﻿60 NEW YORK STATE MUSEUM 



gunk conglomerates, and the last (3) is Postdevonic (probably 

 Postcarbonic, judging from neighboring regions of similar history) 

 and has left as its most important evidence in this district, the 

 excessively complicated sharp foldings and thrusts of the Shawan- 

 gunk range and its extension in the Rosendale cement district. 



Kinds. As to forms produced there are no usually described 

 types that are not to be found here. The simpler forms of anti- 

 clines and synclines, both open and closed, symmetrical and unsym- 

 metrical and overturned, are all common. The isoclinal is common 

 in the gneisses. In each epoch of folding the compression forces 

 were effective chiefly in a northwest-southeast direction producing 

 arches and troughs whose axes trend northeast-southwest. This is 

 the trend of the main structures throughout the region. 



The extent of crustal shortening accomplished by this series of 

 compressions is undetermined, but that it amounts to a total of 

 many miles is indicated by the fact that over broad areas the 

 strata stand almost on edge. Furthermore, in the older Highlands 

 and in portions of the Hudson river districts the folds have been 

 slightly overturned so that commonly the strata on both limbs dip 

 in the same direction (toward the southeast). This seems to indi- 

 cate a strong thrust from the southeast. All stages between the 

 gentlest warping to strongly overturned folds, and from minute 

 crumbling to folds of great extent and persistence are to be seen. 



The effect of all the folding is chiefly to present a series of up- 

 turned strata to erosion and encourage a subsequent development 

 of valleys along the softer beds bordered by ridges of the more 

 resistant types. 



As the axes of the folds lie in a northeast-southwest direction, 

 this gives a marked physiographic development of ridges and val- 

 leys of the same trend, a most conspicuous topographic feature of 

 southeastern New York. 



d Faults. Accompanying the folding in each epoch, and 

 especially the stronger overthrust movements there has been a 

 tendency to rupture and displacement. These breaks are known 

 as faults. Multitudes of them are of minute proportions and prac- 

 tically neglectable in a broad view, but many also are of large 

 extent, traceable across country for many miles and indicating dis- 

 placements in some cases of many hundreds of feet. For the most 

 part these faults are of the thrust type and wholly consistent with 

 the folds in origin. They run generally in a northeast-southwest 

 direction, especially the larger ones, and frequently form the sep- 

 aration planes between different formations. Occasional cross 



