300 H. L. FAIRCHILD POST-GLACIAL SUBMERGENCE OF LONG ISLAND 



slope and material at this level. Even these two varieties of evidence are not 

 always coexistent." ... 



"As the land rose from its 80-foot depression at New York there seems to 

 have been a brief period of less rapid elevation, during which a second series 

 of estuary terraces and alluvial plains were formed which now stand about 25 

 feet above tide level. These have been recognized on Staten Island by Dr. 

 N. L. Britton and may be seen on the Harlem River near Fordham Heights 

 and at various points on the Long Island shore of Westchester County." 



Prof. N. S. Shaler recognized the marine origin of the low sand-plains 

 of Marthas Vineyard and l^antncket and noted the absence of beaches. 

 His explanation was the too rapid rise of the land, stated as follows : 



"The emergence of the drift deposits of this district from the sea must have 

 taken place with singular rapidity, for there are no signs of wear on the sur- 

 face of the moraines or the low-lying kames, such as would inevitably have 

 occurred if their surfaces had been exposed to the action of the waves for any 

 length of time." ^ 



The writer is able to confirm Merrill's views and to prove, from field 

 study, the absence of beach phenomena on sand stretches of strong shore- 

 lines. 



Probably most students of shorelines have found gaps along otherwise 

 strong beaches. It requires a long time at a fixed level for waves and 

 shore currents to bridge unfavorable stretches. On shores as strong as 

 that of the glacial Lake Warren and Lake Iroquois localities are found 

 with only smooth, wave-washed slopes, comparable to the inner edge of 

 the Long Island plain. With changing levels the construction of beach 

 phenomena — ^bars, spits, and cliffs — is more difficult. In postglacial time 

 the land was rising (rapidly or slowly, according to the mental view- 

 point) and the beach-building work was constantly shifted to lower and 

 lower levels. The conditions of changing attitude immediately following 

 the removal of the ice-sheet were unfavorable for beach-building. 



On the open seashores of Staten and Long islands another inhibiting 

 factor existed which was not present with the glacial lakes, namely, the 

 tidal fluctuations of level. A flight change in the water level may pro- 

 duce large variation in the force and direction of shore currents and 

 effectively change the plane of beach construction. 



There is yet another factor in this connection which seems to have 

 been overlooked, but which is of equal or perhaps of more importance 

 than those just mentioned. It may be illustrated by examples from the 

 field. On the north boundary of New York is the projection of high 

 ground from the Adirondack mass locally known as Covey Hill. Around 



8 Geology of Marthas Vineyard. Seventh Ann. Rept., U. S. Geol. Survey, X888, page 

 321. 



