Río Frías Formation (Spanish: Formación Río Frías) is a Middle Miocene geologic formation made up sedimentary rock located in Aysén Region, western Patagonia.

[2] The Friasian period in the South American Land Mammal Ages is named after the formation.

Moreno was director of La Plata Museum and was involved in the Cordillera of the Andes Boundary Case between Chile and Argentina, thus there was both a political and scientific motivation behind the exploration of Patagonia.

[1] Roth sent fossils he collected from the formation to Florentino Ameghino who was active at La Plata.

[4] The formation was deposited in a fluvial environment, characterized by an intermontane valley flanked by Cretaceous basement rocks.