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 [Illustration: uncaptioned] _Illinois State Geological Survey Educational Series 9_ Transcriber’s Notes —Silently corrected a few typos. —Retained publication information from the printed edition: this eBook is public-domain in the country of publication. —In the text versions only, text in italics is delimited by _underscores_.