Longcraig, Port Seton - a dolerite dyke standing proud of a shore platform developed across Carboniferous Limestone
Definition: sheet-like intrusions of igneous rock lying at a high angle to the bedding of the surrounding rocks
Dykes are generally harder than the sedimentary rocks which they cut across and so dykes should form wall-like features. The thinness of dykes, means, however that these walls are only rarely expressed in the relief so that the best examples in East Lothian are to be found standing on the shore platforms. Also, there is often a weakness between the dyke rock and the surrounding sediments where there has been metamorphism or hydro-fracturing. In these cases, the dyke margin will weather quickly and form a trench.
Dykes are sheet-like bodies of igneous rock that cut across sedimentary bedding or other structures in igneous and metamorphic rocks. Dykes may occur singly or in swarms, giving linear or radial patterns.
Most dykes are under 3 m thick but often of considerable length. The Longcraig Dyke is 9 km long at the surface; the Cleveland Dyke in Durham lies 400 km from its source igneous complex, the Tertiary volcanic centre on Mull. The extreme length of many dykes requires that the fluid magma was injected very rapidly into fractures.