419 – 359 million years ago
During the Devonian, Scotland lay to the south of the equator in a semi-arid environment. Scotland was all above sea level and was in parts very mountainous. The Devonian saw the final stages of the coming together of the continents of Laurentia and Baltica (see Silurian). This collision of continents formed a mountain range across the northern half of Scotland. The mountains, named the Caledonian Mountains, were probably as high as the Swiss Alps are today, and may even have been as high as the Himalayas. It is due to 400 million years of erosion that the mountains are no longer this high today.
Scotland hasn’t always been on the same postion on the face of the Earth and has not always had the same outline. This map shows how ‘Scotland’ may have looked during the Devonian. Scotland’s present outline has been drawn on the map to help you visualise where the Devonian rocks that we find today were formed in relation to the surrounding continent and seas.
The Midland Valley and the area around Orkney and Shetland were lower lying areas. In these areas away from the high mountains, sediment that had been eroded and washed from the mountains, such as boulders, pebbles, sand and mud, accumulated in alluvial fans, rivers, lakes and floodplains. This sediment formed the conglomerates, sandstones and mudstones that are found today, which is collectively known as the ‘Old Red Sandstone’. Fish flourished in the rivers and lakes that existed here, we know this from their fossilised remains.
The Devonian was also a volcanically active time in Scotland, following on from the closure of the Iapetus Ocean (see Silurian). In southern Scotland we see the remains of the large lava flows that were erupted, with the most extensive areas near Oban, in north and west Fife, and east of Jedburgh in the Scottish Borders. In northern Scotland we see the remains of magma chambers that once held magma deep within the Earth’s crust. The magma cooled in these chambers to form granite, and the subsequent erosion of the overlying mountains has revealed their existence at the Earth’s surface. Because granite is a hard rock resistant to erosion, it forms many of Scotland’s Highland hills, for example the Cairngorm Mountains.