Fluviatile and Marine Gravels. 495 



accumulate along the entire valley and may be brought below 

 base level and thus preserved. 



If the downwarp be geosnyclinal in character, accumulations 

 of gravel are favored. The material from the rim of the newly 

 formed basin is readily removed since the increased stream 

 gradient gives added power of traction. The process of deposi- 

 tion may go on indefinitely, since transference of load favors 

 sinking of the floor of the trough, and the rising of the border- 

 ing lands. Under such circumstances gravel may accumulate 

 to thicknesses of thousands of feet and will bear variable rela- 

 tions with sands, clays, or adobe forming along the axis of the 

 geosyncline. All such deposits may be carried well below base 

 level and hence ma} 7 be preserved throughout several cycles of 

 erosion. 



Fault valleys, or graben, in which a narrow zone of land is 

 subsiding, furnish conditions for gravel accumulations compar- 

 able with geosynclines. If subsidence is long continued, over- 

 lapping gravel fans may reach thicknesses of hundreds or 

 even thousands of feet, and in the absence of regional uplift 

 and peneplanation may be preserved for indefinite periods. 



Climatic Fluctuations. — Changes in climate by modifying 

 discharge affect stream gradients and hence gravel deposition 

 in much the same manner as tectonic movements. Change 

 from a humid climate to an arid may result in the formation of 

 gravel deposits comparable to terraces incident to upwarp. 

 Decreased supply of water finds the grade of streams formed 

 under humid conditions too flat and gravel accordingly 

 accumulates near headwaters. Change of climate from arid to 

 humid produces opposite effects, for in such a case stream 

 gradients are found steeper than necessary for the effective 

 transportation of load ; gravels may be removed from head- 

 waters and the filling of concavities along stream may proceed 

 at rapid rates. So far as cyclical fluctuations in temperature are 

 concerned a change from temperate conditions to extremes of 

 heat or of cold tends to hasten the formation of gravel, and, as 

 shown by Barrell,* oscillations toward cold favor transporta- 

 tion of coarser materials. 



As regards the place of deposition of the coarser materials, 

 particularly along stream courses, the working rule that peb- 

 bles increase in size and degree of angularity toward their 

 source is subject to important exceptions. In fact, the only 

 conditions under which this hypothesis is wholly applicable are 

 equable climate, perennial streams, and the early stages of an 

 uninterrupted physiographic cycle. If the present arid climate 

 of the Plateau Province became humid, with evenly dis- 

 tributed rainfall and perennial streams, the gravels abundantly 

 * Jour. Geol., xvi, p. 381, 1908. 



