A graywacke characterized by abundant unstable materials; specif., a sandstone containing generally less than 75% of quartz and chert and 15% to 75% detrital clay matrix, and having feldspar grains (chiefly sodic plagioclase, indicating a plutonic provenance) in greater abundance than rock fragments (indicating a supracrustal provenance) (Pettijohn, 1954; 1957). Compare: lithic graywacke.