N.M. Stat. § 4-18-3
The dividing line to be known as the dividing line of the counties of Mora and San Miguel, shall be as follows: taking as a fixed point five hundred yards north of the house of Leandro Sanchez, at Pena Blanca, in the county of San Miguel; thence running a line directly to the west to the eastern boundary line of the county of Santa Fe; thence running from said point five hundred yards north of the house of Leandro Sanchez on a line directly east to the fall and source of the Arroyo de la Jara; thence following the current of the Arroyo de la Jara to its junction with the Sapello river; thence following down the Sapello river to the old government bridge or crossing over the El Sapellocito; thence running on a direct line east, or as nearly as may be, to the Pinos Altos, leaving all settlements on both sides of the Mora river, and within the valley of said river, in the county of Mora; thence from Pinos Altos in a direct line to the east to the limits of the territory of New Mexico.
NMS § 4-18-3