The mud flats are exposed for a short time during low tide, and, as the surface of the water here rises and falls with the tide more than 4 feet, with a maximum fluctuation considerably greater, large volumes of water are continually flowing backward and forward over the flats. As the tide rises, strong currents of sea water set in through the inlets, flow up the main channels and through the thoroughfares, and gradually find their way into the countless small channels and out of them over the broad level stretches of soft mud. As the tide falls, this action is reversed, and the broad sheet of water finds its way by devious paths through the winding watercourses out to sea. The larger channels extend considerably below the surface at times of highest water and may be quite deep even at times of low water. They are, perhaps, stream courses excavated before the region was drowned. Many of the smaller channels also have the general form characteristic of normal stream channels, although others show peculiarities not common to subaerial drainage. The origin of these submarine and tidal features is not well understood, but the photographs of them show their form and furnish some basis for a study of them.
Fig. 28—A stream system of the mud-flat area on the ocean side of the Eastern Shore, Virginia. The light-colored area is beach sand above water. The treelike form is a stream system of subnormally developed pattern. Note the seeming uncertainty of course, some of the branch streams rising close to the mouth of the trunk stream; the junction of branches at the head; and the “frostwork” patterns. Scale not known.
Fig. 29—Mud-flat streams, showing curious frostwork pattern at the head of underwater channels. Note the pools and the veinlike drainage lines from them. Scale not known.
The photographs reproduced as Figures 28 and 29 were taken northeast of Cape Charles, Virginia, in the summer of 1920 at low tide. The light-colored ribbon-like bands represent water-filled channels; and the darker-colored areas, either wet mud exposed to the air or mud slightly submerged. However, photographs taken under certain conditions of light may show the exact line between the exposed and the drowned portions of a land surface.