It has already been remarked that the square and the circle, separate or in combination, were favorite figures with the mound-builders; and a large proportion of their works in the Scioto valley, and in Ohio generally, are of these forms. Most of the circular works are small, varying from two hundred and fifty to three hundred feet in diameter, while others are a mile or more in circuit. Some stand isolated, but most in connection with one or more mounds, of greater or less dimensions, or in connection with other more complicated works. Wherever the circles occur, if there be a fosse, or ditch, it is almost invariably interior to the parapet. Instances are frequent where no ditch is discernible, and where it is evident that the earth composing the embankment was brought from a distance, or taken up evenly from the surface. In the square and in the irregular works, if there be a fosse at all, it is exterior to the embankment; except in the case of fortified hills, where the earth, for the best of reasons, is usually thrown from the interior. These facts are not without their importance in determining the character and purpose of these remains. Another fact, bearing directly upon the degree of knowledge possessed by the builders, is, that many, if not most, of the circular works are perfect circles, and that many of the rectangular works are accurate squares. This fact has been demonstrated, in numerous instances, by careful admeasurements; and has been remarked in cases where the works embrace an area of many acres, and where the embankments, or circumvallations, are a mile and upwards in extent. To facilitate description, and to bring something like system out of the disordered materials before us, the enclosures are, to as great a degree as practicable, divided into classes; that is to say, such as are esteemed to be works of defence are placed together, while those which are regarded as sacred, or of a doubtful character, come under another division.
Other defensive works occupy the peninsulas created by the rivers and large streams, or cut off the headlands formed by their junction with each other. In such cases a fosse and wall are thrown across the isthmus, or diagonally from the bank of one stream to the bank of the other. In some, the wall is double, and extends along the bank of the stream some distance inwardly, as if designed to prevent an enemy from turning the flanks 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 terraces are also steep, and vary from ten to thirty and more feet in height. The rivers are constantly shifting their channels; and they frequently cut their way through all the intermediate up to the earliest-formed, or highest terrace, presenting bold banks, inaccessibly steep, and from sixty 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 first, or latest-formed terrace. This terrace alone, except at periods of extraordinary freshets, is subject to overflow.9 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 ancient Works occur upon the lowest or latest-formed of these, while they are found indiscriminately upon all the others, bears directly upon the question of their antiquity.
In addition to the several descriptions of defensive works above enumerated, there are others presenting peculiar features, which will be sufficiently noticed in the plans and explanations that follow. These plans are all drawn from actual and minute, and in most instances personal survey, and are presented, unless otherwise specially noted, on a uniform scale of five hundred feet to the inch. When there are interesting features too minute to be satisfactorily indicated on so small a scale, enlarged plans have been adopted. This is the case with the very first plan presented. Sections and supplementary plans are given, whenever it is supposed they may illustrate the description, or assist the comprehension of the reader. To shorten the text, the admeasurements are often placed upon the plans, and the "Field Books" of survey wholly omitted. The greatest care has, in all cases, been taken to secure perfect fidelity in all essential particulars. In the sectional maps, in order to show something of the character as well as the positions of the works, it has been found necessary to exaggerate them beyond their proportionate size. Some of the minor features of a few works are also slightly exaggerated, but in no case where it would be apt to lead to misapprehension or wrong conceptions of their character.
9. This observation is confirmed by all who have given attention to the 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, and where the operation of natural causes is rather to elevate than depress the beds of the streams, some of the ancient works are invaded by water.