12 NEW YORK STATE MUSEUM 



is between 600 and 700 feet A. T. From this elevation it declines 

 gently eastward to the valley of the Hackensack river, the in- 

 closing hills of which are less than 300 feet high. A sharply 

 marked ridge, varying in width from one half mile to 2 miles, 

 extends along the eastern border from the state line to Haver- 

 straw. Its hight ranges from a little over 200 feet at the south 

 to a maximum of 832 feet at High Tor, just south of Haver straw. 

 With the exception of the gorge at Piermont, where it has been 

 cut through almost to tide level by a* stream which drains the back 

 country, it is everywhere much higher than the adjacent region 

 on the west. Within the distance mentioned, from the state line 

 to Haverstraw, the ridge rises abruptly almost from the bank of 

 the Hudson, except in the vicinity of Nyack, where for a distance 

 of 3 miles it lies about three fourths of a mile back from the river 

 and is bordered by ground having an elevation of about 200 feet. 

 Just south of Haverstraw this ridge turns away from the river 

 and extends westward for several miles, gradually losing its dis- 

 tinctness, not so much because it decreases in hight as because 

 the adjacent country attains a higher general level. Not only is 

 this ridge higher, wider, more massive, more sharply marked and 

 less interrupted by gaps than any of the others, but its eastern 

 and western slopes are more sharply contrasted. Everywhere the 

 eastern slope is exceedingly steep, not infrequently a vertical cliff, 

 whereas the western is generally long and comparatively gentle. 

 These various topographic features are dependent chiefly on the 

 geologic structure of the region. The north and south ridges, 

 as will be brought out in greater detail below, follow the trend 

 of the rocks, and their existence is due to slightly harder and 

 more resistant beds of sandstone and conglomerate. The general 

 lower elevation of the hills in the western part of the area, border- 

 ing the Hackensack river, is to be explained by the softer char- 

 acter of the rocks, there more largely shales and sandstones. The 

 massive, sharply marked ridge along the Hudson river is pro- 

 duced by a belt of trap, an exceedingly resistant rock, which has 

 been intruded into the sandstones. The steeper eastern slopes 

 are readily explained by the general westward dip of the strata. 



