652 II. L. ALLING GLACIAL LAKES OF CENTRAL ADIRONDACKS 



Morainal damming. — The pregiacial drainage has been profoundly 

 modified by morainal deposits of one kind or another in numerous locali- 

 ties. Old river courses have been filled and valleys dammed. Frequently 

 the streams have been able to cut through such barriers and flow through 

 drift-filled valleys little altered in character. On the other hand, some 

 river courses have been altered and now flow in postglacial channels. 

 Two excellent examples of stream diversion may be mentioned. One is 

 in the East Branch of the Ausable River a little south of Keene. In the 

 comparatively broad valley we note an unnamed hill, around the two sides 

 of which the two highways leading into Keene Valley circle. To the east 

 of this hill the present stream rushes between steep walls of Grenville 

 schist and syenite, experiencing rapids and falls. It is clearly a post- 

 glacial channel and is one of the beauty spots in the central Adirondacks. 

 On the other side of this hill the pregiacial channel is plainly perceptible, 

 now blocked by sand and gravel of a lateral delta. 



A similar state of affairs occurs at the Split Rock Falls of the Bouquet 

 Eiver, south of New Russia, in the Elizabethtown quadrangle. The falls 

 are of recent origin. The old channel lies deeply buried to the west, 

 where Coughlin Brook is attempting to cut its way through the drift. 



Many of the lakes and ponds of the Adirondacks are caused by the 

 damming of pregiacial valleys. Lake Placid, for example, has resulted 

 from the damming of two fault-line valleys that have been joined by 

 channels following cross fault-lines producing a ladder-shaped body of 

 water. In a depression in the surface of this dam Mirror Lake now lies. 



A striking morainal ridge in the Chapel Pond fault pass has retained 

 the surface of the pond on one side of the pass at a higher altitude than 

 Beecle Brook. On this ridge the highway runs, affording the geologist 

 an excellent opportunity to observe this phenomenon. 



Glacial Lakes 

 general observations on the origin of the lakes 



Several important conditions have produced two distinct series of local 

 glacial lakes in the central Adirondacks. The first condition was a valley 

 sloping toward and blocked by the ice-front. The second was the isolation 

 of such a valley by mountain ranges. Both the Keene and Elizabethtown 

 valleys fulfilled these conditions to such an extent that considerable time 

 will be devoted to the description of the different lakes that existed in 

 them. 



There are two series of these lakes — one in the East Branch of the 

 Ausable, called the Keene Valley group, and the other in the valley of 

 the Bouquet River, known as the Elizabethtown group. 



