This is a carbonate hardground from the Upper Ordovician
of the Cincinnati region. The hardground formed a resistant layer
on the seafloor during the Ordovician. It was undermined by currents,
which produced a small cavity, and then a trepostome bryozoan
colonized the underside of the hardground on what would have
been the cave roof. Above the bryozoan, perpendicular to the
bedding, is a thin Trypanites boring. Both the bryozoan
and the boring show that this surface was cemented on the seafloor.
This outcrop of the Grant Lake Limestone is exposed near Washington,
Kentucky. (Back to outline or next
hardground.)
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