320 g. k. gilbert — moulin work under glaciers 



Explanation of Plates 

 Plate 40. — Moulin Work, San Joaquin Canyon, Sierra Nevada 



The view looks up the canyon of the South fork of San Joaquin 

 river toward the mouth of Evolution creek. The rock of the 

 foreground is slate, with vertical structure. The sculpture of 

 a tract in the foreground, especially the spoon-shaped hollows 

 with debris, a, o, c, is ascribed to moulin action. Just beyond 

 this tract the profile of the canyon wall, outlined against 

 trees, is of the normal glacial type. 



Plate 41. — Moulin Work, near Tuolumne Meadows, Sierra Nevada 



The view looks eastward. The general direction of Pleistocene 

 ice movement was from distance to foreground. The pothole 

 in the foreground is nearly filled by debris, among which are 

 highly polished boulders. The pine at the left is rooted in 

 another pothole, and the more distant pine probably occupies 

 a third. At various points beyond the nearer pothole the 

 general slope is interrupted by imperfect potholes. The gran- 

 ite is without joints and without schistosity. The lines, sug- 

 gestive of structure, which descend the slope are surface 

 stains. 



Plate 42. — Moulin Work, Mokelumne Canyon, Sierra Nevada 



The locality is under the north wall of the canyon of the North 

 fork of Mokelumne river, 5 miles northwest of Bloods (north 

 boundary of Big Trees quadrangle of U. S. Geological Sur- 

 vey Atlas). The view looks upstream. 



The direction of ice movement may be assumed as parallel to 

 the canyon wall. It will be observed that a considerable 

 number of the potholes are arranged in rows parallel to the 

 canyon wall. There is a row of four holes between A and B, 

 one of seven holes between C and D, and one of four holes 

 between E and F. The last mentioned row possibly begins at 

 Gr and includes eight holes between G and F. 



The photograph is by H. W. Turner, who described the locality 

 in the American Journal of Science in 1802 (third series, 

 volume 44, pages 453-454). He states that the potholes are 

 about 250 in number, that they are from G inches to 6 feet 

 apart, that few or none coalesce, and that the lowest of them 

 are 20 or 30 feet above the river. 



