238 H. L. FAIRCHILD PLEISTOCENE UPLIFT OP NEW YORK 



bases were located in harmony with the datum plane and consequently 

 were equally spaced. 



It must be understood that the "hinge line" can not be a true line, 

 but is a belt of country ; or, in other words, the hinge is flexible and not 

 a joint. The stationary or non-tilted surface must gradually blend into 

 the uplifted area. In different expression, the tilted surface must slowly 

 flatten out into the undisturbed area. This implies that the gradient, or 

 rate of uplift, increases toward the north for some distance and must be 

 drawn as a curving line. The isobases of lower value, from zero up, will 

 be spaced further apart than those in the area of steeper tilting. In the 

 present map the isobases are placed as accurately as the data permit, and 

 it is believed that they are approximately correct. The isobases of 200 

 to 400 feet uplift are located at practically the same points as in the 

 former map. The zero and 100-feet lines are carried southward, while 

 those above 400 feet, in the region of steeper tilt, are more closely spaced, 

 which carries them also to the south. 



The amount of Pleistocene submergence and subsequent uplift indi- 

 cated on the present map for New York City district and also for the 

 Champlain Valley are slightly more than in the former map. 



The isobases are drawn with inclination from the latitude parallels of 

 20°, 70° divergence from the meridians, which give them a curvature 

 equal to their nearest parallels, on the projection of the map. Further 

 knowledge and extension of the isobasal lines over larger territory, spe- 

 cially eastward, may require some slight changes in position and spacing, 

 and specially in curvature ; for somewhere, both west and east, the lines 

 must bend into sharper curves to lie about the center of maximum uplift. 



The positions of the isobases of 200 to 500 feet were determined by 

 comparison of marine summit phenomena in the Hudson and Connecti- 

 cut valleys. The 100-feet line is located with special reference to the 

 deformation in the Lake Erie basin. Leverett gives the amount of defor- 

 mation of the Maumee beach at the Ohio-Pennsylvania boundary as 10 

 feet (100, page 739). At this point the later and lower Whittlesey beach 

 has an altitude of 746 feet (100, page 756). A rise of 90 feet more, or 

 to 836 feet, evidently gives the total uplift of 100 feet. This point on 

 the Whittlesey shoreline is found about 9 miles east of Dunkirk and 2 

 miles north of Forestville and about 4 miles south of Silver Creek. This 

 point, therefore, is taken for the location of the isobase of 100 feet. 

 Drawn approximately parallel to the 200-feet isobase, it crosses the north 

 boundary of New Jersey and intercepts the Hudson River 3 or 4 miles 

 south of Tarrytown. Precise measurements of the marine plane in the 

 lower Hudson will check this line. But in passing it may be noted that 



