Holocene, Southern Uplands
|Stone, P, McMillan, A A, Floyd, J D, Barnes, R P, and Phillips, E R. 2012. British regional geology: South of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The Holocene began abruptly at about 11.7 ka BP when the warm Gulf Stream current became re-established, providing an ameliorating influence on the climate of the British Isles (P912368). Average summer temperatures rose by about 8ºC within 100 years. The widespread occurrence of bare, unstable soils at the beginning of the Holocene led to intense fluvial erosion and deposition with enhanced debris flow activity on mountain sides and extensive formation of landslips as the ground thawed. The rivers were generally braided with gravelly beds. Soils gradually became more stable following the establishment of vegetation, firstly by pioneering herbs, shrubs and scrub communities similar to the succession of the Windermere Interstadial, and later by woodland. During the Holocene, the river terrace and alluvial deposits developed to shape the present-day valley floors.
Sediment cores from lake basins and bogs throughout the south of Scotland have provided extensive pollen records through the Holocene, which complement coleopteran and pollen records from the Bigholms Burn and Redkirk Point sites. At Loch Dungeon (NX 525 845) on the east side of the Rhinns of Kells, the pollen sequence extends from the late glacial–Holocene boundary to the last 100 years. Based on evidence from this and other sites, trees colonised the south of Scotland from the south, at first mainly via the coastal lowlands, which were much more extensive than today owing to lower sea level in the early Holocene (see below). At the end of the Loch Lomond Stadial, pioneer species-rich, herbaceous assemblages were rapidly replaced by fern-rich juniper and birch scrub. Birch and hazel together with willow, aspen and rowan woodland expanded in the early Holocene by about 9 ka BP. An expansion of elm and oak took place at about 8.5 ka BP, especially in low-lying fertile areas. Pine spread into Galloway from around 7.5 ka BP but was restricted to upland areas free from competition with other species. The arrival of alder at about 7.4 ka BP preceded the transition at about 7.0 ka BP from the drier, ‘Boreal’ climate period to the wetter, ‘Atlantic’ one in the mid Holocene (P912390). All the elements of the mixed mid Holocene deciduous forest were present by 6 ka BP when dense tree cover dominated the landscape of all but the highest parts of southern Scotland. Thereafter, a wetter climate with consequent bog growth resulted in the decline of pine, with both its extinction and a concomitant decline in elm (the ‘Elm Decline’) at about 5.0 ka BP. Expansion of grasses and sedges accompanied deforestation from 5.0 ka BP and lime arrived shortly after with ash, maple, yew and beech all increasing later in the Holocene as a result of human intervention.
Throughout the Holocene the burgeoning human population has had a profound influence on the landscape. The Mesolithic hunter–gatherers began to burn forests in the earliest Holocene, perhaps to increase the availability of food resources, but substantial clearance did not begin until about 6 ka BP with the arrival of cereal farming in the Neolithic. This preceded the ‘Elm Decline’, an important biostratigraphical marker in pollen records that defines the end of maximum forest extension at about 5 ka BP. Forest clearance increased between 4 and 2.8 ka BP in the Bronze Age, with the most extensive deforestation occurring between 2.5 and 2 ka BP in the Iron Age. Clear evidence for these changes is preserved in the many wetland sites that have survived throughout much of the Holocene.
Although the imprint of glaciation and periglaciation remains dominant (and though periglacial processes continue to operate on ground above about 450 m OD) postglacial processes have superimposed distinctive modifications on the landscape of southern Scotland. Tidal estuaries and smaller inlets on the Solway Firth are now occupied by muddy estuarine alluvium and salt marsh deposits. Steep hillsides of the Southern Uplands have been modified by gullying, slope failure, soil creep and debris flow, and valley floors have been sculpted by fluvial erosion and deposition. Extensive gullying associated with both climate change and human impact occurred at intervals during the late Holocene (the Sub-Boreal to Sub-Atlantic climate periods) notably at 2500–2200, 1300–800 and after 500 years BP. Locally, deep differential erosion has taken place, for example along the sides of the valley of the Back Water (Fairy Castle Dean), Oldhamstocks (P001091), which was most probably initiated as a subglacial dranage channel.
Peat growth accelerated at the beginning of the Atlantic climate period with extensive ombrogenous blanket mires forming over the wet uplands and within poorly drained topographical depressions. The single-thread, submeandering stream patterns of the present day chapter nine: quaternary Calibrated radiocarbon years BP 0 5000 10 000 15 000 20 000 became established early in the Holocene once soils had been stabilised by vegetation, but catchments were then profoundly affected by subsequent deforestation, land drainage, cultivation and sheep grazing. New (20th century) forest plantations, mining (metalliferous in the 19th century; coal in the 19th and 20th centuries), gravel extraction and industrial and infrastructure development have all effected further local changes to the landscape.
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