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. 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.