Greywacke

Physical Properties

Hardness: 6-7 (Mohs scale); Color: dark grey to greenish-grey; Luster: dull to earthy; Crystal structure: clastic (poorly sorted framework grains); Cleavage: none, breaks unevenly; Specific gravity: approx. 2.6-2.7

Formation & Geological History

Formed in deep-sea environments (turbidite deposits) from the rapid deposition of poorly sorted sediment. Often dates back to various geological eras from Paleozoic to Mesozoic.

Uses & Applications

Primarily used as high-quality aggregate for road construction, concrete production, and railroad ballast due to its durability and resistance to wear.

Geological Facts

Greywacke is often called 'dirty sandstone' because of its high matrix content. It is a key indicator of ancient submarine fan systems and tectonic activity.

Field Identification & Locations

Identify in the field by its dark color, extreme hardness compared to other sandstones, and the presence of visible angular grains of quartz and feldspar in a fine-grained matrix. Commonly found in folded mountain belts and coastal ranges.

Identified on: 5/3/2026

Mode: Standard