The Great Bahama Bank, from which rise the low coral-built Andros Islands and a large number of crags and rocks, measures about 360 geographical miles from southeast to northwest, and has a width of approximately 200 geographical miles. Throughout its entire submerged portion the water is less than 100, and over much of the area less than 10 fathoms deep. It is invaded and given an irregular shape, however, by a "tongue of the ocean" which curves in from the northward, in which soundings of from 700 to 1,000 fathoms have been obtained. To the north of the Great Bahama Bank, and separated from it by water nearly 2,000 fathoms deep, is the Little Bahama Bank, measuring 50 by 150 geographical miles, from which rises the low islands known as Great Bahama, Great Abaco, and a multitude of islets and crags, while beneath the water, as is the case also on the greater submarine plateau to the south, there are numerous shoals. Southeast from the Great Bahama Bank, and in a general view to be classed with it, are several other shallow areas in the sea, of similar character, and with numerous islands and reefs rising from them. The southeastern terminus of this series of plateaus, the surfaces of which have been built up practically to the level of the surface of the sea, is the Navidad Bank, situated about 50 miles to the north of the eastern end of the island of Santo Domingo (Haiti) and forms the west border of Brownson Deep. The length of the series of banks to the north of the Greater Antilles is about 800, and its average width 120 geographical miles.
The unevenness of the surface of the Bahama Banks (and the same is true also of the southern portion of Florida, the Yucatan peninsula, and of nearly all of the submarine plateaus or banks in West India waters) is largely due to the coral reefs and the coral rock formed on them. While the outer portion of the continental shelf, in most instances, is formed of soft, unconsolidated calcareous mud or ooze, in the tropical seas, where the depth, clearness of the water, etc., are favourable, reef-building coral-polyps become attached and form massive corals. The growth of these corals is irregular, and the surface of the plateaus where they are attached becomes roughened. There is a delicate adjustment between the growth of reef-building corals and strength of current, freedom of exposure to the waves, etc., and they flourish in certain localities, as on the windward border of islands, and die at other localities. The growth of coral "heads" and reefs changes the direction of currents, and the spaces of soft ooze and dead coral between the localities most favourable for coral growth are liable to be scoured out and the bottom lowered. When coral, together with the shells of molluscs and other organic refuse of the teeming life of tropical seas, reaches the surface of the water, fragments and even large masses are broken off by the force of the waves, ground into calcareous sand owing to the movements produced by the waves and currents, and much of it heaped on the borders of the reefs so as to raise them above the fair-weather level of the sea. Much of this material, when it becomes dry, is moved by the winds and built into dunes, thus still further increasing the height of the land. Many of the islands in the Bahamas have thus been formed, but the process has been modified in the greater part of that region by movements in the earth's crust which have produced widely extended elevations and depressions. The larger islands in the Bahama group are coral platforms which have been moderately elevated, and bear on their surfaces extensive accumulations of wind-deposited sand. The Yucatan peninsula is also, to a great extent, an upraised coral platform. The surfaces of such exposed areas of easily soluble calcareous rocks are roughened still more by the action of rain and percolating waters, and if subsequently submerged to a moderate depth would give origin to "banks" with uneven, and possibly conspicuously roughened surfaces. Different stages in this varied history are illustrated throughout the West India region.