Ch. 24] MARINE SANDS AND GRAVELS 455 



undersized material frequently involves the handling of much excess 

 material for the production of a given quantity of aggregate. In 

 deposits that contain large boulders the expense of processing is fur- 

 ther increased. Where crushing of the oversized material is necessary 

 to augment finer size grades, the expense is again increased. 



Large accumulations of glacial erratic boulders may sometimes be 

 used for concrete aggregate. In such cases the boulders are collected, 

 crushed and screened in the same manner as though they were quar- 

 ried rock. This procedure is economically feasible only in exceptional 

 cases. 



In contrast to true glacial deposits, glaciofluvial deposits are widely 

 used as concrete aggregate. Only moderate reworking of glacial mate- 

 rials by fluvial agencies may suffice to form deposits comparable to 

 true river sands and gravels. 



Marine Sands and Gravels 



Marine sands and gravels are typically characterized by hard, firm 

 particles as a consequence of the extreme natural selection of the 

 durable and elimination of the non-durable materials consequent to 

 the winnowing action by waves and currents on a beach. The particles 

 usually are very well-rounded and restricted to a narrow range of size 

 grades. Marine sands and gravels are not widely used as concrete 

 aggregate, but where available in sufficient thickness and extent for 

 economical production and where proper gradations may be obtained 

 by no more elaborate processing than is economically feasible, ag- 

 gregate of excellent quality can be produced. These conditions may 

 be found in thick terraces lying above the level of the present shore. 

 Such deposits not uncommonly exhibit vertical variations of grain size 

 from layer to layer, but selective excavation may be feasible by which 

 is removed material which possesses an approximation of the grada- 

 tion required, thus minimizing the amount of artificial processing that 

 must be done. 



Beach sands frequently exhibit characteristics similar to wind-blown 

 sand, or indeed may owe their origin as much or more to the action 

 of the wind as to the action of ocean waves and currents. These mate- 

 rials, like wind-blown sand, discussed previously, are suitable for use 

 as blending sands in combination with other aggregates. 



Sands and gravels occurring on beaches may be impregnated with 

 deleterious salts and thus require rigorous washing. Such salts may 

 have been entirely removed from older terrace deposits through leach- 

 ing by surface and subsurface waters. 



