The Tertiary Period (65 – 2.6 Ma)
The Tertiary geological period covers the time span 65-2.6 Ma – from the end of the mass extinction of the dinosaurs to the Quaternary ‘Ice Age’. It was a period of immense contrasts, ranging from the violent volcanic activity that gave birth to the Atlantic Ocean to the gentler, slower weathering processes that shaped Britain’s landscape before the arrival of the glaciers. Today, many geologists use the terms Palaeogene and Neogene to describe this time span, with the Palaeogene comprising, from oldest to youngest, the Palaeocene, Eocene and Oligocene epochs and the Neogene consisting of the Miocene and Pliocene. The Palaeogene, Neogene and Quaternary collectively make up the Cenozoic Era. When exactly the Pliocene ended and the Quaternary began, however, still remains a hotly debated geological topic. In general, the Tertiary was a period of climatic cooling. At the start, mammals replaced reptiles as the dominant vertebrates. Hominoids, the earliest relatives of humans, appeared towards the end of the period.
Despite a small outcrop area, Anglesey’s Tertiary rocks have been disproportionately important to our understanding of the geological evolution of Great Britain and to the development of geological science. First, a swarm of Palaeogene dykes bears witness to the plate tectonic processes that split the ancient continent of Pangaea and saw the birth of the Atlantic Ocean around 65 Ma. These dykes were born from the same volcanic activity that formed the spectacular basalt columns of the Giant’s Causeway in Northern Ireland.
The dykes are rich in iron-bearing silicate minerals, and are rather prone to deep spheroidal weathering, so that their outcrops often display the classic “onion-skin” texture. Mapping of the dykes, by aeromagnetic survey, has revealed a series of apparent offsets that have been interpreted as evidence that significant amounts of sinistral faulting have occurred in North Wales in the last 55 million years. However, because some of the dykes are exposed at outcrop, these apparent offsets can be examined in detail. The outcrops show that the intrusions ‘skipped’ from following one plane of weakness to another, and so the offsets appear not to be due to later faulting. The uncertainties require that the outcrops are preserved and subjected to future investigation.
Above: an early Cenozoic dyke cutting older rocks on the coast at Porth Namarch, near Holyhead. Basic in composition, with a lot of iron-bearing silicates, the dyke-rock shows the distinctive, spheroidal “onion-skin” weathering pattern.
A tiny ‘pocket’ of Miocene deposits in the north of Anglesey provides a rare glimpse of the climates and processes that shaped Britain’s landscape and heralded the onset of the Quaternary ‘ice ages’, some 2.6 Ma.