An earthwork is any structure made of earth. They were often built in layers of carefully selected materials: clay, sand, gravel and soil, which were often packed down to withstand erosion. The fact that the earthworks lasted for thousands of years is proof of their careful construction. There are three basic types of prehistoric earthworks: mounds, walls, and excavations.
Walls were long embankments of earth with sloping sides and rounded tops, like a mound that stretches on for a distance. These walls can be as high as 20 feet, but typically are between 3 and 10 feet high. Some walls were miles long. Many walls formed enclosures. Often, these enclosures had burial mounds inside them. Sometimes, long parallel walls, resembling mysterious passageways, connected enclosures or extended out from them for long distances.
- “Geometric enclosures” have walls that form enclosures in geometric shapes, like circles, squares and other polygons. The polygons are often astronomically aligned and are less likely to have burial mounds inside them than circular or irregular enclosures. Large geometric enclosures are almost always found on valley floors, just above the floodplain of a major river or stream.Smaller enclosures can be found on hill tops, as well. Intact geometric enclosures can still be seen at the Newark Earthworks.
- “Irregular enclosures” were usually designed to follow along a topographical contour or a river terrace. Remnants of one of the largest irregular enclosures in the Hopewell Heartland can still be seen at Hopewell Mound Group, part of Hopewell Culture National Historical Park.
- “Hilltop enclosures” were built around the lip of a high plateau. The most impressive pre-Columbian embankment walls in the Hopewell Heartland are at Fort Ancient, a massive hilltop enclosure whose walls extend for just over 3 miles and run 20 feet high.
Mounds may be burial mounds, but not all mounds have burials in them. Some mounds were used for other ceremonial purposes, or used as markers, sometimes for astronomical alignments.
- “Effigy mounds” were built in the shape of living things like animals, and typically do not contain burials. The most famous effigy mound in the Hopewell Heartland is the Great Serpent Mound, though this effigy may not be a Hopewell era construction.
- “Burial mounds” were often built on the site of a special building that was used for sacred ceremonies, including funerals. If someone was buried on the site, the body, bones or cremated remains were often placed on the floor of the building, often entombed in a small mound of clay, or a small crypt of clay and wood. Artifacts were often placed near a burial. The ceremonial building may have been used for many years. At some point, the Native Americans would then burn or tear down the building before they covered over the area with a mound of earth. When archaeologists excavatedsuch mounds, in addition to burials or artifacts, they also found the plastered clay floor of the building and the cylindrical holes in the ground where the building’s wooden posts once stood. The most impressive array of mounds covering buildings and burials in the Hopewell Heartland can be seen at Mound City Group in Hopewell Culture National Historical Park.
- “Gate mounds” are a type of mound often found in large enclosures. They typically stand inside the openings in the perimeter walls, symbolically blocking or guarding the entrances to the enclosure. They usually do not contain any burials or the underlying foundations of any structures. Intact gate mounds can still be seen in the great octagon at Newark Earthworks.
Excavations of unusual shape or size are also often found at earthwork sites. Many nineteenth century maps refer to these as “dug holes.” At Mound City, for example, these excavations are large pits located outside the embankments. Once thought to just be areas for borrowing dirt to build mounds and embankments, many archaeologists now believe that excavated depressions were an integral part of an earthwork site’s architecture. Some excavations appear to have been lined with clay, which would have helped to hold water. Many, but not all, embankments were lined with precisely excavated ditches. Thanks to their being filled in and buried, these ditches are often all that remain today, while the walls they once accompanied have long since been flattened by two centuries of plowing.
The Heartland Earthworks Conservancy is devoted to protecting all types of ancient earthworks, as well as the area around the earthworks, which often contains the remains of the mound-builders’ homes, settlements, temporary encampments or other important clues about the function of the earthworks.