The coast line, or margin of sea and land, is an area rapidly wearing
away under the ceaseless influence of the waves, and of the sand and
rock, they are perpetually hurling to and fro. Coasts may be either flat
or high, composed either of hard or soft rock, and either submerged or
raised. A submerged coast is one where the land has sunk or the sea has
risen, so that the low grounds and valleys are flooded. A raised coast
is one where the land has risen or the sea has retired, and what was
formerly the sea bottom is bared.
A flat coast is usually sandy, often bordered by sandhills and lagoons.
It may be carved into cliffs, as in the clay cliffs of Norfolk, England.
A raised coast is usually flat from the long-continued action of the
waves during the period when it was submerged. Flat coasts have no good
harbors.
A submerged coast differs according to the nature of the submerged
region. If this was hilly or mountainous, with valleys running parallel
to the shore, the coast will be ironbound and harbor-less unless the
sea-level has risen sufficiently to give access to the valleys behind
the first range of heights. If this happens, T-shaped gulfs are formed.
Where the valleys open at right angles to the sea, they become bays,
usually with excellent harbors. The hills between the valleys rise as
peninsulas, or islands. If the land was flat before submerging took
place, a flat coast is the result.
Where the land is composed of soft rocks, a more uniform coast-line
results than where it is composed of harder rocks, or of hard and soft
rocks mixed. The waves, in eating out the softer rocks, often form
magnificent sea-caves, natural arches, and pinnacles.