Page:Popular Science Monthly Volume 53.djvu/33

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PSM V53 D033 Longitudinal sections of three valleys.png

These sections, Fit's. 5 and 6. illustrate gradation plains and steps between the col of the straits of Florida and the floors of the Mexican and Atlantic basins The cañonlike character is shown by the dissection of the continental shelf, whose floor is seen in broken shading. The gradient of the Colorado Canon is also shown in Fig. 5. The depth of submergence is given in feet; and the mean slopes in feet for distances of a given number of miles.

were not depressed by the terrestrial movements which elevated mountain zones or depressed ocean basins. A few of the submarine valleys are parallel to the mountain ridges, and may have been