i66 STUDIES FOR STUDENTS 



though the total amounts of these may hold to each other entirely 

 different proportions. It is to be concluded that the nature of the 

 parent rock is best reflected by the nature of the sediment where this 

 is but slightly transported, but may show in minor degree for long 

 distances. 



Relations of Rainfall and Topography to Erosion 

 interrelations of topographic and climatic causes 



The relation between the topographic character of the land and 

 the resulting sediment has been discussed by Bailey Willis, who 

 points out that in the youthful topographic stages rock-breaking as 

 a method of erosion dominates over rock decay; in topographic old 

 age the reverse is true. The former supplies a maximum of unleached 

 mechanical sediments; the second, a maximum of rock matter in 

 solution. ^ 



Under arid climates, however, and to a lesser extent in cold climates, 

 the material derived from the rocks is characterized by the same 

 dominance of disintegration over decomposition, even in regions of 

 topographic maturity or old age ; sun and wind serving to erode and 

 transport rock material without producing rock decay. When swept 

 to a distance by rivers and laid down upon flood-plains, there may be 

 strong chemical resemblances between the sediments originating under 

 such dissimilar conditions and consequently a resulting confusion 

 between distinct topographic and climatic causes. 



Furthermore, topography and climate not only have independent 

 and sometimes similar effects upon the sediments, but have mutual 

 effects upon each other. On the one hand, topography in its major 

 features modifies climate, on the other, climate materially affects 

 the minor features of topography. 



Different kinds of climate and topography may, therefore, lead to 

 various results, and a change in either the climatic or topographic 

 factor in the regions of erosion may be expressed by a change in 

 sedimentation over a distant flood-plain. In analyzing and separat- 

 ing these interrelations and simulated effects three kinds of topog- 

 raphy may be considered: first, young and mountainous topography, 

 characterized by deep and precipitous valley walls, high elevations, 



I Studies for Students, "Conditions of Sedimentary Deposition," Journal of 

 Geology, Vol. I, 1893, pp. 476-80. 



