Ch. 24] SIGNIFICANCE OF "SOURCE MATERIAL" 441 



Sorting incident to transportation and deposition. This action con- 

 trols the grading of the sand and gravel, subject, of course, to the 

 gradation of sizes furnished at the source, and it usually governs the 

 maximum and minimum sizes of particles occurring in the final deposit. 

 This action also influences the variability of the deposit, its stratifica- 

 tion, and the thickness and kind of overburden. 



Weathering, leaching, and chemical reaction. Through this action 

 the strength or durability of the particles may be decreased, or the 

 absorption increased through the leaching of soluble constituents or 

 the chemical decomposition of vulnerable ingredients. Undesirable 

 particles, if soluble or reactive, may be eliminated through solution, 

 leaching, or chemical reaction; or the same processes may cause the 

 formation of coatings about aggregate particles which may be in- 

 nocuous if they are hard, chemically stable, and firmly adherent, but 

 which may be deleterious if soft, chemically reactive, or loosely bonded. 

 By a further extension of the same process particles may be cemented 

 in a manner to hinder the economical exploitation of a deposit. Clay 

 or other impurities may be formed as a consequence of the chemical de- 

 composition of certain minerals (for example, feldspar) ; such clay 

 may remain within the individual particles (contributing to volume 

 change through wetting and drying) or accumulate as a separate com- 

 ponent of the deposit and contribute to an excess in the very fine size 

 grades. 



Significance of "Source Material" 



The final properties of a sediment, including those properties im- 

 portant in concrete aggregate, are strongly influenced by the kind and 

 condition of the source material. Different source materials will be 

 affected differently by the various sedimentation processes, and a sedi- 

 ment usually exhibits vestiges of initial properties — inherited char- 

 acteristics, variously preserved and variously modified — as well as 

 those superimposed during sedimentation. 



The properties contributed by the parent rock are those related to 

 the mineralogic and petrographic composition of the rock or its initial 

 physical and chemical condition (ASTM, 1948a; Bureau of Reclama- 

 tion, 1949; Rhoades and Mielenz, 1946). For instance, the strength or 

 the elasticity of an individual particle of sand or gravel is primarily 

 determined by its mineralogy and the textural relationships of the 

 component minerals. The initial mineralogy and texture may be 

 altered slightly or profoundly during transportation and deposition, 

 but they will be preserved in some degree unless the particle is wholly 

 decomposed or disintegrated; moreover, the initial mineralogy and 



