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Title: Late- and post-glacial changes of shoreline on the northern side of the Forth valley and estuary
Author: Smith, D. E.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1965
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Abstract:
The generally accepted patterns of raised shorelines on the northern side of the Forth valley and estuary are substantially in error. This investigation has shown that a radically different pattern of raised shorelines exists in the area, and that from this pattern and the associated glacial features there may be interpreted the following general sequence of events: 1. During the Würm period, the Forth area was covered by a great ice sheet which depressed the crust of the earth beneath it. As the ice sheet began to retreat, and as the load decreased, the crust reacted by trying to maintain isostatic equilibrium, thus rising. During the retreat large quantities of water, which had been abstracted from the oceans during the build-up of this and other contemporary ice sheets, were returned to the oceans and world sea level rose. Thus by the time the Forth area began to be released from the ice and the sea was able to gain access, both isostatic and eustatic rise were in progress. This resulted in the formation of several displaced shorelines, which mark periods when isostatic and eustatic movement were in equilibrium. 2. Two (LG1-2) and probably four other (LG3-6) displaced shorelines were formed during this early period while the ice probably retreated without major interruption from Fife Ness to Leven. Each shoreline slopes down towards the east, demonstrating that maximum isostatic uplift lay in the west, and each shoreline is less steeply sloping than the one above it, suggesting regular uplift. 3. The six displaced shorelines are as follows: LG1 is the best developed. It slopes eastwards from a height of 90-92 feet O. D. at Anstruther to 75-78 feet O.D. at Fife Ness. It is associated with a prolonged halt of the retreating ice near the site of Anstruther. • LG2 is also well formed. It achieves 81y83 feet O. D. near Pittenweem, and slopes down to 68-69 feet O.D. at Fife Ness. It displays no definite association with a former ice front, but there are instances of parts having been formed in association with dead ice. • LG3 is less clear, but has been defined as sloping eastwards from a maximum of 81-84 feet O.D. near Elie to a low of 64-65 feet O.D. near Fife Ness. There is evidence that certain fragments were formed alongside masses of dead ice. • LG4 is better defined, sloping eastward from 82-84 feet O. D. near Lower Largo down to 72-73 feet O.D. near Pittenweem. It appears to have been formed while the ice front lay in the vicinity of the site of Lower Largo. The highest Kincraig rock bench (80 feet O. D. ) was probably formed by the sea of this level. • LG5 is better developed than either of its two predecessors, and slopes from 77-81 feet O. D. near Leven down to 60-61 feet O. D. at Fife Ness. The second Kincraig rock bench (70 feet O. D. ) appears to be a part of this level. The ice front associated with this shoreline lay near the site of Leven. • LG6 is least well developed, since its fragments are well scattered. It appears to slope from 72-73 feet O.D. at Leven, where it is associated with a large dead-ice hollow, to 53-56 feet O.D. at Fife Ness. • The pattern of the above shorelines suggests that during this stage isostatic movement was in the form of an expanding dome. 4. The subsequent retreat of the ice is less well documented by the field evidence, and likewise there is little record of local sea level movement. After a retreat of an unknown, but probably not inconsiderable, distance, a major readvance occurred, correlated with the Perth Readvance. A well marked shoreline, LG7, was formed while the ice lay very near the maximum of the readvance, and it would appear that this shoreline represents a transgression. It runs from 106-108 feet O.D. at Kincardine to 66-68 feet O. D. at Burntisland, and a projection of the line of slope eastwardsuggests that the 37-38 feet O.D. Kincraig rock bench is a part of this feature. Apart from the carse, this shoreline is the best marked shoreline in the area investigated. Its inclination is steeper than that of LG 4-6 when these are plotted on a line through the most probable direction of maximum displacement, WNW-ESE. This interruption in the shoreline pattern suggests that the weight of the ice of the Perth Readvance caused a temporary renewed depression of the crust. 5. During further retreat of the Perth Readvance ice, a shoreline seems to have been formed at least west of Stirling, sloping eastwards from 68-69 feet O.D. at Wester Borland, near Thornhill, to 64-65 feet O.D. at Burnbank. This has been called LG8. 6. Some time afterwards, during Zone III of the pollen sequence, or 10,800-10,300 B. P., another major readvance occurred, the Loch Lomond Readvance. In the area investigated it was associated with a low sea level, but subsequently a transgression laid down a deposit of fine grey sand over the outwash gravels. The surface of this deposit slopes down towards the east. Sea level fell, and peat grew on the surface of this marine deposit. Peat in a similar situation in the Forth valley has been Carbon 14 dated at 8421±157 B.P. 7. Sea level rose, and submerged the peat, excepting a small island of peat on the site of Flanders Moss. The carse clays were deposited during this transgression, their altitude reflecting at least four sea levels, and showing that isostatic rise was still in progress. • PG1 slopes from 47-48 feet O.D. at Tarr down to 40-41 feet O.D. near Tullibody. • PG2 slopes from 42r43 feet O-D. near Ballinton down to 38-39 feet O. D. by Menstrie. • PG3 slopes from 31-32 feet O.D. near Bridge of Allan down to 29-30 feet O.D. at Cambus. • PG4 slopes from 23-24 feet O.D. at Alloa down to 22-23 feet O.D. at Kincardine. • Each slopes less steeply than the one above it, suggesting no interruption in the rate of isostatic recovery. Remnants of these shorelines occur eastwards in the estuary, but it has not been possible to make definite correlations. A Carbon 14 date of 5492±130 B.P. from the base of a peat moss lying on the carse of PG1 shows that this shoreline at least was formed by that date. 8. The shorelines defined after LG7 suggest that the movement of an expanding isostatic dome was renewed after the Perth Readvance.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.662113  DOI: Not available
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