CAPE COD GEOLOGY 



97 



Dall appears to have supposed that the species considered were obtained 

 from the lowermost bed at Sankaty, but Wilson states that they were obtained 

 from the upper bed, which lies above the bed containing a Pleistocene fauna. 

 These mollusks may have lived during a long and warm interglacial stage, when 

 the conditions simulated those of late Pliocene time. 



My own work at Sankaty Head has been limited to the attempt to de- 

 termine the stratigraphic relations of the beds in this section to those in other 

 sections on the island. For this purpose a few exposures on the coast immediately 

 north of Sankaty Head were available. 



In the low bluffs between Sankaty Head and Squam Head a section that 

 was visible in the summers of 1915 and 1916 showed a tilted bed of sand con- 

 taining comminuted shells, evidently the upper part of the Sankaty sand, which 

 was unconformably overlain by a bed of till containing striated pebbles. This 

 bed of till came out horizontally from the upper part of the bluff and was flexed 

 downward into a vertical position, its resistance to erosion by the sea causing it 

 to form a small buttress on the beach. In this section the Sankaty sand appears 

 to underlie one of the beds of till in the older Pleistocene, a fact shown also, 

 though less clearly, in the section under the lighthouse. 



In the section exposed in the bluffs at Squam Head in 1915 and 1916 the 

 same gravelly till that overlies the shell-bearing deposits on the south passed 

 northward in nearly horizontal attitude into stratified material. 



All the sections on the east coast of Nantucket exposed in these years showed 

 that a bed of till of the type I have called a stony clay, rather than a boulder bed, 

 lies above the fossiliferous sand. At some places this bed of till is stratified and 

 looks like a bed of gravel containing large quantities of clay, or a bed of clay 

 containing some pebbles (see Plate 10, fig. 2). The deposit has no feature in 

 common with the basal Dukes boulder bed, on Marthas Vineyard, a deposit 

 resembling the thoroughly washed huddles of boulders on a sloping beach of the 

 present coast. On the other hand, the till strongly resembles the Moshup stony 

 blue clay or till of the Gay Head section. I, therefore, conclude that the Sankaty 

 shell beds lie above the base of the local Pleistocene series, defined as marking 

 the beginning of the first ice invasion. 



The time of the deposition of the shell beds and the overlying till is also 

 suggested by a consideration of the geological structure in its relation to the 

 time of the dislocation by ice thrust and to the Vineyard stage of erosion on 

 the island. The front of the Wisconsin ice sheet at its farthest advance stood 

 along the head of the outwash plain on Nantucket, covering the upland at 



