Gneiss from the SW Highlands on the beach at Seacliff
Glacial erratics left from excavations at Fettes College, Edinburgh. The local quartz dolerite dominates but the rocks include Carboniferous sedimentary rocks and one or two Highland rocks.
Definition: An erratic is a boulder transported and deposited by a glacier having a lithology different than the bedrock upon which it is sitting. Erratics are useful indicators of patterns of former ice flow.
The flow of ice across East Lothian has left a trail of erratic boulders. Most of these represent locally-derived rocks but some are from farther afield. For 200 years geologists have recognised occasional stones of Highland origin within the boulder clay or washed out and reworked into beaches. These have been carried from north of the Highland Boundary Fault from around Loch Lomond. On the southern fringes of East Lothian there are erratics from the Southern Uplands, especially in the areas around Humbie. The suite includes the largest known erratic mass in Scotland at Kidlaw.
The minimum thickness of the last or earlier ice sheets is indicated from the elevation of glacial erratics, together with other signs of glacial erosion, on the hills. It has been known since the 19th century (Somervail, 1879) that the Pentland Hill summits carry glacial erratics derived from both the west and southwest at high elevations. This requires not only that ice over-topped these hill summits but also that there was sufficient momentum within the ice sheet to drive ice locally uphill to elevations of ~500 m. Recent models of the last ice sheet suggest that the surface elevation over the Forth lowland was above 1 km.