sedimentary
Silty Shale or Mudstone
Siltstone/Mudstone (Clastic Sedimentary Rock)
Hardness: 3-4 (Mohs); Color: Dark grey, charcoal, with brown limonitic weathering; Luster: Dull or earthy; Structure: Fine-grained, fissile or blocky; Cleavage: Parallel to bedding planes.
- Hardness
- 3-4 (Mohs)
- Color
- Dark grey, charcoal, with brown limonitic weathering
- Luster
- Dull or earthy
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Physical properties
Hardness: 3-4 (Mohs); Color: Dark grey, charcoal, with brown limonitic weathering; Luster: Dull or earthy; Structure: Fine-grained, fissile or blocky; Cleavage: Parallel to bedding planes.
Formation & geological history
Formed through the lithification of silt and clay particles deposited in low-energy environments such as river floodplains, lagoons, or deep marine basins. These rocks can date from any geological era depending on the deposit location.
Uses & applications
Primarily used in the manufacture of cement, brick making, and crushed stone for road construction. Some organic-rich shales are sources of natural gas or oil.
Geological facts
Shales account for over 50% of all sedimentary rocks found on the Earth's surface. They are excellent for preserving fossils due to the fine grain size which can capture intricate details of organisms.
Field identification & locations
Identify by its fine-grained texture, tendency to split into thin sheets (fissility), and muddy smell when wet. Commonly found in riverbeds and coastal cliffs. Collectors should look for signs of fossil impressions along bedding planes.
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