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
