Great Plains First Nations trading networks
The Great Plains First Nations trading networks encountered by the first Europeans on the Great Plains were built on a number of trading centers acting as hubs in an advanced system of exchange over great distances. The primary centers were found at the villages of the Mandan, Hidatsa, and Arikara, with a surplus of agricultural produce that could be exchanged.
Source: Wikipedia — Great Plains First Nations trading networks (CC BY-SA 4.0)