Abstract:
The relief area of the Lake District, Cumbria, and its landscape features are used as evidence to suggest that the successive period of glaciation seen in isotope records did in fact occur within Northern England. The Lake District is situated precisely for it to be likely an ice source point and in the path of descending Scottish ice sheets. Some features however cannot be explained through glacial processes and so a further time span must be assumed. Through an understanding of the underlying geology a better comprehension of how and why the glacial features may be exaggerated or dampened can be inferred. Distinction must be made furthermore between features of glacial erosion and deposition and those which have been acquired in timespans and from processes active when not encumbered with glaciation. Following such predications an interdependency between all physical aspects can be seen and applied to possible current landscape evolution.
Five Key words:
Lake District; topography; glacial features; landscape; Holocene.
The Lake District, Cumbria, is a location of the UK which has experienced great levels of glaciation, figure 1 (Holden, 2017). It is no surprise therefore that it is characterised by distinct glacial features – these being specific geographical definable imperfections in an archetypal basic landscape – which can directly be attributed to processes consistent with glaciation. This, however, cannot describe all features found in the Lake District. It is also the case that a basic archetypal landscape cannot easily be defined and arguably does not exist. Thus, to correctly understand the nature of the area, an investigation considering solely glacial and interglacial processes would not suffice. It is with this predication that this essay will consider the geological origins of the mountainous topography and the interdependent role this topography has with its features. Considerations of how the Lake district’s past has impacted upon its current environments will allow a greater understanding to be made when predicting the impact of any future climate change.
The geological origins of the topography in the lake district will be considered, the timeline of different glacial and inter-glacial periods will be examined and consideration given to the erosional and depositional geomorphological processes that have created landscape features within the study area; features resulting from glaciation, from processes active post-glaciation, and other contemporary processes. Finally, having established changes that might be currently occurring, it will highlight areas of societal interest.
The Lake District is a low relief mountain environment (McDougal, 2013) which is broadly comprised of three bands of rock trending from North West to South West with completely different characteristics. The Skiddaw bedrock forms a broadly triangular, mountainous zone in the northern third of the National Park, reaching maximum heights of 931m on Skiddaw. Formed from muds and sands on the seabed about 500 million years ago during the Ordovician period, the rock was subjected to uplift and faulting during the Caledonian and Devonian orogenies. Generally, Skiddaw group mountains are typically smooth in outline; examples include the fells of the Newlands valley. The brittle rocks break down easily resulting in features such as streams and deep gorges such as Trout Beck. The central Lake District is dominated by rugged mountains, formed from hard rocks of the Borrowdale volcanic group (BVG). 450 million years ago, subduction of the lapetus Ocean under Avalonia caused melting of the Earth’s mantle; magma formed a volcanic arc across Cumbria centred near the Scafell mountain range (McDougall, 2013). Eruptions of lavas, ash and pyroclastic flows formed a layer of rock estimated at 8km. The BVG rocks are mainly andesites, basalts and rhyolites; such hard rocks are resistant to external forces, breaking down into large blocks and comprise extensive areas of the highest upland with rugged, craggy slopes – Scafell (964m) which contrast with the Skiddaw rock. On a smaller scale, variations in detailed form of the individual lakes is controlled by local geology, e.g. Derwent Water’s shape being the result of a valley glacier widening rather than deepening the valley due to the bedrock of less resistant Skiddaw rocks which compares with the ribbon shape of the other BVG lakes (Smith, 2008).
Rock type is important in explaining the detailed form of the glacial troughs; the BNG rocks were heavily influenced by successive periods of glaciation; water at the base of the glaciers on the impermeable rocks led to greater propagation of cracks and loosening of blocks of rock greatly increasing the erosive capacity of the glaciers. Resulting valley sides are rugged and irregular knolls and striations remain. Such steep crags are fewer over the Skiddaw valleys where rocks broke down more easily producing a fine debris forming smoother valley sides closer to a v-shape.
