Ch. 24] RIVER AND STREAM DEPOSITS 447 



derived and to the distance the material may have been transported. 

 For example, the comparatively hard and firm gravels of the Colorado 

 River above the mouth of the tributary Dolores River, in the vicinity 

 of Dewey Dam site (Utah) contrast sharply with the gravels down- 

 stream from the mouth of the Dolores, because of the large quantity 

 of soft sandstone contributed to the main stream by that tributary. 

 Farther downstream the soft Dolores sandstone is progressively re- 

 duced in quantity. 



Improvement of surface texture also results from the attrition, im- 

 pact, and abrasion incident to river transportation, so that river gravels 

 will normally possess rugose surface textures conducive to good bond 

 with cement. However, if large quantities of the pebbles that are 

 furnished to the stream possess potential planes of breakage, as along 

 cleavages, the impact and jostling during transportation may promote 

 the division of larger pieces into smaller ones with comparatively 

 smooth surfaces. 



It must not be overlooked that the environment of river transporta- 

 tion and deposition is conducive to leaching of soft or soluble materials 

 or to chemical reaction with materials susceptible to decomposition. 

 These processes can aid the processes of mechanical attrition in promot- 

 ing the selective concentration of hard and firm particles, but in other 

 cases they can cause softening or weakening of particles that would 

 be relatively invulnerable to mechanical attack alone. 



The field occurrence of river-channel deposits on the upper Missouri 

 River is illustrated in Fig. 2 (see also the petrographic analysis, Table 



1). 



River terraces. River-terrace deposits possess the general character- 

 istics of river-channel deposits and are widely used for concrete aggre- 

 gate. Terrace deposits, in many places, are exploited more economi- 

 cally than channel deposits, since, because of their higher topographic 

 positions, they are normally not so subject to the production diffi- 

 culties attending shallow ground water. In some areas, as along cer- 

 tain reaches of the Colorado River in Texas, the rivers themselves are 

 not now transporting gravelly material, but gravels may, nonetheless, 

 be found in terraces mantling the highlands where they were deposited 

 by gravel-bearing streams of earlier times. 



Terrace deposits commonly, although not universally, differ from 

 channel deposits in exhibiting a greater prevalence of secondary coat- 

 ing or cementation of particles. Coatings resulting from the action of 

 ground water, or waters infiltrating from the surface, may permeate 

 the entire thickness of a deposit, but commonly (particularly in arid 

 regions) they are associated with transpiration of moisture toward the 



