ELIZABETHTOWN AND PORT HENRY QUADRANGLES 75 



Chapter 6 



STRUCTURAL GEOLOGY 



Faults 



Faults are of more than usual importance in the Adirondacks 

 and the Champlain valley. They have been frequently and neces- 

 sarily referred to in the opening description of the general relief. 

 While the discordance produced by these displacements can not 

 always and can not easily be demonstrated in the ancient crystal- 

 lines, yet the tilted block character of the topography, the frequency 

 of precipitous escarpments, the innumerable cross gulches along 

 the crests of the ridges and the crushed and sheeted rock revealed 

 by cascades in the brooks/all leave no alternative to the observer 

 other than to believe in the general presence of these dislocations. 

 There are, howcA-er, some positive forms of evidence which are 

 more tangible, and it will be the purpose here to pass them in re- 

 view. 



Faults in the Paleozoic strata. Dislocations in the Paleozoic 

 rocks have long been known and outside the present area are ex- 

 hibited with diagrammatic clearness. They have been matters of 

 record since the early work of Ebenezer Emmons, and in Chazy 

 township, north of Plattsburg have been mapped and studied by 

 Professor Gushing.^ Near Port Henry and Westport they are not 

 so clearly exhibited between individual members of the Paleozoic 

 as they are farther north, but as between the Paleozoics and the 

 older crystallines, they are well known. 



The clearest case is afforded by the block of Beekmantown lime- 

 stone, just north of Port Henry. As one passes through the two 

 tunnels and across a small inlet, a block of this silicions, magnesian 

 limestone is encountered which rises loo feet or more above the 

 lake. It is extensively quarried for flux for the Port Henry blast 

 furnace and also for macadam. It has a very flat dip of lo north- 

 east and a strike of n. 55° w. If projected across the little em- 

 bayment called Craig harbor on the map, it would abut abruptly 

 against the Grenville. There is obviously a fault which causes the 

 discordance and which passes northeast and southwest. The sheet- 

 ing and crushing of the rock have given rise to the embayment, 

 which has been worked back by the waves through the broken rock. 



Another significant feature is found in the prevailing dips of 

 the Potsdam and later strata. They are almost or quite always in- 



1 dishing-, H. P. Faults of Chazy Township, Clinton County, N. Y. 

 Geol. Soc. Am. Bui. 1855. 6:285. 



