Sentence:  They pass through a cleft that has been made across a low range of hills, like a <WSD>kerf</WSD> in the top of a log, and enter into a lovely territory of subtly swelling emerald green fields strewn randomly with small white capsules that he takes to be sheep. 
Meaning of the word marked with <WSD> tags:  The groove or slit created by cutting or sawing something; an incision.