CAPE COD GEOLOGY 



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till is usually an aggregate of boulders, gravel, and sand relatively free from a 

 clay matrix. The gravel below the Montauk till (the Herod gravel member) and 

 that above it (the Hempstead gravel member) appear to have been laid down 

 during one glacial advance, the Manhasset. 



What appears to be a boulder clay till bed at or near the base of the Gardi- 

 ners clay near Nashaquitsa cliffs is in many respects like the Montauk till, but 

 it is less than 10 feet thick. Unlike the lowest boulder beds, which are invariably 

 water-washed deposits, this bed looks relatively fresh, as does also the Jameco 

 gravel, which lies at nearly the same horizon or just below it. Judged by its con- 

 tent of blue clay and its fresh appearance, as well as its freedom from cementation 

 by secondary alteration and deposition of salts of iron and lime, this bed of till 

 lies above the oldest Pleistocene deposits, with whose physical aspects and wear 

 by water it has nothing in common except its glacial origin. Its exemption from 

 weathering and alteration is probably due to the fact that it was not exposed to 

 leaching and oxidation above ground water or sea level so long as the Jerseyan and 

 Kansan tills. The older beds of boulders, gravel, and drab sand, some of them 

 containing water-worn Miocene fossils, consist of glacial material worked over 

 by water and are comparable with the deposits now being laid down along our 

 coast rather than with the glacial material on the land that has not been worked 

 over by waves and currents. The distinction is as sharp and as important as that 

 between the stratified and the unstratified drift of the last glaciation. 



The lowest and oldest beds of boulders on Block Island and Marthas Vine- 

 yard contain no true boulder clay. The boulders are either embedded in strati- 

 fied gravel or occur in water-washed and rudely bedded masses, having evidently 

 been reassorted under an ice sheet by the action of waves and currents or by 

 streams. Only at one point, in an exposure at Gay Head cliffs, have I found 

 glacial striae on a fragment in these deposits. The appearance of these huddles 

 of boulders and rounded gravel suggests that they were included in a sheet of 

 morainal drift that was long exposed to the action of waves and currents on 

 shores and shoals. In fact, the boulders are as well worn and clean as those that 

 have been long exposed to wave action on existing shores. This characteristic 

 has a diagnostic value in excluding from the earliest glacial origin any drift that 

 proves to be a typical boulder clay. Any such clay is undoubtedly more recent 

 than the Dukes boulder bed. It may lie above that horizon, in the Moshup stony 

 blue clay below the stoneless blue clays of the Gardiners formation, or it may 

 belong still higher up, at the Montauk horizon. No typical boulder clay of the 

 bluish unoxidized type has been found in the Wisconsin moraines on these islands. 



