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  • The Ordovician age Oneota Dolomite underlies all of Illinois except the northmost part of the state. It ranges in thickness from 100 feet in the north to over 500 feet in the south. This formation consists of fine- to coarse-grained, light gray to brownish gray, cherty dolomite that contains minor amounts of sand and, at its base, thin shaly beds.
  • The second earliest period of the Paleozoic era (after the Cambrian and before the Silurian), thought to have covered the span of time between 490 and 443 million years ago; also, the corresponding system of rocks. It is named after a Celtic tribe called the Ordovices. In the older literature the Ordovician is sometimes know as the Lower Silurian.
  • An aerial photograph or satellite scene that has been transformed by the orthogonal projection, yielding a map that is free of most significant geometric distortions.
  • Stratified glacially derived sediment (clay, silt, sand, gravel) deposited by meltwater streams in channels, deltas, outwash plains, on flood plains, and in glacial lakes.
  • The surface of a broad body of outwash formed in front of a glacier.
  • The loose soil, silt, sand, gravel or other unconsolidated material overlying bedrock, either transported or formed in place.
  • A crescent-shaped lake in an abandoned bend of a river channel. A precursor of a meander scar.
  • An erratic of massive, dark, hard sandstone that contains light-toned (generally buff-colored) soft concretions that commonly weather recessively to leave rounded out depressions in the rock.