[Illustration: FIG. 50--A maturely dissected highland: Santa Monica Mountains north of Santa Monica, Cal., as photographed from a height of nearly 10,000 feet at a midday in January, 1919. The light-colored irregular line at the left is Sepulveda Canyon; and the similar line at the right, Stone Canyon (for location, see Fig. 51). These mountains rise nearly 1,600 feet above sea level and about 700 feet above the bottom of the canyons. To obtain the proper impression of ridges and valleys the figure should be reversed. Such photographs as this of the actual ground can hardly be distinguished from photographs of good relief models; they strikingly confirm the correctness of this and similar methods of representing relief on maps, developed intuitively, as it were, such as the Swiss school of hill shading. Scale, about 1:17,000.] [Illustration: FIG. 51--Map of the region between the center of Los Angeles and Santa Monica, Cal., showing the location of the area covered in Fig. 50 (the double-ruled rectangle in the upper left corner). Reduced from the Santa Monica, Cal., sheet, 1:62,500, of the “Progressive Military Map” of the United States being published by the Corps of Engineers, U.S.A. This sheet, which is the equivalent of the Santa Monica topographic sheet surveyed in 1893 and published by the U.S. Geological Survey, was revised in 1920 by airplane photography. A comparison of the 1893 and 1920 editions brings out strikingly the rapid urban development in this region. Scale, 1:123,000.] [Illustration: 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.