A fossil rock glacier on San Mateo Peak, Socorro County, New Mexico
— John W. Blagbrough

A tongue-shaped, fossil rock glacier of Wisconsin age is at the head of a valley at an elevation of about 2,700 m on the northwest slope of San Mateo Peak. It is 285 m long and 75 m wide. Characteristic features include a steep front, a lateral ridge along the northeast flank, and longitudinal and transverse ridges and furrows. The rock glacier is composed of blocks and slabby clasts of rhyolite with lengths of 1-1.2 m. The debris is stable and exposed faces bear a growth of lichen. Surface features indicate that the rock glacier moved by the flow of interstitial ice. The existence of the ice indicates a periglacial climate during the Wisconsin, with a mean annual temperature near freezing and permafrost at about 2,700 m on the northwest slope of the mountain.