Complex sentence: Four large piers of the bridge are ornamented with the prows of Viking ships, carved in granite, which refer to a hypothetical voyage by Leif Eriksson up the Charles River circa 1000 AD, as promoted by Harvard professor Eben Horsford.
Split into simpler sentences: Four large piers of the bridge are ornamented with the prows of Viking ships, carved in granite. They refer to a hypothetical voyage by Leif Eriksson up the Charles River circa 1000 AD, as promoted by Harvard professor Eben Horsford.