N.M. Stat. § 4-21-1
The boundaries of the county of Rio Arriba are as follows: on the south from the Puertecito of Pojuaque, drawing a direct line toward the west in the direction of the mesilla of San Yldefonso; from the mesilla crossing the Rio del Norte toward the west and continuing until it reaches the boundaries of the territory; drawing a direct line from said Puertecito de Pojuaque toward the east until it reaches the last house of the town of Cundiyo, toward the south, continuing the same line until it reaches the highest point of the mountain of Nambe; from thence, following the summit of the mountain toward the north, until it reaches the southern boundary of the county of Taos; this shall constitute the eastern boundary, and on the north the boundary of the county of Taos, and on the west the boundary line of the territory.
NMS § 4-21-1