in the Pleistocene and Post- Pleistocene. 13 



ciation leans toward the view that the amount of water 

 abstracted for the formation of the ice sheets was not a major 

 factor in the control of Pleistocene sea levels. From the 

 estimates which have been given by Woodward, Penck, and 

 Daly it would seem neccessarily to have been a true and 

 important factor, but the evidence of the Pleistocene oscilla- 

 tions of the Atlantic shore suggests that the diastrophic rhythm 

 continuing with accelerated movement from the Pliocene con- 

 stituted a more controlling factor. 



Post-glacial Emergent Cycle Marginal to the Glaciated Areas* 



The third line of investigation which led to an intersection 

 with the problem of Pleistocene and post-Pleistocene crust 

 movements is that connected with the Strength of the Crust, a 

 subject now in progress of publication in the Journal of 

 Geology. If the hypothesis set forth there is valid, — that a 

 thick and strong lithosphere rests upon a thick zone of com- 

 parative weakness, an asthenosphere ; then the weight of a 

 continental ice sheet should tend to depress the crust into this 

 weak zone. The crust up to a certain limit would yield as an 

 elastic plate, the asthenosphere would reach its elastic limit at 

 a far earlier stage and from that point yield by flowage, a 

 flowage, it is thought, which is akin to the recrystallization 

 which explains glacial flow. But as this subcrustal zone is not 

 a fluid, it cannot transmit the excess pressures to unlimited 

 distances. Broad and low pressure ridges would tend to be 

 raised therefore beyond the limits of the ice sheets. A rail- 

 road embankment sinking into a marsh and elevating slightly 

 the adjacent portions of the marsh offers an instructive analogy. 

 Upon the removal of all of the embankment, including that 

 which had sunk below the marsh level, a re-elevation- of the 

 central tract toward the original level would occur. The 

 lateral pressure ridges might rise at first with the central part 

 and then subside. This double motion would be favored if 

 the deeper levels of the marsh were the more nearly fluid and 

 if the removal of the load was very rapid, exceeding the rate 

 of readjustment for the upper levels. 



Does the expectation raised by this hypothesis correspond 

 with the known depression of the crust under the Pleistocene 

 continental ice sheets and the recovery from that depression 

 which followed the retreat and disappearance of the ice ? The 

 question serves in a measure as a test of the validity of the 

 initial hypothesis regarding the distribution of strength in the 

 outer earth. As giving such a test the problem was investigated, 

 though, as it is somewhat apart from the principal subject, it 

 ife not intended to publish the results as a part of the series on 

 the Strength of the Crust, but rather later as a separate article. 



