Serpent Mound was built by one of the cultures known as the
"mound builders." These peoples built mounds in a large area
from the Great Lakes to the Gulf of Mexico and from the
Mississippi River to the Appalachian mountains.
Mounds vary in age; some dating back as far as the early
part of the 6th century, while others (especially those in
the southeastern area) were built in historic times. Stone,
copper, mica, obsidian, and meteoric iron were widely used
by the prehistoric mound builders. Obsidian coming from the
Rocky mountains, mica from the S Appalachian mountains, and
copper from Wisconsin indicate widespread trade. The people
practiced weaving and pottery making.
(Paraphrased from nativeamericans.com, 2004)