Snowdonia's Geological History
The beginnings of Snowdonia - 625 million years ago: Precambrian North Wales
The geological story of Snowdonia, as told by the rocks that crop out on its hillsides and along its valleys, covers over 625 million years. The tale begins deep in the Southern Hemisphere, when what is now Wales was a scattered series of volcanic islands along the northern edge of the ancient, south pole-straddling continent, Gondwana. By the beginning of the Palaeozoic Era, 542 million years ago, these islands had formed together into a small microcontinent that geologists call Avalonia.
The Cambrian Period - 542-485 million years ago: the Welsh marine basin
The Palaeozoic Era is divided up into six periods of geological time, of which the first two – the Cambrian and the Ordovician are those to which the rocks of Snowdonia almost entirely belong. By the start of the Cambrian, the part of Avalonia that is now northern and western Wales was undergoing subsidence and the sea had flooded in, marking the start of the evolution of the marine basin – the Welsh Basin – that was to be the key geographical feature of the area for the next 150 million years. Rivers carried sediment into the basin: at times, sea-levels were low and deltas extended out across the basin, depositing coarse sediments such as pebbly sandstones (image 1). At other times, when sea levels were higher, shallow-water sediments were deposited, some of them containing fossils (image 2) of the creatures that inhabited this ancient environment. In deeper waters, rhythmic banded silts and muds were deposited layer upon layer by turbid underwater currents, whilst occasionally the sediment supply was almost cut off and black mudstones accumulated from fine particles settling out of the water. An unusual period of sedimentation in the Lower Cambrian led to the deposition, as a chemical precipitate, of a manganese ore-bed (image 3).
The Ordovician Period - 485-444 million years ago: the Age of Volcanoes
Sedimentary rocks make up a large part of the strata of Snowdonia, but at times, especially during the Middle Ordovician, the tranquility was interrupted by violent episodes of often explosive volcanic activity. Vast volumes of lava and ash – an incredible sixty cubic kilometres in one case – were erupted from volcanoes that were often situated underwater, but sometimes grew into islands. Evidence of these explosive eruptions is revealed by the tuffs (image 4), ash deposited from violent pyroclastic flows, and the lava-breccias (image 5) where existing rocks were shattered by explosions. The banded, silica-rich rhyolite lavas (image 6) were erupted at surface, but deep underground, some of the magma cooled and crystallised in place to form intrusions, such as dolerite and the micro-granites (image 7) that now crop out near Ogwen, Blaenau Ffestiniog and elsewhere. Igneous rocks tend to be much harder than sedimentary rocks like mudstones, so that they make up much of the craggier ground of Snowdonia.
The Silurian Period - 444-419 million years ago: the death of an ocean
Darwin did not encounter rocks from the Silurian Period during his journey. It is highly likely that they were once present, since they may be found in neighbouring areas of Wales, but in the part of Snowdonia that Darwin traversed they were removed, long ago, by erosion. The evidence from the Silurian rocks of neighbouring districts indicates that the Welsh Basin sea persisted until the late Silurian, when it finally silted up. The district has probably remained land ever since that time.
The Devonian Period - 419-359 million years ago: continental collision
During the Devonian, Avalonia joined with Laurentia (North America, Scotland and Northern Ireland) and, to the south, other small microcontinents rifted away from Gondwana and drifted northwards, resulting in further collisions. The tectonic stresses generated by these events led to the folding of the rocks of North Wales: one of the best-known examples of such folding forms the spectacular backdrop to Cwm Idwal. So great was the pressure exerted on the rocks that many of their constituent mineral grains were recrystallised. This was especially the case in the mudstones, whose recrystallised fabric – or cleavage – allows them to be split into thin sheets. Thus was the famous Welsh slate (image 8) formed, hundreds of millions of years ago.
The Quaternary Period – 2.6 million years ago to the present day: the shaping of today’s landscape
Snowdonia’s rugged landscape was carved by glaciers that grew repeatedly in the mountains during the ‘ice ages’ of the Quaternary Period. Glaciers also moved through the areas now occupied by the Irish Sea and Cardigan Bay, gouging rocks and mud from the seabed and transporting them southward. These glacial deposits include boulders and pebbles of granite, gneiss and other distinctive rock types from as far away as Scotland, Ireland and the Lake District (image 9). Boulders of Lower and Middle Jurassic (201-168 million years ago) limestone (image 10) are found along the coast south of Barmouth and were dumped by the southward-moving ice. The source of these limestones – on the floor of Cardigan Bay – was only discovered by drilling in the 1970s. Boulders of rock types found in the mountains can also be found on the coast. They were transported here by the westward-moving Welsh mountain ice.