Another experiment was made by the Coast and Geodetic Survey off the coast of Florida, where the water is clear, in an attempt to photograph “the small coral heads and pinnacle rocks” which may be disastrous to boats. The report states that the results were unsatisfactory and concludes that airplane pictures are useful in “aerial photo-topography” but not in [Illustration: FIG. 76--Channels, shoals, and terraces in a drowned river valley: Roberts Creek, 9 miles southeast of Yorktown, Va., an estuary tributary to Chesapeake Bay. Under the water in the drowned valley is seen the channel, “braided” in some places, and extending out through sand bars to the deep water of Poquoson River shown at the top (north) of the illustration. At the left the dark-colored forest area is fringed with a narrow strip of white beach sand, then with a belt of shallow water at the outer edge of which the waves form an irregular white line, and beyond this with a belt which represents a submerged terrace. Scale, about 1:14,000.] [Illustration: FIG. 77--The underwater channel in Quantico Bay. Quantico Creek is a tributary of the Potomac River entering from the Virginia side, about 28 miles downstream from Washington. The “Bay” is the widened part of the stream at its entrance into the Potomac, which is entered by the tide. Forested land appears at the right and left, with light-colored fields at the left. The space between the wooded areas is occupied by shallow water, beneath which appears the relatively deep channel, which forks after the manner of streams. The branching channel is wholly under water and differs in some respects from the channel of a normal stream. Among the more obvious peculiarities is the “fanning out,” of the headwaters. Photograph taken from a height of about 18,000 feet. Scale, about 1:21,000.] [Illustration: FIG. 78--Natural channels and shoals near Miami, Fla.: an oblique view eastward to the ocean across Bear’s Cut, which is situated between Biscayne Key and Virginia Key. Because of the greater depth of water, the channels with a maximum depth of 17 feet appear darker than the shoals. The photograph was taken at a height of 3,000 feet, April 20, 1918.] [Illustration: FIG. 79--A dredged channel at Miami, Fla.: an oblique photograph showing part of the city, the boat landing, the sandy beach, and the shallow water, through which a straight ship channel has been cut, which appears dark-colored in the illustration because of the greater depth of water. The photograph was taken April 22, 1918, from 21 height of 3,000 feet.] [Illustration: FIG. 80--A shoal in Hereford Inlet north of Wildwood, N. J., as seen from a height of 10,000 feet. The breaking waves indicate that the shoal is only slightly submerged. (Tidal variation is here about 4 feet.) The sand spit north of the shoal, the south end of Sevenmile Beach, like other spits of the New Jersey coast, is building southward and may in time annex the shoal. Scale, about 1:13,000.]