Ch. 24] RIVER AND STREAM DEPOSITS 445 



during transportation, or through the roughening of surface textures. 

 But the strength of particles may be reduced by leaching or by the 

 loosening of initial microfractures through impact. The overall grad- 

 ing may improve downstream through the abrasive reduction of coarse 

 sizes and the fortification of finer sizes which might otherwise be de- 

 ficient in quantity, but the same processes may impair the quality of 

 the gravel for use as concrete aggregate through so stringent a re- 

 duction of the coarse particles as to leave the gravel deficient in the 

 larger sizes required for economical concrete design (ASTM, 1948b). 

 Quality may be impaired through mixing with unsuitable rock types, 

 but .similar processes may be beneficial if inferior materials are "di- 

 luted" by the addition of sounder material from tributary streams. 

 Usually improvement of certain characteristics accompanies impair- 

 ment in others. In any event the end result will be the summation of 

 effects of the various mechanisms of transportation and deposition as 

 they impinge on the different rocks and minerals — with their various 

 responses and reactions — that are supplied by the parent source. 



TYPICAL OCCURRENCES OF NATURAL AGGREGATES 



Potential sources of natural aggregates may be classified as river 

 and stream deposits, talus deposits, wind-blown sand, glacial deposits, 

 marine deposits, and residual deposits. A given area may contain 

 several of these types of deposits or may contain only a few or no 

 more than one. Geographical location strongly influences the type of 

 deposit available for aggregate production and the character of aggre- 

 gate in general use in any region (ASTM, 1948d; Thoenen, 1932). 



River and Stream Deposits 



Deposits in river channels. Deposits of sand and gravel occurring 

 in channels of rivers and streams will be characterized generally by 

 rounded particles distributed with some uniformity throughout the 

 various size grades. Although large differences in gradation occur from 

 deposit to deposit, as well as differences in the maximum size repre- 

 sented, deposits in the channels of gravel-bearing streams can usually 

 be found which will satisfy the gradation requirements necessary for 

 concrete design or can be made satisfactory in this respect by only 

 moderate processing. Figure 1 indicates a gradation recommended by 

 the American Society for Testing Materials (ASTM Designation C33- 

 46) . Superimposed on the same figure is a curve showing the grada- 

 tion of natural sand and gravel obtained from the North Platte River 

 near Kortes Dam site. In this instance, a gradation suitable for the 



