Page:Popular Science Monthly Volume 53.djvu/26

16 to the ridges or breaking through them, are the result of the denudation of the rock surfaces of the country, and accordingly, when similar forms occur beneath the sea, they suggest the same origin.

—Concerning the magnitude of land valleys, they vary from the smallest ravines to valleys such as the Mississippi, whose flood plains are from thirty to eighty miles wide for the lower five hundred miles of its length. Several comparatively short rivers, crossing the coastal plains of the southeastern Atlantic States, have valleys from two to four miles wide, even at a hundred miles from the sea, and upon nearing the coast

 —Cross-section of a complex valley in the southern Appalachians, showing it to be independent of the geological structure, which is represented by the shading. Dotted lines mark what was the upper limit of strata which Lave been denuded away. The straight lines (F) illustrate some of the faults affecting the region.

they may be from five to ten miles wide, bounded by only broken hills. All the rivers crossing the coastal plains are flowing over deeply buried channels. That of the Savannah River is buried beneath two hundred and fifty feet or more of superficial deposits, and the old Mississippi Valley is now known to reach one thousand feet below sea level at New Orleans. These buried channels prove that in recent times the continent has sunken to a great extent. The valley of the St. Lawrence River differs from that of the Mississippi in being drowned but not subsequently filled with the mud brought down by the streams. In its lower reaches it is seventy miles wide. These examples of continental valleys are greater than any of the drowned ones to be considered in this paper. The examples cited are those of valleys crossing extensive plains at no great elevation above the sea. with their lower reaches depressed even below high tide.

The Colorado River of the West flows from table-lands eight thousand to ten thousand feet above the sea. Its cañon section is about two hundred and twenty miles long. This is not a simple gorge, but a broad valley from five to twelve miles wide, bounded by walls rising two thousand feet above its floor. This floor was an old base level of erosion formed at no considerable altitude, so that the streams meandered sluggishly over it, and the rains and rills widened it to broad proportions; but, owing to subsequent elevation of the region, the river has cut down its channel to a still lower base level, and in doing so it has been deepened thirty-five hundred or four