LAND AND SEA OSCILLATIONS TJLRICH. 323 



preceding continuous distribution established when the climate of 

 the northeastern coast was warmer and its coastal strip higher, wider, 

 and much less broken by water gaps. These required land con- 

 ditions may be readily conceived as having obtained during, and 

 as having resulted from, the ice loading of the glaciated regions to 

 the west and northwest. As the latter sank under their growing 

 load the continental shelf bulged its surface above sea level. But 

 whether the plant migration could have been effected during the 

 maximum extent of the Labrador Pleistocene ice sheet is so doubtful 

 that Barrell 3 thought it necessary to assmne delay in the settling 

 back of the upwarped marginal zone after the removal of the ice 

 sheet. As defined by Barrell, his hypothesis is " that the w r eight of 

 the ice sheets caused crustal depression directly below the load, but 

 moderate elevation in a wide zone beyond the load. Upon the 

 removal of the ice it appears the first isostatic upwarping carried 

 up higher this marginal upwarped zone with it. Being already an 

 upswollen tract the broader regional movement carried it up to 

 a level where it became unstable and a slow settling back occurred as 

 an after effect, coincident with the last stages of upwarping over the 

 centers of glacial load. The actual evidence at hand does not decide 

 between these hypotheses. The association with the close of gla- 

 ciation appears to favor a genetic connection with deglaciation, but, 

 on the other hand, it remains to be- demonstrated why the extra- 

 marginal zone should rise together with the region directly glaciated, 

 or that the cycle was restricted to such an extra-marginal zone." 



That the eastern margin of the continent, south of Labrador, 

 did rise to higher levels than the present during the retreat of at 

 least the last Pleistocene ice sheet seems, with Barrell's interpretation 

 of TToodworth's 4 data and conclusions regarding "Ancient Water 

 Levels " of the Champlain and Hudson Valleys, highly probable. In- 

 deed, supported as this evidence is by the facts concerning the dis- 

 tribution of the coastal plain flora just alluded to, emergence of this 

 marginal area at this time may justly be accepted as reasonably es- 

 tablished. As will have been observed in the quotation, Barrell's 

 hesitancy in adopting this hypothesis arose mainly from the uncom- 

 pleted demonstration of "why the extramarginal zone should rise 

 together with the region directly glaciated." 



In thinking this matter over the possible solution of the difficulty 

 somewhat crudely illustrated in figure 1 has been reached. The 

 diagram represents in generalized profile four Pleistocene stages of 

 eastern North America, the profile running southeastwardly from 

 Labrador to the edge of the continental shelf. The stages are repre- 



3 Idem, pp. 19-21. 



4 N\ Y. State Education Department, Bull. 84, 1905. 



