250 H. L. FAIRCHILD PLEISTOCENE UPLIFT OP NEW YORK 



All the phenomena described in this paper and the facts relating to 

 land nplift are explained best on the theory that the rise of the land did 

 not begin in any district until after the ice-sheet was removed from the 

 region, and perhaps far removed. In common with other geologists, the 

 writer has formerly assumed that some land uplift took place beneath at 

 least the border of the waning ice-sheet. This older view requires that 

 a large land area should lift with considerable rigidity quite promptly 

 after the ice load was only partly reduced, or else that a smaller wave 

 movement should folloAV very quickly. 



If the rising of the land surface several hundred feet may be produced 

 only by the inflowage of the interior, plastic magma beneath the area of 

 diminished weight, it must be a very slow process, and we should expect 

 that it would lag far behind the release of pressure ; and it would also 

 seem most probable that the uplift would be wavelike instead of the rigid 

 lifting of a large area. It is a question, or balancing, of the relative rate 

 of the melting of the glacier and of the restoration of isostatic equilibrium. 



It will be conceded that no uplift would begin in the region of the 

 terminal moraine until release of weight began, which means not until 

 the ice-front had receded; so it is a question if the land uplift will ever 

 overtake the ice-front recession. One might conceive that subsequent to 

 the long time removal of the ice burden from a district some far readvance 

 of the edge of the ice-sheet might lie on rising ground, but such great 

 readvance after the glacial waning was well established and sufficient to 

 produce isostatic uplift seems improbable. 



It will not be held that when uplift began it simultaneously involved 

 the entire great area buried under the ice-cap, unless the rising was very 

 dilatory, because the waning of the continental glacier must have been 

 around the borders for a long time before there was any ablation of the 

 ice-cap and reduction of weight over the central area. The only alter- 

 native is wave uplift, and the question is, therefore, as to the character 

 of the wave, its breadth, and its time relation to the ice-margin. We have 

 some facts bearing on both elements of the problem. 



First, as to the time relation. Certainly the Few York City district 

 did not rise at all until the ice was gone, for not until the ice-margin had 

 withdrawn considerable distance was there any effective reduction of 

 weight. Did the wave uplift ever overtake the receding ice-front up the 

 Hudson Valley? All the evidence is negative. In the Schenectady dis- 

 trict we have positive proof that the land uplift did not overtake the 

 glacier. For a very long time after the Schenectady- Albany district was 

 under the sealevel waters the Glaciomohawk and Iromohawk rivers were 

 building the great delta with its apex at Schenectady and the spreading 

 flow southeastward. When the uplift finally began the river, blocked by 



