This rock cannon consists of a series of holes drilled vertically into a slab of bare rock, and joined by shallow grooves; it is located just below the summit of Garreg Flaenllym, overlooking Blaenau Ffestiniog. Rock cannons were fired to celebrate special events such as visits of royalty. The holes were partly filled with black gunpowder, and a trail of powder was also laid in the grooves and covered with stone dust. The powder at one end was ignited, and the holes fired in sequence, the length of each groove between two holes dictating the time between each explosion.
W J Crompton, RCAHMW, 2 August 2012.