Heretofore the study of beaches, deltas, and other partly submerged land forms has been chiefly confined to the exposed parts, the underwater forms being largely matters of conjecture. By means of air photographs not only can the exposed parts of the delta and beach be studied, but the forms of shoals and terraces, the underwater portions of river deltas, tidal deltas and their underwater distributaries, and many other submerged forms can be shown clearly. Sand bars, terraces, and other submerged forms appear in many of the photographs already presented; but a few so taken that the bars and terraces appear to be the chief objects in the picture may be useful for illustrating the underwater land forms and for demonstrating that these forms can be successfully photographed. Unfortunately not many photographs could be found which were taken with the express object in view of illustrating underwater land features. In most of the available photographs these features were only incidental, the chief purpose in taking them being to photograph the shore.
Much has been written concerning the physiographic history of the Atlantic Coastal Plain of the United States, and the question is still being debated whether the land is rising, sinking, or stationary. To some extent these questions are answered by the exposed land forms. The submarine forms are imperfectly known. The possibility of recognizing shoals and channels from a photograph and of determining in some measure the shapes of the submerged land forms opens a new avenue of approach to the study of submarine geography. In some places, especially in regions of drowned topography, it is possible that, by using the air photograph in working out the physiographic processes that have produced the land forms that are now under water, some of the vexing problems of earth history may be solved.
Fig. 30 (left)—Sand bars and drowned terrace about Stove Point Neck, at the mouth of the Piankatank River, Virginia, as photographed from a height of about 10,000 feet at 11:30 A.M., December 11, 1920. West (left) of the neck, at the outer edge of the terrace, the water is 2 to 3 feet deep at low tide, or 5.7 feet and 6.7 feet at high tide, but deepens abruptly westward, where it is 20 to 30 feet deep in Fishing Bay (see Fig. 32). To the south and east of the point the abrupt descent is at the side of the deep channel of the Piankatank River. To the right, the bottom, having a wavy appearance because of sand bars, fades off more gradually under deep water. The mottled area in the middle of the neck is wooded, and the smoother parts near the point and in the upper part of the neck are cleared land. Scale, about 1:30,000.
Fig. 31 (right)—Drowned terraces at Gwynn Island at the mouth of the Piankatank River, Virginia, as photographed from a height of about 10,000 feet at 11:30 A.M., December 11, 1920. At the right is a part of the island, showing trees, fields, and houses. Bordering the land area is a narrow band of light-colored beach sand, expanded at Cherry Point into a conspicuous sharply recurved hook. Under the shallow water can be seen wave marks resembling large ripple marks. The water is 2 to 3 feet deep at low tide at the outer edge of the light-colored submerged shelf, beyond which the bottom descends abruptly toward the left to a depth of about 20 feet. North of Cherry Point the waxy bottom shades off more gradually to the deep channel of the Piankatank. Scale, about 1:30,000.
The Best Conditions for Photographing Underwater Land Forms
Fig. 32—Part of the Kilmarnock and Mathews, Va., topographic sheets, 1:62,500, published by the U. S. Geological Survey, showing the location of Figs. 31 and 32; and a cross section along the line indicated on the map, showing a terrace 26 feet above sea level at the left, one less than 5 feet above water level on Gwynn Island, one 5 feet or less below water level; and the river channel with abrupt banks between the shoals. Scale, 1:70,000.