From the InterMountain:
The term “mound builders” is applied to a prehistoric race spanning about 2,000 years (1000 BC to 1000 AD). The first group of these people was the Adena people. We know little about how or why the mounds were built, but most historians agree that the Early Americans built the mounds for their honored dead.
The Adena Mounds ranged in size from about 25 feet to 250 feet in diameter. These people lived in an area making up most of Kentucky, Indiana, Ohio, West Virginia and parts of Pennsylvania and New York.
One of my favorite cemeteries in West Virginia is Indian Mound Cemetery, located in Romney. Here are some photos I took on a visit in 2005.