ULRICH: major causes of OSCIIvIvATIONS 59 



ice sheets to effect considerable lowering of sea level ; and the evi- 

 dence indicating warping of the land surface, because of the un- 

 even distribution of the ice load, as first pointed out by Jamie- 

 son, seems to me reasonably compelling. I believe also that in 

 deglaciation the land surface largely re-established itself by elas- 

 tic, or rather, isostatic rebound to preceding relief. 



Though accepting in modified form the idea of glacial control 

 of particularly Pleistocene sea levels, it is not to be denied that 

 the present well-known occurrence in Newfoundland and in re- 

 mote outlying stations along the coast of New England and the 

 Maritime Provinces of many plants characteristic of the Coastal 

 Plain of New Jersey and the south tends, as expressed b}^ Bar- 

 rell,^ "to rule out the hypothesis that emergence was controlled 

 only by the level of the ocean water as controlled in turn by 

 glaciation." The extraordinary distribution of plants referred 

 to could not be brought about by nattu-al processes today. 

 Evidently the northern occurrence of this flora is to be v^iewed 

 as remnants of a 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 conditions may be readily conceived as hav- 

 ing obtained during, and as having resulted from, the ice loading 

 of the glaciated regions to the west and northwest. As the lat- 

 ter 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 BarrelP thought 

 it necessary to assume delay in the settling back of the up- 

 warped marginal zone after the removal of the ice sheet. As 

 defined by Barrell, his hypothesis is "that the weight 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 car- 

 ried up higher this marginal upwarped zone with it. Being al- 

 ready an upswollen tract the broader regional movement car- 



- Amer. Journ. Sci. 40: 17. 1915. 

 ^ Idem. pp. 19-21. 



