Subscribe to our newsletter. The tierra blanca shown in this roadcut belongs to the San Salvador Formation, which underlies the metropolitan region of El Salvador's capital, San Salvador. It consists of tuff, an igneous rock formed by the airfall of volcanic ash.
Pumice is the principal ingredient in tuff.
It tends to form when erupting lavas are stiff and high in silica, which holds the volcanic gases in bubbles rather than letting it escape. After the ash falls, it may be reworked by rainfall and streams. That accounts for the crossbedding near the top of the lower part of the roadcut. If tuff beds are thick enough, they can consolidate into a fairly strong rock. In parts of San Salvador, the tierra blanca is thicker than 50 meters.