graywacke
/ˈɡreɪˌwæk/
noun
- A dark, coarse-grained sedimentary rock composed of sand-sized particles and clay, often found in ancient mountain belts.
- The geologist identified the dark cliff as graywacke from the Paleozoic era.
- The road cut exposed layers of graywacke and shale.
- Graywacke is commonly used as a building stone in some regions.