Colo. Rev. Stat. § 30-5-128
So much of the county of Summit, in the state of Colorado, as is included within the following described boundaries, shall be set apart and is hereby created into a new county, to be called Grand county:
Beginning on the summit of the snowy range of the Rocky mountains, on the west boundary of Clear Creek county, at the point where the dividing range between the waters of Williams Fork and Blue river diverges from the main snowy range; running thence northwest along the summit of said dividing range between Williams Fork and Blue river to a point of intersection with the boundary line between townships one and two, south of the base line; thence west along said township line to the eastern boundary of the Ute Indian reservation; thence north along said boundary to the northeast corner of said reservation; thence west along the north boundary of said reservation to the west boundary of the state; thence north to the northwest corner of the same; thence east along the north boundary of the state to the northwest corner of Larimer county; thence south along the western boundaries of Larimer, Boulder and Gilpin counties, and west, northwest and southwest, along the north and west boundaries of Clear Creek county, to the place of beginning.
C.R.S. § 30-5-128