The greatest production of coal by far is from the Appalachian Mountain and Allegheny Plateau districts, from the western half of Pennsylvania to Alabama, where all the coal is bituminous of Pennsylvanian Age. Here as well as elsewhere the coal beds are interstratified with various kinds of sedimentary rocks, most commonly with shales and sandstones. In the Appalachian field the strata including coal beds are more or less folded toward the east, while they are nearly horizontal toward the west. The famous Pittsburgh coal bed is probably the most extensive important single coal bed known. It covers an area of over 12,000 square miles and is workable, with a thickness of five to fifteen feet, over an area of 6,000 square miles of parts of western Pennsylvania, Ohio, and West Virginia. [Illustration: Fig. 76.--Map of the United States, showing the principal coal fields. Cross-lined areas represent lignitic coals. (After U. S. Geological Survey.)] The greatest production of anthracite coal by far is from central-eastern Pennsylvania, where strata of Pennsylvania Age, including a number of anthracite beds, are mostly highly folded. Most remarkable of all in this district is the so-called "mammoth bed" of anthracite, nearly everywhere present, with a thickness up to as much as fifty or sixty feet. Less than 500 square miles are there underlain by workable anthracite coal. Next to the greatest production of coal in the United States is from the two large areas in the middle of the Mississippi Valley. It is all bituminous coal, associated with nearly horizontal strata of Pennsylvanian Age. The scattering areas through the Rocky Mountains yield all types of coal--anthracite, bituminous, and lignite. In some of these areas the coal beds have been but little disturbed from their original horizontal position, but usually they are more or less folded along with the inclosing strata, the crustal disturbances affecting the coal beds having taken place late in the Mesozoic era and early in the Cenozoic era. Practically all of these coals are of Cretaceous and Tertiary Ages, the best being Cretaceous. Very little of the Rocky Mountain coal is anthracite. On the Pacific Coast coal production is relatively very small. The coals are there bituminous to lignitic of Tertiary Age, usually folded in with the strata. In Alaska there are widely distributed, relatively small coal fields, but they have been little developed. Alaskan coals range in age from Pennsylvanian to Tertiary, and in kind from anthracite to lignite.