some of their sand loads into deeper water. This causes elongated strips of sand
known as spits to build out from the shore.
There are three types of coastlines delineated with respect to sea level. They are
emergent, submergent, and neutral. Tectonic forces make land rise or fall and sea
level can rise and fall because of changes in ocean water volume. At this point in
Earthhistory we observe many submergent shorelines. This is because of the rise
of sea level resulting from the melting of ice of the Pleistocene glaciers. The east
coast of the United States is dominated by submergent coastlines with Chesapeake
Bay a prime example of a river system that was drowned by the ingress of ocean
water. Conversely, the Island of Arran in western Scotland is home to King’s
Caves, which were formed by erosion of water at sea level but are now several
meters above sea level as the result of the isostatic uplift of land resulting from
the unloading of the mass of Pleistocene ice.
Coastlines are not all inorganic. Coral coasts are common in shallow tropical
oceans. Coral polyps are simple, soft creatures a few millimeters tall and a few
millimeters in circumference. They live as huge groups of genetically identical
animals and excrete calcium carbonate, which makes up the hard substance com-
monly called coral. Besides providing support for the coral, the matrix of calcium
carbonate frequently becomes a reef-provided habitat for a rich diversity of life.
Humankind has always been drawn to coasts. We have built prolifically along
shorelines to sometimes disastrous effects. For instance, the population of U.S.
coastal counties has more than doubled since World War II. The building of housing,
towns, marinas, and golf courses has exponentially increased the chances for prop-
erty damage during storms and played havoc with some parts of the natural system.
By building on coastal sand, the local sand supply available to nature is effectively
lessened and beaches disappear because beach drifting far exceeds the local storage.
In the modern era, with good knowledge of the physical geography of coastlines, it
is unfortunate that we do not fully respect the power of nature along the coastline.
Comparative Advantage
First described in detail by the British economist David Ricardo in the early 18th
century, the theory of comparative advantage holds that it may be advantageous
for two countries to trade with one another, even when one holds an absolute ad-
vantage in the production of the traded goods. Comparative advantage is linked
to the notion ofareal differentiation, in that it is assumed that countries that are
potential trading partners have different allocations of natural resources, differing
levels of labor productivity, capital investment, etc.
Comparative Advantage 69