Ch. 24] WIND-BLOWN SAND 451 



or extremely fine gradations, and if screening and washing were done 

 thoroughly and with care. But the resulting concrete would be harsh 

 and difficultly workable because of the angular nature of the individual 

 particles. 



Talus Deposits 



Talus deposits formed by gravity at the bottoms of steep slopes are 

 usually composed of a single rock type or a limited number of rock 

 types — the types that occur on the upper part of the slopes — which 

 have accumulated as particles ranging from very fine to extremely 

 large, of angular and irregular shape, and with a minimum of orderly 

 distribution of the various sizes. Such deposits represent, in effect, 

 simply the original parent rock broken into fragments but otherwise 

 deposited with little of the rounding, sorting, or segregating actions 

 that characterize to a greater or lesser extent most of the other agencies 

 of sedimentary transportation. 



If the parent rock possesses characteristics suitable as concrete ag- 

 gregate, the talus deposits will usually be suitable but will require the 

 crushing and other beneficiation normally applied to quarried rock. 

 Inasmuch as the rock has been fragmented by natural processes, it 

 may on occasion be more economical to produce aggregate from a talus 

 accumulation than from the parent-rock outcrop, from which the frag- 

 ments would have to be produced artificially by blasting. In some 

 cases the spacing and pattern of joints in the parent rock have con- 

 trolled the maximum size of fragment that will form in a talus deposit, 

 and in this way economical production may be facilitated through 

 simplification of the plant installation required for crushing and re- 

 lated processing. 



Four talus deposits have been recently investigated as possible 

 sources of concrete aggregate: a granodiorite accumulation on the 

 slopes of Ragged Mountain north of Spring Creek Dam site (Colo- 

 rado) ; andesite porphyry on the slopes at Platoro Dam site (Colo- 

 rado) ; and rhyolite from Sundance Mountain and phonolite porphyry 

 from Missouri Butte (Fig. 4), both in the vicinity of Keyhole Dam 

 site (Wyoming) . It was found that these materials would make satis- 

 factory concrete but that, because of the expensive processing re- 

 quired, the importation of river gravel and sand from other areas would 

 be more economical. 



Wind-Blown Sand 



The extreme rigor of transportation by wind frequently results in 

 the preservation of only the hardest and most durable rock and mineral 



