372 STUDIES FOR STUDENTS 



carried forward rapidly whenever reached by the tumultuous currents. 

 It will therefore have moved relatively to the adjacent finer material 

 but little farther than the actual distance; whereas, as previously 

 pointed out, the coarser material usually moves relatively many times 

 farther than the finer. Not only, therefore, will the final region of 

 deposit undergo a sudden increase in sedimentation which may be 

 called a veritable flood of waste, but it will be of phenomenal coarseness 

 compared to that which preceded and that which will come after, 

 the preceding sediment being fine in the delta region on account of the 

 small carrying power of the rivers of the semi-arid plains; the suc- 

 ceeding sediment being fine because of the graded character of the 

 mountain slopes and the more decomposed nature of the waste which 

 they give to the streams in a time of more plmdal climate. The coarser 

 material deposited on the delta region will, however, be finer than 

 that previously deposited on the piedmont slopes since attrition and 

 wear are inevitable during transportation. Such a chmatic change 

 from subarid to rainy will thus be marked by a shifting of the stored 

 waste of the earlier epoch from the middle to the lowest portion of 

 the river system. If it be assumed that the area of deposition in the 

 second case is no larger than in the first, the amount will form a 

 deposit on the average of equal thickness to the depth of the erosion 

 in the piedmont plains. Thus it is within the limits of past possibili- 

 ties that widespread sand or conglomerate formations intercalated 

 between others of markedly different nature should be formed upon 

 delta surfaces or over the bottoms of shallow seas, as the result of a 

 rapid and profound climatic change. Such formations in the regions 

 of their greater coarseness would be remarkably clean from clay 

 and where reaching a maximum development might be several hundred 

 or even a thousand feet in thickness. This possibility will become a 

 probability in proportion as such arenaceous or conglomeratic for- 

 mations are widespread, dissociated from the other evidences of origin 

 through crustal movement, and correlated on other lines of evidence 

 with cHmatic changes of the proper nature. 



The most favorable period for the testing of these deductive state- 

 ments is the Pleistocene, since the deposits are still preserved at the 

 surface, they can be traced to their sources in the regions of erosion 

 and great climatic oscillations are known to have taken place. The 



