GEOLOGY OF THE NEW YORK CITY AQUEDUCT 73 



(i) Coastal plain. A district underlain by Cretaceous and later 

 rocks and confined to a part of Staten Island and Long Island., 

 not exceeding 400 feet relief. This zone is characterized by den- 

 dritic drainage, except a narrow belt on its inner margin which 

 is a longitudinal valley of the " inner lowland " type. Long Island 

 sound occupies the position of this old adjusted valley. 



(2) Piedmont belt. A district lying between the coastal plain 

 and the Highlands. It is underlain chiefly by crystalline rocks and 

 metamorphosed sediments. Not exceeding 800 feet relief. It is 

 characterized by adjusted drainage obscured only by drift. The 

 ridges and valleys trend northeast and southwest close together 

 and with very little variation on the east side of the Hudson, 

 while on the west side the gentle dips of the Triassic give broader 

 and more unsymmetrical forms with dip slopes and escarpments 

 wholly independent of the opposite side. The zone is essentially 

 transitional between the simple forms of the coastal plain and the 

 complex mountainous character of the Highlands. 

 • (3) Highlands. The rugged elevated zone formed by the crys- 

 talline gneisses. Reaching elevations of 1600 feet. It is character- 

 ized by irregular mountain masses and lofty ridges of a general 

 northeast trend but with many prominent irregularities both- of 

 form and of drainage. The valleys are deep and narrow. There 

 are many steep escarpments. It is a mountainous zone in which 

 complex structures and rocks have led to the development of com- 

 plex forms. The zone forms a sort of barrier 20 miles wide across 

 the Hudson river which exhibits its most zigzag and narrow and 

 gorgelike development in this district. 



(4) Appalachian folds. Characterized by folded Paleozoic rocks 

 north of the Highlands. Reaching elevations of 1500 feet rarely 

 — general relief 400-800 feet. North of the Highlands the relief 

 is much less pronounced. The softer rocks of the early Paleozoic 

 formations permitted the development of a broad valley with almost 

 perfectly adjusted tributaries, most of which on the west side of 

 the Hudson are reversed. The topographic forms give expression 

 to the universal folding and faulting of the formations. It is 

 essentially a transition from the complex mountain zone of the 

 Highlands to the much simpler Catskill area. 



(5) Catskill Monadnock group. Characterized by undisturbed 

 Paleozoic strata and Ytry strong relief — reaching elevations of 

 3500 feet. The eastern margin is an escarpment facing the Esopus 

 and Rondout valleys which are adjusted to the gently dipping 

 strata of that side. Over the rest of the district the beds lie so 



