sedimentary
Bored Siltstone (Holey Rock)
Siltstone with Pholad borings (Lithophaga/Pholadidae trace fossils)
Hardness: 3-4 (Mohs); Color: Grey, tan, or light brown; Luster: Dull/Earthy; Texture: Fine-grained with smooth, cylindrical perforations; Cleavage: None/Conchoidal fractures.
- Hardness
- 3-4 (Mohs)
- Color
- Grey, tan, or light brown
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Grey, tan, or light brown; Luster: Dull/Earthy; Texture: Fine-grained with smooth, cylindrical perforations; Cleavage: None/Conchoidal fractures.
Formation & geological history
Formed from the lithification of silt and clay. This specific specimen was shaped by bioerosion, where marine bivalves (Pholads) used rotatory movements of their shells to bore into the rock over several years to decades during the Quaternary period.
Uses & applications
Primarily used as decorative pieces in aquarium landscaping (hardscape), garden ornaments, and collected as 'lucky stones' or geological curiosities.
Geological facts
The perfectly circular holes are bioerosion trace fossils; the clams that make them are known as 'piddocks.' When the piddock dies, the holes are often inhabited by other small marine creatures, creating a micro-ecosystem.
Field identification & locations
Identify in the field by looking for smooth, cylindrical holes of varying depths in soft sedimentary rocks along coastal tide pools and beaches. Commonly found on the Pacific coast of North America and the southern coast of England.
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