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.
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 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
picture on the 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.
