Where Land Meets Sea: Ten Coastal Cliffs, Arches and Stacks

Start with the chalk. These Kent cliffs rise about 110 meters and are made of the Cretaceous Chalk Group, laid down between about 100 and 66 million years ago from the remains of microscopic marine organisms called coccolithophores, then uplifted and cut back by waves.

The same Cretaceous chalk continues along the Sussex coast. Here the cliff line undulates because each peak stands between two truncated dry valleys, and the cliffs keep retreating by roughly a third of a meter per year on average.

In Denmark the chalk is about 70 million years old and reaches around 128 meters above the Baltic Sea. Glacial ice sheets folded and thrust it into steep, jagged blocks, so it looks quite different from the evenly bedded cliffs at Dover.

Limestone cliffs in Victoria, Australia, show the next stage. Waves hollowed caves, widened them into arches and collapsed the arches, leaving isolated stacks in the sea; one of them fell in 2005.

This Quebec stack of Devonian limestone is about 88 meters tall. Early explorers recorded several arches in it, but repeated collapses, most dramatically in 1845, left a single arch, which gives the rock its name meaning pierced.

On the Jurassic Coast of Dorset a near-vertical band of Portland limestone runs out into the sea. Waves carved a cave in its weak seaward end and enlarged it into an arch that will one day collapse to leave a stack.

The same arch-cutting process works on volcanic rock in New Zealand. Waves exploited natural lines of weakness in Cathedral Cove Rhyolite to form a tall archway between two beaches, with a freestanding stack called Te Hoho Rock nearby.

A stack of Old Red Sandstone about 137 meters tall, resting on a more resistant volcanic base, was isolated when waves cut through the neck of its headland. It is thought to be young, perhaps only a few hundred years old, and erosion still undercuts its base.

In the Bay of Fundy the sculptor is the tide. A range of up to 16 meters floods and drains the shore twice daily, and with freeze-thaw weathering it cuts narrow bases under conglomerate and sandstone pillars of up to 15 meters, capped with trees.

End with layered rock. Alternating beds of limestone and softer mudstone about 30 million years old were uplifted and eroded into stacked cliffs, and incoming tides force water up through blowholes that show the erosion still at work.