sedimentary

Siltstone with flute casts or scratch marks

Clastic Sedimentary Rock (Siltstone/Fine-grained Sandstone)

Hardness: 3-5 (Mohs), Color: Grey to olive-green with lighter tan markings, Luster: Dull/Earthy (though looks wet/shiny in photo), Crystal structure: Clastic (non-crystalline), Cleavage: None, Specific Gravity: approx 2.6.

Identified March 31, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the lithification of silt-sized particles in low-to-medium energy environments, such as floodplains, riverbeds, or lagoons. The linear markings may be tool marks or 'scratches' caused by debris dragging across the soft sediment surface before hardening.

Uses & applications

Used primarily as aggregate in construction, fill material, and occasionally for flagstones if well-bedded. Of interest to geologists for paleo-environmental reconstruction.

Geological facts

Siltstones are intermediate between sandstone and shale. The markings on this specimen are often used by geologists to determine 'paleocurrent,' or the direction water was flowing millions of years ago when the rock was mud.

Field identification & locations

Identify in the field by its fine-grained texture (feels grittier than shale but smoother than sandstone). Look for sedimentary structures like these linear marks on the bedding planes. Commonly found in river basins and ancient lake deposits.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

Arenite (primarily SiO2)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

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Sandstone (with man-made markings)

Arenite (composed primarily of Quartz, SiO2)

sedimentary