Iztaccíhuatl (the "Woman in White") presents the profile of a sleeping woman as seen from the Valley of Mexico. A series of overlapping cones constructed along a NNW-SSE line to the south of the Pleistocene Llano Grande caldera forms the summit ridge of the massive 450 km3 volcano. Andesitic and dacitic Quaternary volcanism has taken place from vents at or near the summit. Valley headwalls near the summit vent (El Pecho) have lava flows and tuff deposits post-dating a glaciation about 11,000 years ago. The youngest vents are located at the summit and a depression at 5,100 m along the summit ridge midway between El Pecho and Los Pies (White, 1986). Nixon (1989) considered all summit lavas to be Pleistocene, but noted a postglacial dacitic vent on the S flank of Iztaccíhuatl, north of the saddle between it and Popocatépetl.