wacke
Concise Oxford English Dictionary © 2008 Oxford University Press:
wacke /ˈwakə/
▶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 C19: from Ger., from Mid. High Ger. wacke ‘large stone’.
'wacke' also found in these Oxford entries:

