Scotland

The Dalradian Supergroup was originally a succession of sediments more varied in type than the Moine. This has allowed the mapping of distinctive rock types across the country, revealing a complex pattern of folds (some upright, others over-folded) and fracture surfaces, themselves often folded after their original formation. These were formed by complex, multi-phase movements which occurred during a general convergence of the crust in a northwest/southeast direction. Radioactive dating indicates that much of this movement took place 470 million years ago, in the same Grampian episode that also deformed the Moine. It is estimated that the crustal rocks of the northern part of the Grampian Highland terrane were uplifted by some 25–35 km during this event, creating a major mountain range. Note that, despite such large amounts of uplift being indicated by research on the pressures that cause the metamorphism, mountains themselves never reach heights above sea level of this magnitude. The present height of Mount Everest is about 9 km, and this is thought to be some indication of the maximum height to which mountains can be lifted, given the powers of erosion that can be generated in present-day steep and high mountain belts. The mountains being measured in planets and moons may be bigger because of the different gravitational forces present.

Igneous intrusions were also formed during the Caledonian episodes, as heat from the compression produced molten magma that rose in the deforming crust, cooled and solidified, most commonly forming granites. These igneous volumes were emplaced both during and after the various phases of Caledonian movement. Where they have been exposed by erosion, they have given rise to differences in the material properties of the bedrock that have locally influenced the present-day landscapes.

The Great Glen Fault is one of the most obvious features of the landscape when Scotland is viewed from a satellite in space. Unlike the complex forms of the coastline and the river valleys, it represents a simple, straight or perhaps very slightly curved, vertical fracture cutting the crust (Figs 19, 20, 22). This major feature separating the Northern Highland and Grampian Highland terranes, and bisecting the Caledonian core, is now thought to have been part of a system of fractures that formed first in the Scandian phase (mid-Silurian, 430 million years ago) due to compressive continental movements that involved a strong enough oblique component to produce sliding parallel to the bedrock fabric of folds and faults generated by the general compression. A recent estimate of the amount of strike-slip sliding between Laurentia and Baltica (Fig. 25) during this phase is that it was about 1200 km, although this total movement was distributed between numerous faults. In the simple analysis of fault mechanics in Chapter 3 (Fig. 17), a clear distinction was drawn between reverse faulting, resulting from convergence or compression, and strike-slip faulting, resulting from shearing. The present belief is that the Great Glen, and other similar faults, formed as a result of a combination of compression and shearing, sometimes referred to as oblique-slip, or transpression.

Episode 5: formation of the Lower Palaeozoic of the Southern Uplands terrane

Strongly folded, fractured and altered Ordovician and Silurian bedrock predominates in the Southern Uplands terrane. The commonest material is mudstone, often altered to slate. Altered sandstones are also common, with lesser amounts of altered limestone and volcanic material (Fig. 19). In the present landscapes, much of this material has been weathered and covered to some degree with Ice Age deposits, so good exposures of the sediments are rare and the hills of the Southern Uplands are generally more rounded and less rocky than those of the Highlands.

FIG 26. Sketch sections of an accretionary prism forming during subduction: (a–c) the development of thrusting (reverse faulting) within an accretionary prism; (d) age relations within the thrust stack. Thrust sheets get younger towards the southeast (i.e. 1 is older than 3) but, within each sheet, beds get younger towards the northwest. The beds are often very tightly folded and dip steeply.

It is thought that these sediments first formed as an accretionary prism, created when ocean crust in the southeast was subducted (see Chapter 3) beneath the deforming continent to the northwest, now represented by the Highlands. As subduction continued, the newly deposited sediments were folded and scraped up into a number of slices that were made of younger and younger ocean floor sediment as the movement continued (Fig. 26). How much of the Southern Uplands formed as one of these accretionary prisms is uncertain, but it is clear that the setting was marginal to the main Caledonian mountains that lay to the north. The oceanic crust was subducted along a line (locally called the Iapetus Suture: see Fig. 20) that lay to the southeast of the Southern Uplands, roughly along the present Scotland–England border.

Episode 6: formation of the Lower Old Red Sandstone

Old Red Sandstone is the name commonly given to the red sandstones, mudstones and conglomerates that underlie rocks of Carboniferous age. The Old Red rests unconformably on older rocks in all of the Scottish terranes except the Hebridean, where it is absent (Figs 19, 20). Successions of this bedrock have been classified as Lower, Middle and Upper Old Red Sandstone, depending on their fossil content and spatial relationships. Episode 6 concerns only the deposition of the Lower Old Red Sandstone.

