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
Calvert Cliffs State Park during that time. The age of the formation is (19-)18–15(-14) million years ago (Ma), i.e. it extends essentially over the Hemingfordian stage. This formation occurs in Maryland and neighboring Virginia. In addition, rocks of the younger Choptank and the St. Marys Formations are exposed here. This makes Calvert Cliffs State Park extremely interesting for its paleoclimatology and paleontology, because the accessible strata provide a good record of the Middle Miocene Climate Transition and document a minor mass extinction event — the "Middle Miocene disruption." Fossil collecting and "rockhounding" are permitted on the beach; the cliffs are closed due the dangers