This is Gaelic for small rocky hillock (of which there are thousands in the Outer Hebrides!).  It is the home of the gome witch Mhairhidgh Dubh (“Varry Doo”).  Two large boulders hide a cave between them.  A stinking peat bog sits in front of the boulders and out of this swamp Mhairhidgh Dubh emerges, sitting on a rotting tree stump as a throne, and covered in midges.

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We are first introduced to Cnoc Liath in Twilight Cave, Chapter LVI, A Fine Walk on the Moors but it takes on a bigger and bigger significance as the Wullie saga continues.