5 The main lithotectonic units of the British Isles
In previous sections, it was revealed that in the British Isles, the Phanerozoic era was punctuated by two major tectonic periods referred to as the Caledonian and Variscan Orogenies. Both events involved the collision of continental plates, resulting in extensive crustal thickening and isostatic uplift. Over time, the mountain chains formed by these collisions underwent rapid erosion, so that during times of sea-level highs, the sea inundated the land. This allowed new sedimentary units to be deposited, separated from the underlying deformed rocks by a major unconformity, representing a significant change in the tectonic and lithological history of the area. Figure 10 illustrates the steps involved in the formation of such an unconformity, examples of which can be found marking specific episodes in the geological history of the British Isles.
Using this model, the geological history of the British Isles can be interpreted in terms of a series of distinct orogenic units and their overlying covering units. By doing this, the whole of the British Isles geological history can be simplified into five main units – the Precambrian and Lower Palaeozoic Basement, the Caledonian Orogenic Belt, the Older Cover, the Variscan Orogenic Belt and the Younger Cover (see Figure 9). Although these units each have a distinct geological history based on lithology and tectonic structures, they do not correlate with distinct geological periods. Instead, these five units are referred to as lithotectonic units.
Note that ‘Basement’ (with capital ‘B’) implies the Precambrian and Lower Palaeozoic Basement in the sense used in this section, whereas ‘basement’ (with lower case ‘b’) is used to indicate any other rocks underlying a covering lithotectonic unit. For example, the orogenic belt in Figure 10 could be referred to as basement, regardless of its age.