Existing comment:
Volcanic Ash:
About 225 million years ago, Arizona was in the tropics. The climate throughout the Southwest was mostly humid, with alternating wet and dry periods.
Southern Arizona was higher than northern Arizona. As rocks in the southern highlands erodes, streams carried the sediments here and deposited them in lakes and swamps on a flat floodplain.
Interspersed with erosion were pulses of volcanic activity in the southern highlands. Volcanic ash rained down into streams, was carried here and deposited. The ash (now bentonite clay) and the sediments are what make up these formations. |