Outcrops in two areas in the east and west fronts of the Sierra Madre Oriental allow the study of Lower Cretaceous sedimentary and volcanic rocks. In the Fortin-Zongolica area Berriasian-Valanginian sedimentary rocks composed mainly of sandstone, siltstone, and limestone are associated with penecontemporaneous dacitic and andesitic volcanic rocks which were deposited in a shallow marine environment. In the Tehuacan area a section of sedimentary rocks formed by sandstone, shale, and limestone is associated with andesitic pillow lavas and volcaniclastics; the terrigenous fraction contains scattered primitive rudist shells. Correlation of the volcanic rocks is not certain but they may be in part equivalent with the rocks exposed in the Fortin-Zongolica area.

Elsewhere in central Mexico there are other evidences of Upper Jurassic-Lower Cretaceous andesitic volcanic rocks. These, together with the Tehuacan-Fortin-Zongolica volcanic rocks, could form an east-west volcanic arc extending across central Mexico.

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