Plopped on the sands of Koekohe Beach, the Moeraki Boulders are favorite jungle gym for kids and tourists alike. The Maori regarded them as jetsam washed ashore from the wreck of a large sailing canoe. Geologists regard them as concretions caused by the cementation of Paleocene mudstone—the equivalent of a 4-million-year-old stony pearl. Of the various nicknames for them, “hooligans’ gallstones” takes the cake.