144 CONSTRUCTION SOURCES OF GRAVEL, MINTZER 
indicates to an interpreter that glaciated gravel terrain may be expected near by. Where 
a valley-train formed stream terrace, or outwash plain exists in the area, it is likely that 
the area will supply a source of gravel. Granular materials are generally not too far 
distant from rocky hills or mountains, which were associated with water cutting and 
deposition. The interpreter with the areal concept, the elements of the pattern and the 
mental image induced, finds the identification for a given airphoto pattern. 
Photo interpretation techniques. 
In general, the literature shows, and this author's experience confirms, that the areal 
concept method can be applied to almost any geographic or physiographic region, but that 
the success of locating suitable sources of gravel is primarily dependent upon the engi- 
neering training and photo interpretation experience of the investigator [2]. An expe- 
rienced interpreter uses his knowledge of surface geology and general terrain for a given 
area. A knowledge of geology makes it possible for each landform to take on a special 
meaning to the engineer. Geologically the composition of the soil within the landform has 
certain characteristics because of the way it was deposited. By drawing upon field and 
laboratory experience in geology and pedology and integrating this knowledge engineer- 
ing soil surveying experience, the engineer interpreter has an approach to identifying soil 
or aggregate conditions in advance of construction This thinking process is the “areal 
concept” of soils engineering. Thus, as the engineer applies the areal concept of the ter- 
rain his study gives him knowledge of what to expect to find in the vicinity of a given 
construction site. 
The areal concept is based primarily on recognizing a given landform. The landform 
is readily apparent from a photo mosaic and subsequent stereoscopic study. Once a land- 
form is identified, the underlying textural characteristics of the soil may be inferred. 
The shades of photo-gray tone, the streamflow patterns, the gully shapes and gradients, 
the cultivation patterns, and even in some cases the type and manner of growth of the 
vegetation give the interpreter knowledge to infer the soil texture. 
The aim of the photo analysis method is to prepare a provisional map of the area 
surveyed. A likeness in landforms represents a likeness in the kind of soil or aggregate 
to be expected. This common likeness is used as the guidance system for the analysis. The 
analysis is based on the recognition of objects, conditions, and patterns associated with 
common conditions of a given terrain. Each separate pattern is recognized and outlined 
on the photo mosaic. The patterns so delineated are grouped as mapping units; the units 
are then assigned a provisional classification system according to the inferred properties 
of the photo pattern representative of a soil unit. These mapping units represent soil 
associations. All patterns associated with the terrain are then analyzed in terms of ele- 
ments. The analysis sifts the data on the likeness of various elements, which in turn as- 
sociate themselves with the identity of a given photo pattern; thus the inferred soil type 
is induced from the photo study. After a ground control check is made engineering char- 
acteristics of the soils or aggregates represented by a photo pattern are confirmed. The 
soil or aggregate site mapping is then completed by drafting in the symbols representative 
of ground conditions. 
In the investigations of the terrain for the location of suitable sources of gravel 
certain techniques are helpful. The following procedure is economical and general yields 
good results: 
preliminary study; 
placing of orders for photographs; 
mosaic and stereophoto study; 
aggregate source site delineation on photos and highway itinerary map; 
carrying out field reconnaissance by aircraft; 
ground procurement of field samples and possible mineral options; 
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