A geo or gio (/ɡjoʊ/ GYOH, from Old Norse gjá) is an inlet, a gully or a narrow and deep cleft in the face of a cliff. Geos are common on the coastline of the Shetland and Orkney islands. They are created by the wave driven erosion of cliffs along faults and bedding planes in the rock. Geos may have sea caves at their heads. Such sea caves may collapse, extending the geo, or leaving depressions inland from the geo. Geos can also be created from this process.
A surge channel is a narrow inlet, usually on a rocky shoreline, and is formed by differential erosion of those rocks by coastal wave action. As waves strike the shore, water fills the channel, and drains out again as the waves retreat. The narrow confines of the channel create powerful currents that reverse themselves rapidly as the water level rises and falls, and cause violent hydrodynamic mixing. However, there is relatively little exchange of water between channels; experimental studies and mathematical modelling of the coastline near Hopkins Marine Station in California have shown that water is rapidly mixed within each channel, but that it mostly moves in an oscillatory manner. Surge channels have been likened to 'containment vessels', retaining water borne gametes and probably enhancing the effectiveness of external fertilisation of marine species dwelling within them.Surge channels can form in reefs, and the term is sometimes also applied to breaches of coastal dunes by storms.Surge channels can range from a few inches across to 10 feet or more across. They may create tide pools if the conditions are correct, but the rapid water movement almost always creates a dangerous situation for people or animals that are caught in it. The West Coast Trail on the coast of Vancouver Island, B.C., is known for its large number of surge channels, some of which are impassable even at low tide and must be crossed inland.