Late Pleistocene Glacial Sequence for the West Side of Iztaccihuatl, Mexico
A sequence of late Pleistocene glacial deposits on the west side of the volcano Iztaccihuatl, Mexico, represents (oldest to youngest) the Tomicoxco, Diamantes, Alcalican, and Ayolotepito substages, determined by study of stratigraphy of lava and ash alternating with glacial deposits, comparisons of morainal topography and degree of weathering and of soil development, and similarity of sedimentary characteristics. Till-like sediments in older alluvial deposits along the west base of the mountain suggest an earlier glaciation; these extend down to an altitude of 2450 m. Pyroclastic sediments overlie these, and all are severely eroded, weathered, and support a thick yellow Podzolic soil. During the Tomicoxco Substage, of two distinct ice advances, moraines of Nexcualango Till were deposited across lower mountain slopes; outwash reached the Mexico basin and is the only outwash correlated with a former lake level (the highest) in the basin. Moraines on the mountain are eroded and support a strongly developed gray-brown Podzolic soil. Volcanic activity occurred intermittently high on the north end. A belt of striking moraines of Hueyatlaco Till representing the Diamantes Substage, showing evidence of an interval of erosion and of soil formation, was built higher across the mountain, covering in places a new flow. Lava inundated the upper ends of several moraines of this till, and all the region was mantled with ash and lapilli. The moraines have been eroded and bear a moderately developed soil. Glaciation during the Alcalican Substage left small moraines of Milpulco Till in the southwest valleys only. Black ash then fell on the mountain; a weak soil is developed on these latest deposits. During the Ayolotepito Substage, a final but minor episode of refrigeration, huge moraines of Ayoloco Till were built in the mouths of the valley heads. Since then the little glaciers in the valley heads have retreated, leaving three or four recessional moraines.