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 fine-grained sand from the overlying dune facies. This facies is most common at the western extent of the Organ Rock Formation. Plant fossils include "Supaia", "Walchi" and "Yakia" which lived in semi-arid conditions. Fossil vertebrates represent the majority of the faunal assemblage present in the Organ Rock Formation. Animal remains are most commonly preserved within ancient stream channels of the floodplain and channel facies. Within this facies, fossilized bone occurs proximal to conglomeratic lenses. Taxa recovered include: freshwater actinopterygian fishes, temnospondyl amphibians, diadectomorphs, seymouriamorphs, and early synapsids. Fossil fish material, either bone or scale, is infrequent within the Organ Rock