Geographic Continuity Problems!!!!!

GGeographically Incorrect Movie!
My husband and I just watched the john Wayne & Katharine Hepburn classic "Rooster Cogburn." Maybe "geographically incorrect" is too mild. It was geographically CONFUSED!! It takes place in the late 1870s - early 1880s in the "Territories." By then the only "territories" anywhere near to Fort Smith, Arkansas would have been Oklahoma, New Mexico and Arizona! Why was it shot in central Oregon with it's distinctive volcanic topography as an obvious backdrop?

The only people that would have been oblivious would be people who have never been west of the Appalachians
and never been outside the Philly to Boston corridor!

It was a great movie once we stopped laughing at its geographic continuity issues.

Oh, and yes it was pretty much "True Grit" with Katharine Hepburn in the Kim Darby role.