64 ulrich: major causes of oscillations 



deformational effects on the lithosphere only in one respect — that 

 is, in both cases the loaded area sinks. They differ, primarily, 

 in that the ice cap originates on, and spreads outwardly from, 

 normally positive areas whereas the rock sediments are laid only 

 in areas of relatively negative tendencies. Subsidence because 

 of ice loading, therefore, is an abnormal process in that it is car- 

 ried on under unusual conditions, so that normal gravitational 

 tendencies are reversed; in the other case not only the process 

 but the results also are perfectly in accord with the normal 

 gravitational tendencies of the affected areas. Next, they differ 

 in that the ice sheets presently melt away, whereas the water- 

 laid rock deposits commonly remain as a permanent asset of the 

 area covered by them. A third difference is that in the first 

 cases the removal of the ice load tends to re-establish the nor- 

 mally positive tendencies of the deglaciated areas, whereas in 

 the areas loaded with rock deposits their normal negative ten- 

 dency is not reversed. 



Finally, there is the rather generally accepted belief among 

 stratigraphers and students of paleogeography that in the past 

 the advances of the sea usually were slow and gradual, whereas 

 the retreats were more rapid and relatively impulsive. Many 

 facts in Paleozoic stratigraphy are cited in my Revision in sup- 

 port of this belief, and Barrell, in 191 5, expressed himself as 

 favoring the view. 



Now, if we accept this conclusion it certainly does not help 

 the hypothesis of measurable sea level fall by storing of oceanic 

 waters in continental ice sheets. Obviously, the subtraction 

 of water from the seas to make the ice sheets must have been a 

 slow and on the whole gradual process; and the time consumed 

 in the growth of the ice sheets probably was not materially 

 shorter or longer than that required in their melting. 



From these considerations it is clearly evident how exceedingly 

 difficult is the proper determination of the part actually played 

 by glaciation and ensuing deglaciation in the emergence and 

 submergence of the continental borders. The fall and rise of 

 sea level directly resulting from the storing of oceanic water 

 to make a great ice sheet that later is returned to the sea is so 



