The large, thick beachrock of Arrigunaga beach (Bizkaia, Spain) is unusual among beachrocks because of (a) Its location, at 43ºN latitude; (b) The substrate that was cemented, largely consisting on smelter slag mixed with natural beach sediment; (c) The timing and amount of slag dumping at sea, with millions of tons of waste dumped in the short interval AD 1902–1966; (d) The sudden cessation of dumping (AD 1966), followed by i...