Scotland. Peter Friend

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FIG 56. Sea-level curve for the Solway Firth. (Data from Smith et al. 2003, Transactions of the Royal Society of Edinburgh: Earth Sciences, 93, 301–31)

      The number of large bays is one of the more obvious features of the coastal strip of Area 1. The main ones are Loch Ryan and Luce Bay, which together define the Rhins of Galloway, and Wigtown Bay further east. Numerous smaller bays are present along the south coast, including the Water of Fleet and Kirkcudbright Bay on the southeast side of Wigtown Bay. In general, these bays are located in the lower-lying ground of this Area, and therefore do not reflect a large difference in rock strength between headland and bay. Instead, they seem to roughly coincide with the large northwest/southeast faults shown in Figure 47, and with the outlets of major rivers. The exception is the Loch Ryan–Luce Bay pair, which may have been the site of a Tertiary river, now partially flooded by the sea.

      Igneous rocks underlie some of the more prominent headlands on the west coast of Area 1, such as the Carboniferous vent rocks that make up the Heads of Ayr, a prominent headland some 6 km southwest of Ayr. The vent has been intruded into a colourful mix of Early Carboniferous sediments, including limestone, grey-green shales and red and green sandstones. Ash erupted from the Heads of Ayr vent is thought to underlie the knoll on which Greenan Castle has been built, around a kilometre up the shore towards Ayr. Where Silurian and Ordovician sedimentary rocks have been cut by cliff sections, the result is often rather impressive because of the exposed folding within the bedrock. Particularly spectacular ‘textbook’ examples of rock folds and other structures are found on the Machars peninsula, for example at Back Bay, just south of Monreith on the west coast, and around the Isle of Whithorn in the southeast. The fold structures at Back Bay not only illustrate two fold generations with a second set of folds superimposed on a first, but represent one of the most dramatic large-scale exposures of major re-folded folds in the UK.

      Perhaps some of the most dramatic coastal scenery in Area 1 is found on the island of Ailsa Craig (Fig. 50). Despite being only 1.2 km wide, the island is nearly surrounded by 340 m-high cliffs. The height of these cliffs reflects the exposed nature of the island to marine erosion, the strength and resistance of the bedrock to this erosion, and also the topographic high left behind here after the retreat of the last glaciers. For the most part, the foot of these cliffs is now between 5 and 10 m above sea level, and so marks a raised shoreline. A distinctive triangular raised beach is located on the eastern side of the island, fringed by storm ridges and an associated spit. These landform features highlight the importance of prevailing wind and storm direction, as they were caused by the westerly winds and waves since post-glacial times: sediment is deposited on the sheltered side of the island. The importance of exposure is also seen on the Rhins of Galloway, where the exposed western coast is generally rugged with steep cliffs and occasional inlets, in contrast to the calmer eastern coast with its sandy beaches.

      South of the Southern Uplands Fault, the coast is generally rocky, with low cliffs and only small local beaches or small stretches of shingle. Cliffs are particularly well developed along the exposed western coast of the Rhins, reaching a maximum of 120 m in height near Dunman. This cliff line is largely inactive now, having been raised clear of wave action by crustal uplift. On the north coast of the Solway Firth in particular, the rocky coastline is punctuated by a number of large bays, generally river estuaries. The more sheltered conditions within these bays and the ample sediment supply have allowed wide expanses of sand-flat, mud-flat and salt-marsh to accumulate, and together the flats and marshes of the Solway Firth provide one of the largest continuous areas of intertidal habitat in Britain. Remnants of formerly more extensive lowland peat bogs are developed on raised estuarine sediments. These include the nationally important Lochar Moss and Moss of Cree.

      Beaches are generally more common and extensive in this Area than in the Highlands of Scotland. The main reason for this is the relative abundance of sediment, primarily glacial sediment laid down on the near-shore shelf during the Devensian glaciation, and then driven onshore during the Flandrian sea-level rise, where it became stranded as sea levels fell again across the region. The relatively mild winds and waves experienced by the lowlands then meant that this abundant sediment source has remained fairly stable, and many of the beaches in this Area are still accreting today, rather than eroding. Old dune deposits are relatively common on the coastal strip south of Troon on the north edge of the Area, and sands were formerly worked from pits in these dune sands northwest of Monkton. Now, most of the dune deposits on the coast around Ayr are covered by golf courses.

      Large expanses of tidal sand-flats are found along the southern coast of Area 1, and at low tide many kilometres of sand are exposed, such as at Mersehead Sands (20 km south of Dumfries). Further inland, the reworking of raised beach deposits and other sandy sediments by the wind has, in some coastal locations, produced extensive spreads of sand dunes, now for the most part anchored by coarse grass or forestry. The largest beach-dune system in southwest Scotland is found at the head of Luce Bay, home to a complex array of dune-related landforms, and still actively accreting today. The entire peninsula of the Rhins acts as a huge breakwater from the currents of the North Channel, creating the relatively calm waters of Loch Ryan and Luce Bay.

      Salt-marshes are typically developed on low, raised beaches of sand or shingle and display a complex topography of pans, creeks and terraces. The salt-marshes of the Cree estuary in Wigtown Bay are particularly well developed, sandwiched between extensive sand-flats seawards and reed-swamps and emerged estuarine deposits (carse) landwards. The extensive carse deposits of the Cree estuary show that sedimentation has prevailed here over most of the last 10,000 years, despite changing sea levels. The estuary is well sheltered by Burrow Head to the south, and this has produced a largely unidirectional wave climate in which sediment is brought into the bay, with little subsequent removal. It appears that this system still operates, since many of the sand-flat and salt-marsh systems are accreting today. The presence of Sellafield-derived radionuclides attached to the sediment in the Cree and Water of Fleet sandbanks confirms the Outer Solway as a major sediment source, whilst important amounts of mud within the Cree mouth itself suggest a more fluvial source, further enhanced by active reworking of sediment from the carse deposits.

      CHAPTER 7

      Area 2: Southern Borders

      AREA 2 COVERS THE CENTRAL PART of the Southern Uplands, sandwiched between Areas 1 (Galloway) and 5 (Edinburgh) (Fig. 57). It straddles the southern part of the English–Scottish border, extending from Annandale and Moffatdale in the west to the Cheviot Hills in the east (Fig. 58). The discussion provided here is restricted to features on the Scottish side of the border.

      In the northern half of the Area, the River Teviot drains the relatively low-lying ground of Teviotdale, which in turn feeds into the larger Tweed Basin of Area 5 (Chapter 2, Fig. 7). Teviotdale is bounded to the east by the Cheviot Hills and to the west by the Moffat Hills of the Southern Uplands. These rounded hills reach heights of over 800 m, and form a broad region of elevated ground which dominates the landscape in the northwestern part of this Area. СКАЧАТЬ