(no subject)
Summarise your impression of the rate, timing and magnitude of environmental changes in the Langdale Valley within the context of the wider region.
The Langdale Valley is situated in the heart of the Lake District, an area where its famous U-shaped valleys and freshwater lakes are signs of past glacial activity. The Pleistocene epoch is the epoch that covers the most recent period of glaciations - ice advanced during the glacials (such as the Devonian and Anglican) and then retreated during the interglacials. We are currently in an interglacial period known as the Holocene.
This advance and retreat of ice in the form of glaciers carved out the wide U-shaped valleys, of which the Langdale Valley is a good example of. These valleys are one of the largest type of glacial feature, being many kilometres long and potentially, a few kilometers wide. Langdale Valley is approximately 10kms long and 1km wide. The formation of such a big valley would have taken many cycles of glacials and interglacials over the Pleistocene epoch (which lasted from around 1.8 million years ago until 11 500 years ago). The rate of ice advance is slow, but due to the extensive mass and the pressures involved, a U-shaped valley was formed. (Grotzinger 2007)
Unchanged by any valley glaciers however, the Langdale Pikes would have protruded from the ice as nunattaks.
As each glacial period caused ice to advance, more and more glacial features would be formed in the valley. One such feature is a roche moutonnee, where ice has flowed over a small hill of rock, giving it a smooth slope on the upstream side and a rough, harsh angle on the lee side. These are relatively small features and its orientation can show the direction of ice flow in the region. In the Langdale Valley a roche moutinnee can be seen at grid reference (267, 066) showing the direction of ice advance going down the valley.
As well as erosional features, many depositional features can be seen in the valley, hummocky moraines being the most prominent. These form when material carried in the glacier accumulates together and is deposited on the valley floor when the ice melts. These are likely to have been deposited by a recent glaciation, as any further glaciations would 'bulldoze' or erode these out of the way. In the Langdale Valley there is an extensive area south of Rossett Gill, reaching along beside the stream until the confluence of Troughton Beck, that is shaped by hummocky moraines - reaching approximately 10m in height.
Another type of moraine that is prominent, although covers less area than the hummocky moraine, is a terminal moraine that is situated further towards the mouth of the valley. This marks the extent of the ice during the last glacial maximum (21Ka), and is less pronounced in the Langdale Valley, which could suggest that it was deposited significantly before the hummocky moraines and since been subject to erosional processes. The hummocky moraine could therefore have been deposited during the more recent Younger Dryas glacial event (a return to glacial conditions before the beginnign of the Holocene, approximately 12Ka).
Both of the moraines would have been deposited relatively quickly in comparison to other features such as U-shaped valleys, as it would have only taken the time that the ice takes to recede a short distance, rather than that of the whole valley.
However these have all been modified through erosional processes that occurred in the interglacial periods. The melting ice produced a large volume of water that cut through the hummocky and terminal moraines, creating many meandering streams. Due to the high volume of water, the velocity is great enough in some places to produce rapids over the rocks, (which themselves had been deposited there by the receding ice). The water has a high erosional capacity because of its high velocity and also because the large number of sediment it entrains that erodes the riverbed by abrasion. This can lead to plunge pools and extensive erosion of the glacial deposition features.
The high volume of river sediment is due to the high volume of sediment that the glacier was transporting. It erodes this from the valley bottom and sides by either 'plucking' (glacial friction causes small amounts of ice to melt, get into cracks in the rocks causing fragments to break off and become entrained in the glacier) or abrasion (friction between particles already entrained in the glacier and the bedrock) (Holden 2005).
As the glacier retreats and meltwater streams form, this material is transported some considerable distances downstream as glacial outwash. Due to the different size of the particles and the long distances they can travel, the particles can become 'sorted' as the heavier sediments settle first, and these sorted sediments collected in gravel pits in the wider region.
In the Langdale Valley another feature that can be seen in many places is an alluvial fan. This is caused by ephermal water (water that is only present during storms or a wet period) that spreads out at mountain fronts and deposits material into a fan shape (Holden 2005). These fans were seen particularly on the valley sides, suggesting that water enters the valley occasionally from sources other than the perennial tributaries. However this is only a low frequency, and low magnitude event (although it would happen more often than glacial advance occurs in the valley).
As can be seen, the Langdale Valley - as well as the rest of the Lake District - has been influenced by many environmental factors that have changed over time. These all date from no older than the Pleistocene era, as any features present before then will have been intensely modified by the Pleistocene glacial cycles and will be beyond recognition. As each successive glacial occurred, the ice 'wiped clean' any record of previous glacials before it, so the features seen today are remnants of the last glacial maximum that have been further modified (to a lesser extent) by fluvial and aeolian processes.
References
Holden, J. (ed) (2005) An Introduction to Physical Geography and the Environment. Harlow: Pearson Prentice Hall.
Grotzinger (et al.) (2007) Understanding Earth. New York: W H Freeman.
