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CHAPTER 5

Later Surface Modifications

THE PREVIOUS CHAPTER dealt with nine episodes recorded in the bedrock of Scotland. This chapter deals with three more recent episodes (Episodes 10–12; Fig. 21) which have modified the surface, removing bedrock and adding soft material to the surface blanket.

SURFACE-MODIFICATION EPISODES

Episode 10: Tertiary landscape erosion

Dating of the lavas extruded in Episode 9 suggests that Tertiary igneous activity in Scotland lasted for only about 5 million years and finished about 55 million years ago. This was followed by more than 50 million years of Tertiary and Quaternary landscape erosion (Fig. 21), during which time the main valleys of present-day Scotland increasingly approached their present shape and size.

Sedimentary bedrock of Tertiary age (Palaeogene and Neogene) is very largely absent on land in Scotland, even where volcanic and other igneous bedrock is present. This suggests that the crust below the present land area of Scotland was moving upwards and was subjected to net erosion during most of the Tertiary. Part of the evidence for this is the large thickness of Tertiary sandstones and mudstones that are found offshore to the east, north and west of Scotland, as shown by extensive oil exploration.

The valleys and mountains of Scotland, along with the lochs, sea lochs and offshore rock basins, have all been shaped by this erosion, principally by Tertiary rivers but also by more recent glacial ice (Episode 11). The present-day drainage pattern in Scotland (see Chapter 2) represents the latest phase in the evolution of this erosional system, and provides clues to the way it may have developed over the past 55 million years.

Episode 11: the Ice Age

During the nineteenth century, it became generally accepted that much of Britain had been subjected to glaciation by ice sheets and valley glaciers. Since then, this distinctive episode in the history of the British landscape has been referred to as the Ice Age, broadly equivalent to the Quaternary period of the internationally accepted series of time divisions (Fig. 21).

Over the last few years of geological research, one of the most far-reaching developments has been the establishment of the detailed record of fluctuating climate changes that have occurred during the Ice Age. A key step in this advance was the realisation that various indicators (often called proxies) of climate change can be measured at very high time resolution in successions of sediment or ice. The first of these successions to be tackled covered only the last few thousand years, but further work has now provided estimates of global temperature extending back several million years.

One of the best climate indicators has turned out to be variations in the ratios of oxygen isotopes (oxygen-16 versus oxygen-18), as recorded by microfossils that have been deposited over time on deep ocean floors. When alive, these organisms floated in the surface waters, where their skeletons incorporated the chemistry of the ocean water – including the relative amounts of oxygen-16 and oxygen-18. During cold climatic periods (glacials) water evaporating from the oceans may fall as snow on land and may be incorporated within ice sheets. Because oxygen-16 is lighter than oxygen-18 it evaporates more easily, so during cold periods the newly formed ice sheets tend to be rich in oxygen-16, relative to the oceans. The ratio of oxygen isotopes in the world’s oceans, as recorded by microfossils, can therefore be used to distinguish glacial and interglacial periods. Other useful indicators of ancient climate have come from measuring the chemical properties of ice cores, which preserve a record of the atmospheric oxygen composition, to complement the oceanic data from sediment cores.

Ratios of the isotopes of oxygen have turned out to provide one of the most important indicators of climate change, because they depend principally on ocean temperature and the amount of water locked up in the world’s ice sheets. There are, however, numerous other factors that can affect the ratios in ice and sediment cores, so interpretation of the data is rarely straightforward.

Figure 32 shows corrected oxygen isotope ratios as an indicator of temperature over the last 3.3 million years. The numbers on the vertical axis are expressed as δ18O values (pronounced ‘delta 18 O’), which compare the oxygen-18/oxygen-16 ratios in a given sample to those in an internationally accepted standard. The greater the proportion of heavy oxygen-18 in a sample the larger the δ18O value and, as described above, the lower the corresponding ocean temperature. For this reason, the vertical axis on Figure 32 is plotted with the numbers decreasing upwards, so that warmer temperatures are at the top of the figure and cooler ones at the bottom. The pattern shown in Figure 32 is of an overall cooling trend with, in detail, a remarkable series of over 100 warm and cool periods or oscillations. These alternations have been numbered, for ease of communication by the scientific community, with even numbers for the cold periods and odd numbers for the warm periods.

FIG 32.Oxygen isotope ratios track the more than 100 climate fluctuations over the last 3.3 million years. Warm episodes (red lines above the curve) alternate with cold episodes (blue lines below the curve). These have been used as the basis for numbering the global oxygen isotope stages, as shown.

