Calder's Geo, Esha Ness (see map)
Intersecting geo heads, The Ness, North Roe. Note the presence
of three distinct lines of weakness
Intersecting geo heads, The Ness, North Roe. Note the presence of three distinct lines of weakness
NCC map of Christie's Hole and geos
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'.
Geo and gloups on Papa Stour. Google Earth image.
Geos are linear clefts in a sea cliff that reflect marine erosion along a line of weakness. Where faults, joints and other fractures intersect the cliff face then linear zones of fractured rocks are available for wave erosion. Long, narrow slots develop in response to selective marine erosion between adjacent rock buttresses. Geos can also form through roof collapse of narrow sea caves. Many geos have a boulder beach at their head, generally with large amounts of flotsam.
Why are geos so common on Shetland and Orkney? Partly it is reflection of the frequency of fractures in otherwise massive basement and also the development of orthogonal joint and bedding plane systems in the Old Red Sandstone and other rocks. High wave energy is also significant for the deepest geos lie on the exposed outer coast of Shetland. Yet most rocks have fractures and other high energy coasts do not show the same development of geos. The depth of water in geos may be important, allowing water to surge to the head of the geo and so remove blocks.
The restricted width of many geos is a reflection of the narrowness of the line of geological weakness that is being exploited. The processes of landward extension are little understood. Where the geo ends in a rock wall then cavitation and hydraulic action must be dominant. Geos with bay head beaches have the possibility of abrasion but the boulders are only 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.
Christie's Hole on Papa Stour provides a spectacular example of a geo with tunnels and collapsed sections of cavern. Erosion along a line of structural weakness has not only cut geos and caverns but also undermined the overlying ground, producing a sequence of depressions filled by shallow lochs on the surface. The collapse of the floor of one of these depressions into the cave below drained a lochan in 1981.
On the exposed western coasts the geos are actively eroding. Recent rocks falls are widespread, especially in the inner parts of geos, indicating continuing extension of the geo heads. No remains of till plugs are seen on these coasts but on the east coast of Foula and Papa Stour till covers drape the heads of smaller geos and voes, indicating that the geos have a long history of erosion.
The serrated cliffs of Esha Ness, with many deep geos and caves