Ch. 24] RIVER AND STREAM DEPOSITS 449 



as aggregate through decomposition of the individual particles. Ter- 

 races in some parts of the Columbia River Basin, for example, were 

 formed at two different periods; in some localities the older and 

 younger terrace materials appear to occur in continuous sequence and 

 are not distinguishable through superficial observation, nor is the con- 

 tact between them clearly discernible. In the investigation of these 

 materials to determine their suitability for concrete, anomalous and 

 conflicting results arose from a failure to distinguish between the older 

 and younger materials. Subsequent studies disclosed that durable 

 concrete may be made of the firm and hard basaltic particles com- 

 prising the younger terrace deposit, whereas the older, underlying 

 material, which is similar in lithology and appearance, is composed of 

 incipiently decomposed particles unsuitable for use in concrete. Typi- 

 cal gravel terraces are shown in Fig. 2. 



Flood-plain deposits. Flood-plain deposits are characterized by 

 variable and heterogeneous gradations. True flood-plain deposits 

 caused by the periodic overflow of a stream from its normal channel are 

 usually deficient in material larger than sand and hence have only a 

 limited usefulness as concrete aggregate. Frequently such material 

 mantles the upper surfaces of flood plains but is underlain by hetero- 

 geneous assortments of sand and gravel deposited in former times as 

 the river meandered randomwise from side to side. When favorable 

 size gradations are found in flood-plain deposits, they are entirely 

 suitable for aggregate and possess the same general character as chan- 

 nel or terrace deposits. The flood-plain deposits shown in Fig. 2 are 

 mixed with terrace and alluvial-fan deposits. They occur more typi- 

 cally in the broad valleys of more mature rivers. 



Alluvial fans and cones. The principal characteristics of alluvial 

 fan or cone deposits, particularly in arid regions, are heterogeneity and 

 angularity. Since they were formed by successive torrential down- 

 pours with brief but violent runoff, and since the volume and velocity 

 of runoff differed from storm to storm, the deposits are usually rudely 

 layered into zones ranging from very fine material to very coarse ; and 

 the transportation, although frequently violent, is usually too quick 

 and too short for the development of rounded particles. Alluvial fans 

 and cones deposited by intermittent streams may contain material 

 suitable for the production of concrete but usually require elaborate 

 beneficiation, including screening, and possibly other measures such 

 as blending with imported materials to fortify certain deficient size 

 grades. 



The alluvial fans shown in Fig. 2 were deposited under semi-humid 



