Eleanor,

Not all soil or rock reddens from heat, no matter what the temperature. Those
geological materials containing iron oxides will redden from heat. Many rocks
pick up iron oxide from soil matrix and leaching into cracks and voids. Iron
rich rocks and soil will oxidize from fire, which causes the reddening effect
to which you speak. Intense heat can burn out the iron oxide after prolonged
periods.

Ron May
Legacy 106, Inc.