GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



425 



This is no longer the grade of the region, a comparatively 

 recent movement having elevated it, and the streams are now 

 busily engaged in the task of getting their beds down to the 

 new grade and of widening their valleys. They are yet far 

 from being thoroughly adjusted to the new conditions. 



Peneplains 



It would seem inherently probable that the Adirondack region 

 participated in the actions outlined, but several causes conspire 

 to make the evidence less clear than in the other districts men- 

 tioned. In the southern and western portions of the region, 

 however, the hilltops do, in general, rise to quite concordant 

 levels, which are wholly independent of the >rock attitude and 

 structure, and the inference is irresistible that they rise to the 

 level of an old and comparatively even erosion surface, quite 



Fig. 8 Somewhat ideal section from north to south across the Mohawlt lowland. The 

 Cretaceous peneplain surface extended across the region from A to D. Following the 

 uplift, the Intervening region has been worn down, owing to the weak character of 

 the limestones, Sb, and the shales, Su. The harder rocks at A. and from C to D, 

 have resisted wear and remains at the old level. Erosion has not yet reached the 

 horizon of the Potsdam sandstone. Cp. The wearing away of the limestones has bared 

 the underljing Precambric rocks from B to C, exposing the old, Precambric 

 erosion surface, which meets the Cretaceous peneplain at a small angle, which the 

 large vertical exaggeration of the diagram makes altogether too prominent. 



Jig. 9 shows the angle drawn to true scale, the upper line representing the CD, and 

 the lower the BC slope. 



likely a product of the same great erosion period which else- 

 where developed the Cretaceous peneplain. The probability is 

 hightened when it is seen that the prolongation of this surface 

 southward, above the Mohawk lowland, to the plateau of 

 southern New York, finds it in close correspondence with the 

 upland levels there, as if the two were developments of the 

 same great surface [fig.8]. In both districts, too, this surface 

 is now tipped to the west. 



This old peneplain is best preserved in Hamilton county, though 

 well marked also in Herkimer and St. Lawrence. On the 

 west and south margins of the region it is replaced by another 



