Scotland. Peter Friend

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FIG 60.Timeline of bedrock and surface-layer events in Area 2.

      The early Devonian was also a time of igneous activity, and Devonian igneous rocks are often associated with interesting landscape features in Area 2. The largest area underlain by Devonian igneous bedrock is the Cheviot Hills in the northeast. Here, great thicknesses of sub-horizontal lava flows (predominantly mafic andesite) rest unconformably on Silurian marine sediments. This lava was extruded from an igneous centre that today straddles the border between Scotland and England. The centre experienced several stages of igneous activity: an initially explosive volcano was followed by a large out-pouring of lava, which today, after erosion, covers around 600 km2 of the Earth’s surface. This large area of lava was then intruded at depth by felsic granite, which after millions of years of erosion has been exhumed and today underlies the highest part of the Cheviot Hills, over the border in England. Elsewhere in the Area, much smaller Devonian volcanic vents and intrusions are relatively common.

      By the start of the Carboniferous, around 360 million years ago, the Caledonian Mountains had been largely eroded, although the Southern Uplands still formed a considerable upland area. Throughout the following 60 million years of the Carboniferous, deposition in this Area occurred mostly in the lowlands of the Solway Firth Basin and along the border with England. Marine and coastal plain environments dominated, although varying relative sea levels resulted in the deposition of a variety of sediments, including limestones, sandstones, mudstones and coal. The Carboniferous sediments near Glencartholm, just south of Langholm (Fig. 58), contain one of the richest Carboniferous fish faunas in Great Britain, and indeed in the world. The site is remarkable for the number of species of fish discovered, around 35, several of which are unique to the site. Some Carboniferous to Permian-aged igneous rocks are also found in this Area, generally in the form of small intrusions of coarser-grained rock.

      By the end of the Carboniferous, Scotland had drifted northwards from the equator and the climate became more arid. Throughout the Permian (between 290 and 250 million years ago), red desert siltstones, sandstones and conglomerates were again deposited by winds and rivers. Fossils are not particularly common, but at Locharbriggs, just northeast of Dumfries, numerous reptile footprints have been discovered, generally heading in a southwards direction (very possibly towards the nearest source of water, in the Solway Firth Basin). The hot and arid climate of the Permian continued into the Triassic, and boulders, pebbles and sands continued to be washed and blown from the higher ground of the Southern Uplands down into sedimentary basins in the south and west of Area 2. These Triassic units tend to be mostly red, yellow and brown mottled sandstones, and comprise the Sherwood Sandstone Group. Together, both Permian and Triassic units make up the New Red Sandstone Supergroup.

      In this Area, the New Red Sandstone was deposited in a series of basins, preserved in what are today the valleys of Nithsdale and the lower part of Annandale. These basins generally have a north-northwest/south-southeast trend, broadly perpendicular to the regional Caledonian trend. The Dumfries and Lochmaben basins are fault-bounded, and gravity studies suggest that Permian strata in these basins reach thicknesses of over 1000 m. They are flanked to the south by the much larger Solway Firth Basin, whose axis overlies the Iapetus Suture. The basement of this large basin is made up of tightly folded Ordovician and Silurian strata, overlain by up to 6 km of Devonian to Lower Jurassic fill.

      No Mesozoic rocks are preserved in Area 2, although they would certainly have been present once: by the end of the Cretaceous, the sea had risen to cover all but the highest topography then present in this Area, and marine sediments would have been deposited during this time. However, during subsequent uplift and erosion all these younger rocks were washed away, such that during the last ~230 million years this Area has been subject to net erosion. The youngest bedrock present in this Area consists of Tertiary dykes, which despite being very small can be responsible for distinct walls in some landscapes. These dykes are long, thin intrusions of igneous rock, injected from igneous centres in western Scotland, such as Mull, around 60 million years ago.

      MAKING THE LANDSCAPE

      Tertiary erosion

      In early Tertiary times, widespread uplift occurred across much of Scotland, particularly in the west. The Southern Uplands were again uplifted, and, in the warm, wet climate of the time, the result was vigorous weathering and erosion. As in the rest of Scotland, this weathering initiated some of the largest-scale landscape features seen today: the main upland and lowland areas became either defined or enhanced, and most of the main river valleys were initiated. Although there is not an exact correlation, bedrock has obviously played a role in determining the characteristic of the landscape – the high ground of the Southern Uplands is underlain by relatively hard Silurian strata, whilst the softer Devonian to Triassic rocks underlie generally lower ground. The bedrock of the Cheviot igneous centre has also resisted erosion, forming the Cheviot Hills (Fig. 61).