One of the things Thomas Jefferson believed Lewis and Clark might find on their expedition was a Welsh Tribe of Indians.
Yup. However, when Lewis and Clark began to report, he interrupted and said, "I don't want to hear any tales of blue-eyed Indians." His fear was that some other European power would/could lay claim to the western lands.
http://www.state.tn.us/environment/parks/parks/OldStoneFort/
This site in Tennessee was once believed to be built by Welsh immigrants of a very early date. More recent scholarship suggests American Indians built it. Lovely place for a hike!