TOPOGRAPHY 371 



as it affords a basis for calculating the amount of erosion which has taken 

 place since the sea reached the original southern base, the position of 

 which, owing to the constructional origin of the hill, can be readily de- 

 termined. In figure 3 is given a section through the hill showing its anti- 

 clinal structure and former extent. 



A few small spits have been built out at a number of points on the 

 coast of the island, and barrier beaches have been formed by the waves 

 across some of the small reentrants, giving rise to certain of the ponds, 

 but at other points, as in the vicinity of Silver Eel pond, the barriers 

 are composed of boulders and appear to be in part, at least, the work of 

 glacial ice. 



Geology 



general conditions 



Fishers island falls in line with the inner or later of the two Wiscon- 

 sin moraines (figure 1), and as it exhibits a kettle and knob topography 

 and has a surface of till, or at least a sprinkling of large erratic boulders 

 over considerable areas, it has generally been regarded as a morainal 

 island of late Wisconsin age. A careful study of the composition and 

 structure of the deposits, however, brought out the fact, which was 

 already suspected from the topography of the island, that the Wisconsin 

 drift forms a thin superficial mantle resting on folded and eroded Pleis- 

 tocene beds of a much earlier stage. 



Figure 4.— Generalized Section of Fishers Island. 



a, Wisconsin till; b, Montauk drift; c, Herod gravel; d, Jacob sand; e, Gardiner clay;/, Jameco 

 gravel ; g, Mannetto gravel (?) ; h, Cretaceous clay ; i, light-gray granite. 



In working out the geology of the island, cliff sections furnished the 

 greater part of the information, although the big clay pit on the east 

 side of West harbor and an artificial section near the steamboat landing 

 on the west side of West harbor afforded important data. In the field 

 work all prominent bluff exposures along the coast were visited, the 

 highways traversed, and such artificial sections as could be found 

 examined. 



The field work brought out the fact that not only are the greater part 

 of the deposits of pre-Wisconsin age, but that in the upbuilding of the 