In the southern third of the Lake District, the Windermere group comprises a zone of less resistant Silurian slates, sandstones, and limestones, which result in gentler foothills stretching from the Duddon estuary to Kendal and include Windermere and Coniston water. About 350 million years ago, during submergence, carboniferous rocks developed; evidenced by a limestone cuesta in the north and two west-facing scarps: Whitbarrow Scar and Scout Scar. Gentle sandstone hills and till covered undulating plateaus to the west and north of the region result from the Permian and Triassic uplift periods underlie areas of lowlands.
Over the last 200 million years, extensive erosion over several hundred million years exposed the Silurian slate and wore the rock down to low-profile hills. During the Alpine orogeny, a batholith of granite, 10km below the surface, buoyed up the ‘central massif’; subsequent erosion exposing it in the Eastern Skiddaw Fells. The granite batholith combined with hardness of the BVG bedrock has determined the topography and altitude of this system (McDougall, 2013) which when combined with previous fault systems formed the distinctive drainage system whereby valleys radiate from a point centred on Dunmail Raise north of Grasmere. The impermeability of the underlying BVG bedrock has ensured that meltwater and latterly rain water is held in place forming Britain’s longest lakes. The resistant nature of the BVG rocks has further relevance when considering the effect of glaciations. During glaciations, the most efficient ice flows were the valleys (Smith, 2008); the ice used and modified such fault line valleys, deepening their floors and in places straightening their alignments.
Underlying geology and rock characteristics have had a significant impact upon the landscape of the Lake District. The consequential mountainous nature of the area has resulted in high precipitation, which during glacial events resulted in high accumulation of ice and erosive glaciers with resulting landforms.
2.6 million years ago, during the Quaternary period, climatic conditions worsened, resulting in a possible 21 distinct glacial periods separated by interglacials and interstadials. This is highlighted in Figure two. Evidence of repeated cycles is found within the landscape but the precise geological timeline for such events is derived from ocean and ice cores along with pollen analysis. The Woolstonian Glacial period lasted 160,000 years until 128 KaBP (thousand years before present); clays and glacial debris found in boreholes near Sellafield data from this period whilst, vegetative evidence in Scardale peat deposits dates from the Ipswichian interglacial. The Devensian glacial occurred 115KaBP to 12 KaBP with maximum ice coverage circa 18 KaBP; average temperatures declined progressively over the first few thousand years with eventual, rapid ice build-up and intense glacial condition (Smith, 2008). The majority of deposits in the Lake District relate to the Devensian cycle.
Whilst the surface of the area and low lying topography would all have been encased in ice sheets, there is evidence to suggest that within the area some peaks remained untouched. This evidence comes from trimlines found on nunataks. Erosional evidence suggests that valley glaciers in the west and south west Lake District were larger and more active than those further east – possibly due to an increased level of precipitation (Smith, 2008). Evidence on High Street ridge and on Great Gable shows that the ice reached a maximum ice height of 870m (Paterson and Cuffey, 2013). Furthermore, on some of the Central Fells frost shattered rock outcrops indicate that the summits were exposed peaks (Smith, 2008). This contrasts with previous models. The existence of other Devinsian icecaps impacted upon the three-dimensional character of the Lake District. Drumlin’s length to width rations can indicate the rate of ice movement; the more rounded drumlins with ratios of 4.6:1 found near Wigton indicate faster ice moving unimpeded into the Irish Sea basin, whilst ratios of 1.9:1 found near Carlisle are indicative of ice blocked by other masses.
Drumlin location Length:width ratio Characteristic Indication
Appleby
2.5:1 Slow moving Up valley SE through Stainmore Gap
Penrith 3.1:1 Fast moving Travelling N
Carlisle 1.9:1 Slow moving Ice impeded by Scottish ice
Wigton 4.6:1 Very fast Ice streaming into Irish Sea
Figure x
Throughout most of the Devensian glacial period, the polar front was at a latitude off Northern Portugal, but by 15KaBP it receded to a position close to Iceland. The impact of such change was dramatic. By 14.5 KaBp the climate of Britain was comparable with present day. This period was known as the Windermere Interstadial (Bell and Walker 2005). Between 13 and 11.5 KaBp, during the Loch Lomond Stadial (LLS), polar waters spread southwards at a rate of 5km yr-1, with the polar front reaching south of Ireland. Renewed glacier activity occurred in mountainous regions; valley glaciers developed in the Lake district and woodland that had re-established during the warm periods was replaced by tundra in northern and western Europe.
