Uplift and Subsidence. 1'?' 



physical action has resulted, which may cause the expan- 

 sion of the columns. 



What is the cause of an increase in density m the 

 column is, of course, not known, but when a column under 

 a mountain mass is elevated as erosion takes place, we are 

 bringing up material to colder zones than they formerly 

 occupied and this may lead to some chemical or physical 

 action which would contract the materials sufficiently to 

 account for a depression of what was formerly a moun- 

 tain area. 



Horizontal movements may he incidental to vertical uplift. 



There is abundant evidence that there have been hori- 

 zontal forces at work distorting the sedimentary strata 

 in a mountain area and we have evidence of overthrusts 

 which extend for a number of miles. This, on first 

 thought, might lead one to conclude that the mountain 

 formation could not have resulted from vertically acting 

 forces. The apparent answer to this objection seems to 

 be that the base of the mountain system is usually large. 



The Appalachian system is approximately 200 miles 

 wide, on an average. This appears to be a sufficiently 

 large area to permit of development of horizontal move- 

 ments incident to the uplift. It seems reasonable to 

 suppose that the sedimentary material was of widely 

 varying thicknesses, that it was laid down at different 

 rates at different places and that the base of the sediments 

 gave way at different places at different times and at 

 different rates. No doubt the sedimentary strata were 

 somewhat distorted in the process of subsidence. 



When uplift begins it is probable that it takes 

 place in different sections of the area affected at different 

 rates and at different times. There would be more resist- 

 ance to the uplift in some parts of the area than in others. 

 The isostatic shell just beyond the zone of uplift would 

 undoubtedly have considerable effect on the upward 

 motion of the materials of the column adjacent to its 

 edges. It seems probable that the uplifted material would 

 follow the line of least resistance and, at least near the 

 surface, some of the lines followed by the material would 

 be horizontal or nearly so. It is conceivable that in the 

 process of uplift in a large area we should get distortions 

 such as are found in most areas of mountain uplift. 



Am. Jour. Sci.— Fifth Series, Vol. II, No. 7.— July, 1921. 



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