Named for the site of the Inka bridge over the Colca adjacent to Chivay, the Inca formation consists of Andesite and Trachyandesites that occur in exogenous domes and flows that appear to emanate from the north side of the Colca river, just north of the town of Chivay. These domes are composed of andesites and trachyandesites marked by a high percentages of alkali feldspars (Palacios, et al. 1993). Two Potassium Argon dates from these flows by Sandor (1992: 232-235) indicate that they formed during the Middle to Late Pleistocene (64,000 ± 14,000 bp and 172,000 ± 14,000 bp).