sedimentary
Piddock bored rock (likely Mudstone or Shale)
Ichnofossil (Gastrochaenolites borings) in Fine-grained Sedimentary Rock
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.
Identified More sedimentary →
Identify your own rocks.
Get a report just like this from any photo, free.
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.
More like this
Other sedimentary specimens
Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
Sandstone
Arenite (primarily SiO2)
sedimentary
Chert Breccia
Brecciated Chert (Microcrystalline Silica)
sedimentary
Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
sedimentary
Shale or Slaty Mudstone
Argillaceous sedimentary rock
sedimentary
Sandstone
Arenite
sedimentary