Why Is There a Beach Halfway Up That Hill? The Land Is Still Rising

The form is a level or gently sloping platform running along a coast well above the reach of the present sea, backed by a steep slope that was once a cliff. On the platform lie rounded beach pebbles, sand and marine shells, sometimes cemented into rock. The old cliff behind frequently contains caves cut by waves, now dry and high and dark, and occasionally arches and stacks stranded inland. The whole assemblage is unmistakable once recognised, because nothing but a sea produces that combination of flat platform, rounded stones and shell debris.

A shoreline can be stranded in two quite different ways:

• •The sea fell, which happens globally when water is locked into ice
• •The land rose, which happens locally for several reasons
• •Ice sheets press the crust down beneath them while they exist
• •Removing that weight lets the crust rise again over millennia
• •Tectonic uplift raises coasts near plate boundaries
• •Most examples record a combination of the two effects

The rising of formerly glaciated land is the commonest explanation in northern Europe and North America and the mechanism is worth stating carefully. An ice sheet kilometres thick presses the crust down into the slowly flowing mantle beneath, displacing it sideways. When the ice melts the pressure goes, and the crust rises back, but the mantle flows slowly so the recovery takes many thousands of years and is still in progress. Parts of Scandinavia and Canada are rising by nearly a centimetre a year today, and areas on the edge of the former ice are correspondingly sinking as the mantle flows back.

The reverse situation produces a quite different coastline and the contrast makes both easier to read. Where land is sinking or the sea is rising relative to it, valleys are flooded from their lower ends, producing long branching inlets that are river valleys with water in them rather than features cut by the sea. Old cliffs are drowned and attacked afresh at a higher level. Coastal settlements and field boundaries end abruptly at the water, and submerged forests appear at low tide. Many coastlines show both, having risen and sunk at different periods as ice advanced and retreated.

These features are among the most useful records available for reconstructing recent earth history. Shells and driftwood on a stranded platform can be dated directly, so a staircase of several platforms at different heights gives a sequence of shoreline positions with ages attached, which is a direct measurement of how fast the land rose. Comparing the heights around a region maps the shape of the vanished ice sheet, since the land rose most where the ice was thickest. Archaeologists use them heavily, because coastal settlements follow the shoreline and a site's height indicates its age.

A flat platform above the present sea with rounded pebbles, shells and a dry cliff behind it records an old shoreline, stranded either by falling sea level or by rising land. The commonest cause in the north is crust rebounding after ice sheets melted, which is still happening at nearly a centimetre a year in places. Dating a staircase of them measures that rise directly.