Much of the region consists of broad and flat river valleys, such as  western Montana between the mountain ranges.  There are big ranges to cross between sections of the region. 

In one part of Wyoming, between Cheyenne and Laramie, one can cross the continental divide with out seeing a mountain.  It is high plains.  Of course one must get up onto those plains to begin with.  Planning a route through the Rockies can bring a wide variety of experiences, depending on which way one goes.