384 M. L. FULLER — GEOLOGY OF FISHERS ISLAND 



silts, which is characteristic of rapid and only partially assorted deposits 

 such as accumulate at the margin of glaciers. It unquestionably belongs 

 to the Montauk stage, representing certain of the more aqueous phases of 

 deposition. A little farther to the north, although the contact is not 

 seen, the semi-stratified deposits appear to be replaced by till. 



Where well exposed, it is usually not difficult to distinguish the Mon- 

 tauk from the Wisconsin drift, the latter, which is relatively sandy and 

 of a buff color, contrasting quite sharply with the older bluish and more 

 clayey drift. In the ordinary imperfect exposures, however, the two can 

 not readily be distinguished, and although certain parts of the till exposed 

 at the surface and at numerous points in the low bluffs of the island may 

 belong to the Montauk, this can not be established with certainty ; in 

 fact, it seems probable that most of the surface till is Wisconsin. 



Later pre- Wisconsin events. — The accumulation of the Montauk drift on 

 Long island was followed by the deposition of a conformable series, of 

 marine gravels, which are found at many points both in the eastern and 

 western parts of the island. In the latter locality they are probably 

 represented by the lower part of the Manhasset gravels of J. B. Wood- 

 worth,* or that portion below the boulder bed so well exposed along 

 Hempstead harbor. Following the deposition of these beds the ice, which 

 appears to have temporarily retreated for some distance, readvanced 

 over the whole of eastern Long island and the islands to the east, fold- 

 ing and planing the older beds, but, except in western Long island, where 

 the upper Manhasset was laid down, it deposited very little of the 

 materials now to be seen above sealevel. It is at this time that the 

 severe folding on Fishers island such as shown at the clay pit (figure 

 6) and at other points (figures 3, 7, and 8) appears to have been pro- 

 duced. It is believed to be contemporaneous with the folding on Block 

 island, at Gay head, etcetera. 



During the period commencing with the deposition of the Herod 

 gravel and continuing through the Montauk stage and through the sub- 

 sequent stage of deposition and folding a gradual subsidence appears to 

 have been going on, until on western Long island, as indicated by the 

 upper level of the marine glacial deposits, the land was 250 feet lower 

 than at present. Fishers island, if not beneath the ice, must have been 

 covered by the sea if a similar depression existed in that region. 



Following the retreat of the ice an uplift took place, the land gradu- 

 ally rising from a position 250 feet lower than at present until it stood 

 considerably higher. The uplift was not sudden, there being, on the 

 contrary, several periods of halt, the most marked being at the present 



* Pleistocene geology of portions of Nassau county and the borough of Queens. New York State 

 Mus., Bull. 48, pp. 618-670. 



