One of Knoxville's nicknames is The Marble City. In the early 20th century, a number of quarries were active in the city, supplying Tennessee pink marble (actually Ordovician limestone of the Holston Formation) to much of the country. Notable buildings such as the National Gallery of Art in Washington are constructed of Knoxville marble. The National Gallery's fountains were turned by Candoro Marble Company, which once ran the largest marble lathes in the United States. Knoxville was once also known as the Underwear Capital of the World. In the 1930s, no fewer than 20 textile and clothing mills operated in Knoxville, and the industry was the city's largest employer. In the 1950s, the mills began to close, causing an overall population loss of 10% by 1960.