494 H. E. Gregory — Formation and Distribution of 



strands may be tens of miles; and if a group of parallel streams 

 are working under similar conditions the gravels may extend 

 laterally for greater distances. The length of the overlapping 

 and branching gravel lenses will be determined by the net 

 result of all the agents concerned with the transportation and 

 deposition of coarse material. 



Observations of modern rivers indicate that gravel in 

 quantity is carried 30 to 40 miles by streams of high gradient 

 and under favorable conditions portions of a gravel bank may 

 be transported a few hundred miles. Thus 60 per cent of a 

 gravel bar examined on the ITrubamba River, Peru, consisted 

 of pebbles up to 3 inches in diameter which had been ferried 

 down stream for a distance of 33 miles. Twenty per cent of 

 the pebbles and cobbles in certain bars along Chinli Creek in 

 Arizona have their source 36 miles distant, and scattering 

 pebbles were found 50 miles from their parent ledge. In front 

 of the Rocky Mountains gravel is distributed for distances of 

 300 miles, and pebbles in the Nile delta " have been river- 

 borne for at least 400 miles."* Grabauf states that extensive 

 deposits of river gravels in the Triassic of England are found 

 300 miles from their source and that pebbles of the Pottsville 

 conglomerate have made a journey of 400 miles. 



Distribution and Preservation of Gravel during Interrupted 

 Cycles. 



Tectonic Interruptions. — Within a single cycle, as discussed 

 above, climate is assumed to be substantially uniform, and the 

 earth's crust stable. The effect of regional or local downwarp 

 or upwarp with constant volume of water is to modify all fac- 

 tors concerned with the distribution and preservation of gravel. 

 In consequence of crustal movement the stream's gradient and 

 consequently carrying power is changed ; the area from which 

 gravel is collected may be increased or decreased in size and 

 the area over which gravel is deposited may be modified as to 

 extent and position. Since with general regional uplift streams 

 begin to deepen their valleys and to localize their channels, 

 gravel is left on terraces and on interstream spaces. The thick- 

 ness of such deposits is obviously some fraction of the amount 

 laid down during the previous cycle. With the development 

 of drainage the removal of these fringing terraces is favored 

 and the gravel stands little chance of preservation. General 

 regional downwarp results in flattening of stream profile since 

 deposition is facilitated on flood plains in the lower portions of 

 rivers while the headward portions of the valleys are still sub- 

 jected to erosion. On such over-flat profiles gravels tend to 



* Wade, Quart. Jour. Geol. Soc, vol. lxvii, 1911. 

 \ Principles of Stratigraphy, p. 594. 



