Colo. Rev. Stat. § 30-5-155
The county of Pitkin is hereby established, with the legal capacity and functions of other counties in this state, and with boundaries as follows:
Beginning at the point on the summit of the survey or National range where the Elk mountain range intersects the same; thence in a westerly and northwesterly direction along the summit of the Elk mountain range to Snow Mass mountain; thence due west to the divide west of Rock creek; thence along said divide in a northwesterly direction to the southern boundary of Summit county; thence east along the southern boundary of Summit county to the top of the National range being the west boundary line of Lake county; thence along the summit of said range in a southeasterly direction to the place of beginning.
C.R.S. § 30-5-155