Brazos Mountains to 55 million years ago during the Laramide orogeny. To the east and northeast lie the Sangre de Cristo Mountains which share a similar geological history. The Brazos Cliffs, which form a dramatic backdrop to Tierra Amarilla, are formed of Precambrian quartzite. Brazos Mountains The Brazos Mountains is a range in far northern Rio Arriba County, in northern New Mexico in the southwestern United States. A high crest runs from the border with Colorado for over in a south-southeasterly direction. The high point of the range at is on Grouse Mesa, at the Brazos Benchmark. Two miles (3 km) to
Organ Rock Formation seymouriamorph found in the Organ Rock Formation. "Seymouria sanjuanensis" is a unique taxon because of its extensive geographic range. This particular animal was first described by Vaughn (1966) from the Organ Rock Formation. Since this initial find, a number of these individuals have been recovered from localities as distant as Germany. Four early synapsid taxa are present within the Organ Rock Formation. These include Ophiacodon, as well as three sphenacodontids: "Ctenospondylus", "Dimetrodon", and "Sphenacodon ferocior". Most taxa are known from fragmentary material. Early synapsids were dominant members of the terrestrial Permian ecosystem. Some early synapsids adapted an herbivorous diet, however,