Flandrian coastal environmental changes : evidence from three sites in Mainland Orkney, Scotland
The Orcadian coastline is characterised by a high energy paraglacial environment, where remobilisation of abundant glacigenic sediment has favoured the building of numerous barriers. To date, there has been little work on either patterns of relative sea-level change or coastal processes affecting Orkney during the Flandrian. The present research provides an understanding of Flandrian coastal environmental changes in three areas of Mainland Orkney. Detailed morphological, lithostratigraphical and biostratigraphical work was undertaken and has revealed distinctive back-barrier sequences which illustrate the diversity of coastal responses to complex interactions between relative sea-level trends, sediment supply and coastal configuration. Scapa Bay provides direct evidence of relative sea-level rise and coastal retreat during the early Flandrian. There, a freshwater marsh was flooded by the sea c.8.5 kyrs BP at c.-5.4 m 0.0. Abundant sediment supply from nearby drift cliffs was remobilised inland by wave refraction before c.7 kyrs BP, and led to the formation and rapid progradation of swash-aligned barriers (SAB) across the valley mouth. By c.6.6 kyrs BP, direct marine influence was restricted in the back-barrier area, although saltmarshes persisted until c.5 kyrs BP. The enclosed lagoon was then progressively infilled by terrestrial sediments and a freshwater marsh was formed. The multiple barrier complex at Scapa Bay demonstrates land progradation against the backdrop of long-term rising relative sea level. This was facilitated by continuous sediment supply in a sheltered embayment. At the Bay of Carness, a single SAB was built during the early Flandrian. Between c.6.5 and 5.4 kyrs BP, marine influence was at its highest and a saltmarsh formed between c.-3.2 and -2.57 m 0.0. No direct marine flooding has, however, been recorded at the site. Later, a brackish lagoon occupied the coastal depression indicating a rising water table. This lagoon was gradually infilled by terrestrial sediments as soil erosion occurred in the the catchment. Slow inland migration of the SAB during the late Flandrian has been related to complex barrier and lagoon interactions in a context of sediment starvation. The sheltered setting of the site enabled the barrier to keep cohesiveness throughout its inland translocation. However, its present morphology shows clear signs of instability. The Bay of Skaill is the most exposed and dynamic coastal environment of the sites investigated. Abundant sand supply led to the building of a dune ridge during the middle Flandrian and to the pending of a freshwater loch in the back-dune area. By c.6.1 kyrs BP, aeolian processes became dominant and the dune ridge progressively migrated landward, while machair developed inland. After c.4.4 kyrs BP, the bay was gradually formed as the dune ridge retreated to its present position. Moreover, a SAB developed and eroded the seaward dune edge. At present, the SAB is migrating rapidly inland due to sediment starvation and exposure to storm activity. The Flandrian vegetational history around the three sites was also investigated. This accorded with that already established for the islands. Herbaceous vegetation was initially dominant, although a Betula-Corylus woodland, including Salix, Alnus and possibly Quercus, developed during the early Flandrian. This reached its maximum extent c.5 kyrs BP. Anthropogenic impact from Neolithic onwards is believed to have significantly affected the landscape studied, primarily via woodland clearance and mixed farming practices. Between c.4 and 2.5 kyrs BP, a combination of natural and anthropogenic factors led to the spreading of heathland and to a possible decline of anthropogenic activities.