THE GLACIAL MILL 319 



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 -12) 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 debris.* 



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 the 

 inside of a kettle consisting partly of rock and partly of ice." Quart. Jour. Geol. Soc. 

 London, vol. xxx, p. 765, 1874. 



