Scotland. Peter Friend

Чтение книги онлайн.

FIG 55. Tidal marshes showing typical, highly sinuous channels, where the wavelength of the channels cut in the muddy sediments reflects the tidal discharges involved. (© Patricia & Angus Macdonald/Aerographica/Scottish Natural Heritage)

      Man has also altered the landscape by mining and quarrying activities, particularly prevalent in the Midland Valley where large opencast coal workings are still operational today in places, such as east of Patna. Recent clean-up efforts have greatly reduced the impact of colliery tips (bings) on the landscape, such as northeast of Girvan, where they have been landscaped and forested. Local stone has also been quarried for building stone, roadstone and crushed rock aggregate. An important source of building stone is the area around Mauchline, from which the attractive orange-red Permian sandstone has been extracted. Stone from this area has been widely used throughout the UK and Ireland, and even shipped to the USA. The granites have also been economically important for the region – the Glasgow and Liverpool docks, for example, were constructed using Criffel–Dalbeattie granite. Glacial sands, silts and gravels have also frequently been quarried, often leaving their mark on the landscape with flooded gravel pits.

      Today, the Area is generally very wet and mild, as the North Atlantic Drift maintains higher temperatures than those found on the east coast. Indeed, plants normally associated with more southerly latitudes are found on the Rhins, along with dolphins and basking sharks off the coast. Much of the Southern Uplands in this Area lie within the Galloway Forest Park, managed by the Forestry Commission, and as well as rolling moorland, conifer plantations are common on the shallow, poor soils. The main river valleys (such as the Urr, Dee, Cree and Nith) provide a contrast to this rolling moorland, providing much of the good arable land of the Southern Uplands.

      The formation of the coastline

      As described in Chapter 5, sea level in much of Scotland has not been constant, but has risen and fallen according to the interplay between global sea level and the elevation of the land. Global sea level decreases during an ice age, as water is locked up within the ice, and rises again as this ice melts. Meanwhile, during glacials, the crust becomes depressed locally under ice sheets, sinking into the mantle, and slowly rebounds once this ice has melted. The sea level at the coast at any one time therefore depends on the interplay between these two effects, and in the past sea level in Area 1 has been both higher and lower than at present. When sea level has remained constant for long enough, shorelines formed – marked today by erosional features such as rock-cut platforms backed by cliffs, or by beach deposits. Where subsequent uplift has outstripped global sea-level rise, these shorelines now take the form of raised beaches, raised deltas, raised estuarine deposits (known in Scotland as carse), and raised rock platforms and cliffs. Good examples of these features are found in this Area. Shore platforms are relatively common on stretches of rocky coastline, and are particularly well developed, for example, between the Heads of Ayr and Turnberry and around Ballantrae. In the latter area, numerous small caves and gullies often delineate the foot of the cliff, and occasional raised sea stacks rest on the raised platform. Further south, a shore platform can be traced around most of the Mull of Galloway at approximately 10 m above present sea level, and deposits of glacial till on top of it show that it predates the Devensian glaciation. In places, several shorelines are present, such as some 8 km southwest of Girvan, where two old shorelines give the skyline a stepped profile. These shorelines occur as a series of benches, commonly cut into boulder clay. Mafic, dolerite dykes have been etched out by the sea during the formation of each of these shorelines, and today stand as raised sea stacks.

      In the lower-lying coastal areas, past increases in sea level involved the flooding of sometimes large patches of land. For example, along the Ayrshire coast the late glacial sea was nearly 30 m higher than at present, and so the sea would have come several kilometres inland from its present position. Along the Solway coast in the south of the Area, large sections of the present shore are backed by raised estuarine deposits of silt and clay, which provide valuable records of this sea-level change over the last 15,000 years. Good examples occur flanking the heads of the Cree and Fleet estuaries, which in this area lie between 7 and 10 m above present sea level.

      The sediments within cores taken from the carselands flanking the Cree estuary have been analysed and dated, and have proved very useful in reconstructing the sea-level history of the Solway Firth, as summarised in Figure 56. These deposits show that sea level rose to cover this area by around 9600 years ago, followed by a rise to the so-called Main Post-glacial Shoreline by 6500 years ago, when sea levels were between 7 and 10 m above present and some 9 m of estuarine deposits were laid down in the Cree area. Sea level then fell from the uppermost carse surface to its present level as the land continued to rebound, whilst worldwide sea volume changed very little.

      The large-scale shape of the coastline is controlled by a number of factors, many of which are in turn related to one another. Rising sea levels cause valleys to become flooded, forming bays and islands, whilst the location of these valleys often reflects rock type (i.e. hardness) and structure (both the presence of jointing within a rock unit and lines of weakness, such as faults). Climate and tidal energy also play an important role, controlling wave energy environments and terrestrial processes, such as sediment supply. The amount of sediment supply to a coast of course depends on the availability of that sediment, and in this regard glaciation СКАЧАТЬ