The Thirtynine Mile volcanic area, Central Colorado volcanic field, is an extinct volcanic area located in Park and Teller counties, Colorado, northwest of Cripple Creek and southeast of South Park. The area was the site of significant volcanism in the Paleogene Period about 35 million years ago.  Ashfall and lahars (mudflows) from the volcanoes created the conditions for fossilization at what is now Florissant Fossil Beds National Monument.