Volcanoes – worn down to their roots
Some 350 million years ago, East Lothian was a peaceful flat coastal plain, clothed by forests, dotted with lagoons and crossed by meandering rivers flowing into deltas at the edge of a sea to the north-east – much like the American Gulf Coast is today. From time to time, though, this peace and tranquillity was disturbed by the fire and explosion of erupting volcanoes.
Volcanoes are formed when molten rock (called magma) forces its way from depth up a pipe (vent) to the surface. A volcano can be violent, throwing clouds of steam, ash, rock debris and bombs into the air, or less violent, with the flows of lava gradually building up a cone round about the crater. Some volcanoes erupt only one or two flows before becoming extinct, with the activity moving elsewhere. Other volcanoes produce many eruptions over a long time, building up a cone of lava flows separated by layers of ash. East Lothian must have presented quite a sight as volcanoes erupted near and far; near – at North Berwick Law, the Bass Rock, Tantallon and Dunbar, and far – at Arthur’s Seat and the Lomond Hills.
In the many millennia since the volcanoes were active they have been buried and then uncovered by erosion, so today it is only the remnants and roots of some of the volcanoes that are left. As volcanic rocks are harder and more resistant than the surrounding sedimentary strata, they form the main landscape features of the lowland areas of East Lothian.
North Berwick Law and the Bass Rock are the roots of two old volcanoes. The cones of lavas and ash have worn away, just leaving the columns of hard igneous rock – the last of the molten rock that cooled in the pipe, hence their almost circular shape. The Law has a conical shape, whereas the sea-washed sides of the Bass Rock have remained vertical.
Lavas and ashes that erupted from volcanoes such as the Law, can be seen on the foreshore and around the harbour at North Berwick. As the rocks have been tilted to the west by earth movements, different beds are exposed along the shore. The Paddling Pool has been cut in red volcanic ash which settled in water where the layers and the larger ash fragments can be seen. Then three lava flows erupted one after another.
Each flow is about 10m thick, consisting of a dark, structureless lower portion, grading upwards into a honeycomb rock, formed as gas escaped from the erupted lava flow. The lavas are formed of a crystalline rock, called basalt. Lava flows gave rise to the hills from North Berwick on the coast by way of Kingston and the Garleton Hills, Whitekirk Hill and East Linton to Whittingham.