Main image on top: Geo near Scarfskerry (Credit: Vincent Lowe / Alamy Stock Photo / BXTMEX)

Definition: a narrow and deep cleft in the cliff face excavated by marine erosion along a line of structural weakness. Norse gya, with a hard ‘g’.

A small geo at St Mary’s, with a storm beach at the bay head

These linear clefts in the cliff line reflect marine erosion along the line of weakness. Geos are most common in high cliffs developed in gently dipping flagstones and sandstones. Most geos align to trap dykes, crush zones and faults. Where joints and other planes weakness are close together, these long, narrow slots develop in response to selective marine erosion between the neighbouring planes or weakness. They can also form through roof collapse of narrow sea caves. Some geos have a boulder beach at their head, generally with large amounts of flotsam.

A re-excavated geo at Drumhollistan

The processes of landward extension are little understood. Where the geo ends in a rock wall then cavitation and hydraulic action may be dominant. Geos with bay head beaches have the possibility of abrasion but lichen-covered boulders must only be mobilised during major storms. Where fracturing or weathering has reduced the rock into small blocks or even grit then the sluicing effects of spray will remove this debris and may undermine larger blocks. Although no measurements exist, it is likely that incoming storm waves can generate high water and air pressures in fractures at the heads of geos, leading to extension of fractures landwards. The suction of the withdrawing wave water may also allow plucking of loosened blocks.

In some parts of the coast, the presence of till in geo heads indicates that the sea is only now completing the task of cleaning out pre-existing canyons. Elsewhere major rock falls leave no doubt that geo extension continues today.

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