Huddart, D and Glasser, NF. (2002) Quaternary of Northern England. Peterborough
The Langdale Valley is situated in the heart of the Lake District, an area where its famous U-shaped valleys and freshwater lakes are signs of past glacial activity. The Pleistocene epoch is the epoch that covers the most recent period of glaciations - ice advanced during the glacials (such as the Devonian and Anglican) and then retreated during the interglacials. We are currently in an interglacial period known as the Holocene.
This advance and retreat of ice in the form of glaciers carved out the wide U-shaped valleys, of which the Langdale Valley is a good example of. These valleys are one of the largest type of glacial feature, being many kilometres long and potentially, a few kilometers wide. Langdale Valley is approximately 10kms long and 1km wide. The formation of such a big valley would have taken many cycles of glacials and interglacials over the Pleistocene epoch (which lasted from around 1.8 million years ago until 11 500 years ago). The rate of ice advance is slow, but due to the extensive mass and the pressures involved, a U-shaped valley was formed. (Grotzinger 2007)
Unchanged by any valley glaciers however, the Langdale Pikes would have protruded from the ice as nunattaks.
As each glacial period caused ice to advance, more and more glacial features would be formed in the valley. One such feature is a roche moutonnee, where ice has flowed over a small hill of rock, giving it a smooth slope on the upstream side and a rough, harsh angle on the lee side. These are relatively small features and its orientation can show the direction of ice flow in the region. In the Langdale Valley a roche moutinnee can be seen at grid reference (267, 066) showing the direction of ice advance going down the valley.
As well as erosional features, many depositional features can be seen in the valley, hummocky moraines being the most prominent. These form when material carried in the glacier accumulates together and is deposited on the valley floor when the ice melts. These are likely to have been deposited by a recent glaciation, as any further glaciations would 'bulldoze' or erode these out of the way. In the Langdale Valley there is an extensive area south of Rossett Gill, reaching along beside the stream until the confluence of Troughton Beck, that is shaped by hummocky moraines - reaching approximately 10m in height.
Another type of moraine that is prominent, although covers less area than the hummocky moraine, is a terminal moraine that is situated further towards the mouth of the valley. This marks the extent of the ice during the last glacial maximum (21Ka), and is less pronounced in the Langdale Valley, which could suggest that it was deposited significantly before the hummocky moraines and since been subject to erosional processes. The hummocky moraine could therefore have been deposited during the more recent Younger Dryas glacial event (a return to glacial conditions before the beginnign of the Holocene, approximately 12Ka).
Both of the moraines would have been deposited relatively quickly in comparison to other features such as U-shaped valleys, as it would have only taken the time that the ice takes to recede a short distance, rather than that of the whole valley.
However these have all been modified through erosional processes that occurred in the interglacial periods. The melting ice produced a large volume of water that cut through the hummocky and terminal moraines, creating many meandering streams. Due to the high volume of water, the velocity is great enough in some places to produce rapids over the rocks, (which themselves had been deposited there by the receding ice). The water has a high erosional capacity because of its high velocity and also because the large number of sediment it entrains that erodes the riverbed by abrasion. This can lead to plunge pools and extensive erosion of the glacial deposition features.
The high volume of river sediment is due to the high volume of sediment that the glacier was transporting. It erodes this from the valley bottom and sides by either 'plucking' (glacial friction causes small amounts of ice to melt, get into cracks in the rocks causing fragments to break off and become entrained in the glacier) or abrasion (friction between particles already entrained in the glacier and the bedrock) (Holden 2005).
As the glacier retreats and meltwater streams form, this material is transported some considerable distances downstream as glacial outwash. Due to the different size of the particles and the long distances they can travel, the particles can become 'sorted' as the heavier sediments settle first, and these sorted sediments collected in gravel pits in the wider region.
In the Langdale Valley another feature that can be seen in many places is an alluvial fan. This is caused by ephermal water (water that is only present during storms or a wet period) that spreads out at mountain fronts and deposits material into a fan shape (Holden 2005). These fans were seen particularly on the valley sides, suggesting that water enters the valley occasionally from sources other than the perennial tributaries. However this is only a low frequency, and low magnitude event (although it would happen more often than glacial advance occurs in the valley).
As can be seen, the Langdale Valley - as well as the rest of the Lake District - has been influenced by many environmental factors that have changed over time. These all date from no older than the Pleistocene era, as any features present before then will have been intensely modified by the Pleistocene glacial cycles and will be beyond recognition. As each successive glacial occurred, the ice 'wiped clean' any record of previous glacials before it, so the features seen today are remnants of the last glacial maximum that have been further modified (to a lesser extent) by fluvial and aeolian processes.
References
Holden, J. (ed) (2005) An Introduction to Physical Geography and the Environment. Harlow: Pearson Prentice Hall.
Grotzinger (et al.) (2007) Understanding Earth. New York: W H Freeman.
Huddart, D and Glasser, NF. (2002) Quaternary of Northern England. Peterborough