[Illustration: FIG. 33--A drowned valley: Lambs Creek, 8 miles southeast of Yorktown, Va., one of the estuary-streams tributary to Chesapeake Bay, showing the broad mouth narrowing upstream and the irregular margins caused by partial submergence of the valley slopes, eroded before the rise of the water to its present height. Even the vertical photograph, which does not register relative elevations, shows a distinct difference between the shore line of this type of body of water and rivers with broad, low flood plains. The large trees close to the margin of the river and the cultivated fields just back of them indicate a relatively high bank. Scale, about 1:9,000.] times he can see with great distinctness. In trying to ascertain the most favorable conditions for such observation, it was found that submerged objects are seen best when the sky is evenly overcast or when it is uniformly clear. Sometimes when the sky is only partly cloudy the surface of the water seems to act as a mirror and nothing is seen but the reflection of cloud and sky. Waves have less effect on the visibility of objects beneath the surface than was expected, although they diffuse the reflected light to some extent and consequently weaken the image on the negative. But the reflected light from the surface of the water is stronger than that coming from objects under water. Hence, to photograph underwater features successfully, a time should be chosen when direct reflection of light from the sun or from a brightly illuminated cloud will not enter the lens. Experience in both the air and the laboratory shows that the best results are likely to be obtained when the sunlight strikes the surface at an oblique angle. In summer favorable times are mid-forenoon or mid-afternoon under an evenly illuminated sky. In winter the sun is low enough at midday to avoid direct reflection into the lens. But experience also indicates that often photographs taken at moments when the eye caught the image of a submerged object show only the surface of the water. CHAPTER IX THE PLAIN FROM THE AIR (FIGS. 34 TO 41) A RIVER ON THE GREAT PLAINS The difficulty of photographing a plain from a point on its surface needs no emphasis, but its successful representation by means of air photographs is illustrated by many figures in this book. The Great Plains of the west-central part of the United States are illustrated here by a view of the Red River (Fig. 36), which shows the flat surface of the land and the broad sandy bed of the river only partly covered by the intricately woven strands of the braided channels--a scene characteristic of the Great Plains. MEANDERING STREAMS ON THE COASTAL PLAIN