Sentence:  This sloshing was compounded by the fact we were unable to keep in step with each other due to the many and various sized <WSD>niggerheads</WSD> (hard tussocks in tundra) that would attempt to roll from under us as if one was stepping on balls ranging in size from softballs to basketballs, with each step if unable (most of the time we could not) to avoid. 
Meaning of the word marked with <WSD> tags:  A tussock (clump of plant material), found on tundra.