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FIG 36. West-to-east generalised cross-section at the maximum extent of the Devensian ice sheet.
FIG 37. The larger rock basins are the result of erosion by Quaternary ice streams.
There is abundant local evidence in Scotland of the modification of valleys by glaciers and ice streams, which deepened and opened out the valley profiles, removing spurs and side ridges, to produce classic U-shaped glacial troughs. These troughs are very different from the V-shaped cross-sections and sinuous forms typical of river erosion (see Fig. 8, Chapter 2). This modification work is likely to have taken place in every one of the Ice Age glacial stages that occurred in Scotland, and the same processes have also been responsible for the elongate rock basins now recognised in many offshore areas (Fig. 37).
FIG 38. Shrinking of the main Scottish ice sheet over the last 18,000 years.
Episode 12: since the Devensian Late Glacial Maximum
The period of rather more than 20,000 years since the Late Glacial Maximum represents one of the most recent phases of intense landscape evolution (Fig. 38). Because this was a period when ice cover was generally decreasing, local evidence is often preserved that would have been destroyed during a major phase of advancing ice. The last 10,000 years is often referred to as either the Holocene or the Flandrian Interglacial, the latter name emphasising that the ice may well return.
FIG 39. Oxygen isotope variation from (a) Greenland ice cores and (b) Northeast Atlantic sea-surface temperatures, both over the time period from 15,000 to 10,000 years BP (before present).
The record of climate change since the Late Glacial Maximum has been greatly illuminated by the same use of oxygen isotopes as described above for Episode 11. One important advantage in working on these recent times is that it is possible to seek additional, independent information for the ages of samples. Some of this dating may be based on comparison of plant remains, particularly pollen from cores extracted by drilling into lake beds or peat-rich wetlands. Other dates come from the analysis of radioactive carbon, whose rapid decay rate makes it a powerful tool in dating material that is so relatively young.
Although the dominant feature of global climate change over the past 20,000 years has been the general warming trend, detailed research has established a complex pattern of climatic fluctuations. In Scotland, the most important of these fluctuations is the Younger Dryas cold phase, also known as the Loch Lomond Stadial (Fig. 39). During this time, between about 13,000 and 11,500 years ago, the generally retreating ice re-advanced to form an icecap covering much of the western Highlands (Fig. 38, red line). The local effects of this Loch Lomond Advance are particularly clear within the area of western Scotland where moraines were pushed forward.
SEA-LEVEL CHANGE
In Areas with coastlines, some of the freshest features of the landscape have formed since the Late Glacial Maximum as a result of changes in sea level. Two different mechanisms have combined to produce these changes:
(1) Worldwide ocean-volume changes of the water occupying the world’s ocean basins. These have been the direct result of the locking-up or releasing of water from land-based ice sheets as they grow or shrink due to climate fluctuations. The water itself may also have expanded or contracted as its temperature changed. These worldwide processes are often grouped together as eustatic.
(2) Solid Earth local movements which have resulted in the local raising or lowering of the ground surface relative to the level of the sea. These movements were responses to changes in the local temperature or stress pattern within the Earth. Ice-sheet melting unloaded the crust of the Earth locally, resulting in uplift, while ice-sheet growth loaded the crust, resulting in subsidence. These effects are often referred to as isostatic adjustments of local sea level (see Chapters 2 and 3).
Some parts of the world, for example many tropical areas, have been free of ice since before the Late Glacial Maximum and so have avoided any solid Earth movements associated with loading and unloading by ice. Records of changing sea level from these areas can therefore be used to estimate worldwide (eustatic) changes in the volume of the world’s oceans since the Late Glacial Maximum. Figure 40 shows that eustatic sea level has risen by about 120 m over the past 18,000 years, beginning with a slow, steady rise until about 12,000 years ago, followed by a rapid increase until about 6,000 years ago, and then another slow, steady phase up to the present day.
Curves of local sea-level change for any area can be estimated (relative to the present) by recognising and dating various features that indicate elevations in ancient coastal profiles. These features, preserved in the rocks either above or below the present sea level, include former erosional cliff lines, wave-cut platforms and ancient tidal, estuarine or freshwater deposits. The similarity or otherwise of such curves to the eustatic curve (Fig. 40) depends on whether the areas in question have been subjected to any localised solid Earth movements, such as ice loading or unloading.
Two examples of British sea-level curves, relative to the present, illustrate how the local uplift and subsidence history varies for different coastal areas around Britain. In the Thames Estuary, local evidence shows that a rise of some 40 m has taken place through time over the last 10,000 years, at first very rapidly but then more slowly between about 6,000 years ago and the present (Fig. 41, red circles). Modelling of the processes involved, incorporating estimates of eustatic (global) sea-level change and local solid Earth movements, gives a fairly good match to the observational data (Fig. 41, black line).
FIG 40. Generalised change of worldwide (eustatic) sea level over the past 18,000 years. (After Van Andel 1994, Fig. 4.11)
FIG 41.Relative sea-level curve for the Thames Estuary.
FIG 42. Relative sea-level curve for the upper River Forth at Arnprior.
Our second example of relative sea-level change comes from the upper River Forth and is quite different. It shows that there has been a fall of relative sea level of about 50 m over the past 15,000 years, so that former coastline features are now visible well above the present-day coast (Fig. 42, red circles). This type of curve is common in Scotland and, given the ~120 m worldwide rise in sea level shown in Figure 40, it is clear that the crust of the Forth region must have been subjected to significant uplift (~170 m) in order to produce
