Named Ildefonso formation for Barrio San Ildefonso, south-central PR. Consists of volcanic conglomerate, conglomeratic sandstone, tuffaceous siltstone, thin-bedded argillaceous limestone, and massive argillaceous limestone. The massive limestone occurs as lenses throughout the formation; the two largest lenses are herein named the Santa Ana and Jueyes. The total thickness of the formation is difficult to calculate because top of formation is nowhere exposed. Lower part of formation (Santa Ana lens to base) is about 3000 feet. The Ildefonso is intruded by the Pasto andesite. The formation includes part of the Rio Jueyes, Guayama, and Sierra de Cayey "series" of Hodge (1920) which could not be recognized in this study. Part of Hodge's Rio Descalabrado[s] "series" also assigned to the Ildefonso. The Ildefonso disconformably overlies the Robles formation and unconformably underlies the Jacaguas group; it also interfingers with Toa Vaca formation. Unit is of Late Cretaceous age.

Source: GNU records (USGS DDS-6; Reston GNULEX).