Ice Age history of East Lothian
Ice moulded terrain in the Garleton Hills
The scenery of East Lothian bears the strong imprint of glaciation. During the cold stages of the Ice Age or Pleistocene, heavy snow fall led to the build up of glaciers and ice sheets over the southwest Grampians, with discharge into the Forth valley. A subsidiary flow developed out of the Southern Uplands along the flank of the Pentlands. A zone of fast flow or an ice stream developed along the line of the Firth of Forth as the ice sheets thickened and advanced. This ice stream flowed out onto the dry bed of the North Sea. At its maximum extent the ice sheet turned southeast to flow down the western North Sea towards an ice margin in East Anglia. The presence of glacial erratics near the summits of the Pentlands and the Lammermuirs indicate that an ice-sheet many hundreds of metres thick periodically covered the entire area. The ice sheet eroded deeply the soft Carboniferous sediments and streamlined the relief.
The Pleistocene was a period of rapid and major fluctuations in temperature. East Lothian has undoubtedly been covered by many glaciers over last million years but, aside from a few inherited coastal landforms, all surviving deposits appear to relate to the passage of the last ice sheet and to the postglacial erosion.
The last ice-sheet deposited a ground moraine of till which locally blankets the underlying rocks of the coastal strip to depths of 5 to 10 metres. On the flanks of the Lammermuir Hills till occurs mainly in the valleys. The matrix of the boulder clay is a tough well-consolidated silty or sandy clay; the many pebbles and boulders are mainly of local origin, but some are of far-travelled erratics including metamorphic rocks from the hills around Loch Lomond.
Retreat of the last ice sheet began after 21 000 years ago but the proximity of East Lothian to the main zone of ice accumulation probably means that eastern and higher parts of the region only first started to emerge from ice around 16 000 years ago. Initially the margins of the ice-sheet retreated to the north and to the south-east, leaving the summits and upper flanks of the Lammermuir Hills free of ice. Whilst the ice was banked up against the northern slopes of the Lammermuir Hills, the ice margin oscillated. Glacial meltwater from the decaying ice flowed along the margins and under the ice cutting numerous large and small meltwater channels into rock and into boulder clay. The meltwater deposited sand and gravel as kame terraces at the exits of the main channels. Subsequent erosion by meltwater has re-excavated and deepened the channels leaving remnant deposits as terraces on the valley sides. Extensive spreads of sand and gravel were laid down at the mouths of the channels along the coast south-east from Dunbar, now partly reworked into beaches.
Sea cave on Fidra, cut at a higher sea level around 5500 years ago
Thinning of the ice led to isostatic rebound of the land at the same time as global sea levels were rising to give fluctuations in the local sea level. During periods of relative standstill, beach features and deposits formed and subsequently were uplifted. These raised beaches now tilt eastwards away from the area of major isostatic uplift. Late-Glacial sea-level, during the decay of the ice sheet between 16 000 and 14 000 years ago, reached a height of 21 m in the Tyne and east Peffer valleys. Estuarine clay, silt, sand and gravel deposits accumulated there.
The date of final deglaciation remains uncertain but the last glaciers in East Lothian are unlikely to have persisted long into the Windermere Interstadial, a warm phase between 15 and 13.5 ka. The succeeding Loch Lomond Stadial stage from 13.5 to 11 ka brought a final sharp drop in temperatures, with the readvance of glaciers to the Lake of Menteith and the return of permafrost to the lowlands of East Lothian.