After the United States purchased the Louisiana Territory, America’s western boundary shifted to the , which is a(n) boundary.

Rocky Mountains; physical

Explanation:

United States and France came into the agreement of the land which is popularly termed as The Louisiana Purchase. It was the United States that acquired the large part of the land situated in the west of the Mississippi River. The territory of Louisiana covered the area as  Mississippi river in the east to the Rocky mountains in the west. It stretched from the Gulf of Mexico in the south the Canadian border in the north.