Argillaceous interval in the Silurian of Ohio, USA.

Dolostone is a common lithology in the Silurian of America's Midwest. Dolostone is a chemical sedimentary rock composed of the mineral dolomite (CaMg(CO3)2 - calcium magnesium carbonate). Dolomite and dolostone are usually gray in color and readily bubble in acid only when powdered.

Seen here is a shaly/muddy intervals in the upper Tymochtee Dolomite (pronounced "Tie-mock") - the yellowish-colored material has been more intensely weathered than the gray material.

Description of unit from Herdendorf (1993):

Tymochtee - 170 feet thick. Dolomite, dark, bluish-gray to brown, thin-bedded to shaly; calcareous shale partings in upper beds; containing gypsum and anhydrite; uppermost 5 meters exposed on South Bass Island, Catawba Island, and West Sister Island.

Stratigraphy: Tymochtee Dolomite, upper Salina Group, Ludlovian Series, lower Upper Silurian

Locality: lakeshore cliff, South Bass Island State Park, western South Bass Island, western Lake Erie, far-northern Ohio, USA

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