sedimentary
Red Siltstone (or Red Shale)
Clastic Sedimentary Rock (Composition: primarily Quartz, Clay minerals, and Hematite Fe2O3)
Hardness: 3-4 (scratchable with a copper penny or steel nail); Color: Deep reddish-brown to brick red; Luster: Dull/Earthy; Crystal structure: Non-crystalline clastic grains; Cleavage: Often exhibits fissility or planes of layering; Specific gravity: 2.2-2.8
- Hardness
- 3-4 (scratchable with a copper penny or steel nail)
- Color
- Deep reddish-brown to brick red
- Luster
- Dull/Earthy
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Physical properties
Hardness: 3-4 (scratchable with a copper penny or steel nail); Color: Deep reddish-brown to brick red; Luster: Dull/Earthy; Crystal structure: Non-crystalline clastic grains; Cleavage: Often exhibits fissility or planes of layering; Specific gravity: 2.2-2.8
Formation & geological history
Formed through the lithification of silt-sized particles deposited in slow-moving water environments like floodplains, lagoons, or shallow marine basins. The red color suggests deposition in an oxygen-rich environment where iron minerals oxidized (rusted). Typical geological ages range from the Paleozoic (e.g., Catskill Formation) to the Mesozoic.
Uses & applications
Used primarily as low-grade construction fill, crushed stone for road bases, and historically in brickmaking. It is also used as a decorative landscaping stone due to its vibrant color.
Geological facts
The red color is caused by a very small percentage of hematite (iron oxide) coating the mineral grains. Large formations of this rock are responsible for the famous 'Red Rock' landscapes in the American Southwest and the 'Old Red Sandstone' of Great Britain.
Field identification & locations
Identify in the field by its fine-grained texture (gritty but smoother than sandstone), prominent red-brown streak when rubbed on a porcelain plate, and the presence of thin, flat layers. Common in the Appalachian Mountains, the Colorado Plateau, and the UK. Collector tip: Look for fossilized ripple marks or mud cracks on the bedding surfaces.
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