54 



CAPE COD GEOLOGY 



of Block Island, the till is 30 to 40 feet thick and carries some boulders that 

 are over 6 feet in diameter. In parts of the cliffs there appear to be two beds, 

 separated by assorted gravel, indicating two advances of the ice, as at the Wis- 

 consin stage, but this appearance may prove to be due to repetition of the same 

 bed by overthrust. Similar beds of boulder till are seen in the bluffs on the south 

 side of No Mans Land, but in the western part of Marthas Vineyard, owing either 

 to the poor exposures of the Manhasset or the thinness of the till and the associ- 

 ated beds of gravel, the deposit is less well defined. North and east of Marthas 

 Vineyard the till, or boulder bed, has not been recognized, but the reported oc- 

 currence of boulders in the beds of clay encountered in the Cape Cod Canal 

 near its east end, and again in the lower part of the beds of clay in West Barn- 

 stable, on the north side of the lower arm of the Cape, makes it seem probable 

 that the Montauk till, rather than the lower and older Gardiners clay may occur 

 there near sea level. The known facts concerning the till between eastern Long 

 Island and Marthas Vineyard indicate that the ice which produced this de- 

 posit was in the form of a lobe whose axis lay over Rhode Island and eastern 

 Connecticut. 1 



The Montauk till can be distinguished from the Wisconsin till of south- 

 eastern New England by its large content of blue clay. No similar boulder clay 

 is found in the Wisconsin drift in southeastern Massachusetts. The great amount 

 of clay in the Montauk till may be due in part to the erosion of the underlying 

 Gardiners clay, which probably overlapped far upon the lowland of southern 

 Massachusetts and Rhode Island and to a less extent upon southern Connecticut, 

 though not in less thickness in the narrower lowlands of the river valleys and 

 in the Triassic lowland about New Haven. 



The outer limit of the Montauk till is unknown. The ice sheet evidently 

 extended beyond the site of the outer New England islands, as did also the other 

 members of the Manhasset formation, but the action of the sea has commingled 

 on its floor the materials of numerous glacial deposits. The large boulders indi- 

 cated on old charts of the Nantucket shoals near the site of the modern lightship 



1 Woodworth, J. B., Pleistocene geology of the Oyster Bay and Hempstead quadrangles on Long 

 Island, N. Y. State Mus. Bull. 48, pi. 1, Albany, 1901. On use of name "Tisbury beds" to include equiva- 

 lent deposits on Marthas Vineyard, see Woodworth in Seventeenth Annual Report United States Geo- 

 logical Survey (1895-96), pt. 1, pp. 977-978, 1896, correlation table facing p. 988. As originally defined, 

 the "Tisbury beds" prove to have included formations from the Gardiners clay to the top of the Man- 

 hasset formation as now defined. References to the Montauk till are made by Fuller, M. L., Probable 

 pre-Kansan and Iowan deposits on Long Island, N. Y., Am. Geologist, 32, pp. 308-311, 1903; Glacial 

 stages in southeastern New England and vicinity, Science, 24, pp. 467-469, October 12, 1906; The geology 

 of Long Island, U. S. Geol. Survey Prof. Paper 82, pp. 132-149, 1914. Also Veatch, A. C., Outlines of the 

 geology of Long Island, U. S. Geol. Survey Prof. Paper 44, pp. 41-43, 1906. 



