This section is from the book "An Introduction To Geology", by William B. Scott. Also available from Amazon: An Introduction to Geology.
As ancient structures on long-inhabited coasts sometimes show elevation, they likewise sometimes show depression. On the north coast of Egypt ancient rock-cut tombs are now visible beneath the waters of the Mediterranean. The testimony of old buildings shows that the eastern end of the island of Crete is sinking, while the west and south coasts are rising. The Roman mole at Pozzuoli has sunk, as is shown by the mooring-rings for ships, now permanently below sea-level. South of Stockholm, in Sweden, the remains of an ancient hut were found in place, 65 feet below the surface, buried in marine deposits which contain shells of the same species now living in the Baltic. On the west coast of Greenland the sinking is so rapid as to have attracted the attention of the natives.
Buried forests found below sea-level indicate subsidence. Such forests occur in the delta of the Mississippi, on the shores of Chesapeake Bay and at many points on the sea-coast of the southern and middle Atlantic States, notably in New Jersey, where the coast is sinking at a rate estimated at 2 feet per century. Submerged forests are also found on the coast of Holland and along the whole north coast of Germany, both on the North and Baltic Seas.
A river channel invaded and covered by the sea is still another proof of depression, because a river flowing into the sea cannot excavate the sea-bottom below the level of its mouth. Very many such instances are known, of which it will suffice to mention two or three. The ancient channel of the Hudson has been traced by soundings out to the edge of the continental platform, more than 100 miles southeast of Sandy Hook. In the same manner the channel of the St. Lawrence may be followed out through the Straits of Belle Isle, and that of the Congo extends out 70 miles from the west coast of Africa, with a depth of nearly 1000 fathoms.
The apparently contradictory evidence in the case of the St. Lawrence channel, which indicates depression, and that of the Labrador coast, which is rising, is not so in reality, for the movements are successive, not simultaneous.
Coral reefs often give proof of depression, for, as most of the reef-building corals cannot live in water more than 20 fathoms deep, a greater thickness of the reef than this indicates a slow sinking, at a rate not exceeding the upward growth of the coral. Borings made in the South Pacific island of Funafuti show that that reef exceeds 1100 feet in thickness, and must therefore have been gradually depressed. Another obvious proof of subsidence is a great thickness of shallow water deposits; for, if the sea-bottom did not sink, the shallow water would soon be filled up and the coast-line advanced. The study of the materials now accumulating on the ocean-floor enables us to determine the depth of water in which ancient deposits were formed, and applying this knowledge, we learn (to give only one example) that from the Hudson River southward, the coastal plain of the Atlantic States is covered by very thick, shallow-water, marine beds, as is revealed by the numerous artesian wells which have been driven through them.
The fact that these beds are now part of a land-surface indicates, of course, that they have been elevated subsequently to their formation.
Finally, the form and topography of a coast may betray its recent subsidence, as will be more fully explained in Part III.
As regards the oscillations of level which are now going on, it is not definitely known whether they proceed continuously at a uniform rate, or spasmodically with intervals of complete rest. In the case of a succession of marine terraces, one above another, the movement cannot have been uniform, or else a continuous slope would have been produced; each terrace and beach indicates a pause, during which the waves cut the rocky shelf, or accumulated the beach, while the steep slope between two successive terraces points to a relatively rapid rise.
The following table, which exhibits the data gathered chiefly by Kayser, will be serviceable as showing the extent and character of the diastrophic movements which, it is inferred, are still, or have recently been in progress along the principal coast-lines of the world. In this table no account is taken of the movements which have here and there been detected in the interior of the different continents, Such as those already mentioned for North America, and others which have been observed in northern Switzerland.
East coast of Greenland. East coast down to 450 N. lat. Gulf of Mexico and Antilles. Pacific coast.
West coast of Greenland.
East coast from 45° N. lat. to end of Florida. East coast of Central America.
Pacific coast, except that of Peru. Atlantic coast from mouth of La Plata to 200 S. lat.
Coast of Peru.
Atlantic coast, except Uruguay and South Brazil.
Entire north coast and east coast to 300 N. lat.
South coast and Malay Archipelago.
East coast of southern China and
Tonkin. . Laccadive and Maldive Islands.
Australia, south coast, and Tasmania.
East coast of New Zealand.
Pacific coast of New Guinea.
Solomon, New Hebrides, Samoan, Sandwich Islands, and many others.
Australia, northeast coast. West coast of New Zealand. South coast of New Guinea. Caroline, Marshall, Gilbert, Tonga, Society Islands, etc.
Coast of Red Sea; east coast, west coast up to Gulf of Guinea.
Atlantic coast of Morocco, east coast of Tripoli. North coast of Egypt: Gulf of
Peloponnesus, Sicily, Sardinia, Li-gurian coast, Balearic Islands, south coast of Spain; west coast of France, Ireland and Scotland; Scandinavia.
England; north coast of France, the Netherlands and Germany.
From this table it is apparent that few coasts are stationary, but that almost all are, or have lately been, in movement, and further that upheaval greatly preponderates over subsidence. Still another significant result of these observations is that areas of opposite movement may be in close juxtaposition, as on the two sides of the Baltic, the east and west coasts of Greenland, the eastern coast of North America and Asia, and many other regions. In such cases the movement must be in the land rather than in the sea.