wacke /ˈwaki 
/ ▸ noun [mass noun] Geology a sandstone of which the mud matrix in which the grains are embedded amounts to between 15 and 75 per cent of the mass. – ORIGIN late 18th century: from German, from Middle High German wacke ‘large stone’, Old High German waggo ‘pebble’.
EnglishOxford Dictionary of English
