The Cincinnati arch is a platformlike structure located between the Michigan, Illinois, and Appalachian basins. Lower Silurian (Llandoverian) rocks crop out along the flanks of the arch but were removed almost completely from the central part of the structure by pre-Middle Devonian erosion. They consist primarily of marine limestone and dolostone. Paleontologic and lithologic evidence indicates that the Cincinnati arch area was the locale of a marine transgression in the early Llandovery, and regression and transgression during the late Llandovery. These events are summarized as follows.

1. Early Llandovery (A₁-A₄): There is an erosional unconformity between the Lower Silurian and Upper Ordovician; A₁ rocks are missing from this area. Silty A₂-A₄ dolomites are found solely on the east side of the Cincinnati arch.

2. Middle Llandovery (B₁-B₃): The arch was covered by a shallow epeiric sea resulting in the deposition of limestones and dolostones.

3. Late Llandovery (C₁-C₆): The depositional pattern of the area changed with the introduction of shales from the east (C₁-C₂). Shallow-water carbonates were deposited on the northern, western, and southern parts of the present arch. The interval from C₂-C₄ is marked by erosion and nondeposition, except for a few localities in southeastern Indiana where silty dolomites are found. The close of the Llandovery was marked by a second transgression of the sea with the deposition of shales on the southeastern side of the arch and carbonates on the northern, western, and southern sides.

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