Pennsylvanian rocks of the Eastern Interior basin underlie about 53,000 square miles in Illinois, Indiana, and western Kentucky, with small outliers in Missouri and Iowa, attaining maximum thickness of about 2,500 feet in southeastern Illinois. At the beginning of the Pennsylvanian there was a regional southwest slope furrowed by numerous subparallel valleys as deep as 200 feet. An eastward slope prevailed along the western border of the basin with smaller valleys. Subjacent strata range in age from the Middle Ordovician St. Peter sandstone to the Upper Mississippian Kinkaid limestone. Because of topographic unevenness and early Pennsylvanian tectonism, the early Pennsylvanian Caseyville and Tradewater units vary from zero to 1,200 feet. Movements along the LaSalle anticl ne and Duquoin monocline are reflected in an isopach map of this interval. In later Pennsylvanian Carbondale and McLeansboro time the intervals increase regularly toward the southeast, showing that the present southeast border of the basin is due to post-Pennsylvanian deformation. Structural relief on the No. 2 coal ranges from +700 to -913 feet. Similar systems of nomenclature are used in Illinois and Kentucky but different names are employed in Indiana. The significant features of 46 key beds are described.

Clastic ratios average about 8 parts clastic to 1 non-clastic. Sand-shale ratios are extremely variable due to lenticular sandstones at more than 20 positions. Many sandstones occupy channels excavated into underlying strata; others seem to be off-shore barrier beach deposits. There are marine limestones at about 25 different positions, generally thickening toward the western border of the basin, but each has its individual pattern of distribution. Lower Pennsylvanian sandstones are highly quartzose and derived from older sediments; later sandstones are micaceous and feldspathic with much interstitial clay, evidently a first-cycle deposit from a metamorphic terrane. Coals are generally banded, and range in rank from high-volatile to medium-volatile bituminous. Refractory underclays ar found in the Lower Pennsylvanian in northern Illinois and in the area near St. Louis, Missouri.

Cyclic sedimentation is very well displayed with at least 25 alternations of coal, marine limestone, shale, sandstone, and underclay. Early Pennsylvanian sediments came from the east and northeast but the source of the later micaceous sandstones is still uncertain. Most marine invasions came from the west, north of the Ozark uplift, but some in the early Pennsylvanian probably entered the basin from the east and south.

The basin has produced about 5 billion tons of coal and 325 million barrels of Pennsylvanian oil. Other industries based on Pennsylvanian materials are refractories, common brick and tile, road metal, agricultural limestone, portland cement, rock asphalt, glass sand, foundry sand, and whetstone.