Page:Popular Science Monthly Volume 74.djvu/608

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The internal structure of the Okefinokee Ridge is not its least interesting feature. In the big ditch at Camp Cornelia, as well as at the crossings of four railroads (three of them shown on the map and one a little farther north), there occurs beneath a few feet of white sand a chocolate-colored or almost black material of unknown depth, known locally as "hardpan." No analysis of the hardpan is available, but when pulverized in the fingers it feels like nothing but sand. Its dark color is doubtless due to vegetable matter, with a little cement of iron oxide or bog iron ore, which makes it so hard in the mass that dynamite

PSM V74 D608 Outer edge of jacksons bay.png

Jackson's Bay (outer edge).

was used in removing it, it is said. No recognizable organic remains were noticed in it, but a faint horizontal stratification could be detected. The aspect of the hardpan is so similar to that of the subsoil of existing salt marshes that its origin is not hard to guess.

Although this peculiar formation may not be confined to the Okefinokee Ridge, its extent is evidently limited; for in railroad cuts around Waycross and Folkston and in Camden County and elsewhere the ordinary reddish Pliocene loam can be seen near the surface, without any signs of the black hardpan. The hardpan was doubtless formed in some prehistoric swamp or marsh occupying a somewhat larger area than Okefinokee does now—perhaps the "Suwannee Strait" of geologists,[1] which is supposed to have separated Florida from the mainland in Miocene times. It seems to have no effect on the vegetation above it, which is Just like that of the ordinary flat pine-barrens underlaid by the loam.

  1. See Dall, Bull. U. S. Geol. Surv., 84: 111, 121-122, 126, 1892.