The original wooden miter sill and timber floor of Chesapeake & Ohio Lock No. 55 in Maryland are visible in this photo, taken in 1959–60. The recess in the lock wall for one of the lock gates, including the rounded pier for the gate’s quoin post, is visible at left. When the gate was closed, water pressure would have pressed it tightly against the quoin pier and mitre sill, ensuring a tight seal. Original Erie Canal locks were also built this way. (Jack E. Boucher, Historic American Buildings Survey, Library of Congress)