The southern end of the range is ill-defined, being marked by cuestas that merge into badlands to the south.
[1] The mountains are steep on their westward faces, where they rise 2,000 feet (610 m) over the Rio Grande Valley, but slope more gently to the east.
[2] The Los Pinos Mountains consist of a western ridge of Precambrian rock exposed by erosion.
[1] Formations present in the Los Pinos Mountains, in stratigraphic order, include:[1] These are mostly located on the east and south.
The western side of the range is underlain by the Los Pinos pluton, dated at 1655 ± 3 million years old.