GEOLOGY OF THE NORTHERN ADIRONDACK REGION 



409 



surface is similar. The small fault blocks in the intervening strip 

 have been updragged by the faulting, giving them a pronounced 

 dip, in general 10° or more, away from the fault plane toward' 

 the southeast. Hence the faults that cross the strip are quite 

 typical dip faults, and the lateral shifting of corresponding beds 

 on the two sides of a fault is plainly brought out on the map. 

 Owing to the steep dip, the more resistant rock layers involved 

 appear as low, sharp backed ridges, and the lateral shifting of 

 these, as a fault is crossed, is a prominent, minor feature of the 

 topography. In the most northerly pair of these faults shown 

 on the map, the north one throws to the north and the south one 

 to the south, so that the middle block has been upthrown between 

 the two others. Just the reverse is true with the pair just south 

 of Chazy village, the middle block having been downthrown be- 

 tween the two adjoining blocks. On the east and west edges of 

 the map faults are not indicated simply because outcrops are 

 not sufficiently numerous, or sufficiently definite, to permit of 

 their location. That they are there is quite certain. 



Plate 13 shows the faults in a portion of Plattsburg and Peru 

 townships, so far as the outcrops will admit their being located. 

 One very extensive fault of the meridional class, the Plattsburg 

 fault, runs across the map limits from north to south, exposing 

 Beekmantown rocks constantly on the west side and either Chazy 

 or Trenton on the other. The throw of the fault causes the dis- 

 appearance of the major part of the Beekmantown formation and 

 may be safely set down as at least 1000 feet. Toward the north 

 the throw diminishes, and another great fault develops, the Beek- 

 mantown fault, the two coalescing at the north limits of the map 

 and extending on beyond as a single fault of very large throw. 

 Between the two a wedge of Chazy is brought up, with Beekman- 

 town rocks on one side and high Trenton on the other. At the 

 point of junction the Chazy is pinched out and the Beekmantown 

 and Trenton rocks adjoin across the fault. There are two cross 

 faults in this Chazy wedge, two on Valcour island, one on the 

 mainland at Valcour and another just north of Bluff point, as 

 well as several small ones in the shales on Cumberland head, and 

 one on the south edge of the map at Lapham, which brings up the 

 Potsdam against the Beekmantown. Lack of outcrops prevents 

 the location of others. 



