Traprain Law from the SE showing the crags of the laccolith sides and the tail
A model of laccolith intrusion
Traprain Law from the SW, with a glacially-abraded up-ice slope and steep southern flank.
Definition: A significant upward flow of magma along a "neck" that spreads out and bulges upward in a subterranean mushroom shape
Laccoliths are lens-shaped intrusions where magma was emplaced like a sill between sedimentary layers but then bulged up into a dome. Most laccoliths are from 1-15 km across, with the largest laccoliths formerly the most deeply buried. Erosion has subsequently stripped away the overlying and surrounding rocks to leave the resistant igneous rock as a hill or upland.
Traprain Law is one of the most widely-cited examples of a laccolith in the world. The Law is one prominent hills in East Lothian, a crag-bound, egg-shaped and ice-moulded hill rising 120 m above the surrounding lowland. The disused road metal quarries on its flanks show that this small laccolith is made of phonolite, a fine-grained, well-jointed and resistant igneous rock. The magma was intruded into Carboniferous sedimentary rocks at shallow depth about 320 million years ago.