Siccar Point – the famous Hutton unconformity
The Silurian greywackes (formed in an ocean) have been folded in the Caledonian Orogeny as a result of the collision between Laurentia and Avalonia plates. The rocks were extensively folded and then eroded to form a land surface (marked by the line of the unconformity) onto which Devonian sediments were deposited by rivers and lakes.. The unconformity represents a time gap of 65 million years.
Below, the breccia which is the lowest bed of the Devonian rocks. It is made up of angular blocks (clasts) of the underlying Silurian rocks. This is the land surface across which one day a powerful torrent deposited these rocks. Over time they were buried by layers of sand, which eventually lithified (compacted and cemented together) to form the rocks beds we see today.
A conglomerate with angular clasts is termed a breccia.