Although fossil evidence for dating the Lower Old Red Sandstone is not common, the primitive fish and plant fossils that do occur indicate that it was deposited during the late Silurian and early Devonian, about 420 – 400 million years ago (Fig. 21). The weathering properties of these rocks are such that, in their present-day erosional landscapes, the conglomerates (with their associated lavas) have generally resisted erosion, tending to produce distinct ridges and steep slopes.

The processes of surface modification that deposited the Lower Old Red Sandstone took place largely on land, in rivers and lakes, with small amounts of sediment transported locally by the wind. Great thicknesses of lava are also important, particularly in the Midland Valley, Grampian Highlands and the Cheviot area of the Southern Uplands. The andesitic composition of these lavas suggests they were formed by internal Earth movements related to the plate subduction associated with Episode 5, and they are the earliest Scottish rocks to have yielded reliable measurements of their magnetism at the time of their formation. This information has been used to show that Scotland was located roughly 20 degrees south of the equator at this time, and it is believed that the Scottish terranes had moved into approximately their present-day positions, relative to one another, by the end of this episode (Fig. 25).

FIG 27.Geography of Scotland during deposition of the Lower Old Red Sandstone. (After Trewin 2002)

It seems likely that many of the late Silurian and early Devonian sediments and igneous rocks accumulated in distinct subsiding basins, separated by a series of northeast/southwest-trending uplifting areas that formed during the later phases of the Caledonian mountain building. Although much of the sediment in these basins was derived locally from these actively moving uplands, there is evidence that some of it was transported here by large rivers flowing from other areas of active movement in Scandinavia. The fact that the Lower Old Red sediments are predominantly non-marine in nature shows that most of the crustal surface of Scotland had been raised above sea level by this time (Fig. 27).

POST-CALEDONIAN EPISODES

Episode 7: formation of the middle to late Devonian, Carboniferous and Permian

It is convenient to group together as one episode the deposition of the Middle and Upper Old Red Sandstone (Devonian), the rocks of the Carboniferous and those of the Permian. The total time period represented by these units extends from about 395 to 290 million years ago, by which time Scotland had moved north to equatorial latitudes. The rocks of this episode consist largely of mudstones and sandstones, deposited by rivers in lakes, on coasts and in shallow seas. They vary considerably in age and extent, lying on the eroded top of the deformed Caledonian bedrock and often reaching thicknesses of many kilometres.

Although there is plenty of evidence of internal earth movements during this episode, their intensity and regional geography indicates a change from the strongly compressive regime associated with the Caledonian mountain building and the closing of the Iapetus Ocean (Episodes 4 to 6). By the mid-Devonian, extension had begun through much of Scotland, resulting in the formation of subsiding basins. The Middle Old Red Sandstone formed in a particularly large basin often referred to as the Orcadian Lake Basin (Fig. 28). This extensional tectonic regime continued to characterise Scotland during much of the Carboniferous.

During the Devonian and Permian, sandy, wind-blown dune fields and evaporating groundwater conditions existed at times when local deserts developed under arid climatic conditions. The Carboniferous by contrast lacks evidence of such arid climates: river mouths were often deltaic, and the regular movement of river channels deposited distinctive cycles in the sedimentary succession, consisting of vertical changes in sediment type – most obviously between sheets of sandstone and mudstone. Limestones are also sometimes dominant where sources of sand and mud were absent. Coal-forming conditions developed repeatedly during the Carboniferous, particularly in parts of what is now the Midland Valley, and hydrocarbon-bearing mudstones were briefly but vigorously exploited west of Edinburgh. Both these had an important influence on economic and social development both locally and nationally. Carboniferous limestones, ironstones and certain sandstones have been economically important as well, at least in local terms.

FIG 28. Geography of Scotland during Middle Old Red Sandstone times. (After Trewin 2002)

Because of their economic significance, many of the Carboniferous deposits formed in this episode have been studied in great detail: tracing individual marker beds and attempting to date them by painstaking analysis of the fossil fauna and flora contained within them. This work has revealed that the Carboniferous sediments were deposited in large numbers of subsiding basins, usually only a few kilometres or tens of kilometres across (Fig. 29). These basins formed due to vertical movements of the Earth’s crust along faults, the continued activity of which caused thickening and thinning of the sediments as they accumulated.

FIG 29. Geography of Scotland during early Carboniferous times. (After Trewin 2002)

As well as sedimentation, this episode also involved considerable Carboniferous igneous activity, creating volcanoes and extensive lava fields and injecting large bodies of molten rock into the crust. This igneous bedrock has had a profound effect on the present-day landscape of the Midland Valley, and also on parts of the Southern Uplands. The weathering and erosion of the landscape has preferentially picked out the igneous bedrock because it is generally more resistant than the neighbouring sediments.

