wacke /ˈwakə, ˈwækə 
/ ▸ noun Geology a sandstone of which the mud matrix in which the grains are embedded amounts to between 15 and 75 percent of the mass.
– ORIGIN early 19th century : from German, from Middle High German wacke ‘large stone’, Old High German wacko ‘pebble’.
EnglishOxford New American Dictionary
