Are relict landscapes useful for reconstructing past ice mass extent and movement?
Assemblages of relict landscapes are very useful when it is possible o carry out inversion modelling in combination with a morphological mapping of post-glacial areas via analysis of glacial deposits
How can Roche moutonees be used to reconstruct past ice mass movement or extent
Roche moutonees are large rocks with distinct erosional features lee and stoss ends:
- stoss end is relatively smooth with a polished rock surface indicative of glacial abrasion which always points up slope
- Lee end end is rough and jagged which is evidence of glacial plucking, this end always points down slope
- Some examples of of roche moutonees with these features are the Grasmere roche moutonee and the Steilimiggletscher in the Bernese alps. Both of which indicate a south easterly movement of their respective glaciers
- Roche Moutonees can also indicate direction for striations which are always parallel to ice flow. However they only indicate local movement and can be hard to distinguish between other scratch like features
- Roche Moutonnes cannot help us indicate past ice mass extent
How can Glacial Troughs be used to reconstruct past ice mass movement or extent
- If entirety of the trough is mapped then we can trace the entire path of the valley glacier from its corrie
- Locating a trim line can be useful for indicating the maximum extent of the glacier, below trim line we can see evidence of glacial erosion such as abrasion or plucking, above we do not see this
- -This may not that useful however as it only indicated height (one of 3 dimensions of a glacier) which varies due to compressional and extensional flows which change the height, with ratio of the glacier. For example the mere de Glac in Chamonix has a peak height of 200m during compressional flow but a peak height of 50m during extensional flow
- in some cases such as the nanty-y-llyn valley in the bergwyn mountain, the trough ahs no obvious trimline, above the trough we see blockfields and scree fall which is evident of periglacial erosion which suggests the glacier extended beyond the valley, meaning when it comes to the extent we can only know a size of the glacier when it was no greater than its valley
How can Drumlins be used to reconstruct past ice mass movement or extent
- Sinusoidal hills with lee and stoss end where the tapered lee points up slope and blunt stoss points down slope
- elongation of rumlin indicated rate of ice movement (geater the length the faster the movement), somemay rach up to 1500m in length
- sometimes possible to do till fabric analysis if all clusts point in one predominant direction which could infer the direction of ice mass movement
- However we cannot use drumlins to determine extend accuratley due to equifinality:
- -Bolton Menzies and Shaw theory both proposed however we do not know which one is more accurate and consequently cannot use them as a reference to make predicitons
- Drumlins are subject to human interference such as farming, construction and urbanisation which alter or destroy dumlins and reducing their value to reoncstruciton
- Glasgow is built on a drumlin field, those drumlins now cannot be used for reconstructive purposes
How can terminal moraines be used to reconstruct past ice mass movement or extent
Prominent ridges of glacial debris which have been deposited at the end of a glacier sustained advance (indicating its maximum extent).
- An example of this is the Valparaiso moraine south of Chicago which indicated the maximum extent of the glaciers from the Pleistocene period
- However due to the polycyclic nature of glacial landscapes, glaciers may advance past terminal moraines and cause them to become push moraines and consequently make it difficult to discern moraine extent
- Terminal moraines are also subject to subaerial weathering and can be modified by fluvial action
How can Eskers be used to reconstruct past ice mass movement or extent
Long sinuous ridges of sand and gravel - deposited by a steam in a subglacial tunnels
- have very high hydrostatic pressure which allows them to carry large amounts of debris, when glacier retreats then debris is deposited at a consistent ridge and forms a sinuous ridge. Since debris are deposited at a constant rates, eskers such as Comforlessbreen in Svalbard can be used to find the maximum extent of glaciers
- If we assume melt water and ice flow were parallel to each other then we can use eskers to indicate ice movement, although it is normally the case, this is nonetheless an assumption and therefore may not always be entirely accurate
How can Kame Terraces be used to reconstruct past ice mass movement or extent
Result from sediment accumulating in marginal glacial lakes and becoming trapped between glacial lobes at the valley sides. Sediment is typically fine and gravel like
- When a glacier retreats the sediment is left on the valley side to form a pair of kame terraces. The distance between them can be used to indicate ice mass width, in combination with a trimline we can use it to calculate the two dimensions of the glacier (width and height)
- They are commonly used alongside moraines and out-wash plains to map out consecutive areas of glacial retreat
- Example of kame terraces are the Cairgons where kame terraces occur on all flanks wherever meltwater has pressed against hillsides
- They can however be subject to weathering and human interference such as mining the sediment for building materials which devalues their reliability
Conclusion to the question
They are useful especially when used in combination with each other as to mitigate their respective limitations
- It is however important to consider that most glaciated regions are polycyclic meaning they are the product of many glacial cycles and could have faced modification during intergalcial, paraglacial and post glacial conditions
- weathering and mass movement may have altered these landscapes post glaciation, this must be taken into account
- Relict landscapes are useful for reconstruction in terms of approximation but should never be used as a method of certain reconstruction due to the inherent limitations present in all glacial landscapes in regards to their polyciclic nature and post glacial alterations
- Good for approximation but cannot provide a 100% fullproof reconstruction of past ice extent and movement
