the cliff it probably ought 

 to have left sediment or other sign on an up slope that rises 

 at least 200 feet within a mile. The north slope of the hill is covered 

 deeply with the ordinary granitic till of the region, without glacial gravel 

 or erosion channel or any other sign of a glacial stream. The only admis- 

 sible interpretation of these facts that occurs to me is that a superficial 

 stream here tumbled down a crevasse that formed as the ice passed up 

 the clifts. 



Can potholes be formed at the foot of a moulin shaft? Professor Dana 

 has suggested that the change in position of the waterfall due to the 

 advance of the ice would produce an elongated rather than a round cavity. 



It is a fact that as each crevasse moves onward a new crevasse is pro- 

 duced in the rear of the former, and a new well is soon excavated down 

 the crevasse last formed. It will naturally result that the water will not 

 continually fall in the same place, but over an area as long as the distance 

 between the successive shafts. In other words, each shaft begins at a cer- 

 tain place and moves on, subjecting all the rock over which it passes to the 



