Dictionary.net

a name given to a prehistoric race the principal remains of which are extensive earthworks found in the Mississippi Valley extending from the lakes southward to the gulf. Many of these are clearly defensive works or places of sepulture. Fort Hill, Ohio, has a line of circumvallation about four miles in extent. These defensive works also include structures used for religious purposes. Many mounds are of regular outline assuming the form of various geometrical figures. In Newark, Ohio, works of this character cover an area of more than two square miles. A mound near St. Louis is 700 feet long by 500 broad at the base and ninety feet high. Some mounds of this character contain skeletons. Mounds, such as those near Wheeling, WV., and Miamisburg, OH, are evidently the graves of distinguished personages. In Wisconsin and Iowa are earthworks which assume the outline of men and animals. One in Adams County, OH, has the form of a serpent. It is over 1000 feet in length and its mouth is partially closed around an egg of perfectly regular dimensions. The figure reaches a height of about five feet. Various theories prevail as to the question what race built the mounds. It is now frequently thought to have been a race related to the Indians.