It is built of granite from a nearby quarry at Towednack .
It is has three piers supporting four 40 feet ( 12 m ) arches , giving a total length of 78 yards ( 71 m ) ; it is 78 feet ( 24 m ) high .
It is built of granite from a nearby quarry at Towednack and has three piers supporting four 40 feet ( 12 m ) arches , giving a total length of 78 yards ( 71 m ) ; it is 78 feet ( 24 m ) high .