Quaternary Geology and Geomorphology

The present landforms and landscape of St Kilda owe a great deal to the modifications that occurred during the Ice Age. These modifications include the effects of glaciation, frost weathering on the rocks and slopes and changes in the level of the sea and thus the level at which wave processes operated.

During the Ice Age, which began 2.5 million years ago, the climate fluctuated dramatically between conditions similar to those of today and conditions of intense cold. Vast ice sheets expanded on a number of occasions across the northern regions of North America, Europe and Asia, including the adjacent continental shelves. During these intensely-cold episodes, ice sheets from the Highlands of Scotland swept westwards out across the continental shelf that was then dry land due to the general lowering of world sea levels. Whether or not these ice sheets reached St Kilda has been a matter of speculation, but some evidence from the presence of erratic material in the deposits around Village Bay suggests that this was possible. The age of such an ice invasion is unknown, but it pre-dates the Late Devensian glacial maximum (approximately 22,000 years ago).

The Hebrides Shelf has been subjected to a number of glaciations in the Late Pleistocene (approximately the last 700,000 years). However, with the exception of the last phases in the late Devensian (approximately 25,000 years and younger) there are few conclusive data constraining the ages of events.

Figure 3.5: Schematic reconstruction of ice sheet limits and associated deposits on and around the Hebrides Shelf during the late Pleistocene (modified after Stoker et al., 1994).

It is likely that the majority of the Hebrides Shelf was covered by ice during Anglian (Elsterian) times (approximately 480,000 to 380,000 years ago), during which time sediment was supplied to the slope and basin. Although there is no Wolstonian (Saalian) age (approximately 300,000 to 130,000 years ago) sediment proven on the Hebridean margin, this does not preclude shelf glaciation during this period. Consequently, the presence of reworked red sandstone on St Kilda might be attributed to transport from the east during either or both of these glacial phases.

In Devensian (Weichselian) times (approximately 110,000 to 11,500 years ago), the balance of evidence suggests that St Kilda was not covered by glacial ice derived from the Outer Hebrides or Scottish mainland, though the limits of this ice sheet appear to have been close. However, there is evidence for local glaciation on St Kilda in both the early and late Devensian. It is probable that to the north and south of St Kilda, ice reached the shelf break in the early Devensian and possibly also in the late Devensian.

During shelf glaciation large volumes of sediment were transported across the shelf and on to the slope, where they were deposited as fans (the Sula Sgeir and Barra fans) comprising glacial debris flows. Major ice streams sourced from the Scottish mainland supplied both of these fans and their equivalents north of the Wyville-Thomson Ridge. By contrast, the area west of the Outer Hebrides does not appear to have been crossed by major ice streams, although it is probable that smaller, slow-moving glaciers did transport sediments across this part of the shelf. The upper Pleistocene on the outer shelf and slope west of the Outer Hebrides comprises ice-distal mud that was deposited in a low energy, contour current regime.

Stac LeeThough the impact on St Kilda of ice sheets spreading out from the mainland is uncertain, the effects on the landscape of local glaciers that formed in the glen behind Village Bay are readily evident. At least two periods of valley glacier development can be recognised. An older, as yet undated, ice advance is represented by glacial deposits at the base of the cliffs fronting the east side of the bay. A younger ice advance associated with the intensely cold conditions of the last glacial maximum around 18,000 years ago produced moraine ridges along the west side of the glen and deposited a till or boulder clay in the floor of the glen (which can be seen in exposures along the Abhainn Mhor). The latter deposit extends across to the east side of the glen where it forms a distinct drift limit on the hillside. During this cold phase, when the mean summer temperatures may have been as low as 4ºC, about 9ºC lower than at present, the slopes beyond the ice were subject to intense frost weathering and breakdown of the rock under periglacial conditions. These conditions produced periglacial deposits, good examples of which can be seen on the resistant ultramafic rock outcrops on the ridge out to Mullach Bi.

The weathered debris, where it accumulated on the slopes, was moved downhill by solifluction, producing solifluction sheets and lobes, for example on Mullach Sgar. On the east side of Village Bay on the path out to Ruaival, there are good exposures of these slope deposits. Similar deposits can be seen in the cliffs at the head of Glen Bay. On the evidence of these deposits it appears that Gleann Mor was not occupied by ice during the last main glaciation although it may have supported a small glacier during an earlier phase of the Ice Age.

A further set of landforms relates to a short, intensely-cold episode at the end of the last glaciation, between about 12,500-11,500 years ago, and known as the Loch Lomond Stadial. At this time, permanent snow or ice patches formed in the glen behind Village Bay. Rockfall debris from the cliffs above accumulated at the foot of these snow/ice patches forming clear ridges, known as protalus ramparts.Two striking examples occur at the base of the slopes below Mullach Sgar and Conachair.

Figure 3.6: Geomorphology map of Hirta (Reproduced from ‘Late Quaternary glaciation and environmental change on St Kilda, Scotland, and their palaeoclimatic significance’ by Sutherland, Ballantyne and Walter from Boreas, www.tandf. no/boreas, 1984, 13, 261-272, by permission of Taylor &Francis AS.)

The associated climate change also affected the wider environment and vegetation. The sediments which infill a small topographic basin in Gleann Mor on Hirta contain a valuable pollen record of the vegetation changes that occurred during the interval since the end of the last glaciation (the last 11,500 years). During the Loch Lomond Stadial, an open tundra landscape existed and was characterised by the presence of willow, purple saxifrage, mountain sorrel and wormwood. Following the climatic amelioration at the end of the stadial and during the middle part of the postglacial period, around 6,000 years ago, the vegetation comprised a mix of floristically diverse grassland (plantains, cinquefoils, docks, Compositae and Polypodium) and heathland (crowberry, ling) communities. However, there is no evidence for the development of a cover of wood scrub or colonisation by trees. The climatic shift associated with the establishment of this floristically diverse vegetation is a reflection of a change to more oceanic conditions in north west Europe, following the post-glacial rise in sea levels. Later, there was a marked decline in the presence of the heathland and an expansion of plantain grassland, possibly indicating wetter and stormier conditions with salt spray being blown across the island. These records probably reflect the marked deterioration in climate, which occurred throughout north west Europe around 2,500 ago.

Hydrographic Office charts of St Kilda: a) 1831 b) 1900 c) Chart 2524 (1985)

A change to drier and less stormy conditions is then indicated by a decline in maritime grasses and an expansion of heathland, at the expense of the formerly extensive maritime communities on Hirta around 2,000 ago when a period of more stabilised climate persisted in the North Atlantic. The last 400 years or so saw a return to the dominance of maritime plantain grasslands at the expense of heathlands reflecting wetter and stormier conditions. This may coincide with the climatic deterioration associated with the Little Ice Age, in the north west of Europe during the 16th to18th centuries. The anthropogenic effects on the vegetation succession on Hirta have been negligible until very recently. The clearest evidence of human activity occurs in the uppermost 0.2m of the pollen profile where higher frequencies of cereal type pollen are encountered. Therefore, the pollen profile at Gleann Mor provides an excellent and extremely rare proxy of climatic change over the last 6,000 years, in an environment largely free of anthropogenic impacts.