THE STRENGTH OF THE EARTH'S CRUST 33 



Regional igneous activity has poured out lavas and breccias, 

 burying previous mountainous topography and adding thousands 

 of feet to the outer crust. Lack of simultaneous erosion, as in the 

 Miocene flows of the Columbia plateau, shows that subsidence 

 progressed, perhaps with approximately equal pace. The present 

 altitude of the Columbia plateau is youthful, as shown by the steep 

 canyon walls and undissected interfluvial areas. The initial sub- 

 sidence accompanying igneous outpouring and the distinctly later 

 upwarping without compression suggest that here isostasy has pre- 

 vailed. But in such regions the geologic evidence points toward a 

 minimum strength of the crust. The wide area of activity, the nu- 

 merous vents, the general absence of localization, all are suggestive 

 of widespread fluid rock beneath, magmas which are probably far 

 above the level where the accompanying temperatures are normal. 

 Such conditions would seem to imply the impossibility of the outer 

 crust carrying over such regions the stresses which are possible in 

 regions long free from igneous activity. More reliance as maxi- 

 miun measures of the strength of the crust should be placed there- 

 fore upon those external changes which are entirely independent 

 in origin from the interior of the earth locally beneath them. 



SHIFTINGS OF LOAD DUE TO CLIMATIC CHANGE 



Some of the most striking examples of loading and unloading 

 of the crust are those connected with the climatic fluctuations of 

 the Pleistocene. The continental ice sheet formed, advanced, and 

 retreated rather rapidly, as viewed from the geologic standpoint. 

 As it retreated, the lacustrine and estuarine shores show that the 

 land was rising with the melting of the ice. The upwarping accom- 

 panying deglaciation was limited to the approximate region of 

 maximum glaciation and was greatest in the direction where the 

 ice was thickest, in the St. Lawrence valley the maximum uplift 

 being more than 600 ft. These relations suggest strongly an iso- 

 static response to the relief of load. It is not known, however, to 

 what degree the previous downwarp compensated for the burden 

 of the continental ice sheet and what degree of regional stress the 

 crust was able to bear. The lack of close response is seen in that 

 the upwarp continued as a residual movement after the ice departed. 



