ulrich: major causes of oscillations 6r 



"Ancient Water Levels of the Champlain and Hudson val- 

 leys," highly probable. Indeed, supported as this evidence is by 

 the facts concerning the distribution of the Coastal Plain flora 

 just alluded to, emergence of this marginal area as this time may 

 justly be accepted as reasonably established. As will have been 

 observed in the quotation, Barrell's hesitancy in adopting this 

 hypothesis arose mainly from the uncompleted demonstration 

 of "why the extramarginal zone should rise together with the 

 region directly glaciated." 



In thinking this matter over the possible solution of the diffi- 

 culty somewhat crudely illustrated in figure i has been reached. 

 The diagram represents in generalized profile four Pleistocene 

 stages of eastern North America, the profile running southeast- 

 wardly from Labrador to the edge of the continental shelf. The 

 stages are represented separately, showing relief of land surface 

 in each and the extent of the ice sheet in the maximum and two 

 partly deglaciated stages. The fourth represents the present 

 condition. One of the new features is that as the ice retreated 

 the normally positive strip bordering the present eastern shore 

 responded at once to the release from directly applied weight 

 pressure by rising. Emergence of this Piedmont and Coastal 

 Plain strip would be further insured by the necessity of main- 

 taining isostatic balance with the outer strip of the continental 

 shelf which had bulged to emergent status by subterranean 

 flow from beneath the ice loaded land. In consequence, as the ice 

 sheet retreated the emerged outer part of the continental shelf 

 began to sink, whereas the strip along the landward side of the 

 present shore rose. Among the physiographic changes that may 

 be supposed to have occurred at the time of this southwardly 

 decreasing elevation of the coast lands north of Baltimore is the 

 cutting of the now buried deep channel of the lower Hudson; 

 also the sharp southward deflection of the Delaware and Sus- 

 quehanna rivers. During the preceding maximum extent of the 

 ice sheet Maryland is supposed to have stood higher than at 

 present and the lower stretches of these rivers either flowed north- 

 eastwardly or they emptied more directly and much sooner 

 into the sea, which then probably covered the New Jersey part 



