The Pittsburgh Coal Seam is the thickest and most extensive coal bed in the Appalachian Basin, an Upper Pennsylvanian coal bed that extends over 11,000 square miles through 63 counties from Allegany County, Maryland, to Belmont County, Ohio, and from Allegheny County, Pennsylvania, to Putnam County, West Virginia. Named by H.D. Rodgers of the First Geological Survey of Pennsylvania, it was first referenced on a 1751 map, and by the mid 1700s coal was being mined at Fort Pitt on Coal Hill, now Mount Washington, where drift mines worked an outcrop about 200 feet above the Monongahela River and the coal was moved by trench and canoe across the river to the fort. Around 1765, fire broke out in the mines and burned for several years, causing part of the face of the hill to collapse, and by the end of the 18th century coal mines ran along the face of Mount Washington for 1,800 feet. By 1830, Pittsburgh consumed more than 400 tons of coal per day for domestic and light industry use as coal replaced wood largely because of cost, with wood at two dollars a cord and delivered coal at six cents a bushel, while miners within a mile of Pittsburgh earned about $1.60 a week and could produce up to 100 bushels a day. After 1860, mining of the seam expanded rapidly as surging demand for iron, steel, and coke in the late 19th and early 20th century made Pittsburgh-seam coal, especially in the Connellsville District, the best coal for coke in America, raising production from 4.3 million tons in 1880 to 40 million tons in 1916 during the period known as the Golden Age of King Coal, Queen Coke and Princess Steel. Beginning in the 1910s, technological and industrial improvements, by-product coke ovens, the Great Depression, alternative fuels, more efficient iron furnaces, and the late 1970s and 1980s steel mill closings reduced demand and production, but after two centuries of mining, over 15 billion tons of coal remain, with the largest block in southwestern Pennsylvania and adjacent areas of West Virginia, while much of the remaining southern and western coal is unlikely to be extensively mined because it is high in ash and sulfur.