Hopewell Rocks lets you walk on ground that will lie under 14 metres of seawater by nightfall. The Bay of Fundy carries the highest tides on Earth, and at low water you take a staircase down and walk out across red, rippled mud among a line of sea stacks 12 to 21 metres tall. They are pinched at the waist and still carry soil, spruce and birch on their heads, which is why they are called the flowerpot rocks. Six hours later that floor is gone. The stacks have turned into tree-topped islets, and kayakers glide over the exact spot where you stood. The point is to come twice. The rock they are cut from is a Carboniferous conglomerate, coarse natural concrete of pebbles and boulders shed from mountains some 350 million years ago.
The tides here are a matter of rhythm rather than size. The bay's water rocks from end to end on a natural beat of about 12.5 hours, almost exactly the interval at which the Atlantic tide returns to push again. Every push lands in time with the last, like a child pumping a swing. Later upheavals tilted the rock beds to 30 or 40 degrees and split them with vertical cracks. The tide has prised those cracks open ever since, with ice taking its turn each winter, eating the base faster than the crown. In late July and August the flats between the rocks fill with tens of thousands of sandpipers fattening on mud shrimp. Nothing here is fixed: on 14 March 2016 the most photographed stack of all, Elephant Rock, dropped 100 to 200 tonnes of itself in a single fall. The park opens in mid-May and closes in late October.