Ice-sheet upon Slender Projecting Roclt Masses. 403 



unlike practically all of the other ridges of the valley, opposed 

 its long face and, therefore, its weakest direction to the ice 

 mass, which was directed against its thin vertical walls. Owing 

 in part to dip and in part to the manner of faulting, its western 

 end (the present peak) projected above its general wall-like 

 mass. This part also was unprotected by the higher mass of 

 Rattlesnake Hill. In this way, it is believed, the great blocks 

 which are now found over a mile and a half away and con- 

 nected by the train with this elevation were separated from 

 their parent mass. 



Castle Rock, a prismatic block of nearly equal basal dimen- 

 sions and now having a sheer cliff upon its northern face 

 nearly one hundred feet in height, is located in the north- 

 eastern part of the area.* Like the west end of Sherman Hill 

 its position left it unprotected by other masses from the north- 

 west, where a considerable valley allowed the ice to sag well 

 below the crests of the elevations surrounding the valley. The 

 row of basalt blocks located southeast of White Oaks is 

 believed to have been derived from this elevation much as the 

 other train has been from Sherman Hill. The slender nature 

 of the prism of Castle Rock, its considerable elevation above 

 the general level, and its exposure upon the northwestern 

 edge of the area all favor such an hypothesis. The direction 

 of this train (S. 13° E.) would indicate that the long valley east 

 of the basalt masses has modified the direction of movement of 

 the lower portions of the ice mantle. 



That the ice found a depression in the Pomperaug Yalley, 

 into which its substance sagged, is indicated by the present 

 altitudes of the higher basalt ridges which have been decapi- 

 tated, in comparison with those of the gneiss hills of the vicin- 

 ity (the basalt ridges are some 200 to 300 feet the lower). 

 It must be assumed that erosion in post-glacial time has been 

 more effective upon the gneiss than upon the basalt, though 

 this would doubtless be modified by the soft arkose members 

 above the main basalt. 



To ascribe these trains to the separation of blocks from the 

 lee side of ledges due to the frictional action of overriding, is 

 to leave unexplained their large size and the distance to which 

 some have been carried, more especially, however, the restric- 

 tion of the trains to ledges which the geological study shows 

 to have been slender in form and with deep valleys northwest 

 of them into which the ice could sink and act in a horizontal 

 direction against their walls. 



Professor Chamberlin has informed me that he has observed 

 in Greenland an undoubted instance of such degradational 

 action of the ice from lateral pressure upon rock walls. 



University of Wisconsin, Madison, Wisconsin, 

 *Loc. cit., fig. 41, p. 111. 



