Page:Popular Science Monthly Volume 74.djvu/606

602 stretch of straight track in Georgia, from a few miles southwest of Waycross to a few miles beyond Valdosta, sixty miles in all, crosses the head-waters of the Okefinokee.

The geology of this flat pine-barren region is comparatively simple. The surface is a few inches or a few feet of Columbia sand, and under that is the clay, loam or coarse sand of the Altamaha Grit or Grand Gulf formation to a depth of three or four hundred feet. None of these formations are fossiliferous, but they are believed to be quite recent, probably of Pliocene or later age. Under them is a limestone believed to be Miocene', and below that presumably all the older coastal plain formations in succession. There is every reason to believe that the whole swamp is underlaid by the same formations, from the Columbia down.

Immediately east of the Okefinokee is one of the most interesting topographic features of the region, which would scarcely be noticeable but for the general flatness of the country. It is a broad low ridge, exactly parallel with the coast and just about forty miles distant from it. This ridge has been traced by the writer from a few miles west of Jesup southward into the great bend of the St. Mary's River, and about thirty miles into Florida, where it is known as the "Trail Ridge," and happens to coincide in part with the Atlantic and Gulf divide and with the eastern boundary of Baker and Bradford counties, though still maintaining its parallelism with the Atlantic coast. It is not an important divide in Georgia, though no streams intersect it between the St.