Lithology of the Horton Creek is variable and may consist of siltstone, shale, sandy limestone, micritic limestone and dolomite, oolitic limestone, and limestone conglomerate. Includes Hamburg Oolite Bed at top according to Cluff and others (1981). Maximum thickness of the formation is more than 60 ft in Bond and Montgomery Cos. where it consists mostly of siltstone. Where the limestone of the Horton Creek is not present, the shale and siltstone cannot be differentiated from the overlying Hannibal Shale and a vertical cutoff marks the limit of the formation. Beyond the area underlying the Louisiana Limestone the unit cannot be differentiated from the Saverton Shale. In outcrop areas, conodont faunas show that the formation is Mississippian in age and that the base of the formation is the Devonian-Mississippian boundary, but Cluff and others (1981) believe the unit may be Devonian or Devonian and Mississippian in the subsurface.

Source: GNU records (USGS DDS-6; Reston GNULEX).