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��HISTOliY OF RICHLAND COUNTY
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��depressions and cavities having no outlet, and indicating that the present contour of the sur- face is not the result of recent erosion. The surlace drainage is now filling up and obliterat- ing these cavities, some of which are still swanii)s. and generally the wash from the hills is carrying the silt and humus into these de- pressions, so that surface erosion is steadily diminishing instead of increasing these inequal- ities. Over large areas the clay includes such an alnnidance of rock fragments that, wherever surface erosion is facilitated down the slopes of the hills by road-making or otherwise, the wash is arrested as soon as a shallow channel is formed l)y the accumulation of rock fragments on the surface. If erosion by rainfall excavated the depressions and ravines, the water would have had force sufficient only to carry away the clay, sand, and finer gravels, and the surface would now be covered Avith bowlders and fragments of rocks, but such a condition of the surface is nowhere found. A comparatively few isolated bowlders are scattered over the surface as though dropped upon it. In the deeper ravines, which should he filled with a mass of these bowlders, they are Aery rarely found, and are no more abundant upon the slopes than upon the tops of the hills.
On the margins of the streams there is fre- quently at the bottom a deposit of laminated or finel.y stratified clay, with rudely stratified gravel and bowlders above. The fragments of the local rocks are here rounded and globular; no striated granitic fragments are found. In places, all the fragments of the local rocks are ground to powder, and, with all the clay and finer gravels of the drift, have been washed away, leaving only coarse, well-rounded gran- itic pebliles, with occasional bowlders of the corniferous limestone. In this material, also, cavities are occasionally found having no out- lets, the character of the underlying rocks, and the form of the surface, indicating that they are not properly sink-holes, such as are often found
��in limestone regions. A little east oi' the I'ail- road station at Lexington, two such cavities are quite conspicuous. They are on a long bil- low}' ridge, filled with coarse gravel and bowl- ders, and covered with a foi'cst of hard maple. In the deepest cavity the depression is twenty- five feet, in the other fifteen feet. The slopes in each are smooth, without rock fragments, and covered with the native forest trees. In both there is an accumulation of humus at the bottom, and the deeper one contains a little water. They afford a ready explanation <jf the origin of the small ponds having no outlet, found in other places along this divide, with dead forest trees standing in the water. In the original cavity the drainage through the porous bottom was free, and the forests occupied the liottom and the slopes. The wash of the slopes and the fine material of the decomposed vegetation gradually accunudated in the gravelly bottom, which, like a filter long used, gradually l)e- came impervious to the water, which encroached more and more upon the vegetation, ultimately destroying it, and the dry cavity became a pond. The accumulation of vegetable debris, and the growth of water plants upon the mar- gin, will finally convert the pond into a marsh, which in the end will be filled up and ol)literated. To account for the facts exhibited in the pro- file of Richland County, an agency is required which shall 1 )ring from their home in the far north the granitic bowlders and pelibles. the cornifer- ous limestone, and other hard rocks interven- ing; shall pulverize to clay the soft, argil- •laceous rocks; shall leave the hard rocks brought in from the north rounded and striated; shall mingle all this material intimately with the debris of the friable local rocks, which are neither water-worn nor striated, but are sharp, angular fragments; and leave the whole entirely unassorted upon the high lands in undulating ridges, but upon the margins of the streams often washing away all the finer material, wear- ino- to a sand the debris of the soft local rocks.
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