This page was edited from the BBC's Essential Guide to Rocks website. The information is not really needed for GCSE but it is an interesting story geological story.

SLICES OF SCOTLAND

1998 year saw the agreement for Scottish devolution. But in fact Scotland and England have always been different. The rocks from which these two countries are made from formed in very different parts of the world and it was plate tectonics which brought them together.

Oceans apart
Although joined at the hip today, 550 million years ago Scotland and England were both in the southern hemisphere, separated by a vast ocean called the Iapetus. To the south of the Iapetus Ocean lay the North American continent included the rocks which now form England, Wales and southern Ireland. 5,000 kilometres to the north lay the American continent, and the rocks of Scotland. As permanent as a large ocean may seem, they don't last forever. About 500 million years ago both European and American continents started to close in on each other. Underneath the ocean, cold dense oceanic crust was diving down under the lighter continental crust moving the continents ever closer- the process is called subduction. Slowly the Iapetus ocean began to shrink. Around 430 million years ago, the ocean had been squeezed out and the continents collided. Scotland and England were fused together. The seamless join occurs, rather amazingly, not far from Hadrian's wall.

Piecing together the evidence
This is a remarkable theory and you may be wondering how can geologist tell all this happened. Remarkable theories need remarkable evidence - and that evidence certainly exists. Geologists know the Iapetus ocean existed because of fossils called trilobites found in the rocks on either side of the Scotland-England join. But that's not all. The trilobites on the Scottish side are totally different from those seen in England and Wales. Why? It's thought that the width of the Iapetus was far too wide for trilobites to cross. Only when the ocean had shrunk enough could trilobites swim across which is recorded later on in the rock record.

Subduction is happening today under the Pacific ocean, where crust is diving down under Japan. The volcanoes found on Japan are the result of the this subduction. The melting crust forms large underground vats of molten rocks called magma chambers which feed the volcanoes. If you look at a geology map of Scotland you'll notice some large red blobs. These blobs are granites. They formed when the magma chamber cooled and froze. The granites are further evidence of the closing Iapetus ocean - they were formed from the subducting crust between 500 and 400 million years ago.

Rucked Rocks
Perhaps the most convincing clues to the crunching of the continents can be found at St Abbs Head in south-east Scotland. Exposures along the coast show rocks which are tilted and folded. The rocks are called greywackes are made of mud, silt and sandstones which formed at the edge of the Iapetus ocean basin. Geologists know they formed under water because of structures found in the rocks. One feature is that the fragments of rock from which they are made are well sorted. The larger, heavier bits sank quickly while the finer bits took longer to settle to the bottom. Nearby rocks contain marine fossils called graptolites which floated in the ocean.

Originally the greywackes were laid down horizontally, but today show spectacular folds. These folded rocks are found over many tens of kilometres. What gave rise to such large scale folding? Large scale mountain building forces. These forces buckled and rucked the greywackes as the continents came together.

The final evidence comes from the composition of the Scottish rocks - they match those found in Newfoundland today. Newfoundland was also part of the American plate which collided together with the European plate. So how come it's so far away now? Well, on a planet like ours nothing stays still for long. Since the collision, further land movements put play to our close encounter with America. Slowly as the Atlantic ocean opened the American plate drifted away, leaving behind a large chunk of rock which today forms Scotland.

Back to the tectonic page