Physical Geography Page 1 of 3 Geology and Physical Geography of Snowdonia Geology and Structure The main structural feature is the broad belt of rugged and dissected highland which runs diagonally across the area through the mountain groups of the Carneddau, the Glyderau and Snowdon. It corresponds to the complex downfold of Ordovician rocks known as the Snowdon syncline. To the south of the Snowdon area is the geological structure known as the Harlech dome. This is a folded sequence of lower Palaeozoic rocks forming the thickest and most complete sequences of Cambrian, Ordovician and Silurian strata to be found in Europe. Geological History Oldest rocks (Pre-Cambrian) found to the North West, in Anglesey. They consist of metamorphosed sedimentary and volcanic rocks over 6000m thick, highly deformed by earth movement. Lower Palaeozoic rocks mark the development across the area of a great depositional trough in which accumulated thousands of metres of marine sediments (coarse grits, fine sandstones and slates). The lower rocks (Cambrian) rest unconformably on the older formations. This trough marks the establishment of a sedimentary basin that covered all of Wales, much of England, parts of Ireland and extended into Belgium. The basin was situated on the edge of a continental mass which lay to the south-east, with the Iapetus ocean on the north-west. At the end of the Cambrian, slight uplift exposed the deposits to erosion. New submergence (Ordovician) led to more deposition and more unconformity. Later sediments became finer (shales and mudstones) and included slates (early to middle Ordovician). These Ordovician sediments are characterised by an abundance of trilobites and also contain many brachiopods and graptolites. Volcanic activity is the outstanding feature of the Ordovician period. Vulcanism spread throughout the region and eventually culminated in eruptions of central Snowdonia (giving rise to the Snowdon Series). The volcanoes were partly submarine, the larger ones being islands in the sea. The lavas vary from acid to basic types and as they poured out on the sea floor they may have formed pillow lavas. This volcanic activity coincided with the continued deposition of marine sediments and was a prelude to the later closure of the Iapetus ocean. Though volcanic activity ceased at the end of the Ordovician, marine sedimentation continued into the Silurian. The Silurian sediments show much variation in lithology, and were affected by slight earth-movements recurrent from late Ordovician times. At the end of the Silurian, earth movements culminated in the Caledonian orogeny, a major period of uplift, folding and fracturing which partly destroyed the marine trough. The Lower Palaeozoic seas retreated from North Wales and the strong earth-movements transformed the landscape, and the marine rocks of the Cambrian, Ordovician and Silurian periods were replaced by the continental rocks of the Devonian period. The Caledonian orogeny was the result of the closure of the Iapetus ocean and the resultant continent-continent collision. Subsequent erosion has revealed the major Caledonian structures such as the Snowdon syncline and the Harlech dome. The Cambrian and Ordovician rocks were regionally metamorphosed to low greenschist facies and locally folded, faulted and cleaved to form slates. Intense denudation (erosion) occurred after the Caledonian upheaval and is represented by the deposition of continental rocks in Devonian times. These rocks are locally represented in Anglesey where the basal sediments are conglomerates containing boulders, up to 30cm in diameter, derived from local sources. To the south it was estimated that 6000m had been worn away. The Devonian period is followed unconformably by strongly transgressive Carboniferous rocks. The marine transgression in the Carboniferous covered most of North Wales with only the highest mountains probably remaining unsubmerged. In the North East, limestone forms the prominent headland of the Great Orme. In Anglesey it includes 300m of limestone. A second period of major earth movement (Hercynian) affected the Carboniferous and older rocks, with the main movements occurring along existing fractures. The Hercynian faulting was accompanied, or was closely followed, by mineralisation of many of the fault belts. Apart from Pleistocene and recent sediments, no post Hercynian sedimentary rocks are found in the North Wales (Snowdon) area. Terrestrial and marine sedimentary rocks of the Mesozoic era can be found in the Welsh borderland. The sole relics of the Tertiary period are certain minor intrusive dykes and some off-shore sediments in Cardigan Bay. | 1 | 2 | 3 | Page 1 of 3