The largest work is an enclosure marked _A_ in Plate I, and shown on an enlarged scale in Plate II. It is situated on the margin of a bluff overlooking the Little Iowa and an intervening bog-bayou, probably the former channel of the river. It is almost exactly circular, the curve being broken on the east side, where it touches the brink of the bluff, being here made to conform to the line of the latter, though probably never thrown up to the same height as the other portion. The ends at the southeast overlap each other for a short distance, leaving at this point an entrance way, the only one to the enclosure. A ditch runs round on the inside from the entrance on the south to where the wall strikes the bluff on the north, but is wanting along the bluff and overlapping portion. The north and south diameter, measuring from outside to outside, is 277 feet; from east to west, 235 feet; the entire outer circumference is 807 feet, the length of the portion along the bluff 100 feet, and of the overlapping portion at the entrance 45 feet. The wall is quite uniform in size, about 4 feet high and from 25 to 27 in width, except along the bluff, where it is scarcely apparent; the entrance is 16 feet wide, and the ditch 5 to 6 feet wide and 3 feet deep. On the north, adjoining the wall on the outside and extending along it for about 100 feet, is an excavation (_c_, Plate II) 35 feet wide at the widest point and 3 feet deep. As this ground, including the circle, has been under cultivation for fifteen years, it would be supposed the height of the wall is considerably less than it originally was, but this is probably a mistake. On the contrary, it was originally probably but 20 feet wide and not more than 3 feet high, composed mainly of yellowish brown clay obtained, in part at least, from the ditch, but during occupancy the accumulation of countless bones of animals used as food, stone chips, river shells, broken pottery, and dirt, and, since abandonment, the accumulation of sand drifted by the winds from the crumbling sandstone butte (_C_, Plate I) overlooking it, have not only filled the ditch but elevated the wall and whole interior area 2 feet or more. This accumulation of sand is so great and so uniform over the plateau that fifteen years of cultivation have not sufficed to reach the clay of the original surface nor to unearth or even penetrate to the bones, pottery fragments, and other refuse matter covering the original surface in the circle.