THE ADIRONDACK MOUNTAINS I9 



The Champlain valley bounds the Adirondacks on the east, being 

 a great depression which separates the Green mountains on the east 

 from the Adirondacks on the west. Much of the valley bottom 

 is filled by the waters of Lake Champlain, whose altitude is loi feet. 

 Along the western shores of the lake the topography is characteris- 

 tically hilly, though seldom above 500 feet in elevation. The 

 transition to the higher and rugged Adirondacks is generally rapid. 



Bounding the region on the south, the Mohawk valley clearly 

 separates the Adirondacks from the highlands of the Catskills and 

 the great southwestern plateau of New York. The comparatively 

 narrow inner valley through which the river flows is often erro- 

 neously called the Mohawk valley, but in reality the whole depres- 

 sion, from 10 to 30 miles wide and fully looo feet deep, between 

 the northern and southern highlands of the State should be called 

 the Mohawk valley. The bottom of the valley is only 300 to 400 

 feet above sea level. At Little Falls the inner valley narrows to a 

 gorge several hundred feet deep where the river has cut its way 

 through an old divide. The principal rocks of the valley are 

 shales, sandstones and limestones of Cambrian and Ordovician ages, 

 with Ordovician shales predominating. The great depression owes 

 its existence largely to the presence of this belt of soft shales lying 

 between the hard ancient rocks of the Adirondacks on the north and 

 the relatively hard limestones immediately south of the valley. The 

 work of erosion has made rapid progress in this belt of weak rocks, 

 and at two places. Little Falls and Yosts ("The Noses"), the 

 river has cut through to the underlying pre-Paleozoic (Adiron- 

 dack) rock. In general, the strata of the valley tilt only slightly 

 southward and show little signs of folding (fig. 7), though from 

 Little Falls eastward there are various faults (fractures) in the 

 strata. 



On the west, the Adirondack highland is bounded by the Black 

 River valley, which is about 60 miles long and has a maximum 

 depth of nearly 1500 feet. Immediately west of the valley the Tug 

 Hill plateau stands out as a distinct, isolated geographic province. 

 The top of the plateau, covering many square miles, is remarkably 

 flat, swampy and densely wooded with altitudes ranging from 1800 

 to 2100 feet. This Tug Hill plateau is merely an erosion remnant 

 of a great upraised plain which formerly covered the area of New 

 York State (see page 50). Near Boonville, at an altitude of about 

 HOG feet, occurs the division of drainage between the Black and 

 Mohawk rivers, this divide not only forming the highest connection 

 between the Tug Hill plateau and the Adirondacks, but also being 



