The site of Caltonac is located in the state of Puebla, at the eastern edge of the Mexican Central Altiplano. Preliminary mapping in the course of a geologic survey shows that it is built over a compound andesite flow and can be divided into three sections, each of which covers an area of ca. 2 sq km. The southern section has a well defined public area, surrounded by what appear to be domestic structures arranged along four major roadways that radiate from the public area. In the northern and NW sections the stone-wall habitational structures are arranged in isolated groups surrounding open spaces or clustered at the rim of the andesite flow. A major rhyolite flow is located ca. 10 km NW of the site. The glassy portions of this flow prove to be the source of the D or Zaragoza type obsidian found in Formative to Postclassic Mesoamerican sites, whose provenance was until now uncertain. The chronology of the site is unknown, but an increase during the Classic of the number of sites in which D-type obsidian artifacts are found suggests that by this time local control of the source had been established.