to within a few feet of high
tide, they inclose it with a stout barrier or dike, which has openings
for the free admission of the tidal waters. Entering this basin, the
tide, moving with considerable velocity, bears in quantities of
sediment. In the basin, the motion being arrested, this sediment
falls to the bottom, and serves to raise its level. In a few months
the sheet of sediment is brought near the plane of the tidal movement,
then the gates are closed at times when the tide has attained half of
its height, so that the ground within the dike is not visited by the
sea water, and can be cultivated.

[Illustration: Fig. 19.--Map of Ipswich marshes, Massachusetts, formed
behind a barrier beach.]

Along the coast of New England the ordinary marine marshes attain an
extensive development in the form of broad-grassed savannas. With this
aspect, though with a considerable change in the plants which they
bear, the fringe of savannas continues southward along the coast to
northern Florida. In the region about the mouth of the Savannah River,
so named from the vast extent of the tidal marshes, these fields
attain their greatest development. In central and southern Florida,
however, where the seacoast is admirably suited for their development,
these coastal marshes of the grassy type disappear, their place being
taken by the peculiar morasses formed by the growth of the mangrove
tree.

In the mangrove marshes the tree which gives the areas their name
covers all the field which is visited by the tide. This tree grows
with its crown supported on stiltlike roots, at a level above high
tide. From its horizontal branches there grow off roots, which reach
downward into the water, and thence to the bottom. The seeds of the
mangrove are admirably devised so as to enable the plant to obtain a
foothold on the mud flats, even where they are covered at low tide
with a depth of two or three feet of water. They are several inches in
length, and arranged with booklets at their lower ends; floating near
the bottom, they thus catch upon it, and in a few weeks' growth push
the shoot to the level of the water, thus affording a foundation for a
new plantation. In this manner, extending the old forests out into the
shallow water of the bays, and forming new colonies wherever the water
is not too deep, these plants rapidly occupy all the region which
elsewhere would appear in the form of savannas.

[Illustration: Fig. 20.--Diagram showing mo