Coal the heat and pressure of deep burial causes the loss of water, methane and carbon dioxide and an increase in the proportion of carbon. Thus first lignite (also called "brown coal"), then sub-bituminous coal, bituminous coal, and lastly anthracite (also called "hard coal" or "black coal") may be formed. The wide, shallow seas of the Carboniferous Period provided ideal conditions for coal formation, although coal is known from most geological periods. The exception is the coal gap in the Permian–Triassic extinction event, where coal is rare. Coal is known from Precambrian strata, which predate land plants—this coal is presumed to
Coal Palace red mortar. Vitric coal was also used to spell "Coal Palace" above the main entrance. Sioux City architect Charles P. Brown was contracted to design the Coal Palace; he had previously designed the first Corn Palace in Sioux City and was at the time planning the second. Born in Salem, Massachusetts, Brown moved to Sioux City in 1886 and had designed much of its downtown. Brown recognized the inherent medieval quality of walls covered in uncut coal and capitalized on this theme by adding turrets, crenellated walls, and spired roofs. Ten flags with the names of the ten coal-donating counties