Ch. 24] SIGNIFICANCE OF "SOURCE MATERIAL" 443 



by the processes of transportation and deposition, as, for example, 

 through mixing and dilution with other rock types that are non-re- 

 active; or a rock originally innocuous may become reactive by the ad- 

 dition of secondary opal or chalcedony as coatings or intergranular 

 cement during sedimentation. 



Many different geological formations may contribute to a sediment- 

 ary deposit and supply a large variety of rock types representing many 

 different physical and chemical "source" conditions. Table 1 indicates 

 the complex composition of a gravel deposit on the upper Missouri 

 River. Such mixing of materials is most pronounced and complicated 

 in the sediments of rivers draining large basins. Although a sand and 

 gravel deposit occurring near the head of a river may contain a limited 

 number of rock types, the complexity of the mineral and petrographic 

 assemblage will increase downstream as each successive tributary con- 

 tributes the rock types available within its own watershed. Con- 

 tributions of sound rock by a tributary to a stream whose sand and 

 gravel load is mainly composed of inferior material will naturally be 

 beneficial; but one major tributary contributing a large amount of 

 inferior or deleterious material may render deposits farther down- 

 stream less suitable as concrete aggregate (Spain and Rose, 1937). 



This relationship is well illustrated by the Colorado River between 

 Hoover Dam and Parker Dam. Sand and gravel obtained upstream 

 in the vicinity of Hoover Dam contain a complex assemblage of rock 

 types, some of which are susceptible to alkali-aggregate reaction. 

 However, the quantity of reactive types at this point is small, and 

 Hoover Dam concrete containing this aggregate exhibits no distress 

 from alkali-aggregate reaction after 20 years of service. The Colorado 

 River below Hoover Dam traverses terrain that is predominantly vol- 

 canic, and its sand and gravel become progressively enriched in deleteri- 

 ous volcanic rocks. At Davis Dam, 67 miles downstream, the sand 

 and gravel are sufficiently reactive that special measures were re- 

 quired (use of low-alkali cement and pozzolanic admixtures) to fore- 

 stall alkali reaction in the concrete. Farther downstream (156 miles 

 below Hoover Dam) Bill Williams River contributes copious amounts 

 of highly reactive aggregates — mainly andesites and rhyolites. Parker, 

 Gene Wash, and Copper Basin dams built in this vicinity, and using 

 local aggregate and high-alkali cement, exhibited extreme evidences of 

 alkali reaction within 2 years after their completion. 



Any initial characteristic of a source material may be either im- 

 proved or impaired by the rigors of sedimentary transportation and 

 deposition. For example, as previously noted, improvement can result 

 through the elimination of soft particles or the rounding of particles 



