Tarlton Cross is one of the many enigmatic effigy-type mounds in southern Ohio. Archaeologists, historians, and others have posited that this mound was built during the Middle Woodland period (200 B.C.-A.D. 400) by people belonging to a cultural group referred to as the Hopewell by archaeologists. However, no known evidence exists to support this claim. Nevertheless, the mound was likely built during or after the Middle Woodland period, but not before. Those who came before the Hopewell, archaeologists refer to them as the Adena, built small-to-large conical mounds and circular earthen enclosures. Sometimes the mounds were surrounded by the enclosures. While the Adena were not the first to build mounds in Ohio (the oldest mounds in Ohio are currently dated to about 800-900 B.C.), they were the first to build earthen works other than mounds. Unusually shaped mounds, such as Tarlton, were first built by the Hopewell, as far as is known today. Recently, some of Ohio's unusual mounds, such as the Serpent mound and Alligator mound, were radiocarbon dated to post A.D. 1000, indicating that they were built by the Fort Ancient, a culture group in the Middle Ohio Valley known for corn agriculture and relatively large villages.