GEOLOGY OF THE LAKE PLACID QUADRANGLE 93 



at an altitude of 833.5, and another one on " Lower Jay hill " at 857 

 feet. 



Lower lake levels. At still lower altitudes several more of these 

 perplexing levels were found, but as they are largely outside the 

 Lake Placid quadrangle they will not be discussed. Suffice to say, 

 the most prominent level in the lower Ausable valley is the marine 

 plain in the vicinity of Ausable Forks. Here the measured altitudes 

 agreed within a foot of the value called for by Fairchild's figures. 



Cause of the large amount of material available for the 

 formation of terraces. One of the striking features of the glacial 

 geology of the Adirondacks is the small amount of true morainal 

 material^ unmodified by water" as contrasted with the vast quanti- 

 ties of sand and gravel in deltas, terraces etc., when compared with 

 other districts, such as the Catskill mountains. The following hypo- 

 thesis is offered to account for this. It is generally conceded that 

 with the return of warmer climate the Adirondacks were completely 

 surrounded by a vast ring of ice that isolated the Adirondack high- 

 land from the rest of the state.^ It was during this stage that the 

 glacial lakes here described existed. The great ice sheet undoubtedly 

 destroyed all vegetable life in both the Adirondacks and the Catskills, 

 but in the latter case the ice retreated northward as an irregular edge 

 which allowed vegetable life to follow the ice in its withdrawal. 

 This condition was not possible in the Adirondacks where the ice 

 ring prevented much if 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 newly growing 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. On the contrary, the glacial debris in the Adirondacks 

 was not anchored and most of it has been carried down into the 

 valley bottoms and there worked over into lake deposits. 



Postlacustrine deformation and tilting. It has been pointed 

 out that at the maximum extent of the Wisconsin ice-body, the 

 load upon the land surface must have been tremendous and must 

 have compressed the land below its former level.* Since the ice was 



'Gushing, H. P., N. Y. State Mus. Bui. 115, p. 495. 

 2 Ogilvie, I. H., Jour. Geol., 10:307-412. 1902. 

 " Fairchild, H. L., N. Y. Stale Mus. Bui, 160, pi. 11. 

 * This subject of deformation has not received the attention of struc- 

 tural geologists in the light of isostasy. 



