This section of the park also contains the geological formation that gave the park/forest its name: Chapultepec Hill. It is a formation of volcanic rock and andesite, which is common in the Valley of Mexico and contains small caves and sand deposits. "Chapultepec" in Nahuatl means "grasshopper hill" but it is not clear whether the "Chapul" (grasshopper) part refers to the shape of the hill, or the abundance of grasshoppers in the surrounding woods. This hill was considered special during the pre-Hispanic period from the Toltecs in the 12th century to the Aztecs up to the time of the Conquest by the Spanish. Remains of a Toltec altar have been found at the top of the hill, a number of burials and its use was reserved only for Aztec emperors and other elite.