Much of the Southern Uplands are comprised of Ordovician and Silurian rocks that were once sands and muds deposited on the floor of the Iapetus Ocean. Deposited over a 70 million year period, between 490 and 420 million years ago, these rocks bear testimony to the closure of the Iapetus Ocean, with the coming together of what is now the North American continent and Northern Europe.
Raven Gill in the Southern Uplands – the landscape here is underlain by 485 million year old sediments dating from the Ordovician geological period, including shales, and chert from the floor of the lapetus Ocean. Exposures of basalt and gabbro that formed the ocean crust also occur. © Scottish Natural Heritage.
This continental collision known as the Caledonian Orogeny joined together the crustal foundations of both Scotland and England. During the collision, sediments were scraped off the floor of Iapetus and heaped onto the edge of the northern continent.
At around 400 million years ago, at the start of the Devonian period, there was the intrusion of major granite plutons within the Southern Uplands such as the Criffell granite. The granite in these masses was derived from the actual partial melting of rocks lower within the crust, where the heat and deformation caused by the continental collision was most intense.
Two major junctions within Scotland’s crustal foundations occur within the area and are associated with the closure of the Caledonian Orogeny and the closure of the Iapetus Ocean. In the south, running in a NE-SW direction under the Solway and border area of Scotland and England, there is the ‘Iapetus Suture’, which marks the zone of closure of the ancient ocean. Across the top parts of the region, running in a similar NE-SW direction, there is the Southern Uplands Fault, which marks the northern limit of the Southern Uplands and its contact with the Midland Valley.
Carboniferous sedimentary rocks of the Midland Valley, and separate English basins, overlap onto the Southern Uplands. Permian-age (250 million year old) desert deposits occur in basin within the Region around Dumfries and Lochmaben.
The area is notable for its glacial deposits, particularly drumlins, and for its coastal landforms, particularly salt marshes, sand flats and mudflats.
The landscape of this area reflects the underlying geology, with the whole area having been modified by the effects of glacial erosion and deposition during the Ice Age. Glacial erosion shaped the main valleys and moulded the western uplands which are extensively ice-scoured. The valleys have been deepened and the hills moulded by the passage of the ice. Particularly striking examples of glacial streamlining are represented along the margins of the Tweed valley.
The lower ground is mostly covered by drift deposits, comprising till, sand and gravel, from the last ice sheet. These deposits include outwash terraces and drumlins. The drumlin fields of the Solway lowlands, the area around New Galloway and along the Tweed valley, are good examples. Meltwater channels are common along many of the lower hillslopes, particularly bordering the northern flanks of the Lammermuir Hills. Many of the valleys on the south side of the uplands and the area around Stranraer contain large spreads of glaciofluvial deposits.
Back Bay on the Solway Firth – these sedimentary rock layers, have been folded as a result of the Caledonian Orogeny. The rocks were formed from mud that accumulated on the floor of the lapetus Ocean. © Scottish Natural Heritage.
In the west, rocky shorelines contrast with the wide expanses of sand flat, mudflat and salt marsh in the inner Solway Firth. Raised estuarine deposits occur extensively along the Solway coast and provide valuable records of sea-level changes during the last 14,000 years.