The Variscan mountain building (Fig. 25) is clearly represented in southwestern England and southern Ireland. In Scotland, it appears to be represented only by a change from Carboniferous deltaic sedimentation to undoubtedly freshwater or aeolian sedimentation in New Red Sandstone times, ushering in the Mesozoic.

FIG 30. Geography of Scotland and its surroundings during the Jurassic.

Episode 8: Mesozoic sedimentation

There are only relatively small volumes of Mesozoic sediment preserved as bedrock within the land area of modern-day Scotland, but large offshore areas of the sea bed are underlain by sediment of this age. The simple explanation for this is that the approximate map-shape of present-day Scotland was already becoming established by the beginning of the Mesozoic, resulting in extensive erosion of much of today’s landmass, followed by deposition in areas that are still offshore. Reconstructions of the geography of Jurassic times, say 175 million years ago, show an upland area roughly the shape of present-day eastern and northern Scotland. This area was surrounded by basins along the Hebridean and Atlantic margins to the west and by the North Sea to the east, into which sediments accumulated (Fig. 30). Conditions varied between the areas of accumulation, but this broad pattern continued from the Triassic, through Jurassic and Cretaceous times.

The sandstones and mudstones of the Triassic are often red due to oxidisation of their iron minerals, indicating a dry, desert-like climate. Conditions at this time were influenced partly by the global climate, but also by the general pattern of plate movement which, by the end of the Triassic, saw Scotland at about 30 degrees north – equivalent to the present-day latitude of the Canary Islands.

In Jurassic times, where river deltas fed into shallow seas, a wide variety of rock types was deposited: mudstones, sandstones and limestones, along with rare ironstones and coals. Organic material – largely algal – formed locally in some of the muddy seas and was particularly abundant in the case of the Late Jurassic Kimmeridge Clay. This unit has been the main ‘source rock’ for the North Sea hydrocarbons that have had such a critical influence on the British economy over the last 40 years. Key points in the trapping and preservation of the hydrocarbons are the presence of sandstone with a suitable porosity, and earth movements that have subsequently stretched the crust, faulting it to seal the hydrocarbon reservoirs. Meanwhile, fault-related Jurassic landslide deposits are a spectacular feature of outcrops on one stretch of the east coast of the northern Highlands (see Areas 16 and 17), while in some parts of the Hebrides Jurassic sandstones have provided resistant bedrock that has influenced the development of the landscape.

Cretaceous bedrock is very rare on land in Scotland and is generally only preserved as isolated fragments in areas of Tertiary volcanism, where sheets of lava have protected the Cretaceous rocks from the erosion that has removed them elsewhere. Small amounts of sandstone and chalk (the Late Cretaceous algal limestone that is such a dominant feature of the landscape of southern England and northern France) are preserved in some of the volcanic centres, but do not tend to influence landscapes on a scale that can be considered in this book. On the other hand, the offshore record of the Late Cretaceous around Scotland is much more complete, and the lack of mud and sand (derived from the erosion of land-based bedrock) in these deposits suggests that Scotland had been eroded down to a largely flat landscape by this time.

Episode 9: Tertiary volcanism

About 60 million years ago, in the earliest Tertiary, a dramatic episode of igneous activity took place along the western seaboard of mainland Britain. The resulting bedrock has played a major role in forming features of the landscape of the western Hebridean, Northern Highland and Midland Valley terranes. Successions of lava flows formed volcanic lava fields tens to hundreds of metres thick in many areas of the Inner Hebrides and northern Ireland. Distinct fields have been dated around Eigg and Muck at 60.5 million years old, around Skye and Canna at 58 million years old and around Mull and Morvern at between 58.5 and 55 million years old. The layered (‘stepped’) landscapes eroded in the bedrock of these lava fields are striking, and are due primarily to differences in erosional resistance between the lower and upper parts of each lava flow.

FIG 31. General pattern of processes thought to underlie a typical igneous centre.

Even more striking are the centres of volcanic activity and igneous intrusion that developed in a scatter of localities shortly after the lava fields formed (Fig. 31). The coarsely crystalline intrusive rocks of these centres dominate the landscapes of their surroundings, because of the resistance of this material to erosion. The eroded remains of these ancient igneous centres now form the remarkable Cuillin and the Red Hills of Skye, the mountains of Rum, the hills of the Ardnamurchan peninsula and the main mountains of Mull and Arran, not to mention the islands of St Kilda and Ailsa Craig.

In wider geographical terms, these Tertiary igneous activities, along with the associated uplift and erosion, were responses to the tectonic plate divergence movements that created the Atlantic Ocean, with additional igneous input related to ‘hot-spot’ activity in east Greenland, Iceland, the Faroes, western Scotland and northern Ireland.

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.