POTD September 19, 2013: St. George basin, Washington County, Utah
St. George basin, Washington County, Utah
Photographer: Robert F. Biek
Beyond the 350,000-year-old (Pleistocene-aged) Sullivans Knoll (Volcano Mountain) cinder cone near the town of Hurricane, the snow-covered Pine Valley Mountains are the eroded remnants of one of the world’s largest laccoliths, a shallow, mushroom-shaped igneous intrusion that formed about 20 million years ago. The red, Jurassic-aged Navajo Sandstone in the middle ground is on the northwest-tilted limb of a large upwarp called the Virgin anticline.