Page:Popular Science Monthly Volume 53.djvu/34

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deepened by earth movements, although such is not apparent in the many tributaries. It has been found that the greatest amount of upward warping is in the mountain regions, and the greatest depression is supposed to be along the margins of oceanic abysses. Such movements would have a tendency to somewhat increase terrestrial and submarine declivities; but if transverse upward warping became exaggerated, the valleys would become barricaded so as to form basins, like that of Lake Ontario, or even greater sea basins. Such, however, is not the case with the valleys crossing the submerged coastal plains.

Comparison of Land and Drowned Valleys.—Since the submarine valleys, wherever traceable to the shores, are found to be continuations of the PSM V53 D034 Longitudinal section of the cazonan channel cuba.pngFig. 7.—Longitudinal section of the Cazonan channel (south of Cuba), showing a similar but shorter fiord dissecting the land mass, the submerged floor of which is shown by the broken shading. PSM V53 D034 Longitudinal sections of the Atozac valley mexico.pngFig. 8.—Longitudinal sections of the Atozac Valley, descending from the Mexican plateau (eight thousand feet above the sea), on the same scale us that of the drowned valleys, but which is here too small to show the numerous steps, except the mean declivity for comparison with that of the abrupt slopes of the submarine steps. PSM V53 D034 Longitudinal sections of a valley south of monterey mexico.pngFig. 9.—Longitudinal section of a similar valley south of Monterey in Mexico. buried valleys crossing the coastal plains, they have considerable magnitude where first detected in the soundings. Parenthetically it may be stated that during the late minor oscillations of the continent the old valleys have become filled with sand, etc., for some distance seaward; but beyond this fringe they are always found where the soundings have been taken sufficiently near together. Submerged valleys gradually increase in size until they enter the oceanic embayments indenting the margins of the continental mass. These embayments have been found to vary from perhaps ten to forty miles in width, or within the limit of size shown in the modern Mississippi and St. Lawrence Valleys. Even the upper cañon of the Colorado River has a width of twelve miles. Consequently, the breadth of the drowned valleys