Relative Sea-level Change and Submerged Coastal Landforms

During the coldest stages of the Ice Age, sea levels around St Kilda were up to 120m lower than they are today. Two distinct erosion surfaces (platforms) have been formed during the period of lower sea level, each backed by a line of cliffs. A superb assemblage of rock coast landforms including submerged caves, stacks and arches is associated with these cliffs, and platforms, in particular with the surface at –40m. These features have been subsequently drowned by rising sea level following the melting of the last ice sheet so that the platforms now lie submerged at 120m and 40m below present sea level. The lower surface covers at least 40km2and is believed to have been cut at the time of the maximum extent of the last Scottish ice sheet, around 22,000 years ago when sea level was around –120m. The upper surface is more complex, with two sub-surfaces occurring at around –80m and –40m. The higher stepped platform is thought to have been cut during the last cold spell of the Ice Age, around 12,600-11,500 years ago when local sea levels were 40m below present levels. Possibly both features partly represent inherited surfaces which were cut during earlier periods of relatively lower sea levels and re-occupied and extended during the Last Glacial Maximum and during the Loch Lomond Stadial, respectively.

Thus the large-scale form of the cliff line that characterises the present coastline of the St Kilda archipelago is essentially a relict feature. Although active today, as indicated by cliff-foot caves, geos, notches and overhangs, it has only been slightly modified by marine processes since the end of the last glaciation.

 

Figure 3.8: Bathymetric map of the St Kilda archipelago. A. The outline of St Kilda with sea-level 120m below that of today. This scenario would have applied during periods of maximum ice sheet glaciation (viz. c. 22,000 years ago). B. The outline of St Kilda with sea-level 80m below that of today. This scenario would have applied during periods of less extensive ice sheet glaciation. C. The outline of St Kilda with sea level 40m below that of today. This scenario would have applied during periods of less extensive glaciation (viz. c. 12,000 years ago)(modified from Sutherland, 1984).

The islands of Hirta, Soay and Boreray would have been one large island at the time of the Last Glacial Maximum, linked by the shore platform surfaces. The rise in relative sea level from 120m to 40m below present would have led to the separation of Hirta, Soay and Boreray. To the west, a pronounced 40m high cliffline separates this lower platform from the higher one which slopes gently between –80m and –40m. The present coastal edge of the St Kilda islands and sea stacks formed contemporaneously with the cutting of this upper platform so that the present clifflines and stacks continue 40m underwater. Hence around 11,000 years ago, the cliffs at Conachair would have risen 470m above sea level and Stac an Armin would have towered 231m above the platform surface!

The present cliffline around the islands is exposed to extremely high energy swell and storm wave action so that coastal erosion might be expected to be highly effective today. However, the cliffline extends uninterrupted to –40m with no evidence of a clear platform or notch at or close to present sea level. This contrast with the submerged cliffs and platforms may reflect the involvement of cold-climate processes (particularly intense frost shattering and sea-ice processes in the inter/tidal zone) in the formation of the latter, as on high-latitude shores today. Thus, the absence of a modern shore platform does not signify the absence of erosion, but rather that present conditions are not optimal for shore platform formation. In effect the large-scale form of the St Kilda coastline is essentially a relict feature which was formed earlier in the Ice Age by a combination of frost shattering, sea ice and wave action. The submerged platforms, cliffs, caves, stacks, arches and tunnels and boulder beaches that form the dramatic underwater scenery of St Kilda therefore reflect a particular combination of conditions that is not exactly replicated today.