[Illustration: FIG. 64--A tidal delta in Shark River Inlet, Belmar, N. J., as photographed from a height of 10,000 feet, showing shoals at the left, which appear shadowy because they are under water, and the small channels which radiate outward from the narrow inlet like the ribs of a fan. They are the distributaries of this underwater delta. Scale, about 1:11,000.] USE IN COAST CHARTING [Illustration: FIG. 65--A tidal delta built up until it is partly above water: Popes Creek, Virginia, on the right bank of the lower Potomac River, 4 miles southeast of Colonial Beach, as seen obliquely from a height of 4,000 feet at 3:30 P.M., August 31, 1920. The tidal currents from the Potomac have built hook-shaped bars nearly across the outlet of the creek, and the inflowing currents have built a delta from the mouth of the creek upstream.] It is fortunate for those engaged in the study of shore features and the mapping of coasts that, being flat, shore features are particularly well adapted to representation by air photographs, for on coasts exposed to the wind and waves the channels, shoals and bars are continually changing. Air photography offers a quick and convenient means of keeping charts up to date. The intricacies of the water line in some places makes accurate charting by the ordinary survey methods a slow, laborious process. When bluffs or relatively steep slopes, like those of York River, Virginia, near Gloucester Point, shown in Figure 60, [Illustration: FIG. 66. (For explanation, see next page.)] [Illustration: FIG. 66--A double tidal delta at Barnegat Inlet, New Jersey, as photographed from a height of 10,000 feet. To the east (right) the breaking waves and shadowy depths indicate the position of shoals. West of the surf belt are the light-colored beach sand, shading off from the conspicuous hook at the southern end of Island Beach into underwater shoals and bars, and older surfaces made dark-colored by the growth of plants. South of the hook are the inlet leading into Barnegat Bay and the northern end of Long Beach, at the point of which stands a lighthouse whose long shadow is to be seen across the beach sand. The mottled appearance of the bay to the left is due to shoals slightly submerged or perhaps exposed at low tide, where dark-colored drainage lines appear, and shading off to deeper water, where the submerged land forms have a shadowy appearance. The distribution of the shoals indicates that this is a double tidal delta, an infacing part west of the inlet and an ocean-facing part to the right. Scale, about 1:17,000.]