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CHAPTER 7

Area 2: Southern Borders

AREA 2 COVERS THE CENTRAL PART of the Southern Uplands, sandwiched between Areas 1 (Galloway) and 5 (Edinburgh) (Fig. 57). It straddles the southern part of the English–Scottish border, extending from Annandale and Moffatdale in the west to the Cheviot Hills in the east (Fig. 58). The discussion provided here is restricted to features on the Scottish side of the border.

In the northern half of the Area, the River Teviot drains the relatively low-lying ground of Teviotdale, which in turn feeds into the larger Tweed Basin of Area 5 (Chapter 2, Fig. 7). Teviotdale is bounded to the east by the Cheviot Hills and to the west by the Moffat Hills of the Southern Uplands. These rounded hills reach heights of over 800 m, and form a broad region of elevated ground which dominates the landscape in the northwestern part of this Area. The Solway Firth with its adjacent low-lying plains is the main feature of the southwestern corner of Area 2, along with the broad valleys of the Annan and Esk, which penetrate northwards into the hills of the Southern Uplands.

FIG 57. Location map for Area 2.

FIG 58. Natural and man-made features of Area 2.

STORIES FROM THE BEDROCK

Area 2 is principally made up of hard Ordovician and Silurian sediments, which are also the oldest rocks in the Area (Figs 59, 60). These sandstones (greywackes), siltstones and shales were deposited between around 490 and 420 million years ago on the floor of the extensive Iapetus Ocean that, during this time, formed an Atlantic-scale ocean with Scotland and England on opposite margins. Around 490 million years ago, this ocean began to become smaller, as oceanic crust became subducted beneath the Grampian Highlands, and the Ordovician and Silurian sediments were scraped off the ocean floor and stacked up in a pile against the Midland Valley (Fig. 28). This deformation created a fold system across the uplands of this Area, with the fold axes trending northeast to southwest. Superimposed on this larger structure are countless minor parallel folds, which give a ‘grain’ to the oldest bedrock of the Area. During these movements, the sediments were altered to brittle sandstones and slates.

FIG 59. Simplified geology and hill-shaded topography for Area 2.

By the Devonian (around 410 million years ago), the major uplift and deformation of the Southern Uplands had ceased and the crustal materials of what are now Scotland and England became ‘welded’ along the Iapetus Suture. The mountainous ground of the Southern Uplands then began to be eroded, and large rivers flowed across the Area, depositing sandstones, siltstones and occasional conglomerates on lower-lying plains fringing the mountains. This Area lay in the interior of a large continent, and sediments accumulated on river flood plains where oxidising conditions led to the deposits being stained with red iron oxide. These Devonian sediments were deposited on the eroded surface of older, deformed Silurian and Ordovician rocks. This surface was far from uniform – in places, it had a relief of elevation of over 100 m. As a result, the thickness of the Devonian sediments varies from place to place, in part reflecting the fact that these river sediments were filling hollows in the topography. These deposits, collectively termed the Old Red Sandstone, today underlie a sizeable area just west of the Cheviot Hills.

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).

The Southern Uplands would have been a relatively high table land at the start of the Tertiary. The subsequent erosion cut into this plateau, carving out valleys and watersheds which today define the hills. Although glacial erosion has been very important in creating the ultimate shape of the landscape, the remnants of this plateau surface are still visible in the flat or rounded hill tops, which all lie at similar elevations.

In places, weathering has clearly been concentrated along lines of geological weakness, such as faults and softer sedimentary units. The remarkably linear valley of Moffat Water, for example, coincides with a large Caledonian fault and a thin band of softer Ordovician shale. Likewise, the major valley of Annandale appears to have been eroded in the softer Permian sedimentary rocks, whilst the thin band of Silurian rocks separating the Permian of Dumfries and Lochmaben is obviously more elevated. The main rivers in Area 2 are the Tweed, Annan, Esk, Ettrick, Yarrow and Teviot, and the sources of all but the last lie in the higher hills in the northwest of the Area. Many of these rivers flow along northeast/southwest-trending valleys, parallel to the underlying folded strata, the water having taken advantage of either faults or weaker bands within the succession. Another important valley is found in the southern part of the Area: Liddesdale runs approximately northeast to southwest, with Liddel Water defining the Scottish–English boundary for several kilometres before joining the Esk. The development of this valley, too, appears to have been influenced by the underlying geology, as its location coincides with an area of heavily faulted and soft Carboniferous sedimentary rocks.

FIG 61. Digital elevation model of Area 2 with the main river valleys shown, as well as main upland areas. Sub-areas discussed in later maps are indicated by rectangles with red borders.

