GEOLOGY OF THE NORTHERN ADIRONDACK REGION 435 



those trending with the ridges, indicates that they were not the 

 controlling factor, but that the direction was determined by some- 

 thing else, and, once determined, this joint set becomes of greater 

 importance than the others. By a process of elimination, the 

 faults seem to remain the most probable controlling factors in 

 the original determination of the trend. 



The present valleys were excavated below the Cretaceous base 

 level in the Postcretaceous erosion cj^cle. The comparative 

 weakness of the Grenville rocks determined valley location 

 where they were present in any force, and a respectable number 

 of the valleys of the region are of this origin. They are most 

 numerous in western Essex and St Lawrence counties, being 

 comparatively linfrequent elsewhere. There are also many 

 valleys in which one or more small patches o"f Grenville rocks 

 may be found, surrounded by others of a different nature; and 

 in these it is quite possible that the Grenville patches are merely 

 the final remnants of much larger Grenville masses, which de- 

 termined the location of the valley and have disappeared in its 

 formation. But even where a very large allowance is made for 

 possible instances of this sort, it yet remains true that Grenville 

 rocks make small show in moist of the region, and that the 

 larger number of the valleys can not have been located on Gren- 

 ville belts; are developed in fact in rocks identical in kind with 

 those which make up the neighboring ridges. To account for 

 tlu^se. it seems necessary to invoke some structural cause, and 

 such may be found in belts of rubble rock along the faults, in 

 belts of excessive jointing and slip faulting, and in the location 

 of streams by the original fault scarps; also in the production 

 of actual fault valleys {Grahen). These are truer in direction 

 than the Grenville b'elts and best explain the prevalent trend. 



The larger number of the ridges of the northern Adirondacks 

 show a certain type of configuration which calls for explanation. 

 They have a long, gentle, even incline to the northeast, a summit 

 well toward the south end, and a steep back slope, often in part 

 a perpendicular cliff and in general steepest at the top. These 

 features are seen most typically on the smaller ridges, but the 

 larger ridge masses show a tendency to the same type. At the 

 backs of these an amphitheater is apt to be developed, sometimes 



