358 J. BARRELL FLUVIATILE ORIGIN OF OLD RED SANDSTONE 



Lastly, from a larger point of view, shallow-water deposits, where of 

 great thickness and built of land waste, must very often be dominantly 

 terrestrial, especially on the side of the deposit toward the source of 

 supply. This may be seen by reflection on the broader physiographic 

 relations which attend them. If subsidence were the dominating regional 

 feature of the tectonic movement which results in erosion and sedimenta- 

 tion, then there would be a passage to lacustrine or marine conditions. 

 If, however, uplift in the regions of erosion dominates either areally or 

 vertically over subsidence of the regions of deposition, then the basins 

 will be kept filled to the level of the river grade; there will result an 

 excess of sedimentation. The rate of deposition in any section consisting 

 throughout of shallow-water beds is determined not by the rate of erosion, 

 but by the rate of subsidence. The excess of sediment will be carried 

 farther off, the balance being finally deposited in marine formations. 

 But in all except the most arid regions the great carrying agent is river 

 water. Where sedimentation is in excess of subsidence the deposits will 

 consequently bear usually the marks of fluviatile deposition- The condi- 

 tions of sedimentation can then be divided into two broad classes depend- 

 ing on the ratio of sediment to subsidence. There will only rarely and 

 temporarily occur that delicately balanced condition where subsidence 

 takes place at the same rate, but keeps slightly ahead of deposition, giving 

 rise to permanent yet shallow bodies of water. Nevertheless this balanced 

 state, maintaining a thin yet permanent cover of water, is the one usually 

 assumed to have existed during the Paleozoic when marks of shallow 

 water, of currents, and of exposure to the air are found through great 

 thicknesses of mechanical sediments. It is seen from this analysis that 

 such features must be much more commonty the marks of floodplains and 

 without any necessary relation to shores. 



The emphasis of discussion has been placed thus far on the distinctions 

 between fluviatile and lacustrine deposition. There remain to be con- 

 sidered criteria of another category — those which serve to distinguish the 

 fluviatile deposits of seasonally dry and semi-arid climates from the 

 dominantly torrential and wind-borne deposits of true deserts. 



In desert mountains there is a maximum of rock exposure. Eock- 

 breaking is due to sun and frost. The unweathered rubbish washes and 

 creeps down the slopes and is broken finer until it is within the reach 

 and greater power of the occasional cloudbursts. These sweep along 

 coarse and fine materials together, but soon lose power, and torrential 

 action, as marked by this heterogeneity, is confined to the perceptibly 

 steep slopes of streams within a few miles of the basin margin. 



.On the desert basin plains the wind is the chief agent of transportation. 



