296 



SCIENCE 



[N. S. Vol. XXXVII. No. 947 



pears that the land at the time of the ice 

 recession was at, or perhaps somewhat 

 above, sea-level. Northward the land was 

 increasingly below sea-level. The upraised 

 and tilted water plane which indicates the 

 amount of Pleistocene submergence or of 

 Postpleistocene uplift rises steadily from 

 zero or present sea-level in the district of 

 New York City to over 750 feet on the 

 Canadian boundary. 



The supposed absence of marine fossils 

 in the Hudson Valley is doubtless due to 

 the freshening of the waters by the copious 

 glacial and land drainage. Until the epi- 

 sode of the Second Iroquois the flood of 

 glacial watei^ of the St. Lawrence basin 

 was poured into the Hudson inlet at 

 Schenectady. During the Second Iroquois 

 the glacial flood was merely shifted to the 

 north, and during all the long life of the 

 Hudson-Champlain inlet all the fresh 

 waters were forced south. However, ma- 

 rine fossils are abundant in the Cham- 

 plain Valley and are found at altitudes the 

 planes of which carry over the Fort Ed- 

 ward divide into the Hudson portion of 

 the great valley. 



The detrital deposits formed in the ma- 

 rine waters are large in volume and area. 

 Up the Hudson as far as Catskill the ter- 

 races of clay and sand are very conspicuous 

 and afford the materials for brick manu- 

 facture on an immense scale. North of 

 Catskill, in the widening valley, the sum- 

 mit sandplains lie back from the river, 

 though lower terraces may yet be seen. 

 While much of the deeper deposits and 

 those in the middle of the valley or be- 

 neath the present waters are of glacial 

 origin, the heavy visible deposits are 

 chiefly the deltas of tributary land 

 streams, the greatest being that of the 

 Iromohawk at Schenectady- Albany. 



From Troy to Glens Falls the borders of 

 the lower valley are buried in a deluge of 



sand, sloping down in terraces toward the 

 axis of the valley. Saratoga lies in the 

 midst of a vast area of detrital marine ac- 

 cumulations. The slow lifting of the val- 

 ley out of the waters gave the latter an ex- 

 cellent chance to produce level stretches 

 and conspicuous terraces, the latter being 

 more prominent as the steeper slopes ap- 

 proach the middle of the valley. The 

 Champlain portion of the great valley also 

 holds vast sandplains, especially on the 

 larger rivers, as the Ausable, Saranac and 

 Big Chazy. 



EPEIROGENIC MOVEMENT. DIASTROPHISM 



The great changes in altitude of the sur- 

 face of the state, both before and since the 

 glacial occupation, has already been noted. 

 The relation of the land movement to the 

 burden of the ice cap should be briefly dis- 

 cussed. If the earth's crust is sensitive to 

 long-continued pressures, then the thick- 

 ness and weight of the ice body becomes an 

 important matter. 



Again our lack of knowledge of the dura- 

 tion and diastrophic effects of the Prewis- 

 consin ice caps limits our discussion to the 

 effects of the Laurentian ice body. 



At its maximum the thickness of the ice 

 cap over the Adirondacks and the Cham- 

 plain Valley was probably not less than 

 10,000 feet. This is equal in weight to 

 over 3,000 feet of rock. Southward the ice 

 decreased in thickness and weight to zero 

 in the region of New York Bay. The 

 amount of postglacial iiplift increases 

 from zero in the district of New York Bay 

 to over 750 feet on the north boundary of 

 the state. The correspondence between the 

 thickness of the ice cap and the amount of 

 postglacial uplift of the land is very strik- 

 ing and significant. All about the Lauren- 

 tian basin the tilted shores of the extinct 

 glacial lakes afford us evidence of the dif- 

 ferential uplift of the glaciated territory. 



