A study of well samples from central Iowa shows that a distinct rock-stratigraphic unit is present beneath the Devonian Wapsipinicon Formation and above the characteristic dolomites and cherts of the Silurian Niagaran Series. This subsurface unit consists primarily of medium- to dark-gray chert and light-gray limestone. The unit has been traced into southwestern Iowa and its boundaries have been fairly well defined. The name La Porte City Chert, derived from the town of La Porte City in south eastern Blackhawk County, is proposed for this formation.
The lithologic characteristics and distribution of the La Porte City Chert in Iowa are shown by graphic well sections and a thickness map.