N.M. Stat. § 4-31-1
That the county of Union is hereby created out of that portion of the territory of New Mexico lying and being situate within the following metes and bounds, to wit: commencing at a point on the line between the state of Colorado and the territory of New Mexico where the range line between ranges twenty-seven and twenty-eight east crosses said line between the territory of New Mexico and state of Colorado; thence from said point, running south on said range line between ranges twenty-seven and twenty-eight east, to the south line of the county of Mora, in said territory; thence east along the south line of the said county of Mora to the east line of what is known and called the Pablo Montoya grant; thence in a southeasterly direction along the easterly line of the said Pablo Montoya grant to the north line of what is called and known as Baca location number two; thence east along the north line of said Baca location number two to the northeast corner of said Baca location number two; thence south along the east line of said Baca location number two to the southeast corner of said Baca location number two; thence continuing south to the township line between townships eleven and twelve north; thence east along said line between townships eleven and twelve north, to the east line of the territory of New Mexico; thence north on the east line of the territory of New Mexico to the northeast corner of said territory of New Mexico; thence west along the north line of said territory of New Mexico, to the range line, between ranges twenty-seven and twenty-eight east, the point of beginning.
NMS § 4-31-1