CENOZOIC “Recent Life” Age of Mammals Quaternary 0-500′ Pleistocene or Glacial Age Recent—alluvium in river valleys Glacial till, glacial outwash, gravel, sand, silt lake deposits of clay and silt, loess and sand dunes; covers nearly all of state except northwest corner and southern tip Tertiary 0-500′ Pliocene Chert gravel; present in northern, southern, and western Illinois Eocene Mostly micaceous sand with some silt and clay; present only in southern Illinois Paleocene Mostly clay, little sand; present only in southern Illinois MESOZOIC “Middle Life” Age of Reptiles Cretaceous Mostly sand, some thin beds of clay and, 0-300′ locally, gravel; present only in southern Illinois PALEOZOIC “Ancient Life” Age of Amphibians and Early Plants Pennsylvanian Largely shale and sandstone with beds of coal, 0-3,000′ limestone, and clay (“Coal Measures”) Mississippian Black and gray shale at base; middle zone of 0-3,500′ thick limestone that grades to siltstone, chert, and shale; upper zone of interbedded sandstone, shale and limestone Age of Fishes Devonian Thick limestone, minor sandstones and shales; 0-1,500′ largely chert and cherty limestone in southern Illinois Age of Invertebrates Silurian Principally dolomite and limestone 0-1,000′ Ordovician Largely dolomite and limestone but contains 500-2,000′ sandstone, shale, and siltstone formations Cambrian Chiefly sandstones with some dolomite and shale; 1,500-3,000′ exposed only in small areas in north-central Illinois ARCHEOZOIC and PROTEROZOIC Igneous and metamorphic rocks; known in Illinois only from deep wells As shown by the diagrammatic rock column (fig. 2), rocks placed in the divisions called early Cenozoic and Mesozoic are next in age to the Pleistocene. The map (fig. 1) shows that the Cenozoic and Mesozoic rocks occur only in the extreme southern tip of Illinois because only that part of Illinois was covered by a northward extension of the forerunner of the Gulf of Mexico in which the deposits of sand, gravel, and clay were laid down. The next older division of Illinois rocks is called Pennsylvanian—or “Coal Measures”—because during the last century they were first extensively described by geologists working in Pennsylvania.