SAND AND GRAVEL 



563 



and gravel will result from uneven distribution of 

 deposits, depletion of local deposits, urban encroach- 

 ment, increased cost of property acquisition, and 

 higher transportation costs (Sheridan, 1967). 



Goldman (1966, p. 369), an authority on sand and 

 gravel in California, stated that "there are no un- 

 discovered deposits near the metropolitan areas that 

 can be developed to meet demand * * *. The State 

 faces depletion of its major sources within the next 

 3 decades unless sand and gravel deposits can be 

 set aside as natural resource zones for future use." 

 Thus, it may become necessary to import sand and 

 gravel from beyond the present economic limit of 

 50 miles. Consequently, we are faced with the task 

 of correctly and effectively utilizing the known de- 

 posits. This will, in large part, be controlled by the 

 zoning and planning officials of city governments. 

 The geologist's role, therefore, may be merely to 

 draw attention to the limits of the deposits and to 

 point out that these deposits are the total resource 

 existing within the economic radius of the popula- 

 tion center. 



Crushed rock materials other than the conven- 

 tional unconsolidated river and glacial deposits are 

 potential sources that should be considered; how- 

 ever, they have been neglected for specific reasons. 

 Hard-rock mine dumps are potential resources of 

 aggregate; however, although such material has 

 had some use as road base, it is generally unsuitable 

 for concrete aggregate because it lacks proper size 

 and gradation and contains harmful sulfide min- 

 erals (Goldman, 1956). Artificially crushed stone is 

 also useful for road base where angular shape is 

 desirable, but when crushed stone is used in con- 

 crete, more cement is required, and more care is 

 necessary in mixing and pouring than with conven- 

 tional gravel. In the Tucson, Ariz., area, however, 

 it has been forecast that "probably within 10 years 

 coarse aggregate will be supplemented by broken 

 and crushed rock quarried from nearby mountains" 

 (Williams 1967). 



Dredge tailings generally consist of coarse gravel 

 overlying fine-grained material, from which it is 

 costly to obtain a "good blend"; however, many of 

 these deposits are an excellent source of coarse 

 aggregate. 



Pre-Quaternary rocks may be potentially useful 

 but are generally too firmly cemented or contain too 

 much weathered clayey material (Goldman, 1956). 



Marine and lake environments may offer some of 

 the best potential source areas. Such environments 

 should be target areas in a search for large blanket- 

 type aggregate deposits. Much of the continental 

 shelf from New England to New Jersey is mantled 



by sand associated with lesser amounts of gravel 

 "* * * in a quantity probably sufficient to constitute 

 an economic asset" (Schlee, 1968). As the popula- 

 tion continues to increase along the east coast 

 (Washington, D.C.-Boston), demand for offshore 

 supplies of sand and gravel will markedly increase 

 (Emery, 1966). 



The underwater Great Lakes sand and gravel de- 

 posits continue to grow in importance as upland 

 sources diminish by depletion, zoning, and high 

 transportation costs. Large quantities of high-grade 

 aggregate are already being extracted in western 

 Lake Erie and along the Lake Ontario and Erie 

 shorelines (Woodrov/ and others, 1971). 



EXPLORATION TECHNIQUES 



Surface geologic mapping is perhaps the simplest 

 and most common method by which sand and gravel 

 deposits are located. Data are gathered by geological 

 examination of streambanks, roadcuts, excavations 

 for building foundations, and any other natural or 

 manmade exposure of surficial material. With the 

 aid of aerial photographs and topographic maps, 

 and a knowledge of the types of landforms com- 

 monly underlain by sand and gravel, the geologist 

 can pinpoint potential resource areas for detailed 

 field examinations. After a deposit is located, the 

 geologist must determine thickness, lateral extent, 

 possible tonnage, and overburden depth, and he 

 must collect representative samples for specific de- 

 terminations. Records of drill holes from oil, gas, 

 or water wells can be used when available to give 

 valuable information in the subsurface dimension, 

 particularly pertaining to the thickness of sand and 

 gravel deposits. 



In the absence of drilling data, information about 

 the depth and lateral extent of sand and gravel can 

 be obtained with the aid of several different geo- 

 physical tools. Seismic refraction is a particularly 

 sensitive method for determining depths and thick- 

 ness of sand and gravel in certain geologic situations 

 (Peterson and others, 1968). This method works 

 best where less dense materials (sand and gravel) 

 overlie denser materials (bedrock or fill). If a very 

 dense overburden overlies an aggregate source, the 

 presence of the aggregate is not likely to be deter- 

 mined by use of the seismic method because the 

 shock waves will be diffused downward into lesf 

 dense material. In areas of shallow bedrock, shallow 

 seismic devices employing a sledge hammer or fire- 

 crackers rather than a dynamite charge can be used 

 (Criner, 1966). 



Another geophysical method used to determine 

 thickness of sand and gravel is electrical resistivity. 



