Molten magma forms many shapes
When molten rock or magma forces its way through the Earth’s crust, some reaches the surface and erupts from volcanoes as lava flows and ashes, known as extrusions. However, some magma is forced sideways along weaknesses in the strata, and cools down and solidifies before making it to the surface. This is called an intrusion, which is only seen when erosion has removed the overlying rock and uncovered it. Having cooled more slowly than a lava and not being exposed to weathering, intrusive rocks are generally even harder than lavas and so even more likely to form prominent features in the landscape. Because of the many ways in which the molten magma flowed, there are many different shapes of intrusions – some particularly well seen in East Lothian.
A dyke is a thin (usually only a metre or two) slab of vertical igneous rock. When the surrounding strata were stretched by earth movements, molten rock flowed into the cracks that these upheavals created. The heat of the molten rock baked the strata on either side of the dyke making it harder than normal. Dykes are common throughout the area. A fine example forms the Yellow Man on the North Berwick shore beside the East Links Golf Course. Another dyke cuts a red conglomerate in the Fairy Glen, near Oldhamstocks; the hardest rock here is actually the baked conglomerate, seen to the left of the dyke.
A sill is an intrusion formed where the molten rock forced its way sideways between the layers of strata, wedging them apart. There it solidified, baking the strata with which it came in contact. Because most strata have been tilted, the sills are usually seen in cross-section, exposed by erosion. Many such sills can be seen along the shores of East Lothian, as the one which forms the twin points of Fairy Ness and Craigielaw Point, and also at Gullane Point, Hummel Rocks and Black Rocks.
The offshore islands of Fidra and the Lamb are parts of another sill. The strata above have been completely removed; the strata below are underwater.
Visitors may marvel at the spectacular columnar jointing of the Giant’s Causeway in Ireland, or Fingal’s Cave on Staffa, but many fine examples of this geological phenomenon can be seen in East Lothian. When molten rock cools slowly and solidifies, it shrinks and cracks. Normally the cracks are at right angles, but in ideal conditions, nature creates hexagonal columns.
You need go no further than Dunbar harbour to see a fine set of columns. Others can be seen on the shore between Yellowcraigs and Gullane opposite the tidal slet of Eyebroughy. The island of the Lamb is entirely made of basalt columns, as is much of nearby Fidra.
Traprain Law is an unusual intrusion called a laccolith. Some magma is very thick, so it can only force its way a short distance sideways into a mushroom shape, arching up the strata above before solidifying.
Erosion of these softer sedimentary strata has laid bare the original shape of the intrusion we see today as Traprain Law. Joints in the quarry confirm that this is very much the original shape of the intrusion. Here also you can see flow structures in the magma, frozen into the rock, on the east side of the quarry.