REPORT OF THE DIRECTOR I917 137 



retreated northward as an irregular edge which allowed vegetable 

 life to follow the ice in its withdrawal from the region. This con- 

 dition was not possible in the Adirondacks where the ice ring pre- 

 vented to any great extent any encroachment on the part of plants 

 into the ice-deforested area. In the Catskill region the glacial 

 drift was anchored by the roots of shrubs etc., and thus it was not 

 easily washed by the streams into the standing waters in the valleys 

 below, so a large amount of the drift still remains on the slopes. 

 The glacial debris in the Adirondacks, on the contrary, was not thus 

 anchored and most of it has been carried down into the valley 

 bottoms, there worked over into lake deposits. This condition has 

 an important bearing upon the forestry of the region. It explains 

 the cause of the thin soil covering the slopes, which is easily removed 

 by torrential streams when the mountain ranges are visited by 

 destructive forest fires. This situation necessitates the employment 

 of different methods in combating a raging fire in the Adirondacks than 

 in regions that were nearer the ice front, such as the Cat skills, which 

 were not surrounded by a glacial ring and hence retained more of 

 their soil. 



Postlacustrine Deformation 



It has been pointed out, under the head of Adirondack topography, 

 that at the maximum extent of the ice sheet the load upon the land 

 surface must have been tremendous, and compressed the land below 

 its former level. Since the ice was thicker in the north than in the 

 south, the amount of depression was greater in the northern part of 

 the State. With the removal of the load by the melting of the ice 

 the land sprung back, thus elevating the surface and tilting the 

 shore line features of the glacial lakes. It has been shown 1 that 

 the character of the postlacustrine uplift was a lifting in the 

 form of a warped plane with the amount of warping greater 

 to the north. The lines of equal uplift since the ice period 

 (isobases) incline south-northwest to east-southeast (20 from 

 the latitude parallels). The zero isobase passes far south of 

 New York City. The 560 foot isobase is near Lake Clear. This 

 figure gives the approximate total uplift since the ice period for the 

 area. The amount of tilting in our district, taken perpendicularly 

 to the isobases, is about 2.85 feet a mile, declining southward. Thus 

 all the sandy plains, their beaches and wave-cut shores are tilted to 



1 H. L. Fairchild, " Pleistocene Uplift of New York and Adjacent Territory," 

 Amer. Geol. Soc. Bui., 27:235-62; " Post-Glacial Waters in Vermont," Rep't 

 of Vermont State Geol., 1915-16, p. 1-41, 1917. 



