Sentence:  Long observation has taught Mason that most landscape can be reduced to three essential planes : a foreground in sharp focus - either a light <WSD>area</WSD> with dark accents or a dark one with lights ; a middle distance often containing the major motif ; and a background , usually a silhouetted form foiled against the sky . 
Meaning of the word marked with <WSD> tags:  a particular geographical region of indefinite boundary (usually serving some special purpose or distinguished by its people or culture or geography)