sedimentary

Fossiliferous Limestone with Brachiopod Mold

Calcium Carbonate (CaCO3) matrix containing Brachiopoda fossil impressions

Hardness: 3 (Mohs scale); Color: Cream to light tan matrix with translucent calcite-filled regions; Luster: Dull (matrix) to vitreous (fossil cast); Structure: Bioclastic/Fragmental; Cleavage: Rhombohedral (in calcite crystals); Specific Gravity: 2.71

Hardness: 3 (Mohs scale)
Luster: Dull (matrix) to vitreous (fossil cast)

Identified March 7, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in shallow marine environments during the Paleozoic era (approx. 250-500 million years ago). Created by the accumulation of shell debris and lime mud on the seafloor, followed by lithification over millions of years.

Uses & applications

Primarily used as a scientific learning specimen for paleontology and geology. On a larger scale, limestone is used for cement production, building stone, and soil conditioning.

Geological facts

The distinct 'finger-like' or ridged patterns are the internal molds or casts of Brachiopods, specifically from the order Spiriferida or Rhynchonellida, which dominated sea floors before the Great Dying extinction event.

Field identification & locations

Identify in the field by looking for ridged, shell-like patterns in outcroppings of grey or tan rock that fizz when exposed to dilute hydrochloric acid. Commonly found in road cuts or creek beds in the Midwest and Appalachian regions of the US.

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