The Outer Hebrides and St Kilda

The Outer Hebrides PlatformOuter Hebrides Platform (OHP) extends west from the present island chain towards the Atlantic continental shelfContinental shelf edge and represents a fragment of ArchaeanArchaean crust (Lewisian gneissGneiss) thatLewisian gneiss was differentially uplifted during the PalaeogenePalaeogene and tilted westwards during the NeogeneNeogene. An extensive planation surfacePlanation surfaces developed close to sea levelSea level in the PliocenePliocene and was subsequently modified by PleistocenePleistocene marine and glacial erosion to form an extensive, partly submerged strandflat. During Pleistocene cold stages the Outer Hebrides supported an independent ice capIce caps, 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) theGlacial stages Outer Hebrides Ice CapIce caps remained cold-based and non-erosive above 450–700 m and on lower ground in NW LewisLewis, permitting the preservation of summit blockfields and over-ridden raised beach gravels. During the last glacial stageGlacial stages (~35 to 14 ka) the ice capIce caps fed the MinchMinch, The and Hebrides Ice StreamsHebrides Ice Stream and extended westwards across the shelf but did not reach St KildaSt Kilda, which supported only small cirque glaciers. Readvance of glaciers during the Loch LomondLochLomond Stade (~12.9 to 11.7 ka) was limited to the mountains of North Harris and SW LewisLewis, 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 KildaSt Kilda represent some of the most iconic coastal scenery in Scotland.