The valley of the Mississippi is a vast sedimentary basin extending from the Alleghanies to the Rocky Mountains. Through this the great river and its numerous tributaries have made their way for countless ages, working out shallow depressions in the plain, on which are recorded successive epochs of change in the terraces that mark the deserted levels of ancient channels. The edges of these table-lands bordering on the valleys are indented by numerous ravines; and the junctions of many lesser streams with the rivers have formed nearly detached peninsulas, or in some cases tracts of considerable elevation insulated from the original table-land. Many of those bluff headlands, peninsulas, and isolated hills presented all the requisite adaptations for native strongholds. They have, accordingly, been fortified with great labour and skill. Embankments and ditches enclose the whole space, varying in strength according to the natural resources of the ground. The approaches are guarded by trenches and overlapping walls, more or less numerous in different forts; and have occasionally a mound alongside of the other defences of the approach, but rising above the rest of the works, as if designed both for out-look and additional defence. In some few cases the walls of these enclosures are of stone, but if they were ever characterised by any attempt at regular masonry all traces of it have disappeared, and there seems little reason for supposing that such walls differed in essential character from the earthworks. No cement was used, and in all probability we have in them only the substitution of stone-heaps instead of earth-banks, owing to special local facilities.