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 (grā′wăk′, -wăk′ə)
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 Indo-European roots).]
References in periodicals archive ?
In my line of sight, the castle is just a rocky outcrop, St Giles' Cathedral is a wall of dirty sandstone and the different colours of the Grassmarket shops are hardly noticeable in the periphery.