Effigy mounds, earthworks in the shape of animals and birds, were raised in North America in areas that now correspond to parts of Wisconsin, Minnesota, Illinois, Iowa, and Ohio. The profile images, seldom more than six feet high, include felines, bears, and deer, and they suffered considerably with the increase in farming settlements in the nineteenth century, when many ancient Native American mounds were plowed under. Fortunately, the extraordinary size and recognizable depictions saved many of the effigy mounds from such a fate. The grandest of the representational mounds is the depiction of an undulating snake, perhaps a stylized rattlesnake, in Adams County, Ohio, known as the Great Serpent Mound. Over 1,300 feet long, with an average height of four to five feet and a width of 20 to 25 feet, the serpent is located on a wedge-shaped, slightly convex ridge in the rolling hills of southern Ohio where it overlooks a waterway called Brush Creek. It first came to scholarly attention in the 1840s. Early researchers suggested that the very shape of the bluff upon which the serpent was worked evoked a giant reptile, thereby explaining the original selection of the site. The exact function of the great mound, within the clay structure of which no culturally identifiable objects have yet been located, is uncertain. While burials have been discovered in the effigy mounds further to the northwest, none have been found in the Ohio mound.
The period in which Great Serpent Mound was erected is currently debated among archaeologists. Long thought to have been made by the Adena peoples (1000–100 B.C.) based on the contents of burials discovered nearby, recent archaeological research into the structure of the mound itself has suggested that the serpent is later in date (1000–1200 A.D.). If so, its serpent imagery might relate to the rattlesnake of Mississippian iconography, a prominent image in that cultural manifestation. The oval at the mouth of the serpent was initially considered to be something being swallowed, perhaps an egg, but another view is that it is the eye of the serpent.