Raimey,
Not sure which disturbance you are asking about, as I see at least two.
The circular defect near the midddle of the photo appears to be from forge scale on the anvil being pounded into the damascus, causing a displacement of the edges of some of the layers.
There is an H shaped defect very near the breech end. This is easier to explain. This is caused by decarburization of the steel element in the damascus, due to it being exposed to the forge fire for an extended amount of time. Whenever steel is in the forge fire, carbon is being burnt out of it. The decarbed steel becomes simple iron and etches white. The H shape was caused by welds that were not quickly closed and were open in the fire for several more heats than adjacent areas.
