and produced welded ignimbrite. The Needle Range ash flow tuff is the most extensive, covering 13,000 square miles in southwest Utah and eastern Nevada. While most tuffs are rhyolite, the Needles Range tuff includes amphibolite, biotite and plagioclase phenocrysts. Volcanism was related to the subducting Farallon Plate and took place from 35 to 19 million years ago, predating the block faulting of the Basin and Range Province making tuff useful for stratigraphic comparisons. Several unusual laccolith intrusions formed including the Henry, La Sal, Abajo and Three Peaks mountains. Volcanic activity lessened from 19 to 16 million years ago, followed by basalt flows, cinder cones and the Topaz-Spor Mountain rhyolite into the Miocene, Pliocene and Quaternary. During this time and continuing to the present, block faulting created the basin and range terrain in western Utah and Nevada. Lake Bonneville formed during the Quaternary, covering 20,000 square miles. The Great Salt Lake is the remnant of the deepest part of the lake, where the depth was once as much as . During the Pleistocene, glaciers formed in the mountains and in the case of the Temple Lake Stade and Gannett Peak Stade, advanced 4,000 years ago in the Holocene. Natural resource geology In the 1800s, coal extraction at Coalville was used to supply the Salt Lake Valley, Cedar City and iron mines, with a shift to Carbon County in the 1900s. References Utah