GEOLOGY OP FRANKLIN COUNTY 85- 



tops of the rock ridges of the valley floors, which lay underneath 

 the course of the stream where it was not over its old channel, 

 were uncovered. The rapid down-cutting would be at once- 

 checked at these points, and the laborious process of sawing 

 through the ridge substituted. But on the down stream side of 

 the ridge the cutting in the drift would continue active, thereby 

 producing a marked change in level at such points and causing 

 a fall or rapid down the slope of the ridge. The eventual hight 

 of this fall would mainly depend on the difference in altitude be- 

 tween the summit of the ridge and the summit of the next suc- 

 ceeding ridge uncovered by the stream below. By means of the 

 slow cutting back of the fall a gorge would be formed below, 

 which would correspond in depth with the hight of the fall. At 

 the High falls of the Saranac in Clinton county, for example,, 

 the impressiveness of the fall and gorge are due to the fact that 

 the next rock encountered by the river, at Cadyville 10 miles dis- 

 tant, is at an altitude 450 feet lower. 



Up stream from a rock obstruction the drift could not be cut 

 out to a greater depth than the level of the obstruction, except 

 perhaps locally at the base of a fall. It would, however, be- 

 quickly worn out down to that level. By this process the stream 

 courses would be divided into sections of slight declivity and of 

 sluggish water, commencing and terminating at rapids over rock 

 ridges. As soon as this stage has been reached, the rate at which 

 erosion can go on becomes wholly dependent on the rapidity with 

 which the stream can saw through the hard rock obstructions. 



AH the principal streams of the northern Adirondacks illustrate 

 these general principles. Their headwaters are in chains of lakes, 

 and their courses below consist of reaches of " stillwaters " or 

 " levels ", usually distinguished from one another locally by their 

 lengths, as the 16 mile level, the 8 mile level and so on, which 

 commence at the gorge below one rapid and terminate at the 

 brink of the next. The divides between two lakes belonging to 

 two different drainage systems are often of the most trivial char- 

 acter. Upper St Regis lake is less than a mile distant from Lake 

 Clear, which belongs to the Saranac system, and the divide be- 



