Sentence:  The physical assumption underlying Firey's model is that the abraded particle (pebble) undergoes a series of small collisions with a very large, smooth <WSD>abrader</WSD>, and this might be the case when pebbles are carried by a fast river and collied repeatedly with the riverbed ... 
Meaning of the word marked with <WSD> tags:  Something that abrades; a tool or machine for abrading. (First attested in the mid-19th century.)