Serpent Mound This mound was most likely constructed sometime between 950 and 1200 C.E. by the Mississippian society found in present-day southern Ohio. A sun-worshipping culture, the Mississippians aligned the head of the serpent with the sunset of the summer solstice (June 20 or 21). Goods found at such sites indicate that Hopewell networks were further extended by the Mississippians. Mark C. Burnett/Photo Researchers