Geo A geology term. 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'. BGS: Esha Ness Volcanic Formation - Andesite, Augite. Igneous Bedrock formed approximately 385 to 398 million years ago in the Devonian Period. Local environment previously dominated by eruptions of silica-poor magma. Setting: eruptions of silica-poor magma. These rocks were formed from mobile magma, poor in silica. It rose to the surface and erupted as fluid lava from shallow-sided volcanoes and fissures. These eruptions are associated with the spreading of the Earth's tectonic plates and are often accompanied by feeder dyke and sill intrusions.