The Black Hills form a vast dome-shaped landform running roughly northwest from Hot Springs, South Dakota, to Sundance, Wyoming, about 120 miles long and 60 miles wide, with an ancient inner core of highly deformed and steeply dipping sedimentary and igneous rocks ranging from 1.7 to more than 2.5 billion years old. Their geology reflects a long, complex history dating to some of the earliest known rock formations on earth. The central crystalline core is surrounded by younger Paleozoic, Mesozoic, and Tertiary sediments that dip away from the uplift, along with a distinctive ring-like red band of sediments known as the Red Valley or Racetrack. Spearfish Canyon is a notable feature of this uplift, an approximately 800-foot-deep channel cut through the gently dipping sediments of the Western Limestone Plateau, where thinly layered sediments across the creek are among the oldest fossil-bearing layers in the region and the massive cliffs above are formed from the Pahasapa or Madison Limestone. This limestone contains many caves, including Wind Cave and Jewel Cave, and serves as a major water-bearing aquifer for much of the upper Midwest. East of Spearfish Canyon, the Black Hills gold region has produced more than 42 million ounces of gold since discoveries near Deadwood in 1875, while other mining districts have yielded gold, silver, lithium, beryllium, tin, feldspar, and mica. In the northern Black Hills, Devils Tower and Bear Butte are examples of Tertiary igneous intrusions that pushed up through older rocks and are known to bear other precious metals and rare elements.