324 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



sented 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 b}^ rising. Emergence of this 

 Piedmont and Coastal plain strip would be further insured by the 

 necessity of maintaining isostatic balance with the outer strip of the 

 continental shelf which had bulged to emergent status by subterra- 

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



LABRADOR 



NEW ENGLAND 



COASTAL PLAIN AND 

 CONTINENTAL SHELF 



Fig. 1. — Generalized profiles of eastern North America in Pleistocene stages, indicating 

 isostatic vertical movements of surface of lithosphere in process of deglaciation : 1, Dur- 

 ing maximum extent of ice sheet, when the outer part of continental shelf was emerged ; 

 2, When the ice load had retreated from the present coastal strip ; 3, A later stage when 

 the ice sheet had been reduced to the area of Labrador ; 4, Present relief of land, with 

 submei'gence of continental shelf. Approximately similar conditions may be supposed to 

 have obtained in the growing stages of the ice sheet. 



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 decreas- 

 ing 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 Susquehanna Eivers. Dur- 

 ing the preceding maximum extent of the ice sheet Maryland is sup- 

 posed to have stood higher than at present and the lower stretches of 

 these rivers either flowed northeastwardly or they emptied more 

 directly and much sooner into the sea, which then probably covered 



