EXPLANATION OT MAPS AND ISOBASBS 305 



Plate 11 is slightly modified from the one published in this Bulletin, 

 volume 25, plate 11. These maps are the result of field-work extending 

 over several years and examination of both sides of the Hudson-Cham- 

 plain Valley from N'ew York City to Canada. The detailed description 

 of the shoreline in New York is awaiting publication by the New York 

 State Museum, but the Vermont shore is described in the Eeport of the 

 Vermont State Geologist, 30 of the list of writings, page 308. 



The lines of equal uplift (isobases) drawn on the accompanying map 

 were originally projected across Long Island in accordance with the data 

 of submergence of the Hudson and Connecticut valleys, and were found 

 to be in close accordance with the facts ap observed on the ground. The 

 difference favors slightly greater submergence than the projected isobases 

 indicated, and the lines are here drawn in accordance with the field studj^ 



It is recognized that the isobases must eventually curve northward to 

 lie somewhat parallel with the eastward margin of the glaciated and de- 

 pressed area. The direction of the lines as plotted carries them far out 

 to sea (see also plate 12, in volume 27, of the Bulletin), and suggests a 

 deeper submergence of New England than has usually been recognized. 

 But it should be noted that Shaler thought there had been submergence 

 of Marthas Vineyard to a depth of 300 feet, which is more than the 

 isobases indicate.^ 



Thus far in this writing no movement of the island has been suggested 

 except the postglacial uplift to the present altitude. It is possible that 

 the rise in recovery from depression carried the land up higher than it 

 now stands. This might explain some features on which Veatch, Fuller, 

 and Crosby postulated the higher attitude of the island at the close of 

 Wisconsin time. 



There are other possible factors affecting the water level which are not 

 here taken into account. One is the gravitational pull of the ice-sheet 

 on the marginal waters. Another is the rise of ocean level due to the 

 return of the ice-caps to the sea. The effect of the 'latter factor is to 

 reduce the apparent submergence. The diastrophic and marine-level 

 problem is complicated by these indeterminate factors. The present land 

 attitude may be only the final averaged result or sum of all positive and 

 negative elements. But the maximum difference of postglacial land level 

 could not be less than now exists, as shown by the isobases ; it might have 

 been more. 



The former shorelines of the time of land depression are mostly ap- 

 proximate on the map and very generalized. As beaches are absent on 

 the smooth sand-plain, for reasons already given, the broken boundary 



= Loc. cit., page 320. 



