At the Hopewell Rocks you can walk the sea floor in the morning and paddle over it in the evening. The Bay of Fundy, in New Brunswick, Canada, carries the highest tides on Earth, and around low water a staircase drops from the clifftop onto red rippled mud. The stacks standing on it run from twelve to twenty-one metres, each pinched at the waist and carrying soil on its head with spruce and birch growing out of it, hence flowerpots. You can follow the whole line of them along the base of the cliff. Where the sea has driven clean through the bottom of a stack, you walk under the rock and out the other side. Up close it is stacked beds of pebbles and sand tipped thirty or forty degrees, and those tilted seams are the lines the tide has been working ever since. Six hours later the floor is gone. The stacks have become tree-topped islets, and a guided kayak carries you over the exact patch of mud that was under your boots.
The mud is packed with a shrimp no longer than a grain of rice, Corophium volutator, at densities that can pass a hundred thousand in a square metre. From mid-July to mid-August the flats fill with tens of thousands of semipalmated sandpipers eating it shoulder to shoulder. They feed in furious shifts, because the same tide that lays the larder bare buries it again within hours. When something spooks them the whole flock lifts as one and flashes dark, then white, as it banks. The stairs come out in the autumn and the shore is left to the ice. Every flowerpot here is temporary, and spring does most of the killing. Warm days and freezing nights work water into the fissures until the ice levers the rock apart, as it did to Elephant Rock in 2016. Behind them, though, the cliff holds enough conglomerate to go on making new ones for roughly the next hundred thousand years.