4,560 – 541 million years ago
In the immense time period that is the Precambrian, Scotland as a landmass did not exist. It was at this time that the very foundations of Scotland’s crust were being formed. Where precisely on Earth the various bits of crust that were to form Scotland formed is unclear, but at least in the late Precambrian, it is fairly certain that what is now the north of the country, lay far from its present position
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 Precambrian. Scotland’s present outline has been drawn on the map to help you visualise where the Precambrian rocks that we find today were formed in relation to the surrounding continent and seas.
Excluding the Dalradian rocks which are mentioned in the Cambrian, three major groups of Precambrian rocks occur in Scotland.
One suite of rocks, known as the ‘Torridonian Sandstone’ can be found in the far north-west of Scotland. When these rocks formed between 1200 and 1000 million years ago, Scotland bathed in a hot, arid climate that was interspersed by torrential rains and flash floods. To the north and west, rivers eroded land that now corresponds to the area of north-eastern USA and Greenland. The sediment derived from these areas was deposited in lakes and on riverbeds to form a huge pile of sediment several kilometres in thickness. These sediments eventually turned into the sedimentary rocks that we see today.
The second suite is known as the ‘Moine’. These rocks were originally sedimentary rocks, formed approximately 1000 to 870 million years ago. They began life as sand and mud laid down in an ocean environment. Over time they formed sandstone and mudstone. They have been metamorphosed twice, firstly about 1000 million years ago, and secondly about 420 million years ago when the Iapetus Ocean finally closed (see Silurian). When the rocks were metamorphosed, sandstone became quartzite and mudstone became schist. Some of the rocks were highly metamorphosed to form gneiss. Some people consider that the Moine rocks represent the marine equivalent of the Torridonian but this is yet to be proved.
The oldest rocks in Scotland are also some of the oldest rocks in the world and were formed up to almost 3000 million years ago. Collectively they are called the ‘Lewisian Gneiss’ and the Outer Hebrides forms the largest outcrop. They are metamorphic rocks that have been metamorphosed several times during their long history. From analysing the chemistry of the rocks it appears that the Lewisian Gneiss was predominantly granite-like rocks with some volcanic and sedimentary rocks mixed in. At the time the earliest Lewisian rocks were being formed, the Earth’s crust consisted of many more tectonic plates than it does today and life on Earth at its most complex consisted of bacteria and algae.