Achenbach Trail (ND)
This route encompasses a large swath of wilderness in the North Unit of Theodore Roosevelt National Park. https://www.nps.gov/thro/planyourvisit/maps.htm Steep climbs and descents between river valley and butte plataeus plus two river crossings make this a challenging and interesting trail. Along the way you might see prairie dogs, elk, mule deer, pronghorn, bighorn sheep, and of course bison.