PLEISTOCENE FORMATIONS OF SANKATY HEAD 731 



The Sankaty Head deposits seem to be best accounted for as 

 having accumulated in one of these inlets or lagoons. The lower 

 beds were undoubtedly deposited in a shallow body of water con- 

 nected with, but well protected from, the open sea. This is well 

 shown by the prevalence of such species as Ostrea, Venus, and Mya, 

 and especially numerous specimens of mud crabs, and the presence 

 of our edible crab, CaUinectes sapidus Rathbun, which is found 

 today in some of the little creeks connected with the harbor on the 

 north side of the island. ■ 



Although the total thickness of the fossiliferous beds is but 8 feet 

 a great difference is found between the lower and upper beds, showing 

 changes in the physical conditions. At the time of deposition of 

 the upper beds, the waters had become much colder, probably due 

 to the return of glacial conditions, the fossils being chiefly of the 

 northern fauna driven southward in front of the advancing ice-sheet. 

 The fossils, besides being more northern, and in some instances 

 even arctic, in their range, are also of species which are generally 

 found in somewhat deeper water. 



A noticeable unconformity is found between the fossiliferous beds 

 and the overlying 10 feet of white sand (No. 16) at the point where 

 the section was exposed, as shown in Fig. 10. These sands are very 

 pure, and are finely stratified and assorted, the bedding being nearly 

 horizontal. The sand grains are mostly well-rounded and worn, 

 and at rare intervals, minute fragments of shells occur. In fact 

 this bed, to all appearances, seems to consist of wind-blown sand 

 derived from dunes which have been destroyed and redeposited. 



It will be noticed also from the general section, Fig. 4, that there 

 has been a gradual decrease of dip from the lowest of the beds exposed, 

 to the uppermost of the fossiliferous beds, above which the uncon- 

 formity occurs. 



The upper fossiliferous beds, with their change of fauna, the 

 apparent unconformity above, and the overlying white sands, show 

 changes in the physical conditions which require special explanation. 



There seem to be but two hypotheses which are at all applicable 

 to the facts, and while the phenomena can hardly be reconciled to 

 one, it will be given first, and the points wherein it fails to satisfy 

 the conditions will be explained. The first hypothesis may be stated 

 as follows: 



