Piddock bored rock (likely Mudstone or Shale)

Physical Properties

Hardness: 3-5 (varies by mineral content), Color: dark grey to black but weathers to brown, Luster: dull to earthy, Crystal structure: microcrystalline, Features: circular cylindrical holes bored by bivalves.

Formation & Geological History

Formed through the lithification of silt and clay. The distinctive holes are caused by piddocks (boring clams) like Pholadidae, which use their rough shells to mechanically drill into soft submarine rock for protection during the Holocene era to present.

Uses & Applications

Primarily of interest to collectors of oddities, beachcombers, and for educational purposes regarding marine biology and ichnology.

Geological Facts

The holes are biological trace fossils known as Gastrochaenolites. These clams can drill into very hard material, including wood, concrete, and limestone, using rotating movements of their shells.

Field Identification & Locations

Look for smooth, perfectly circular holes of uniform diameter in rocks found in the intertidal zone. These are common on coastal beaches with soft sedimentary outcroppings.

Identified on: 5/3/2026

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