318 G-.K.i GILBERT — MOULIN WORK UNDER GLACIERS 



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. 



The Moulin, or glacial Mill 



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 way as 

 to determine scour at a particular spot, and a shallow hollow is made. 

 As successive moulins 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 



