Ch. 34] CONSTRUCTION MATERIALS 647 



were utilized in the last war. An example of this is the location of 

 gravel during the Leyte campaign (Gilluly, unpublished manuscript) . 

 In this campaign, Gilluly reports the application of the following simple 

 rules to the location of gravels: (a) decrease in grade size of gravel, 

 from the upper reaches to the lower, of streams leaving the mountains 

 for the plains; (b) lateral variation of grade size away from the river, 

 with the coarsest gravel forming a low natural levee directly along the 

 stream; (c) the tendency of streams to deposit on the inside of mean- 

 ders, which led to location of at least three deposits in abandoned 

 stream channels, each buried beneath 2 or 3 feet of soil. 



Fortunately for the military engineer, his major construction proj- 

 ects are likely to be situated on lowlands, especially flood plains, where 

 the chances of obtaining sufficient supplies of suitable sand and gravel 

 are good. These source areas can be readily located by reconnaissance, 

 especially if they have been delineated in advance on aerial photo- 

 graphs or on geologic maps. 



An effective means of reconnaissance, although not particularly suit- 

 able for combat conditions, is the helicopter. One was used with signal 

 success in locating gravel pits in Quaternary terraces on the Tokyo 

 Plain, when the United States occupation forces were expanding Jap- 

 anese airfields in 1946 (personal communication from Allen H. Nicol, 

 Geologist, U. S. Geological Survey) . 



In addition to sand and gravel, crushed rock is a constant military 

 need. It is used especially for base course and for road metal. The 

 properties desirable in crushed rock are discussed in many texts (Leg- 

 get, 1939; Ries and Watson, 1936) ; suffice it to say that resistance to 

 wear is the most important. 



As the best crushed rock is of igneous origin, a full discussion of the 

 subject is not appropriate here. Well-cemented sandstones (especially 

 those in which the grains are not in contact), quartzites, and some 

 particularly resistant limestones make effective substitutes, and it has 

 sometimes been necessary to crush large boulders from coarse gravels. 

 The latter expedient is a particularly difficult one; because of the 

 selective process of water transportation, the boulders are likely to be 

 very hard and tough, and the relatively light crushers available to most 

 Army Engineer units are almost unable to cope with them. The use 

 of such materials was especially common in the tropics, where great 

 depth of soil weathering made location of quarries difficult and, in 

 places, seemingly impossible. 



The examples cited here emphasize two of the major difficulties of 

 the military engineer: the necessity for using substandard materials, 



