Photographs of the The San Rafael Swell. the Swell is a large geologic feature located in south-central Utah, USA about 30 miles (50 km) west of Green River, Utah. The San Rafael Swell, approximately 75 miles (121 km) by 40 miles (64 km), consists of a giant dome-shaped anticline of sandstone, shale, and limestone that was pushed up millions of years ago[vague]. Since that time, infrequent but powerful flash floods have eroded the sedimentary rocks into numerous valleys, canyons, gorges, mesas and buttes. The Swell is part of the Colorado Plateau physiographic region.