The mounds are of all dimensions, from those of but a few feet in height and a few yards in diameter, to those which, like the celebrated structure at the mouth of Grave Creek in Virginia, rise to the height of seventy feet, and measure one thousand feet in circumference at the base. The great mound in the vicinity of Miamisburgh, Montgomery county, Ohio, is sixty-eight feet in perpendicular height, and eight hundred and fifty-two in circumference at the base, containing 311,353 cubic feet.
[Illustration: Fig. 1. Great Mound at Miamisburgh, Ohio.
The truncated pyramid at Cahokia, Illinois, has an altitude of ninety feet, and is upwards of two thousand feet in circumference at the base. It has a level summit of several acres area. The great mound at Selserstown, Mississippi, is computed to cover six acres of ground. Mounds of these extraordinary dimensions are most common at the south, though there are some of great size at the north. The usual dimensions are, however, considerably less than in the examples here given. The greater number range from six to thirty feet in perpendicular height, by forty to one hundred feet diameter at the base.[4] [p006]
All the above-mentioned constructions are composed of earth or stone; though a combination of these materials in the same work is by no means rare. When there are no ditches interior or exterior to the embankments, «pits» or “dug holes,” from which the earth for their formation was taken, are generally visible near by. These are sometimes very broad and deep, and occasionally quite symmetrical in shape.[5] In the vicinity of large mounds such excavations are common. The earth and stone composing these works are sometimes foreign to the locality which they occupy, and must have been brought from considerable distances.
A large, perhaps the larger, portion of these enclosures are regular in outline, the square and the circle predominating. Some are parallelograms, some ellipses, others polygons, regular or irregular. The regular works are almost invariably erected on level river-terraces, great care having evidently been taken to select those least broken. The irregular works are those which partake most of the character of defences, and are usually made to conform to the nature of the ground upon which they are situated,—running along the brows of hills, or cutting off the approaches to strong natural positions. The square and the circle often occur in combination, frequently communicating with each other or with irregular works directly, or by avenues consisting of parallel lines of embankment. Detached parallels are numerous. The mounds are usually simple cones in form; but they are sometimes truncated, and occasionally terraced, with graded or winding ascents to their summits. Some are elliptical, others pear-shaped, and others squares or parallelograms, with flanking terraces. Besides these, there are others already alluded to, most common in the extreme north-west, which assume the forms of animals and reptiles. Another variety of remains are the causeways or “roads,” and the graded descents to rivers and streams, or from one terrace to another. These several classes of works will be described at length, under appropriate heads.