Difference between revisions of "Holocene, Northern England"
Revision as of 12:32, 9 April 2016
From: Stone, P, Millward, D, Young, B, Merritt, J W, Clarke, S M, McCormac, M and Lawrence, D J D. 2010. British regional geology: Northern England. Fifth edition. Keyworth, Nottingham: British Geological Survey.
The Holocene began abruptly at 11.55 ka BP when the warm Gulf Stream current became re-established, providing an ameliorating influence on the climate of the British Isles. 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 mountainsides 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.
Trees colonised the district 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). Later arrivals came via the Lancashire lowlands and the Vale of York. Most species spread relatively late into the Cumbrian Mountains, Pennines, Cheviots and particularly the Isle of Man. A distinct early phase of juniper dominance was quickly replaced by birch and willow woodland. Hazel became firmly established on the fringes of the Irish Sea and around Teesside by about 10 ka BP, possibly aided by human immigration. An expansion of elm at about 9.6 ka BP generally preceded oak, which arrived between 9.2 and 8.35 ka BP, especially in low-lying fertile areas. Pine spread into southern Cumbria between 9.5 and 8.7 ka BP, and then became established on the Zechstein Group limestone outcrop in County Durham. The arrival of alder at about 7.8 ka BP coincided with the transition from the drier, ‘Boreal’ climate period to the wetter, ‘Atlantic’ one in the mid Holocene (P916117). Lime arrived shortly after and all the elements of the mixed mid Holocene deciduous forest were present by 6.7 ka BP when dense tree cover dominated the landscape of all but the highest parts of northern England. Ash, maple, yew and beech all increased later in the Holocene as a result of human intervention.
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 6.7 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 5.6 ka BP. Forest clearance increased between 4.3 and 2.9 ka BP in the Bronze Age, with the most extensive deforestation occurring between 2.4 and 1.8 ka BP in the Iron Age. Despite the spread and retreat of the great Holocene forests, many species of the late glacial tundra flora of the district persist to the present day on the limestone soils of Upper Teesdale, including Dryas octopetala (the mountain avens, a small, white-flowered plant of the Rosaceae), after which was named the ‘Younger Dryas’ stage — the alternative name for the Loch Lomond Stadial used in continental Europe. Many wetland sites have also survived throughout the Holocene and preserve a valuable record of climatic change. For example, analyses of protozoan spores, plant macrofossils and humification in peat cores from raised bogs such as Walton Moss, Cumbria, have identified cyclical wetter and drier episodes (P916106).
Although the imprint of glaciation and periglaciation remains dominant, postglacial processes have superimposed subtle, but distinctive modifications on the landscape of northern England, particularly around the coasts. Steep hillsides have been modified by gullying, slope failure, soil-creep and debris flow; valley floors have been sculpted by fluvial erosion and deposition; tidal inlets have become choked with muddy estuarine alluvium and salt marsh deposits. Periglacial processes continue to operate on ground above about 450 m OD, particularly on Cross Fell but also elsewhere, creating a range of similar, but smaller features to those formed during earlier periglacial episodes. 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. Diatomaceous deposits accumulated locally in lake basins where there was relatively little sediment influx, such as at Kentmere, near Kendal. The single-thread, submeandering stream patterns of the present day became established early in the Holocene once soils had been stabilised by vegetation, but catchments have been profoundly affected by subsequent deforestation, land drainage, cultivation, overgrazing, mining, gravel extraction and industrial development.
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