COASTAL MUD FLATS
(FIGS. 28 AND 29)
Of frequent occurrence along the Atlantic Coast of the United States are
low mud flats which are practically at sea level and which are covered
with water at times of high tide. Where these tracts are exposed to the
air during ebb tide for so short a time that plants have not taken root
and where the surface material is fine-grained and soft, the tracts are
known as mud flats. In the part of the peninsula between Delaware and
Chesapeake Bays belonging to the state of Virginia which is called the
Eastern Shore a low barrier beach of sand has formed on the ocean side
several miles off shore, and the space between this and the mainland is
occupied by mud flats, broad, shallow lagoons, and an intricate maze of
interlacing channels and winding, branching, interlocking, vermicular
streams.
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.
[Illustration: 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.]
[Illustration: 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.