Page:Popular Science Monthly Volume 53.djvu/32

20 The Savannah Valley, deeply buried on reaching the present sea-shore, crosses the submarine coastal plains at a depth of sixteen hundred and fifty feet lower than the floor of the plateau itself, which is already submerged at that point to nineteen hundred and fifty feet beneath the surface of the sea. The canon becomes still deeper upon nearing the edge of the continental shelf. The Altamaha becomes a canon with a depth of fifty-three hundred feet at the point where the continental shelf is submerged twenty-five hundred feet. Its length is about three hundred miles. It terminates in an embayment thirteen thousand five hundred feet below the surface of the sea. Among the Bahama Islands, and between them and Cuba, there are several similar fiords or drowned valleys, reaching to depths of from two thousand to twelve thousand feet or more. The straits of Florida, and the fiords extending from them, have afforded special opportunities for studying the submerged valleys. The shallowest parts of the straits of Florida are two thousand and sixty-four feet beneath the surface of the sea. From this col, or divide, the Floridian channel extends for a distance of three hundred and fifty miles to a point where it has a depth of ten thousand three hundred and fourteen feet, upon nearing the floor of the Gulf of Mexico. From the same divide the deep Bahaman valley trends in the opposite direction, skirting the northern side of the Bahaman group, and becomes a fiord reaching to a depth of about twelve thousand feet near the edge of the continental shelf. The Abacan channel, crossing the Bahama banks, may be followed to a similar depth. (See map, and Figs. 5 and 6, page 21.)

The valley of the Mississippi (buried to a depth of one thousand feet before reaching the present mouth of the river) is well marked across the submerged plateau to near the floor of the Gulf of Mexico. The drowned valleys of many other Southern rivers are similarly traceable to the floor of the Gulf of Mexico. The same is true of the submarine channels dissecting the drowned plateaus of the Honduras and Caribbean Seas, as shown on the map.

Many of the submerged valleys have tributaries converging from all possible directions, as, for example, those of the Floridian channel. There are also numerous short fiords, tributary to those of greater proportions, but these come from the abrupt margins of the continental shelves or islands, like the amphitheaters indenting the high table-lands.

Of the numerous submarine valleys discovered, a considerable number is shown upon the map, but only a few are cited in the text; by far the larger proportion lie along directions transverse to the trend of the coast lines and mountain ranges, and consequently they