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As sea levels continue to rise today, coastlines around the world are changing before our eyes. Cliffs retreat, beaches migrate and storms re-shape the shoreline. However, these processes are not new.

Scotland after the last glacial maximum

Around 20 000 years ago, Scotland and much of Britain were buried beneath the British–Irish ice sheet. As the climate warmed, the ice gradually retreated, leaving behind a landscape sculpted by ice. Among the most distinctive features are drumlins: teardrop-shaped hills with rounded crests, commonly formed of glacial sediment deposited beneath flowing ice and often occurring in large ‘swarms’.

The contraction and eventual disappearance of the ice also triggered changes in relative sea level. Globally, melting ice sheets caused sea levels to rise while locally, the land itself slowly rebounded (the amount varies with geography) after being released from the enormous weight of the ice sheets. These competing processes meant that coastlines continually shifted position through time, with some areas submerged while others emerged from the sea.

Hidden coastlines beneath the sea

Recent advances in high-resolution seabed mapping have revealed extensive fields of submerged drumlins across the Irish Sea and along Scotland’s west coast. Although these landforms were created beneath the last British–Irish ice sheet, many no longer preserve their typical rounded crests. Instead, numerous drumlins display flat-topped or truncated morphologies associated with gravel tails, tombolos, spits and gravel barriers. These assemblages closely resemble modern eroding drumlin coasts observed in western Ireland and eastern North America, where wave action progressively removes sediment from drumlin coasts and redistributes it along the shoreline.

���ճԹ� scientists have therefore interpreted the submerged drumlins as the product of wave erosion during periods when relative sea level remained lower than today. As sea levels subsequently rose, the former shorelines were drowned and preserved beneath the sea. They provide geomorphological evidence of past coastal positions and former relative sea-level lowstands.

Why do these ancient shorelines matter?

Reconstructing past relative sea level is essential for understanding how the British landscape evolved following deglaciation. Existing reconstructions are based largely on glacial isostatic adjustment (GIA) models, which simulate the combined effects of global sea-level change and uplift of the Earth’s crust following ice-sheet retreat. The submerged, truncated drumlins provide an independent geomorphological record against which these models can be tested. At several locations, the depths of the truncated surfaces indicate sea-level lowstands that are lower than those predicted by current GIA models, suggesting that regional reconstructions may require refinement.

These findings demonstrate the value of submarine geomorphology for improving our understanding of post-glacial coastal evolution. As increasingly detailed seabed mapping becomes available, submerged landscapes are providing new evidence to refine sea-level reconstructions and improve our understanding of how coastlines responded to environmental change following the last ice age.

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Dr Riccardo Arosio

Marine geoscientist

���ճԹ� Edinburgh

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