30 NEW YORK STATE MUSEUM 



and crumpled strata; monoclinal and isoclinal, open and closed, 

 anticlinal and synclinal, symmetrical and overturned, horizontal and 

 pitching folds; joints, crevices, caves, crush zones, shear zones, and 

 contacts ; normal, thrust, dip, strike, large and small faults ; veins, 

 segregations, inclusions, dikes, sills, bosses and bysmaliths. 



With such variety of natural conditions it is not surprising that 

 the problems of the aqueduct are also of great variety. No two 

 have in all respects the same factors in control and no two can be 

 explored and interpreted upon exactly the same lines. 



I Geographic features or districts. (Physical geography^) 



It will be convenient at this point to think of the surface topog- 

 raphy by districts — not wholly distinct from each other, but still 

 with essential differences of origin and form. From south to north 

 they are: (a) New York-Westchester county district. The area 

 of cr}^stalline sediments. South of the Highlands, (b) Highlands 

 of the Hudson (Putnam county), (c) Wallkill-Newburgh district. 

 From the Highlands to the Shawangunk range, (d) Shawangunk 

 range and Rondout valley, (e) Southern Catskills. 



All have been sculptured by the same forces and with similar 

 vicissitudes, but the difference of history and structure and condi- 

 tion, already established when the physiographic forces began on the 

 work now seen, have caused the variety of surface features indi- 

 cated in the divisions made above. The more noticeable character- 

 istics of these five districts are here given. 



a New York- Westchester district. The area south of the 

 Highlands proper is characterized by a comparatively regular suc- 

 cession of nearly parallel ridges separated by valleys of nearly equal 

 extent (^ to 5 miles wide), making a surface of gently fluted 

 aspect and of moderate relief (0-500 feet) sloping endwise toward 

 the Hudson and the sea. The controUing factors in producing this 

 topography are involved in a series of folded, foliated, crystalline 

 sediments, of differing resistance to destructive agencies. 



b The Highland region is one of rugged features, with a 

 range of elevation of 0-1600 feet A. T., forming mountain masses 

 and ridges separated by very narrow valleys all having a general 

 northeast and southwest trend across which the Hudson cuts its 

 way in a narrow, angular gorge, forming the most constricted and 

 crooked portion of its lower course. The bed rock is all crystalline, 



iThe physiographic history of a region is not understandable without a 

 comprehensive knowledge of its geologic features and structures and history. 

 It is therefore treated in a later paragraph. 