Evidence that the Solway Firth area acted as a basin for sedimentation in Permian and Triassic times has been outlined above. It seems very likely that it was acting as a river valley during Tertiary times.

Landscape development during the Quaternary glacials

The final form of the landscapes in Area 2, as in the rest of Scotland, owes much to the action of ice and ice meltwater. During the last 2 million years, ice sheets have repeatedly expanded to cover much of Scotland. These ice sheets flowed radially outwards from centres in the Highlands, the Southern Uplands and the Lake District, and were powerful agents of erosion and deposition.

The most recent glacial stage reached its coldest in the Late Glacial Maximum about 20,000 years ago, at which time Area 2 was overrun by Southern Uplands ice flowing roughly eastwards across the Area from an ice divide located over the outer Solway Firth (Fig. 62). This flow is recorded by west/east-oriented drumlins along the low ground adjacent to the Solway Firth, and by a beautifully preserved array of ice-streamlined ridges along the Teviot valley. This flow direction must have been influenced by the presence of a strong ice dispersal centre in the Lake District that deflected the Southern Uplands ice to flow eastwards along the inner Solway Firth. Ice also accumulated in the Moffat Hills in the northwest of this Area, and a small but independent ice centre, powerful enough to withstand the pressure of the main ice mass, was present in the Cheviot Hills.

After the glacial maximum, the Lake District ice centre decreased in size and strength and Southern Uplands ice became dominant throughout the region. In the northeastern half of the Area, ice continued to flow roughly northeastwards down Teviotdale, but in the southwestern half, flow directions were reversed: drumlins and glacial striae indicate that ice flowed from the uplands around Moffat southwestwards down into the Solway Firth. As deglaciation continued, ice flow became valley-contained, and a final flow phase is recorded by north-northeast/south-southwest-oriented drumlins in the lowlands around the Solway Firth. Mapping of glacial deposits west of Annan and north of Gretna shows that ice then receded up the Solway Firth, as the Solway glacier retreated.

As mentioned above, the Cheviot Hills acted as a small but independent centre of ice accumulation and glacier dispersion during the Devensian glaciation. During this time, Cheviot ice flowed radially outwards from the centre, shielding the higher parts of the massif, particularly those above about 300 m, from the ice from the Solway Firth and Tweed basins. The lower peripheral hills, meanwhile, were overwhelmed and smoothed by this ice, and blocks of bedrock from the Tweed and Solway areas were deposited by the ice up to altitudes of around 300 m. Above this level in the central parts of the massif, soft, deeply rotten bedrock is relatively common and there are tors, erosional relics, over the border in England. These areas have clearly been glaciated, as they are often overlain by till, and yet they have survived erosion. Perhaps at the centre of Cheviot ice dispersal, the preservation favoured net deposition rather than erosion.

FIG 62. Ice flow in Area 2 at different times during the Devensian (LGM is Late Glacial Maximum).

The brief return to glacial conditions which occurred during the Loch Lomond Stadial (around 13,000 to 11,500 years ago; see Fig. 39) had only a limited effect in this Area. Glaciers returned only to the highest ground, such as around Broad Law, but even here the largest was only a few kilometres long. The position of these glaciers is clearly represented by terminal moraines – ridges of glacial till bulldozed by the Loch Lomond Stadial glaciers. Good examples are found, for example, at Loch Skene (Fig. 63), where a series of terminal moraines records fluctuations in glacier growth during retreat. The terminus of one ice lobe from this time has been located at the head of the Grey Mare’s Tail waterfall, at an altitude of around 450 m. Another terminal moraine forms a prominent ridge some 250 m long that runs parallel to the northeastern side of the loch (‘the Causey’). Indeed, Loch Skene owes its presence to these moraines, which effectively dam the loch outlet.

Glacial modification of the uplands

Glacial erosion has played an important role in creating the final shape of the landscape. Most of Area 2 was extensively ice-scoured throughout the course of the Quaternary glaciations, and the land surface present at the end of the Tertiary became heavily modified. In the uplands, moving ice lowered the valley bottoms and smoothed the mountains, creating the high broad hills seen today. This erosion was not as intense as that to which the western Southern Uplands were subjected, and whilst the main valleys were deepened by powerful ice streams, the intervening ridges and plateaus escaped deep scouring. In general, the uplands of this Area therefore lack the ‘Alpine’ form seen in the Highlands of Scotland: corries, arêtes and troughs are not as common nor, when present, as pronounced (Fig. 63