36 W. UPHAM — giants' kettles eroded by MOULIN TORRENTS 



of 5 and 10 feet. * Only one side of these holes is rock, with shallow 

 rock basins at the bottom, the other side having been probably the ice- 

 wall of the moulins through which the eroding torrents fell. Professor 

 W. 0. Crosb}^ has also described this localit}^ referring the pothole ero- 

 sion to the closing stage of the Glacial period.f At two other localities 

 in Cohasset, Bouve and Crosby report waterworn places on rock ledges 

 where their origin must likewise be referred to a nioulin or a subglacial 

 stream. 



Professor Crosby further supplies me the following notes, in a letter, 

 concerning several localities of glacial potholes in Massachusetts. 



At East Braintree a potliole, doubtless of subglacial formation, was 

 discovered in excavating for a foundation, and later was destroyed by 

 blasting. It was about 12 feet long, 3 to 6 feet wide, and 3 feet deep, 

 shaped somewhat like a bathtub, but opening outward at one end, its 

 smoothed sides there blending graduall}^ with the irregular surface of 

 the rock. 



At Newton Upper Falls, on the abrupt southern slope of a high ledge 

 of conglomerate, is a very distinct half of a pothole, some 2] to 3 feet 

 in diameter and 6 or S feet deep, to speak from memory. The a})pear- 

 ance suggests at first that the southern half may have been split off by 

 the action of the ice and the edges subsequently rounded ; or possibly 

 the pothole was formed between the rock on one side and the ice on the 

 other. At the bottom the waterworn surface curves and runs off hori- 

 zontally along the face of the ledge for several feet, gradually fading out. 



Several small and large glacial potholes, up to 8 or 10 feet in diameter, 

 are worn in granite ledges close to the Fitchburg railroad between 

 Roberts and Stony Brook stations. They are nearly filled witli drift, 

 and their depth therefore cannot be stated. 



In the village of Clinton, a very perfect, but small, glacial pothole, 

 about 1 foot in diameter and 2 feet deep, is eroded in a beautifully 

 glaciated slate ledge. 



During the work on the Wachusett reservoir, at a point about a mile 

 southwest of Clinton, two potholes were temporarily exposed last sum- 

 mer on the lee slope of a prominent ledge of contorted })hyllite, which 

 rises above the surface of a widely extended glacial sand plain. They 

 were I to 2 feet in diameter and of corresponding depths. 



At West Berlin station are several small potholes, which Professor 

 Crosby attributes to erosion by the outlet stream of the glacial lake 



♦"Indian potholes, or giants' kettles of foreign writers," Proc. Boston Soc. Nat. Hist., vol. 

 24, 1889, pp. 219-226; with discussion by Warren Upham, pp. 22G-228. 



t" Geology of the Boston Basin" (Occasional Papers of the Boston Soc. Nat. Hist., iv), vol. 1, 

 1893, pp. 148-159. 



