gray·wacke 
(grāwăk′, -wăk′ə)
(grāwăk′, -wăk′ə)n.
Any of various dark gray sandstones that contain shale.
[Partial translation of German Grauwacke : grau, gray + Wacke, rock (from Middle High German, from Old High German waggo, boulder; see wegh- in the Appendix of Indo-European roots).]