The Outer Hebrides and St Kilda

Abstract

The Outer Hebrides Platform extends west from the present island chain towards the Atlantic continental shelf edge and represents a fragment of Archaean crust (Lewisian gneiss) that was differentially uplifted during the Palaeogene and tilted westwards during the Neogene. An extensive planation surface developed close to sea level in the Pliocene and was subsequently modified by Pleistocene marine and glacial erosion to form an extensive, partly submerged strandflat. During Pleistocene cold stages the Outer Hebrides supported an independent ice cap, but mainland ice periodically over-ran the extremities of the island chain and flowed through the sounds separating individual islands. Ice-roughened knock-and-lochan terrain is extensive across lowland areas, but during the last (and probably earlier) glacial stage(s) the Outer Hebrides Ice Cap remained cold-based and non-erosive above 450–700 m and on lower ground in NW Lewis, permitting the preservation of summit blockfields and over-ridden raised beach gravels. During the last glacial stage (~35 to 14 ka) the ice cap fed the Minch and Hebrides Ice Streams and extended westwards across the shelf but did not reach St Kilda, which supported only small cirque glaciers. Readvance of glaciers during the Loch Lomond Stade (~12.9 to 11.7 ka) was limited to the mountains of North Harris and SW Lewis, and resulted in the deposition of end, lateral and multiple recessional moraines. Postglacial sea-level rise drowned the structurally guided and glacially modified coastline, creating numerous rocky inlets; on Atlantic coasts, large volumes of sediment were moved shoreward onto beach, dune and machair (calcareous shell sand) land systems. The beach, dune and machair features of the Outer Hebrides and the exceptionally high sea cliffs and stacks of St Kilda represent some of the most iconic coastal scenery in Scotland.