Fig. 52—A young mountain gorge showing an erosional hollow developing headward into the less deeply eroded highlands: San Joaquin Hills, a coastal range in Southern California about 45 miles southeast of Los Angeles, near the mouth of Aliso Creek. North is at the left (see Corona, Cal., topographic sheet). Scale, probably about 1:10,000.
CHAPTER XI
AIR CRAFT IN THE STUDY OF ROCKS AND ORES
(Fig. 53)
The admirable manner in which air photography lends itself to the observation of geographic relations and physiographic processes suggests its use as a valuable addition to the instruments of geologic reconnaissance; for, not only is the study of geology inseparable from that of physiography, but, in large measure, geology is applied physical geography and many conclusions of a geologic nature are drawn from observed surface relations.
Probably, in most cases, the actual character and composition of rocks cannot be determined from air photographs; but, just as on a good map an area of crystalline rocks can be distinguished from one of sedimentary rocks by means of the topographic expression, so areas of different rocks can be distinguished on photographs. For instance, an area of upturned sedimentary rocks would be readily distinguished from one of horizontal rocks. Figure 42 shows how the character of glaciated mountains is revealed, and Figures 37 to 41 of the Michigan area show well the familiar features of continental glaciation.
It is perhaps premature to say much of the use of the airplane in the study of geology until it has been thoroughly tested. But it should be possible from the air to locate and map ore bodies, metalliferous veins, and outcrops of rock: for it is well known that rocks at the outcrop differ in color, in the forms of erosion developed in them, and in the kind of plants which they support. It is of interest that Colonel Alfred H. Brooks, who was Chief Geologist of the American Expeditionary Forces in France during the war, found that geologic boundaries could be recognized on air photographs and that by means of these photographs he could correct existing geologic maps and identify
