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.
