Baked Soil! The thin reddish layer below the thick, massive basalt lava flow is an ancient soil (paleosol) that was cooked and oxidized - hence the reddish color - by the emplacement of the ~1,100 degree Celsius lava flow. This exposure is on Interstate-17S about 10 miles south of Flagstaff, Arizona. The basalt lava flow is one of thousands erupted in the Quaternary (Pleistocene and Holocene) San Francisco volcanic field. Lava flows here on the SW side of the field are probably on the order of 3-5 million years old.