Sentence:  Responsible in large part for the difficulties of movement to which the geologists referred were tussocks, still known as “<WSD>niggerheads</WSD>” in the 1930s. The “<WSD>niggerhead</WSD>,” according to Bob Marshall, “among the gifts of nature, ranks as the most cursed....” 
Meaning of the word marked with <WSD> tags:  A tussock (clump of plant material), found on tundra.