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 OF THE MISSISSIPPI VALLEY. 17

defence, of the middle ages. The usual defence is a sim- ple parapet thrown up along and a little below the brow of the hill, varying in height and solidity as the declivity is more or Jess steep and difficult of access.

Other defensive works occupy the peninsulas formed by the streams, or cut off the bluff points formed by their junction with each other. In such cases a fosse and wall are carried across the isthmus, or diagonally from the bank of one stream to that of the other. In certain instances the wall is double, and extends along the bank of the stream for some distance inwardly, as if designed to prevent an enemy from turning the flank of the defence.

To understand clearly the nature of the works last mentioned, it should be remembered that the banks of the Western rivers are always steep, and, where these works are located, invariably high; the banks of the various ter- races are also steep, ranging from ten to thirty and more feet in height. The rivers are constantly shifting their channels, and frequently cut their way through all the intermediate up to the earliest formed or highest terrace, presenting bold banks, inaccessibly steep, and from fifty to one hundred feet high. At such points, from which the river has in some instances receded to the distance of half a mile or more, works of this description are oftenest found.

And it is a fact of much importance and worthy of special note, that within the scope of a pretty extended observation, no work of any kind has been found occupying the latest formed terrace.* This terrace alone, except at periods of extraordinary freshets, is subject to overflow. The formation of each terrace constitutes a sort of semi- geological era in the history of the valley ; and the fact that none of the works occur upon the lowest or latest formed

subject in the Ohio and Upper Mississippi valleys. Along the Gulf and at points on the Lower Mississippi, where the entire country is low and subject to inundation, some of the ancient monuments are invaded by the water.
 * This observation is confirmed by all who have given attention to the