MANNETTO AND JAMECO GRAVELS 375 



Jameco, on western Long island, at the point where the deposits were first 

 recognized in a deep well, is applied to the stratified glacial gravels rest- 

 ing unconformably on the Mannetto beds and overlain by the Gardiner 

 clays * They commonly vary from 40 feet upward to possibly 100 feet 

 in thickness, and are rich in granitic material, which is only moderately 

 weathered. These gravels unquestionably underlie the clays of Fishers 

 island, or, where these are cut out, occur beneath the old Montauk drift. 

 They were not seen anywhere in the bluff sections, and presumably do 

 not occur above sealevel, but undoubtedly form a considerable part of 

 the " gravel and sand " recorded in the Ferguson well. 



Gardiner clay— Use of term.— This term, which is applied to the thick 

 clay bed overlying conformably the Jameco gravels throughout Long 

 and the other New York and New England islands, takes its name from 

 Gardiner island, near the east end of Long island, where it is strongly 

 developed in many of the bluffs. Its position in the Fishers Island 

 section is shown in figure 4. 



Conditions of deposition.— The moderate submergence which charac- 

 terized the period of deposition of the Jameco gravels continued without 

 material change during the accumulation of the basal portion of the Gar- 

 diner clay, the actual altitude of the land apparently being slightly higher 

 than at present, the normal position of the clays, except when brought 

 up by folding, being somewhat below the present sealevel. The deposi- 

 tion appears to have taken place during a true interglacial period, as is 

 shown by the absence of recognizable glacial materials and by the pres- 

 ence of a fossil fauna (and on Long island and elsewhere of lignite), 

 indicating a period of sufficient length to permit the reoccupation of the 

 former glacial waters by a fauna characteristic of temperatures not 

 greatly different from those of the Maine coast at the present time. The 

 thickness of the clays themselves, taken in connection with the known 

 slowness of accumulation of such materials, likewise points to a period 

 of great length and one which is more compatible with an interglacial 

 stage than with a temporary retreat. 



The lignitic clays of western Long island appear to have accumulated 

 as marsh deposits bordering the old land masses; but in eastern Long 

 island and on Plum and Fishers islands the clays seem to have been off- 

 shore deposits. 



The source of the materials of which the clays are composed is some- 

 what difficult to determine. On western Long island, where the latter 

 reach their maximum thickness, they may be conceived to have been 

 derived from the erosion of the Cretaceous deposits from which the 

 mantle of Mannetto gravel had been eroded and which there rose well 

 above sealevel. In eastern Long island, however, and eastward at least 



LI— Bull. Geol. Soc. Am., Vol. 16, 1904 



