For thousands of years, Native Americans inhabited the region where the Allegheny and the Monongahela join to form the Ohio. Paleo-Indians conducted a hunter-gatherer lifestyle in the region perhaps as early as 19,000 years ago. Meadowcroft Rockshelter, an archaeological site west of Pittsburgh, provides evidence that these first Americans lived in the region from that date. During the Adena culture that followed, Mound Builders erected a large Indian Mound at the future site of McKees Rocks, about three miles (5 km) from the head of the Ohio. The Indian Mound, a burial site, was augmented in later years by members of the Hopewell culture.