Sentence:  The smallness of the damage done at Purfleet is remarkable, when it is remembered that the impact of a large steamship is sufficient to cut deeply into solid masonry piers, and that only in 1907 the effect of a collision against the souther breakwater of the Admiralty Harbour, Dover, was to sever the <WSD>roundhead</WSD> entirely from the body of the pier, in addition to causing other serious injuries. 
Meaning of the word marked with <WSD> tags:  A circular platform at the end of a pier or breakwater.