Years of operation of the C+O Canal produced some spots where the tow ropes that connected the mules to the canal boat happened to rub on a cut stone of one of the locks. The next 2 photos show this:
Rope burns can occur in another situation, where the canal used river navigation immediately upstream of the dams which were built to supply water to the canal.
The mules remained on the towpath but the canal boats would actually be pulled in the river itself rather than in the water-filled ditch that is the C+O Canal. River navigation obviated the need for a separate canal waterway at that point.
If there was a sharp bend in the towpath, the long tow rope would tend to make a tangent across the arc of the bend and eventually wear a groove in the rocks of the cliff. In this example, looking downstream, the towpath makes a gentle bend right, then in the distance turns sharply left along the cliff at Milepost 107 (the stubby post on the right side of the towpath in the center of the shot).
The tow rope tended to rub along the cliff and cut grooves as seen in this shot:
Man, I love this historic archeology! Especially seeing little things like this that most people would totally miss.