378 STUDIES FOR STUDENTS 



uct of the erosion of the High Plains, the fact still remains that the 

 erosion was concentrated into certain stages and that somewhere 

 sediments of increased volume and coarser nature must have been 

 deposited. 



The same principles may be applied to the more distant past. 

 Wliere the sediments by the chemical conditions of erosion and 

 deposition give evidence of profound climatic change, such for instance 

 as ushers in glacial periods, equally profound textural changes in 

 regions beyond the reach of glaciation, whereby sand or even pebbles 

 were swept to the delta regions or the seas beyond, may properly be 

 ascribed to the change of climate and do not necessarily require any 

 crustal movement for their origin, though they may be complicated 

 by the presence of such movements. In this way periods of glacia- 

 tion may make their existence felt far beyond the actual limits of the 

 ice invasion. It is thought by the writer that the Pottsville con- 

 glomerate or Millstone grit which so commonly underlies the coal 

 measures of the eastern United States and western Europe, sharply 

 separating the Carboniferous from the sub-Carboniferous of those 

 regions, owes its origin primarily to such climatic change. 



CLIMATIC CHANGE FROM RAINY TO SEMI-ARID 



Having given the previous analytical discussion, the reverse side 

 of the problem may more briefly be stated. At a time of marked 

 change from rainy to semi-arid the waste poured into the upper part 

 of the streams may be rapidly increased in amount and in coarseness, 

 owing to less efficient vegetation and more concentrated floods. In 

 crossing the plains of the middle course, however, the rivers will 

 diminish instead of gaining in volume. They will find their previous 

 valleys much below grade with the result that a larger proportion of 

 river sediment will be deposited upon the piedmont slopes than would 

 even be the case under the same climate after the river grades had 

 acquired slopes in equihbrium with the conditions. An accentuated 

 deficiency of sediment will consequently be noted at the terminus of 

 the river system. The stratigraphic effect of such a climatic change 

 will depend, as in the previous instance, upon the quickness and 

 amount of the change, the distance of the headwaters from the delta, 

 and the height of the source. These factors measure the difference 



