The tail of North Berwick Law, looking east
Ice-roughened summit in red-mottled trachyte
The tail, with excavations showing at most a few metres of till resting on bedrock. The difference in height between the sandstone on the stoss and lee slopes gives a minimum estimate of the depth of glacial erosion around the hill. In the case of North Berwick Law, this represents >30 m of erosion.
Edinburgh Castle Rock
Archibald Geikie's (1887) line diagram of the key features of a crag and tail
The crescent-shaped depression on the NW flank of North Berwick Law. The depression is partly in-filled partly by glacial till and glacial erratics are plentyful in the heavy clay soils.
The term "craig and tail" was first used by Sir James Hall in 1815. He recognised from the orientation of striae and crag and tails in central Edinburgh that a powerful agent of erosion had moved from west to east across the city. He attributed the erosion to a tsunami.
Crag and tail
Definition: a tadpole-shaped landform developed by glacial erosion of rocks on unequal resistance. The crags are cliffs developed in near-cylindrical masses of strong rock. The tail is formed in softer rocks sheltered from erosion in its lee. In East Lothian, the tail may carry a thin cover of till but it is essentially a rock-cut feature.
East Lothian shows a number of examples of this classic landform of lowlands which have experienced deep glacial erosion. This frequency is a reflection of the many resistant volcanic necks and plugs intruded into the relatively soft Carboniferous sedimentary rocks. North Berwick Law is the most prominent and well known example.
The basal layers of the Pleistocene ice sheets were forced to deform around large bedrock obstructions. The quickening of flow and the high stresses on the up-ice side of the hill led to the erosion of crescent-shaped depression in front of the hill. The sides of the hill were over-steepened and crags were formed. In the lee of the rock boss, the velocity of glacier flow and the effective pressure on the glacier bed was reduced. This allowed the preservation of relatively softer rock but the sheltering effect diminished with distance from the crag and so the tail tapers away from the hill. A thin layer of till was probably deposited as the ice thinned during deglaciation.