60 NEW YORK STATE MUSEUM 



will be seen that the central Adirondacks, the last to be buried, were 

 the first to be freed from the ice, the much thicker ice in the sur- 

 rounding valleys requiring a longer time for melting. Taken in 

 reverse order, these maps in a general way show three stages in 

 the advance of the ice over New York State. 



Ice erosion. Glacial ice, like flowing water, has very little erosive 

 elTect upon rocks unless it is properly supplied with toob. When 

 flowing ice is shod with hard rock fragments, the power to erode is 

 often pronounced because the work of abrasion is mostly accom- 

 plished by. the rock fragments rather than by the ice itself. A little 

 search will reveal polished and scratched rock surfaces in the 

 Adirondacks, and the freshness and hardness of the surface rock 

 proves that the ice eroded off all the preglacial soil and rotten 

 rock and often more or less of the fresh rock. During the very long 

 preglacial time, rock decomposition must have progressed so far 

 that rotten rock, including soils, had accumulated to considerable 

 depths as is the case in the southern states today. Such soils are 

 called " residual " because they are derived by the decomposition 

 of the very rocks upon which they rest. But now one rarely ever 

 sees rotten rock or residual soil in its original position in the 

 Adirondacks because such materials were nearly all scoured off by 

 the passage of the great ice plow, mixed up with other soils and 

 ground up rock materials and deposited elsewhere. Such are called 

 transported soils. 



Ice shod with hard rock fragments, and flowing through deep, 

 comparatively narrow valleys of relatively soft rock, is particularly 

 powerful as an erosive agent,- because the tools are supplied, the 

 work to be done is easy, and the increased depth of ice where 

 crowded into such a valley causes greater pressure on the bottom 

 and sides of the valley. Many of the valleys of northern New York 

 were thus favorably situated for ice erosion, as, for example, the 

 Champlain, St Lawrence and Black River valleys as well as many 

 of the north-south valleys of the Adirondacks. 



Most of the Adirondack mountain peaks, especially the more 

 isolated ones, were thoroughly scraped oft' and rounded down to the 

 very fresh rock, while the favorably situated valleys were vigor- 

 ously glaciated by the removal of all the rotten rock and at least 

 some of the fresh rock, especially when this latter was comparatively 

 soft, Grenville limestone. Such phenomena are particularly well 

 exhibited in Warren county where the landscape is characterized by 

 numerous glaciated rock domes which rise conspicuously above the 

 vallevs of weak Grenville. In some cases where the ice moved 





