[2] The Variscan orogeny pushed Devonian and earlier layers upwards by more than 700 metres to form a long, elliptical groove running from east of Maidenhead for more than 100km west-north-westwards, up beneath what is now the Ridgeway towards Aston Tirrold in Oxfordshire.
At Stratfield Mortimer and Beech Hill, the igneous rocks are dolerite and form seams between Coal Measures layers.
Another possibility is that in all three places we are seeing 'intrusions' into gaps in Carboniferous layers, comparable to igneous intrusions in Nottinghamshire and Leicestershire, where dolerites cut through some beds of Westphalian age but are overlain by others.
[10] The early Eocene London Clay (Thames Group) generally gets thinner as we proceed westwards, though the thickness of beds can vary considerably over short distances.
[2] Where rivers have cut through these beds Lambeth Group layers are found (notably, the [Palaeocene] Reading Formation, used for brick-making since Roman times but now increasingly scarce in the area after which it was named).
After the Thames broke through the Goring Gap that river and its tributaries the Loddon, Emm Brook, Blackwater and (to some extent) Wey[9] shaped the geography of eastern Berkshire but have not yet eroded away its Eocene cover.