GEOLOGY OF THE NORTHERN ADIRONDACK REGION 277 



igneous intrusions may also have aided or initiated the upward 

 movement. The great thickness of rock removed indicates the 

 probability of more than one upward movement, since it is unlikely 

 that the region ever had an altitude equal to the bulk of removed 

 rock. Periods of depression beneath the sea may have alternated, 

 though it is improbable that these could have had any great dura- 

 tion, or we should surely find traces in the region of the deposits 

 formed, and these we do not find. It is impossible to state posi- 

 tively the amount of rock removed during this great denudation, 

 but in all likelihood at least from 3 to 5 miles of rock thickness 

 were worn away from the surface, and perhaps considerably more, 

 specially locally. 



Surface topography at the close of the erosion interval. The land 

 surface left at the termination of this long period of wear is of 

 such nature that it could have been produced in no other way than 

 by long protracted erosion under conditions of stability of level. 

 After the last uplift of the region the streams sawed their valleys 

 down to grade; and the slow processes of valley widening con- 

 tinued at their work of broadening the valley bottoms and narrow- 

 ing the upland divides between the valleys, till the latter were 

 largely obliterated, and the resulting surface was one of small 

 relief, broad, shallow valleys, largely adjusted to the weaker rock 

 beds and structures, separated by low, gently sloping divide 

 ridges, with occasional low, rounded hills of extraresistant rock 

 protruding- above the general level, with elevations of only a few 

 hundred feet above the valley floors as a maximum. To produce 

 a land surface of this sort, specially on such resistant rocks as 

 those of the Adirondacks, requires a vast lapse of time. The sur- 

 face was not equally planed down in all parts of the region, but 

 was somewhat more irregular on the present northern and eastern 

 borders than on the southern and western, though the discrepancfy 

 is not marked. Over much of the surface the rocks were deeply 

 weathered and decayed, forming a deep soil, but the evidence in 

 this regard is conflicting, and apparently decay was less advanced 

 on the northeast than elsewhere. 



