Pettico Wick near St. Abb's Head, Northumberland.

By late Silurian early Devonian times, approximately 410-380 million years ago, the Iapetus Ocean had closed causing Laurentia to collide with Avalonia and Baltica. This continental collision resulted in the formation of the Caledonide mountains in Scandinavia, northern Great Britain and Greenland, and the Northern Appalachian mountains along the eastern seaboard of North America. The collision  led to the deformation, folding  and uplift of the Ordovician, Silurian and older rocks of Northern England, as can be seen most dramatically at Pettico Wick near St. Abb's Head, Northumberland.

During the Devonian, a period lasting about 50 million years, fold mountains of Himalayan proportions, rugged uplands, and intervening basins were formed over most of Britain. To the south, an ocean called the Rheic existed, part of its shoreline extending across southern Britain.
Renewed plate movements as the two continents collided resulted in some volcanic activity, e.g. the Cheviot volcano as well as the injection of granite magmas into the sedimentary pile. Some of these granite intrusions are now exposed such as those in the Lake District, e.g. the Shap and Skiddaw granites. Others remain at depth such as the Weardale and Wensleydale Granites, both proved to exist by borehole drilling as well as by geophysical surveys.

Hammers show contact between underlying Skiddaw Granite and overlying Skiddaw Slates

Devonian conglomerates seen on the Sedgwick Geological Trail

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 deg. 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 hills of  Great and Little Mell Fell in the Lake District are composed of Devonian conglomerates to be seen at the roadside near Pooley Bridge.


The unconformity at Siccar Point:The lady is sitting on Devonian rocks, her feet on Silurian rocks.

The famous unconformity at Siccar Point, Grid Ref. NT813710, about 20km SE of Dunbar, discovered by the 18th. century geologist James Hutton consists of near horizontal Devonian red sandstones overlying almost vertical Silurian mudstones and sandstones (greywackes). The photo above right shows the lady's feet resting on almost vertical Silurian while she is sat upon near horizontal Devonian (O.R.S.) rocks.
The deeply dissected valleys of the Cheviot Hills are fashioned from lavas and agglomerates erupted from a volcano of Devonian age. The lavas were later intruded by granite, also of Devonian age, which forms a higher standing plateau in the centre of the volcanic rocks.

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