372 M. L. FULLER — GEOLOGY OF FISHERS ISLAND 



island a considerable number of stages, including both those of deposi- 

 tion and erosion, have been represented. Of these at least three may be 

 recognized in the portion of the deposits above sealevel, the first being 

 marked by thick deposits of dark-colored clay, the second by the depo- 

 sition of glacial gravels, followed by the deposition of till or by a period 

 of folding, and the third by a long period of erosion. These events are 

 recorded by the succession of strata and unconformities, as illustrated 

 by the accompanying section (figure 4), and will be described in detail 

 in the following pages, beginning with the lower or oldest bed. 



CHARACTER AND HISTORY OF THE DEPOSITS 



Granite foundation. — The formations exposed above sealevel on Fishers 

 island are all of Pleistocene age, but in the Ferguson well, drilled by C. L. 

 Grant, certain older materials were encountered. 



Record of Ferguson well 



Feet 



Gravel, boulders, and sand to 260 



Blue clay 260 to 281 



Light-gray granite 281 to 485 



In character the granite corresponds to the rock which is extensively 

 quarried at Westerly and vicinity in Rhode Island, a few miles to the 

 northeast, and with which it may be genetically connected. 



The record indicates a slope of the rock surface of about 100 feet to 

 the mile from its outcrop along the Connecticut shore to the well, which 

 is about the same as that found in western Long island and along the 

 Atlantic coastal plain generally. The rock surface, judging from its 

 character elsewhere, is doubtless undulating, but of this there is no local 

 evidence. The tilting of the surface, which had previously been nearly 

 horizontal, probably commenced about the beginning of the Cretaceous 

 period, and doubtless continued during the deposition of the Cretaceous 

 and subsequent deposits until its present inclination was reached. 



One of the interesting features brought out by the well is the occur- 

 rence of fresh water in the granite at a depth of 328 feet, or about 300 

 feet below sealevel. As the overlying drift yielded salt water, it is not 

 likely that the supply came from the island itself, but rather from the 

 mainland to the north, being conveyed along north-south joints or possi- 

 bly faults. The overlying bed of blue clay doubtless serves as a capping, 

 preventing both the escape of the fresh water and the ingress of the salt.* 



Basal blue clay. — Resting on the granite, as indicated in the record of 

 the Ferguson well, with its base about 260 feet below sealevel, is a bed 



♦While unusual, this lateral transmission of water through joints in rocks covered with clay is 

 not unknown elsewhere, the writer having worked out an example of such transmission for a 

 distance of more than 20 miles in southeastern Michigan. 



