The view of Rio Loa shows exposed ignimbrite and lake deposits. The lowest unit is Sifon Ignimbrite, a crystal-rich dacite deposited 8.1 Ma. In this northern area, the Sifon is a single, non-welded, homogenous deposit with a thickness of roughly 8-10 m (de Silva, 1989). Abundant plagioclase and biotite exist within the Sifon and lithics are rare. Small pumice clasts (~3 cm) have silky textures and contain plagioclase and biotite (de Silva, 1989).
The Sifon is overlain by Carcoté Ignimbrite, ~5.6 Ma along the Rio Loa. The uppermost units are younger lake deposits from Chiu Chiu. In some areas, the Lower Rio San Pedro Ignimbrite, 9.66 Ma outcrops below the Sifon. As the Andes were being uplifted the river was downcutting to form the valley that we see today.