Colo. Rev. Stat. § 30-5-114
Costilla county: Commencing at a point on the southeastern boundary of the state, where the range line between ranges sixty-nine and seventy intersects said boundary; thence north along said range line to the point where the same intersects the Sangre de Cristo pass or road; thence in a southwesterly direction on said road to the summit of the Sangre de Cristo range; thence in a northerly and westerly direction along the summit of said range to the head of the main branch of the Mosco creek; thence in a southwesterly direction down the center of said Mosco creek to where said creek enters into the San Luis valley; thence in a westerly direction to the most easterly point of La Loma del Norte; thence down the center of the Rio Grande del Norte to the southern boundary of the state; thence east along said boundary to the place of beginning.
C.R.S. § 30-5-114