| Earth movements
at the end of the Silurian saw the final closure of the Iapetus Ocean
that had separated northern and southern Britain. With the final
elimination of the sea from most of the UK, and the creation of mountain
chains across Scotland, Wales and Northern England, Britain was now
part of a semi-arid continental landmass.
In the Mendips, only the youngest part of the Devonian is represented
in the rock succession. The thick sandstones, pebbly sandstones and
mudstones of the Portishead Formation were laid down by meandering
river systems at the southern margin of a large landmass. The sea
covered southern England south of the Mendips.
|| During the
Devonian land plants evolved, and the microscopic spores they produced
are sometimes found as fossils. Towards the top of the largely unfossiliferous
Devonian succession in the Mendips, there are the remains of plants
and fish, heralding the dramatic change in environment that occurred
across the region at the beginning of the Carboniferous.
The Mendip region formed part of a broad
coastal plain in the Devonian. Meandering rivers crossing this plain
deposited sands, pebble beds and muds.