To the southeast lay the Midland Platform (a part of the micro-continent of Avalonia) and to the northwest, within the Iapetus Ocean, through what is now Ireland and the Lake District, was an island arc; a northeast-southwest aligned Irish Sea landmass which was associated with volcanic activity.
Examination of the sediments and associated fossils allows the deeper centre of the basin to be distinguished from shallower 'platform' areas along its southeastern margins.
From the middle Silurian onwards, collision of Avalonia with the more northerly continent of Laurentia occurred giving rise to the Caledonian Orogeny.
[2] In addition to the sedimentary rocks of the Welsh Basin, there are a number of volcanic formations within the overall sequence, particularly within the Ordovician system.
[3] Typical of the sediments of the Welsh Basin are the Llandovery age Aberystwyth Grits which outcrop along the middle coastal section of Cardigan Bay and comprise alternating mudstones and turbiditic sandstones.