sedimentary
River Cobble (Conglomerate context)
Unconsolidated Fluvial Alluvium (comprised of various lithologies like Quartzite, Granite, or Basalt)
Hardness: variable (typically 6-7 if quartz-rich); Color: shades of tan, grey, and brown; Luster: dull to earthy when dry; Crystal structure: various (clastic texture); Cleavage: none; Shape: sub-rounded to rounded due to water erosion.
- Hardness
- variable (typically 6-7 if quartz-rich)
- Color
- shades of tan, grey, and brown
- Luster
- dull to earthy when dry
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Physical properties
Hardness: variable (typically 6-7 if quartz-rich); Color: shades of tan, grey, and brown; Luster: dull to earthy when dry; Crystal structure: various (clastic texture); Cleavage: none; Shape: sub-rounded to rounded due to water erosion.
Formation & geological history
Formed through the erosion of upstream bedrock, followed by transportation and rounding in high-energy river environments. These deposits are often Holocene to Pleistocene in age, found in alluvial fans and riverbeds.
Uses & applications
Used primarily in the construction industry as aggregate for concrete, road base, and decorative landscaping. Larger smooth stones are popular in xeriscaping.
Geological facts
The rounded shape of these stones is a result of 'attrition,' where rocks collide with each other in moving water, wearing down sharp edges over thousands of years.
Field identification & locations
Identify in the field by their smooth, 'river-worn' rounded surfaces and presence in layered soil banks (alluvial deposits). Common in mountain valleys and glacial outwash plains.
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