Britain and Scotland united. Devonian 408-362Ma.(Upper Palaeozoic times)

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 and Silurian rocks of Northern England, as can be seen most dramatically in the cliffs 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 andesite volcano, subsequently intruded by granite magma. Other Devonian granite intrusions are also 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 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, sandstones and greywackes.

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.

Diagram to the right shows disposition of the continents during Early Devonian and Carboniferous times.
Notice how Gondwana has “collided” with Laurussia.

From:

Phanerozoic Polar Wander, Palaeogeography and Dynamics

Trond H. Torsvik , Rob Van der Voo, Ulla Preeden , Conall Mac Niocaill , Bernhard Steinberger, Pavel V. Doubrovine, Douwe J.J. van Hinsbergen, Mathew Domeier ,Carmen Gaina, Eric Tohveri, Joseph G. Meert, Phil J. A. McCausland, L. Robin M. Cocks

Above photo: Looking south easterly down Hawsen Burn, Cheviot Hills. Remnant Tors of andesite overlying the granite are visible on the horizon.

Above photo: Looking south from Mindrum Mill at the dissected plateau of Cheviot lavas.

Kirk Yetholm Parish Church is built almost entirely of Cheviot andesite lavas.

Thin section of porphyritic andesite (crossed polars). Plag. Feldspars with subordinate altered pyroxenes within a felsitic groundmass.Sample from near base of waterfall (~NT967251) at Carey Burn, Harthope Valley, Cheviot Hills.

Thin section (crossed polars) of microgranite in contact with andesite in Hawsen Burn exposure (about 300m upstream from road, ~NT952227.) Fine grained “saccharoidal” (granular) texture of quartz and feldspar with accessory biotite. Average grain size = 0.5mm

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