Which mountain range, found in eastern North America, was seen as the western frontier prior to about 1800?
Western United States:  Prior to about 1800, the crest of the Appalachian Mountains was seen as the western frontier. Appalachian Mountains: The Appalachian Mountains ( , French: "les Appalaches" ), often called the Appalachians, are a system of mountains in eastern North America.
Appalachian Mountains