Tag Archive for: St. George Basin

St. George basin, Washington County, Utah
Photographer: Robert F. Biek

Ruins of the Harrisburg town site, established by Mormon pioneers in 1861, consist of blocks of Early Jurassic-age Springdale Sandstone, which a mile to the north is the main ore-bearing horizon of the Silver Reef mining district. The mining district is famous among geologists for its uncommon occurrence of silver ore in sandstone. In the distance, the Pine Valley Mountains tower 7,000 feet above the surrounding red-rock country.

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.