Amelioration occurred in the early Holocene; an increase of 15 degrees from the LLS took around 1500 years. The initial increase of 10 degrees was rapid and was accompanied by increased snow accumulation, a decline of wind-blown materials and the rapid expansion of wetlands. Evidence in Icelandic sea cores shows that climatic optimum occurred 9KaBP to 4KaBp, with temperatures in the order of 1 – 3 degrees above present. Pollen analysis shows the northwards migration of woodland in Europe and advancement of the treeline to above present-day levels. This change in climate, along with environmental change established different soil mineral components (Ashman and Puri, 2015) which would eventually contribute to the environment and wildlife seen in the area. This change in climate meant that previously glaciated areas became subjected to other forms of weathering with associated landscape evolution.
Circa 6KaBP, in the late Holocene, a decline in temperatures occurred with an accompanying trend towards wetter conditions around 4.5 KaBP. A slight climatic recovery followed during the historic period with fluctuating but generally warmer conditions, but with slightly cooler and wetter conditions during the middle of the first millennium (Lamb and Ballentyne, 1981). The medieval warm period followed; AD 800 to AD1200, which allowed the advance of cultivation. During a period known as the Little Ice Age from AD 1400 up to circa 1850, slow cooling and colder winters with higher levels of precipitation occurred which resulted in vegetation changes, crop failures, and the abandonment of settlements.
Such a timeline of recurrent climatic change has resulted in a significant number of landforms that can be attributed to climatic-related geomorphological processes; glacial processes being prominent. Such glacial landforms can be attributed to the glacial processes of erosion and deposition. Erosion is the process where material that comes into contact with the glacier is removed; this is largely done from the pressure of the glacier – 44MPa (Holden, 2017) – and the force such a large pressure exerts on the landforms. The ice under pressure plucks or crushes weaknesses in the rock. Transported rock particles also scour the bed through abrasion resulting in features such as striations. A further agent of erosion found in glaciers is that of Nye-channels; sub glacial streams eroding into the rock surface through the mechanical and chemical action of the water (Holden, 2017). Freeze-thaw also contributes significantly to glacial erosion. During periods of glacial recession, deposition is made directly from the glacier whereas during periods of positive mass balance a sub-glacial stream’s ability to entrain is reduced and so deposition occurs – both situations will create depositional landforms. Deposition is the act of a glacier laying down its transported material. As glacial streams gradually decrease competence, outwash sediment will be deposited in accordance with the calibre of the particle resulting in stratigraphically sorted sediment. Conversely, when a glacier directly deposits it material from either a massive change in momentum or total recession, this is done without sorting.
The landscape of the Lake District is a classic example of an area that has undergone long-term glacial erosion. The Scafell massif has an alpine landscape; for example Striding Edge arête on Helvellyn formed between Red Tarn and Nethermost Cove. Well-developed cirques exist in the valley heads (Brown et al, 2011). Authorities list circa 200 cirque basins in the Fells and 19 well-developed tarns with other peat bogs possibly being former tarns (Smith, 2008). Fig 2 demonstrates the influence of aspect where cirque development is concentrated on NE and N facing slopes but also shows that development is concentrated on the BVG. This area contrasts with the more ‘plateau-like’ relief of High Raise in the central Lake District where fewer than 20 cirques developed on Skiddaw bedrock. Seathwaite valley is a classic u-shaped valley with hanging valleys; Styhead Gills a stream filled ravine ends as a waterfall. The characteristics of the specific valleys are determined by bedrock; the distinctive shape of Buttermere glacial trough with steep sides cutting into BVG bedrock contrasts with valleys in the N and NE Fells where ice was eroding over rocks from Skiddaw group; rocks which break down easier resulting in valley sides with smoother sides. Striations are caused as the bedload of the glacier abrades the rocks and roches moutonnees, classic glacial erosional landforms are common along the glacial troughs of the main Lakeland valleys, e.g Wastwater. Similarly, the crag and tail at Castlehead stands 70m above the surrounding area.
