GEOLOGY OF THE NORTHERN ADIRONDACK REGION 443 



by 300 feet in a somewhat shorter course. The Raquette and St 

 Regis have a slope somewhat less, but yet over 15 feet to the 

 mile. These are considerable slopes for streams of the size, and 

 excavation of their beds is going on at a fairly rapid rate. The 

 drift deposits were rapidly cut into and the top of the rock knolls 

 of the valley floors, which lay underneath the stream where it 

 was not over its old channel, were uncovered. The rapid down- 

 cutting would be at once checked at these points, but on the down- 

 stream side of the obstruction the cutting in the drift would 

 continue actively, causing a fall or rapid at the point, which 

 would commence to saw back a gorge into the rock obstruction. 

 Upstream, however, the drift could not be cut out to a greater 

 depth than the level of the obstructing rock ledge, though it 

 would be quickly worn down to that level and a wide valley 

 rapidly eaten out in the yielding drift materials. In this way 

 the stream courses would be divided into sections of slight decliv- 

 ity and of mature character, commencing and terminating with 

 rapids over rock ridges. 



Most of the Adirondack streams illustrate well these general 

 principles. Their head waters are in chains of lakes, and their 

 courses below consist of reaches, or stillwaters or levels as they 

 are locally called, interrupted by rapids and gorges. The Saranac 

 serves well as a type. It rises in Lake Clear, passes thence into 

 Upper Saranac lake, Eound lake and Lower Saranac lake and 

 leaves the latter near the middle of its eastern side in a wholly 

 postglacial channel. At the rapids at Saranac village the river 

 is only 6 miles distant from Lake Clear in an air line, while by 

 water it is from 25 to 30 miles distant. Below the village the 

 first considerable rapid is at Franklin Falls some 20 miles away, 

 where the river falls 40 feet within the space of half a mile. In 

 the 20 miles above it has fallen less than 100 feet, or only about 

 4 feet a mile. Below the falls it flows through a gorge half a mile 

 long, with walls 100 feet high, which apparently marks the chan- 

 nel of a small preglacial stream, or else a low divide between two 

 small streams. Below the gorge a wide marshy valley opens out, 

 through which the river flows in a beautiful series of meanders. 

 Heavy drift filling turns it aside over the rock ledge in the gap 

 at Union Falls. At Clayburg. 8 miles below, it meets the north 

 branch and turns abruptly into the larger valley occupied by 