Our next step involves looking in greater detail over roughly the last 400,000 years (Fig. 33). Over this period, there has been a distinctive pattern of increasingly highly developed 100,000-year-long cold stages, separated by 10,000-year-long warmer stages. This temperature curve (also calculated from isotope ratios) is saw-toothed in shape, representing long periods of cooling followed by rapid warming events. The most recent of the four glacial episodes covered in this diagram (the Devensian) has left abundant fresh evidence on the landscapes of Scotland and obliterated most of the evidence of the earlier ones. In this important respect, the Scottish evidence differs strongly from that of southern England, where the much earlier Anglian glacial episode has left abundant evidence of ice as far south as London. This is because later glaciations, such as the Devensian, did not reach so far south. Not surprisingly, the older evidence in southern England is not as fresh as that of the younger glaciation in Scotland.

An even closer look at the last of these cold-to-warm changes (Fig. 34, black line) allows us to appreciate better the glaciation which has been responsible for much of the recent modification of Scottish landscapes. Starting with the Ipswichian interglacial, the Greenland curve shows fluctuations in the oxygen isotope ratios that were frequent and short-lived, though generally implying increasingly cool conditions. This part of the record is helping to define the Devensian glaciation and shows clearly the Late Glacial Maximum (LGM) at between about 30,000 and 20,000 years ago. Following this, the beginning of the Holocene warm period (about 10,000 years ago) is also clear.

FIG 33. Isotopic temperature of the atmosphere changing through the last 400,000 years, measured from ice cores taken from Vostok, Antarctica.

The link between oxygen isotopes, temperature and sea level becomes clear if we compare oxygen isotope ratios from the Greenland ice (Fig. 34, black line) with sea-level data from tropical reefs in Papua New Guinea (Fig. 34, red line). The data show how colder climates are generally associated with lower sea levels, reflecting the locking up of oxygen-16-rich water in land-based ice sheets during these colder times.

FIG 34. Black line: oxygen isotope ratios sampled from cores taken in the Greenland ice sheet. Red line: sea-level determinations from tropical reefs in Papua New Guinea.

At its maximum extent the Devensian ice sheet covered the whole of Scotland, including the western and northern islands. It also covered most of Wales and northern England and extended as far south as the Midlands, the Bristol Channel and the Wash. Maintaining a thickness of many hundreds of metres, it joined Norwegian ice on the Norwegian side of the northern North Sea (Figs 35, 36).

FIG 35. One estimate of the maximum extent of the Devensian ice sheet, with generalised ice-flow directions. At a later stage the Scottish and Norwegian ice became separated.

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 the curve shown in Figure 42. This uplift is largely the result of isostatic rebound due to unloading of the crust as the ice retreated.

In some curves, as with the upper Forth, oscillations in the curve represent changes in the rates at which the two mechanisms of change were operating. Such changes may leave characteristic coastline features in the landscape, which will be considered in the Area descriptions.

At larger scales, it is useful to consider average rates of crustal movement over a given time period, which can then be plotted as contour maps. The contours shown on Figure 43 are based on estimates of local elevation changes averaged over roughly the last 5,000 years, attempting also to allow for the effects of eustatic sea-level variations. Additional support for this approach comes from data from tide-gauge studies, collected over the last 200 years, which show some consistency with this pattern.

Although our two local studies in the Thames region and the upper Forth (Figs 41, 42) involve a rather longer timescale than the regional analysis (Fig. 43), all three studies highlight the clear contrast between crustal movements in southeastern England and those in western and central Scotland. These variations have been produced by differences in ice-sheet thickness and extent during the last (Devensian) glacial.

The distinctive rise of the land of Scotland relative to sea level lends itself to another approach to the study of sea-level change. Figure 44 shows a plot of the elevations and gradients of various old shoreline features that are now above present-day sea level across Scotland and northern England. These old shoreline features have clearly been uplifted, and those further inland have generally risen more than those near to the present-day coast, so that they now form a dome-like structure. This dome is broadly centred on Rannoch Moor, which was one of Scotland’s main ice centres during the last glaciation. We can therefore usefully identify a Rannoch Rebound Dome as an active feature of local Earth movement, resulting from unloading of the crust of western Scotland in response as the Devensian ice melted.

FIG 43. General trends of crustal movement, relative to sea level, averaged over the last 5000 years.

FIG 44. The elevations and gradients of various old shoreline features along a horseshoe-shaped traverse across northern Britain, suggesting the Rannoch Rebound Dome.