52 NEW YORK STATE MUSEUM 



over the other along the fracture, we have what is called a fault. 

 In short, a fault is a fracture in the earth along which there has been 

 slipping. If one portion of the earth simply drops down with 

 respect to the other we have a normal fault, and if one por- 

 tion has been pushed over the other we have a reversed or thrust 

 fault. Actual movements along a fault surface are sudden, the 

 amount of slipping generally varying from a fraction of an inch 

 to 20 or even 50 feet at a time, though the sum total of all slipping 

 (called "displacement") along large faults, during a long time, 

 often amounts to hundreds or even thousands of feet. Each sudden, 

 notable slipping produces an earthquajce. Definitely traced faults 

 vary in length (that is, across country) up to a hundred miles or 

 more. Along such fractures the rocks are often more or less broken 

 or crushed thus affording relatively easy work for stream erosion, 

 so that streams often follow such fault zones of weakness. The 

 eastern and southeastern Adirondacks have been literally chopped 

 to pieces by numerous faults, all of the ordinary normal fault type 

 with fracture surfaces in practically vertical position. No single 

 fault has been proved to extend across the whole region, but rather 

 there is a prominent series of many roughly parallel fractures few 

 of which have been definitely traced as much as 20 miles and none 

 more than 30 or 40 miles. The greatest number are only a few 

 miles long. The total amount of displacement along these fracture 

 lines in the very ancient Adirondack rocks can not be determined, 

 but frequently it is at least 500 to 2000 feet. This series of faults 

 cuts through the early Paleozoic strata along the shores of Lake 

 Champlain and in the Mohawk valley, and in these regions, due to 

 marked differences in the character of the strata, it has been possible 

 to determine the amounts of displacement with considerable 

 accuracy. 



In addition to this great series of north-northeast by south-south- 

 west faults, there are many others, mostly minor ones, which trend 

 in various directions, though chiefly approximately at right angles 

 to the major series. Accordingly, the eastern and southeastern 

 Adirondack region is really a mosaic of earth blocks and ridges. 

 Since their origin most of these blocks and ridges have of course 

 been appreciably modified by erosion. Sometimes there is no 

 positive evidence of any notable displacement, though the rocks in 

 the fault zone are badly broken. The geologic map of the Lake 

 Pleasant quadrangle affords an excellent illustration of numerovis 

 faults and their evident influence upon the topography in the very 



