Geology Group Diary (11)

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    These are the notes for the session on 13 July 2016 at Merlin's Bridge. The topic was GEOLOGY & SCENERY IN NORTH WALES

    The oldest rocks in North West Wales are found in Anglesey and the Lleyn peninsula. These rocks of late Precambrian age are known as the Monian Group (at least 615 Ma) and they are well displayed at South Stack and Rhoscolyn on Holy Island off the west coast of Anglesey. The Monian Group contains turbidite sandstones, volcanic ash and pillow lavas and a remarkable unit called the Gwna melange that is made up of a chaotic mixture of blocks and boulders some of which are several hundred metres across. This represents an enormous submarine slide down the continental slope known as an olistostrome. Pillow lavas are well exposed at Newborough on the south east side of Anglesey along the Menai Strait. Lava flows of this type represent submarine extrusions of basaltic lava that formed large globules or ‘pillows’ as they react with sea water. Much of the Monian sequence has been strongly metamorphosed into schist and gneiss and intensely folded during orogenesis at the end of Precambrian times when a subduction zone existed in NW Wales.

    Rocks of Cambrian age ( 544-510 Ma) in North Wales are represented by a thick sequence of marine sandstones and shales (most of which are now metamorphosed into slates). The Llanberis Slate outcrops on the NW side of Snowdon where the formation is over 1000 metres thick. The slates were formed as a result of pressure exerted on the Cambrian mudstones during the Caledonian earth building movements at the end of Silurian times. The platy minerals like mica and chlorite in the mudstones were re orientated by pressure that created cleavage planes often perpendicular to the original bedding planes. The slates cleave into thin layers that make excellent roofing material. In the late 19th C at the peak of production, the slate quarrying industry exported Welsh slate across the world. A more modern use of the disused Dinorwic quarry is the pumped storage hydro electric scheme that was opened in 1984. The turbines are hidden in the old slate caverns and they are powered by water released from a high level reservoir which is refilled at night by pumping water up from the lower lake using cheap off peak electricity.
    The Harlech Dome is essentially an inlier of Cambrian rocks uplifted as a result of the Caledonian orogeny. The oldest rocks in the centre of the eroded dome are surrounded by a rim of the Rhinog Grits which are topographically higher (Rhinog Fawr 720m). One of the best walks in the Rhinog National Nature Reserve starts at Roman Steps near Cwm Bychan. Manganese and iron pyrites occur in the shales above the Rhinog Grits and gold was mined in the Lower Cambrian rocks between Barmouth and Dolgellau during the 19th C. Note that the oldest rocks are in the centre of the dome and the youngest rocks that are stratigraphically above the older rocks outcrop on the periphery of the dome.

    The Snowdon mountain range is composed largely of Ordovician mudstones and volcanic rocks. In early Ordovician  times (Tremadoc age 510-493 Ma) marine conditions  existed and great thicknesses of muds were deposited in a geosynclinal basin. These muds were later converted by regional metamorphism into the Tremadoc slates. It is interesting to note that many of the trilobites found in the Tremadoc slates have been distorted by pressure during metamorphism. During most of the Ordovician an island arc was established above a subduction zone on the south side of the Iapetus Ocean and numerous volcanoes spewed volcanic ash and lava into the surrounding seas. Vulcanism continued throughout the Arenig and Llanvirn times (493-464 Ma). The most intensive period of volcanic activity produced some highly explosive eruptions which ejected masses of felsic pyroclastic materials and resulted in the creation of a huge caldera of subsidence in Central Snowdonia. The pyroclastics are of two main types: Airfall materials represent the products of nuée ardente eruptions where a glowing volcanic cloud deposited fine ash into the surrounding sea where it was reworked as a sediment known as air fall tuff. By contrast, pyroclastic ash flow materials roll down the volcanic slopes as incandescent avalanches of gas, steam and lava. They became welded together to form ash flow tuffs or ignimbrites. These often contain glass shards and flattened pumice fragments known as fiamme.

    One of the best localities to view some of the volcanic rocks is at Cwm Idwal on the north side of the Glyder Range. This is a National Nature Reserve with a well marked path around Llyn Idwal. Near to the start of the trail there are exposures of the Pitts Head Tuff which lies at the base of the Snowdon Volcanic Group. This contains numerous spherical siliceous nodules 2-3cm in diameter. These represent infilled gas bubbles within a pyroclastic ash flow. At the head of the cwm the Idwal Slabs represent another example of a rhyolitic ash flow (Lower Rhyolitic Tuff formation). In the Devil’s Kitchen the (Twll Du) in the axis of the Idwal syncline there are bedded pyroclastics overlain by thick pillow lavas. This is clear evidence of the marine environment into which the pillow lavas were extruded during mid Ordovician times.

    The mountainous scenery of Snowdonia has been shaped not only by its rock formations but also by glaciation during the Pleistocene Ice Age. An ice cap covered Snowdonia during the last glaciation (Devensian glacial phase 120,000 – 10,000 years ago) and glaciers were nourished in corries near the mountain summits. These glaciers flowed down the pre existing valleys and widened and deepened them into U shaped valleys. Surface rocks were polished and striated by moving ice and much of the eroded material was deposited on the surrounding lowlands as moraines (ridges formed where the glaciers stopped) and glacial till (boulder clay).

    It is of historical interest to note that in the early 19th C North Wales was surveyed by some eminent geologists. Professor Adam Sedgwick from Cambridge University defined the Cambrian system and on one of his visits he was accompanied by a young student called Charles Darwin who was eventually to become the famous evolutionist. At the same time Roderick Murchison was working on rocks in Wales which he defined as belonging to the Silurian system. However, the two men argued strongly about the boundaries between the two systems and some 40 years later in 1879 Charles Lapworth settled the controversy by proposing a new system called the Ordovician to cover the disputed rocks between the Cambrian and Silurian.

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