30 W. UPHAM — giants' KETTLES ERODED BY MOULIN TORRENTS 



tracts. Some of the potholes are on the slopes and tops of low cliffs, 5 

 to 20 feet above adjoining hollows. The height above the almost still 

 river in the adjoining Dalles varies from 25 to 50 or 60 feet, rising highest 

 northward. 



In general, the rock surface has angularly broken slopes and cliffs, 

 more or less glaciated, but since so weathered that the glacial stride are 

 effticed. Occasionally a little place, 10, 20, or 50 feet in extent, is 

 smoothly waterworn ; but more commonly the surface adjoining the pot- 

 holes on this area and wherever they occur in this neighborhood, except- 

 ing in and near the river channel at the Saint Croix rapids, is roughly 

 fractured or has been abraded only by the ice-sheet. The rim of the 

 potholes, of whatever size, is usually abrupt, not merging with water- 

 worn curving outlines of the adjacent rock, as at subaerial rapids a;id 

 falls of our present rivers. 



The giants' kettle of greatest diameter, which may be named '' the cal- 

 dron," is situated about 30 feet northeast from the east end of the wharf 

 and 25 feet above the river. It is nearly circular, 25 by 27 feet in trans- 

 verse diameters. Its sides descend vertically 8 to 12 feet, below which 

 depth this kettle is filled with many angular rock masses from 1 to 5 feet 

 long. Some of these may have been dislodged by frost action from a 

 much-jointed cliff which rises at the northeastern brink of the kettle to 

 a height of 10 feet above other parts of its rim. More probably, how- 

 ever, as in the case of the only other pothole observed to be similarly 

 almost filled by rock masses, they are attributable to falls through the 

 moulin, having previously been contained in the lower ])art of the ice- 

 sheet as englacial drift. 



Near the northern end of this area and 6 feet northwest of a foot-bridge, 

 the most northwestern of a series of bridges which span little gorges and 

 lead to Angle rock, is the other pothole mentioned for its contents of 

 rock blocks and boulders. Its diameter is about 12 or 15 feet, and its 

 depth, to be proportionate, should reach far below the surface of the 

 many angular trap fragments, up to 3 or 4 feet long, which fill it to 

 within 4 feet below the rim. ^- No high nor fractured rock surface adjoin- 

 ing this kettle could yield its contents, which must therefore have come 

 from the ice-sheet. They are unworn examples of the rock masses which 

 in other instances, being less plentifully supplied, were gyrated in the 

 bottom of the pothole by the torrent, wearing it deeper and grinding 

 themselves to powder, until new masses of rock, falling in, took their 

 places as the work of kettle erosion continued. Two very large kettles 

 thus appear to have been interrupted in their formation by the abun- 

 dance of their supply of grinding stones. 



