FIG 48. Timeline of bedrock and surface-layer events in Area 1.

      Prior to the Caledonian mountain building, the crustal foundations of Scotland and England were separated by the Iapetus Ocean. Around 490 million years ago, this ocean began to be destroyed by subduction: oceanic crust moved down into the mantle beneath the Grampian Highlands, and then beneath the Midland Valley (see Chapter 4). A small fragment of this oceanic crust escaped subduction, being instead thrust up onto the margin of Scotland, to be ‘welded’ onto the Midland Valley by around 470 million years ago. Today, this small but intensively studied area of complexly interfolded rock units outcrops around Ballantrae, the so-called the Ballantrae Complex. Rocks characteristic of the deep sea and oceanic crust are found – sediments such as black shale and chert, basalt lavas with pillow structures, ash, sheets of dykes and upper mantle rocks. The latter originated at depths of up to 40 km in the Earth’s crust, and are today coarse-grained (mafic) gabbros and serpentinite.

      The main deformation of the Southern Uplands terrane occurred during the later stages of the Caledonian mountain building, between the mid-Ordovician and the early Devonian. As oceanic crust continued to be subducted, the sediments which today make up the Southern Uplands were scraped off the ocean floor along a series of thrust faults and stacked up in a pile against the edge of the Midland Valley (Fig. 28). During this deformation, the sediments became tightly folded and weakly metamorphosed: fine-grained mudstone and siltstone became slate, while cement within sandstones recrystallised to produce a tough, hard rock (greywacke). Today, bedding in the Southern Uplands is aligned in a general northeast/southwest direction and dips very steeply to the southeast, and northeast/southwest faults divide the region into numerous fault blocks.

      By the start of the Devonian (around 415 million years ago), the major deformation of the Southern Uplands had ceased and Scotland and England were welded along the Iapetus Suture. It was around this time that the major granite bodies of the Southern Uplands (Fig. 47) were emplaced: partial melting at the base of the thickened crust produced liquid magma, which then rose up into the upper crust where it slowly solidified to form coarse-grained igneous bodies (plutons). As the overlying rocks were subsequently removed by erosion, three major plutons were revealed in the Southern Uplands. The most northerly of these is the hourglass-shaped Loch Doon intrusion, said to be one of the finest examples in Scotland of a concentrically zoned pluton: the interior of the body is silica-rich (felsic) granite, separated from the outer silica-poor grey granodiorite which makes up most of the body by a transition zone. Similar well-developed concentric zonation is seen in the eastern half of the Criffel–Dalbeattie body on the south coast, although overall this body is much less compositionally evolved (i.e. it has a lower silica content) than the other Southern Uplands granites. It is also the most deformed: originally oval, its western part has been distorted southwards by complex faulting. Porphyrite dykes and sills commonly surround the main intrusion (e.g. at Black Stockarton Moor), made up of large crystals embedded in a fine, glassy groundmass. Between the two, the roughly oval Fleet pluton was intruded around 390 million years ago (Devonian) into a broad ductile shear zone, making it the youngest reliably dated Caledonian pluton in mainland Scotland (Fig. 49). It is also the most evolved of the Southern Uplands intrusions, consisting entirely of granite, and is the only intrusion whose magmas were sourced wholly from the melting of metamorphosed sediments (rather than igneous rocks). Because of both its young age and its evolved composition, this pluton has more in common with the Lake District and Northern Ireland granites than with those of Scotland, and it has been suggested that these areas shared a magma source.