An 119-square-mile area within the Heavener 15' quadrangle, Le Flore County, in the eastern part of the Arkoma Basin, Oklahoma, was investigated to determine the geology of the coals in the Hartshorne Formation (Desmoinesian) The Lower Hartshorne coal lies about 60 feet above the base of the Hartshorne Formation, and is low to medium-volatile bituminous in rank (70-84% fixed- carbon, mmf). It is low in sulfur (0.5-3.0%) and ash (8.6-14.2%), is 1.5 to 6 feet thick, and has a calorific value of about 14,000 Btu. A free-swelling index of 9 indicates that the coal makes a strong blast-furnace coke. Approximately 13,356,000 short tons of coal has been mined and lost-in-mining, leaving an estimated 313,478,000 tons in the remaining-resources category. The northeastward increase in rank of the Lower Hartshorne coal was probably caused by a deep-seated heat source in eastern Arkansas. In the Arkoma Basin, folding induced by the Ouachita orogeny apparently did not affect rank of the coal. coal lies between 60 and 120 The Upper Hartshorne coal lies between 60 and 120 stratigraphically above the Lower Hartshorne coal. The coal is low- to medium-volatile bituminous in rank, in 11/2 to 3 feet thick, contains 0.8-2.6% sulfur, and it contains 126.161,000 tons of coal in the remaining-resources category. Hundreds of fossil tree trunks, sterns, and leaves are evidence for deposition within a crevasse splay and overbank environment. A southwestward flowing distributary stream was active during the deposition of the upper Hartshorne Formation. The Upper Hartshorne coal is thin where it overlies a channel-fill sandstone in the distributary; the coal is thicker in areas adjacent to the channel because of greater subsidence. It is proposed that the sediments which formed the Hartshorne Formation were deposited within a prograding deltaic-plain environment.