What motive induced the people to expend so much labor on this mound? It is not probable that this was a burial mound, though it may ultimately prove to be so. The most probable supposition is that the mound was erected so as to secure an elevated site, perhaps for purpose of defense, as on these platforms there was abundant room for a large village, and an elevation or height has always been an important factor in defenses. In this connection, Prof. Putnam has called our attention to a fact which indicates that a very long time was occupied in the construction of the mound, and further, that a numerous population had utilized its platforms as house sites—that is, that “everywhere in the gullies, and over the broken surface of the mounds, mixed with the earth of which it is composed, are quantities of broken vessels of clay, flint chips, arrow-heads, charcoal, bones of animals, etc., apparently the refuse of a numerous people.” The majority of writers, however, think that this elevated site, obtained as the result of so much labor, was utilized for important public buildings, presumedly the temple of their gods, and no one can help noticing the similarity between this structure and those raised by the ancient Mexicans for both religious purposes and town sites.
Mr. Foster thinks that “upon this platform was reared a capacious temple, within whose walls the high-priests gathered from different quarters at stated seasons, celebrated their mystic rites, while the swarming multitudes below looked up with mute adoration.”35 Mr. Breckenridge, whose writings we have already referred to, at the time of his first visit, “everywhere observed a great number of small elevations of earth, to the height of a few feet, at regular distances apart, which appeared to observe some order: near them pieces of flint and fragments of earthen vessels.” From this he concludes that here was a populous town, and that this mound was a temple site. It is doubtful whether we shall ever pierce the veil that lies between us and this aboriginal structure. The pyramids of the Old World have yielded up their secret, and we behold in them the tombs of Egypt’s kings. But this earthen pyramid on the western prairie is more involved in mystery, and we do not know even its builders. If the result of religious zeal, we may be sure that a religion which exacted from its votaries the erection of such a stupendous piece of work was one of great power.