File:Guard Lock 4 on C and O Canal From NPS.jpg

English: Pic of Guard Lock #4, entering Big Slackwater, on the Chesapeake and Ohio Canal. The NPS site states: Guard Lock 4 was completed in 1834 out of a smooth cut, gray-blue limestone on the face. This lock was used as a pass for all barges, as the river becomes the canal for the next 3.3 miles. The upper end of the lock passes through the guard dyke and the dyke is confined by vertical stone retaining walls. The walls are 15' apart, with a 15" recess on each side for a length of 10' for gate pockets. The high walls through the guard dyke (which is also the upper gate pocket) is stepped down in 12" increments of layers of stone (with 12" treads) for 10 levels, thereby decreasing the lock walls by 10' to the normal height. The lock is 91'-5" between gate pockets. The lock gate, at the edge of the river, was an unusual 20' tall. Rather than a normal top and bottom iron gate post strap that lets into the stone quoin to support the wood posts, a third strap was added midway between the top and bottom to help take the added weight off the particularly heavy gates. Above the upper lock gate pocket, the towpath walls extends for about 2', makes a ninety degree turn with a radius of 2' and runs for 21' along the river bank, then tapers into the guard dyke. The berm side extension is 15½' long. A service bridge spans above the upper portion of the upper lock gate pocket. The bridge is probably one of the many replacements of the original bridge that tow mules had to cross. Some lock hardware remains.