The custom of mound building was widespread in eastern North America prior to European contact, and it was long-enduring. Earthen burial mounds were built as far back as the fourth millennium b.c.e. The tradition spread and developed until reaching a culmination that lasted from the late second millennium b.c.e. until well into the mid-second millennium c.e. It was characterized by the construction of huge earthworks typically several hundred meters (more than a thousand feet) across—comparable in size to the largest British henges, such as Avebury. These earthworks are found down the Mississippi valley from Wisconsin to Louisiana, as well as eastwards into Ohio in the north and Alabama in the south. A distinctive phenomenon within the mound-building tradition, particularly from the late first millennium c.e. onwards, was the construction of effigy mounds— earthworks in the form of birds, animals, (occasionally) people, and more abstract designs. Serpent Mound in southern Ohio is one of the best-known examples of these.
The number of mounds built by ancient native North Americans is staggering: over fifteen thousand examples have been documented within the state of Wisconsin alone. Sadly, many have been obliterated and others only survive as almost imperceptible bumps in ploughed fields. Still others have met more distinctive fates—such as the enclosure in the shape of a conjoined circle and octagon at Newark in Ohio, which has been landscaped into a golf course. The largest were on a grand scale, and in just a few cases one can still gain a sense of how impressive they must originally have been.
The peoples who adopted the practice of mound building were many and various, each doubtless turning the practice to their own purposes. These peoples occupied a variety of woodland and valley environments and, over the centuries, developed new modes of subsistence. One of the clearest transitions was the adoption of maize as a staple food in the late first millennium c.e. Archaeologists identify a number of distinctive cultural assemblages: Poverty Point, named after the earliest known large ceremonial earthwork in the lower Mississippi valley, dating to around 1200 b.c.e.; Adena, in the Ohio valley in the mid-first millennium b.c.e.; Hopewell, which flourished later in the same area; and Mississippian, in the first part of the second millennium c.e., its most famous site being the great center of Cahokia, sited close to modern St. Louis.
The Hopewell assemblage is characterized by enormous enclosures in the shape of circles, squares, and other clear-cut geometrical shapes, generally thought to have been built between about 200 b.c.e. and c.e. 400. These are often found in association and even connected together, as at Newark. From an engineering perspective they are awe-inspiring, which has led to much speculation about other aspects of their construction, such as the use of precise measurement units and the incorporation of astronomical alignments. The latter include a number of solstitial alignments: for example, along the diagonal axis of several square earthworks, such as Hopeton, Anderson, Dunlap, Hopewell, and Mound City. Even the Serpent Mound—belonging to the later effigy mound tradition—has its head pointing at least roughly toward midsummer sunset. Various alignments upon potentially significant horizon rising and setting positions of the moon have also been claimed. The best known of these is the axis of the circle-octagon at Newark, which is aligned upon the most northerly rising position of the moon.
One thing is certain: these sites were not astronomical observatories. They served a variety of purposes. The geometrical earthworks demarcate large spaces, and it is generally assumed that they functioned, among other things, as ceremonial centers. One suggestion is that rituals related to burying the dead changed over time into grander ceremonials aimed at ensuring the continuance of world order and seasonal renewal. This explanation certainly fits the archaeological evidence of burial mounds gradually becoming transformed into large ceremonial earthworks. If so, it reflects a modification in the way people expressed and acted upon their beliefs relating to death, ancestors, and regeneration.
If these great earthworks were focuses for ceremonial activity preserving the world order, then they would surely reflect that perceived world order in their overall design. But how do we go about trying to understand this in any detail? One possibility is to examine modern indigenous worldviews in the region, on the basis that some aspects of early indigenous American cosmological beliefs may have survived through to modern times. For example, in the case of Wisconsin effigy mounds it has been suggested, by comparison with modern indigenous beliefs there, that the bird-shaped mounds were representations of powerful spirits inhabiting the upperworld, while land and water animals represented spirits of the lowerworld. Clusters of effigy mounds, then, both reflected—and preserved—the balance between the two realms of nature, earth and sky. (Other modern groups conceive of three realms—sky, earth, and a watery underworld—but the same general principles apply.)
Hopewell mounds are more distant in time from the indigenous present. Yet, just as modern traditional houses of many native North American groups reflect the fundamental perceived division of the (horizontal) world into four quarters, demarcated either by the cardinal or solstitial directions, so many of the Hopewell earthworks have a fourfold symmetry and are cardinally or solstitially aligned. More speculatively, it has been suggested that circles and squares (or octagons) might themselves represent the earth and sky. When conjoined, as at Newark, they would have represented the whole universe in microcosm, defining spaces in which ceremonies or other activities could be performed that were suited to each realm, dedicated to keeping each in its natural balance with the other.
Part of the Serpent Mound, a solstitially oriented effigy mound in southern Ohio.
However close to the truth such speculations might be, it is clear that any astronomical alignments that were intentionally built into the Hopewell mounds formed part of a much wider set of symbolic associations that helped to affirm the perceived order of things. Solstitial alignments, if deliberate, were not very precise. But if their purpose was merely to demarcate the four divisions of the cosmos, they did not need to be. The horizon rising and setting positions of the moon, on the other hand, have no place in modern native American thought, which is one reason why many ar-chaeoastronomers remain unconvinced that such alignments were intentional. The other is that the alignments do not seem to occur in any consistent way.
