The correct answer is: The Rocky Mountains.
Plains Native Americans lived in a combination of sedentary settings and nomadic lifestyle. They planted crops and established villages in the spring, they focused on hunting in the summer and winter and harvested their crops in the fall. Plains Native Americans used two primary methods for establishing boundaries, warfare and geography.
Usually natural land formations or rivers served as a marker of tribal territory. The Great Plains lie between Rio Grande in the south, Mackenzie River in the north, between the Interior Lowland and the Canadian Shield on the east, and the Rocky Mountains on the west. (The Rocky Mountains served as a barrier and the Indian Americans ruled the land).
The Rocky Mountains range over 3,000 mi from British Columbia to New Mexico and reach a height of 14,440 ft.