CAPE COD GEOLOGY 



57 



recognized on the north shore of Long Island in the towns of Oyster Bay and 

 North Hempstead, where a terrace north of the terminal moraine, at an elevation 

 of about 200 feet, composed of the Manhasset formation, was eaten into on the 

 north by streams that drained into the sound, whose valleys were later occupied 

 by the Wisconsin ice sheet. 1 



Similar topographic features are seen at Vineyard Haven, in the eastern part 

 of Marthas Vineyard, and in other partly drift-masked valleys on that island, as 

 well as on the south side of Cape Cod near Chatham. Correlated with these 

 features in time is a narrow, moraine-covered bench on the north side of Marthas 

 Vineyard, which is probably a remnant of a terrace that stood at an altitude of 

 about 150 feet. Remnants of such a surface are recognizable in the southeastern 

 part of Cape Cod peninsula, beyond the outwash and drift of the Falmouth 

 moraine, as well as in the highlands of Truro. 



Large tracts of glacial and older deposits laid down in the eastern islands 

 before the Vineyard stage present surfaces about 100 feet above the present sea 

 level. Such a surface is well displayed at the eastern side of Marthas Vineyard, 

 where it merges into the "outwash" plain of the Nantucket substage of the 

 terminal moraine. Nantucket island appears to have been leveled before or 

 during the Wisconsin stage, so that now only one point on it attains an altitude 

 of 100 feet. Southern Cape Cod appears also to have been reduced before Wiscon- 

 sin time to levels below 100 feet above the present sea level. 



After the terraces that now stand at altitudes between 60 and 100 feet were 

 formed there was an uplift of the land, which was followed by the excavation of 

 valleys on the north side of the eastern islands of the group. Something like a 

 common expression is seen in the narrow coves about Hempstead Harbor and 

 Oyster Bay, on Long Island, and those in Lagoon and Vineyard Haven Harbor 

 and Chappaquonsett and James Ponds, on Marthas Vineyard. The valleys that 

 drain into the sounds north of them appear to have been formed just before the 

 advent of the first Wisconsin ice sheet. On the isostatic hypothesis a depression 

 of the earth's crust by a great regional glacier such as covered northeastern North 

 America should have been compensated in the zone of the ice front by a negative 

 deleveling 2 that increased the effect of the withdrawal of water from the oceans 

 to form the ice sheets through snowfall. 



1 Woodworth, J. B., Pleistocene geology of portions of Nassau County and the Borough of Queens, 

 N. Y. State Mus. Bull. 48, pp. 634-637, Albany, 1901. With colored geological map, pi. 1. 



2 Delevel, to change the level of a body, as of land in relation to sea level, or vice versa. Cf. French 

 denivellement, for which deleveling is an exact equivalent, defined as a change of level. The terms negative 

 and positive are used in this connection in the sense proposed by Edouard Suess in "La Face de la 

 Terre," in accordance with the mode of stating tidal measurements by hydrographers. See note on the 

 term deleveling in Bull. Mus. Comp. Zool., geol. ser., 31, p. 100, 1912. 



