Devonian (408-362 million years ago)
By the early Devonian, two parts of Great Britain had collided in the Caledonian Orogeny, creating folding and faulting across the region. This can be seen most dramatically at Pettico Wick near St. Abb's Head, Northumberland. Renewed plate movements as the two continents collided resulted in some volcanic activity, for example the Cheviot volcano as well as the injection of granite magmas into the sedimentary pile.
It was during the Devonian period that thousands of metres of rocks were weathered and eroded away. Although the climate was hot and desert-like, seasonal rainfall resulting in flash floods led to the deposition of sands and conglomerates. This resulted in a series of Devonian age rocks known as "Old Red Sandstone" as opposed to the "New Red Sandstone" of Permian-Triassic age. Britain would have been 20 degrees south of the equator, with only limited seasonal rainfall.
Exposures of Devonian conglomerates can be seen at the Sedgwick Geological Trail near Sedbergh and at Eyemouth, about 10 miles north of Berwick on Tweed. The famous unconformity at Siccar Point, about 20km SE of Dunbar, was discovered by the 18th Century geologist James Hutton. It consists of almost horizontal Devonian red sandstones overlying almost vertical Silurian mudstones and sandstones (greywackes).