Conglomerate (River Rock)

Physical Properties

Hardness: 6-7 (based on quartz/chert clasts); Color: Multi-colored clasts in a gray/brown matrix; Luster: Dull/Earthy; Structure: Clastic, consisting of rounded pebbles cemented together; Specific Gravity: 2.6-2.8

Formation & Geological History

Formed in high-energy environments like riverbeds or alluvial fans where water current is strong enough to carry large rounded clasts. Over millions of years, these sediments are buried and cemented by minerals like calcite or silica.

Uses & Applications

Used primarily as aggregate in construction, road base material, and decorative landscaping stone. Not typically used in jewelry unless it contains high-quality semi-precious clasts like jasper.

Geological Facts

Conglomerates are often called 'pudding stones' due to their appearance of large 'ingredients' (clasts) stuck in a 'batter' (matrix). They are essential indicators for geologists to locate ancient river channels.

Field Identification & Locations

Identify by looking for rounded, water-worn pebbles of various sizes embedded in a finer-grained sandy or silty matrix. Generally found in areas with history of glacial movement, mountain basins, or active/ancient river systems.

Identified on: 4/19/2026

Mode: Standard