Geological structure of Great Britain

Each of these terranes form a part of the former continent of Laurentia whose southeastern margin is defined by the Iapetus Suture Zone, which runs parallel to the English-Scottish border, though some miles to its south.

The majority of the rest of England and Wales north of the Variscan Front are considered to constitute the Avalon composite terrane.

[1] Northern England is characterised by a series of fault-bound blocks separated by sedimentary basins whose origin can be traced back to the Carboniferous period.

Thus lines of crustal weakness commonly associated with, for example, the Caledonian Orogeny will often predate this particular mountain-building period, much as some of those created during this phase were reactivated during later events.

The Caledonian orogeny took place between about 490 and 390 million years ago as the former micro-continent of Avalonia collided obliquely with the former continent of Laurentia along a line approximating to the modern English-Scottish border.

This long drawn-out, multi-phase event resulted in innumerable geological structures, many of which have persisted to the present day and help to shape the landscapes of much of Britain, from South Wales northwards to the Shetland Islands.

In Britain it resulted in a variety of geological structures across the southwest from Pembrokeshire and South Glamorgan in Wales to Devon and Cornwall.