www.CPR.org/2020/03/16/guess-what-colorado-the-front-range-isnt-where-you-think-it-is
Essentially the first range of mountains those traveling from the east need to pass over. From Laramie down to near Canyon City.
Full size image:
www.FrontRange4x4.com/misc/USBGN-Front-Range-Map-scaled.jpg