barrell: movements of the strand line • 419 



sible also that a slight change in slope of the sea level at the time 

 that river courses were established on a newly emerged coastal 

 plain may be at least a partial cause of the southward deflections 

 of rivers which were established along the Atlantic margin in 

 late Tertiary time. 



Conclusions. First, we must recognize the special factors 

 connected with Pleistocene climates. These consist in the 

 abstraction of vast quantities of ocean water during the develop- 

 ment of the ice-sheets; the direct depression under the load of 

 ice; a possible compensatory elevation in a broad zone beyond; 

 a deferred, intermittent, and possibly oscillatory readjustment 

 upon the removal of the burden of ice. 



But these factors must not be used alone. The second group 

 of causes includes the isostatic factors. They tend to main- 

 tain the equilibrium of large sections of the crust, affect the whole 

 ocean level, and locally warp the lands, but do not involve earth 

 shrinkage. The general rise of lands which has marked the 

 later geologic times may most probably be placed in this cate- 

 gory. The movements are due either to changes of external 

 load or to changes in crustal density. Continental rejuvenation 

 is the chief effect, but they include a possible enlargement or 

 deepening of portions of the ocean basins. 



The third group of causes is thought to be found in great com- 

 pressive movements in the lithosphere which seem to be due 

 in turn to a shrinkage of the centrosphere. Locally they may 

 work against isostasy, more broadly they may start isostatic 

 movements. As their ultimate expression is in mountain build- 

 ing along lines of weakness, they may be classed as orogenic 

 causes. 



Fourth are the planation factors, working in opposition to 

 isostasy. They tend to erode the lands, fill the seas with sedi- 

 ment, and raise their surface level. The combination of iso- 

 static and compressive forces has operated through geologic 

 time. The causes are complex and the result is seen in the 

 composite diastrophic rhythms. 



To sift apart the factors comprehensive investigations must 

 be prosecuted over different parts of the earth. The sequence 