Depositional features which can be found in the landscape include moraines which are found in till-plastered valley floors that have paraglacially modified sediments, scree and thick fluvial gravels (Brown et al, 2011). In some valleys, ice marginal moraines record successive positions of the glaciers which actively backwasted towards their source. In Rosthwaite, two ridges result from a glacial retreat depositing a second moraine. Older moraines from main glaciations often show intense periglacial weathering during the LLS (Mc Dougall, 2013). Drumlins found in the Eden Valley typify deposition.
Landforms typical of periglacial environments can be identified in the Lake District. During the Windermere Interstadial and early Holocene, rapid wastage of great ice sheets and glaciers occurred. The release of enormous quantities of meltwater resulted in both erosional and depositional landforms. Initially drainage was impeded and uncoordinated as water emerged from beneath the ice. Meltwater channels are distinctive periglacial features. In the Lake District some form open, abandoned channels cut into the landscape e.g the entrance to Whitehaven. The Nannycatch river system, which allowed melt waters from the Ennerdale Valley to escape southwards, cut into solid rock and is 90m deep. Enormous amounts of material were deposited; the rock material now lies thick over the surrounding lowlands; Solway Lowlands, the plains of West Cumbria and covers the floor of the Irish Sea. Predominantly it is glacial till mixed with strategraphically sorted sands and gravels flushed out by meltwaters. Classic kettlehole lakes feature over much of the lowland areas of West Cumbria, for example Martin Tarn near Wigton, where in the final stages of a glacial, ice sheets broke up and thick ice patches became stranded, stagnating in-situ. The deglacial phase of a glacial cycle is accompanied by increases in sediment yield resulting in huge quantities of outwash material and spreads of gravel on valley floors. Mosedale and Eskdale have a smooth appearance owing to glaciofluvial alluvium covering the valley floors; along with fine material, large quantities of gravel are indicative of high post-glacial river discharges (Brown et al, 2011).
The most common surfaces for paraglacial slope failure are trough walls; after ice recession valley sides can become susceptible to failure. Wasdale, a trough-valley in the southwest lakes with BVG bedrock, is characterised by numerous rock-slope failures which have occurred paraglacially. Wilson (2005) considers that all sites of slope failure would have been below the maximum height of the ice sheets – 830m (Lamb and Ballentyne, 1998). The process of a rock-slope failure following glacier recession is shown in figure xxxx
Over successive glacial and inter-glacial cycles, the interplay of glacio-eustatic and glacio-isostaic changes have resulted in relative sea-level changes. Around 30 to 19 KaBP, when ice masses were at their greatest global sea levels were reduced by around 135m. Glacio-eustatic lowering, accompanied by the isostatic depression of Northern Britain resulted in shorelines being progressively raised. Glacio-isostatic recovery since the LLS has been 60m with the main uplift occurring in the early Holocene. Later eustatic sea-level rises of about 10mm yr -1 exceeded isostatic recovery. River systems in the Lake District testify to changes in sea level over 25 Ka; meander incision, river terrace development, channel change and sediment loading. Figure one shows a river cross profile at a field study site of Lower Mosedale Beck which is consistent with river terracing and an incised meander, the probable cause being isostatic rebound as the river base level dropped and rejuvenation occurred.
Throughout the Holocene, climatic improvements have also impacted on the landscape. Changes to the periglacial environment initiated the succession whereby arctic tundra changed to deciduous woodland. During the Devensian glaciation, vegetation belts had migrated over several thousand kilometres with accompanying changes in soil properties. This was accompanied by dramatic changes in fluvial regimes, erosional activity and pedogenesis (Bell and Walker, 2016). The last cold stage was characterised by a vegetation which was sparse with large areas of bare ground and moving soils (Iversen 1958). Circa 10.5 Ka to 6 KaBP, temperate deciduous woodland began to become established and fertile brown earth soils developed. From 6 KaBP, anthropogenic influences combined with climatic deterioration disrupted the natural vegetation pattern as heaths and grasslands progressively replaced deciduous woodland. Wetter conditions led to accelerated leaching of soils and massive areas of peat bog formed in depressions eroded during glaciation (Huntley and Birks 1983).