WORK or JOSEPH BARRELL OX SEDIMENTATION 33 



or regioDal, is that surface toward which the external forces strive, the sur- 

 face at which neither erosion nor sedimentation takes place." ® 



Any change in the surface of the lithosphere that causes a change in 

 the position of the land surface or of the sea-bottom with reference to 

 baselevel will obviously produce corresponding changes in sedimentation. 

 Barrell states a principle accepted by most geologists, although he con- 

 sidered it desirable to present the evidence on which it is based, as follows : 



"The rate of denudation increases through the stage of topographic youth, 

 reaching a maximum when all of a drainage basin has become dissected and 

 given a maximum of sloping surface. From this mature stage the rate de- 

 creases as the elevation of the interstream areas are lowered, and finally 

 nearly ceases in topographic old age." ^ 



Firmly grasping this principle, the characteristics of continental de- 

 posits are discussed.^ He classifies these as follows : 



"Formations made upon the land may be classified under several divisions, 

 as follows : 



''Desert deposits. — Typically where the evaporation exceeds the precipita- 

 tion and no outflowing drainage results. 



'■'Piedmont river deposits.- — Built up by rivers or shallow lakes upon the 

 foreland plains or piedmont belt fronting high mountain ranges. 



"Basin deposits of pluvial climates. — The deposits laid down by rivers or in 

 lakes in down-warped basins, such as those of the Great Lakes, situated in 

 continental interiors, but not necessarily associated with mountains. If a 

 large river, laden with sediment, flows across such a region, a lake condition 

 can hardly arise, but, on the contrary, a broad river plain is more likely to be 

 found, constantly built up as subsidence takes place. 



"Sul)aerial delta deposits. — Where powerful and sediment-laden rivers meet 

 the sea, especially if the latter is shallow and protected from tides and storms, 

 a delta is rapidly developed, a considerable portion of which is a land surface 

 reclaimed by the river from the sea.^ 



Each of these classes of deposits is discussed with reference to its stage 

 in the physiographic cycle, but the conclusions can not be summarized in 

 this place. 



The effect of both orogenic and epeiric crustal movement on sedimenta- 

 tion are discussed in considerable detail, particularly in his "rhythms 



^ Khytlims and the measurements of geologic time. Bull. Geol. Soc. Am., vol. 28, 

 1917, p. 778. 



' Op. sup. cit.. p. 756. 



* Illustrative quotations are not in chronologic order, for it can be inferred from 

 Barren's writings that conclusions had been definitely reached and were being utilized 

 by him before he actually expressed them in the words best adapted for purposes of 

 quotation. 



'J Relative geological importance of continental, littoral, and marine sedimentation. 

 .Tour. Geol., vol. 14, 1906, pp. 328, 329. 



Ill — Bull. Geol. Soc. A.m., Vol. 34, 1922 



