Type locality and use of name: The terms Eau Claire Grit and Eau Claire trilobite beds were first used in a casual manner (Wooster, 1878) for clastic rocks exposed near the Dalles of the Chippewa River in Wisconsin (Trowbridge and Atwater, 1934). More formal use dates from 1914, when Walcott (p. 354), crediting an unpublished manuscript by E. O. Ulrich, applied the name Eau Claire Sandstone to about 100 feet (30 m) of thin-bedded and shaly sandstones overlying the Mount Simon Sandstone in exposures along the Eau Claire River, Eau Claire County, Wis.
Description: The Eau Claire Formation in Indiana is known only in the subsurface and has been described by Gutstadt (1958a), Becker, Hreha, and Dawson (1978), and others. The Eau Claire consists of the following rock types: (1) dolomite, feldspathic, and partly glauconitic siltstone; (2) very fine grained to fine-grained, generally well sorted sandstone; (3) maroon and dark-brown micaceous shale; (4) silty dolomite; and (5) oolitic limestone. The Eau Claire everywhere overlies the Mount Simon Sandstone, the contact being conformable. In northwestern Indiana the Eau Claire rocks are predominantly sandstones that grade upward into the Galesville Sandstone. Elsewhere the other rocks mentioned above compose the formation, and the Eau Claire is generally overlain conformably by the Davis Formation. The oolitic limestone zones in the Eau Claire occur near or at its top and are distributed only in southern Indiana. In southwestern Indiana the Eau Claire grades upward into the Potosi Dolomite of the Knox Supergroup (Droste and Patton, 1985). The Eau Claire Formation ranges in thickness from about 400 feet (122 m) in northeastern Indiana to more than 1,000 feet (305 m) in southwestern Indiana.
Correlation: The Eau Claire Formation is known by the one name in Indiana, Illinois, Wisconsin, Michigan, western Ohio, and western Kentucky its equivalent in Easter Missouri is the Bonneterre Dolomite. Cores of the Eau Claire taken from Vermillion County in western Indiana have yielded a trilobites assemblage consisting of representatives of the Cedaria Zone (early Dresbachian age) and the Crepicephalus Zone (middle Dresbachian) (Palmer, 1982).