Page:Popular Science Monthly Volume 53.djvu/28

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that they can not be scaled. The depths vary from a few hundred feet to-a couple of thousand feet, or even more. The same features are found beneath the sea.

On the Declivity of Land Valleys.—The declivity of the greater land valleys crossing regions of continental extent is usually very gentle—a foot per mile, more or less. Smaller valleys may have a declivity of five or ten feet per mile. In the short amphitheaters, descending from the high plateaus, the gradients may be two hundred or even five hundred feet or more per mile. The valleys dissecting recently elevated table-lands, such as those of Mexico, have not uniform slopes, but the descent is characterized by a great series of steps, the surface of each often appearing nearly level to the eye, but with abrupt margins. The vertical heights of such steps vary from five feet to even five hundred feet. The character of the slopes is illustrated in Fig. 4. If stretches of several miles be taken, so

PSM V53 D028 Longitudinal section of a valley atoyac mexico.png
Fig. 4.—Longitudinal section of a Mexican valley (above Atoyac), showing the descent in steps.

many gradation plains occur that the mean declivity of the valley may be from seventy-five to one hundred and fifty feet per mile. Each of these represents pauses in the elevation of the region, while the streams were flowing at levels so low that they could not further deepen their channels, but were widening them out into broad valleys or plains—that is to say, the gradation plains represent base levels of erosion. The valleys on the surface of the table-lands have usually low gradients, which may be as small as those of rivers crossing continental plains. Similar gradation steps are found in valleys beneath the sea.

Submarine Plateaus.—The low coastal plains of the Southeastern States do not terminate at the seashore, but pass beyond, forming shoals and banks, and eventually submarine plateaus, overlooking the edge of the continent, which is fifteen miles eastward of Cape Hatteras, but three hundred miles distant from the coast of Florida. (See map, Plate III.) They extend to and include the Bahamas and other islands. These submarine plateaus have various depths. An elevation of one hundred to three hundred feet would greatly enlarge the Southeastern States, and raise the Bahama banks into broad plains (in reality a continuation of the coastal plains of the Southern States), separated from Cuba and Florida by only narrow channels. A lower broad plateau occurs in this same region, at