Graywacke
“The turbidity current survivor”
Sedimentary
Graywacke is a dark, hard, poorly sorted sandstone formed from sediment rapidly deposited by underwater landslides called turbidity currents. Its mix of angular grains, rock fragments, and fine matrix records dramatic deep-sea events where sediment cascaded down continental slopes.
Composition
quartz, feldspar, rock fragments, clay matrix, chlorite
Where Found
Harz Mountains (Germany), New Zealand, Scottish Highlands, Appalachian Mountains (USA), Wales
Mohs Hardness
6-7
Fun Fact
Some of the oldest known rocks on Earth — the 4.28-billion-year-old Nuvvuagittuq rocks in Quebec — are ancient graywackes from the early ocean floor.
Test Your Knowledge
What depositional process creates the poorly sorted, angular grain texture characteristic of graywacke?
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