732 



/. HOWARD WILSON 



After the lower beds were deposited, and the Wisconsin ice-sheet 

 had attained considerable extension, the land began to subside as 

 the ice advanced, while a deeper water fauna, of arctic type, driven 

 southward by the advancing ice, lived in the region, and its shells 

 were imbedded in the upper beds. As the ice reached Nantucket, 



the pressure of its front against 

 obstructing surfaces produced a 

 crumpling or folding of the strata, 

 as has been observed in other 

 regions, such as shown in Fig. 

 12. This folding would account 

 for the gradual increase of dip 

 from the top to the bottom of the 

 p IG . I2 section at the point indicated by 



the diagonal line AB, represent- 

 ing the face of the bluff in Fig. 12. After the folding had taken place, 

 the barrier beach brought below sea-level by the subsidence of the 

 region, was destroyed by wave-action and redeposited as the upper 

 white sands. As the Wisconsin ice-sheet reached and passed beyond 

 this point, it is likely that a portion of these sands was removed; 

 but soon deposition of glacial material began to take place, and 

 these beds were buried under the vast amount of kame sands and 

 gravel which form the upper part of the bluff. 



This hypothesis seems to fail in the following particulars: (1) It 

 seems improbable that the difference in dip can be accounted for in 

 the way mentioned, because (a) the earliest reports on the dip of 

 the various beds agree almost exactly with present observations, 

 while a cutting back of the bluff, such as has taken place since the 

 locality was first visited, should show a very different dip for the 

 same beds as will be seen from a study of Fig. 1 2 ; and (b) the lateral 

 pressure sufficient to produce this folding would be indicated by 

 minor crumplings of the tough upper clay (No. 6), but, so far as 

 observed, these crumplings do not occur. (2) In order to produce 

 the folding, we must assume the ice-front to have been in the imme- 

 diate vicinity, which assumption cannot be reconciled with the 

 deposition of the upper white sands, which are of seaward origin, 

 very pure and unmixed with any such heterogeneous material as 

 could not fail to be present in proximity to the ice-front. 



