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GLACIATION in SNOWDONIA by Paul Sheppard

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GLACIATION in SNOWDONIA by Paul Sheppard GeoActiveGeoActive 350 OnlineOnline GLACIATION IN SNOWDONIA by Paul Sheppard This unit can be used These remnants of formerly much higher mountains have since been independently or in Conwy conjunction with OS 1:50,000 eroded into what is seen today. The Bangor mountains of Snowdonia have map sheet 115. N Caernarfon Llanrwst been changed as a result of aerial Llanberis Capel Curig Snowdon Betws y Coed (climatic) and fluvial (water) HE SNOWDONIA OR ERYRI (Yr Wyddfa) Beddgelert Blaenau erosion that always act upon a NATIONAL PARK, established Ffestiniog T Porthmadog landscape, but in more recent SNOWDONIA Y Bala in 1951, was the third National NATIONAL Harlech geological times the effects of ice PARK Park to be established in England have had a major impact upon the and Wales following the 1949 Barmouth Abermaw Dolgellau actual land surface. National Parks and Access to the Cadair Countryside Act (Figure 1). It Idris Tywyn 2 Machynlleth Glaciation covers an area of 2,171 km (838 Aberdyfi 0 25 km square miles) of North Wales and Ice ages have been common in the British Isles and northern Europe, encompasses the Carneddau and Figure 1: Snowdonia National Park Glyderau mountain ranges. It also with 40 having been identified by includes the highest mountain in important to look first at the geologists and geomorphologists. England and Wales, with Mount geological origins of the area, as The most recent of these saw Snowdon (Yr Wyddfa in Welsh) this provides the foundation upon Snowdonia covered with ice as reaching a height of 1,085 metres which ice can act. recently as 10,000 years ago. (3,560 feet). This is one of 14 A brief geological history of For an ice age to develop, all that is Welsh peaks exceeding 900 metres needed is a small drop in (2,800 feet) in height. Snowdonia temperature as a result of which Snowdonia is an upland area that winter snowfall does not fully melt A National Park is classed as an area owes its origin to the geological during the following summer. This of outstanding natural beauty. The events that occurred during leads to the accumulation of snow aim of this classification is to Cambrian times 500 million years (firn). Accumulated snow preserve and enhance such places of ago through the Ordovician and eventually compresses into ice, beauty and provide access to the Silurian eras, between 430 and 395 known as nevé. If temperatures public for their enjoyment through million years ago. At that time the remain low and accumulation recreation and tourism while area experienced volcanic activity continues, this leads to an ice age, maintaining these areas’ natural and folding as well as sedimentary with the formation of ice sheets beauty. Snowdonia is also a working deposition. At the end of the and glaciers. These cover the environment, where 27,500 people Silurian period there was major landscape and flow under the force work in farming, forestry, quarrying uplift, known as the Caledonian of gravity from their source to and the tourism industry. Activity in earth movements, which where they either meet the sea or the Park is overseen by The Nature transformed the landscape into the melt in a warmer environment. Conservancy Council which upland area as seen today. Initially However, during an ice age, sea operates in conjunction with the this landscape would have been level is much lower as the water is Park’s inhabitants to preserve the similar to that of the Himalayas. stored in a frozen state and is not unique environment which tourists However, a further 395 million part of the normal water cycle. come to see. In particular years of geological activity, Snowdonia is attractive because of including submergence below the Being an upland area, ice in its spectacular upland environment seas of the Carboniferous era Snowdonia accumulated on higher, that is the product of its geological between 340 and 280 million years cooler slopes and under gravity past and, more recently, the effects ago, and more recent geological flowed down the former river of the last Ice Age. activity which has not left its mark valleys, fundamentally altering the on Snowdonia, have resulted in a landscape. The Snowdonia ice cap To understand the effects of ice on much lower, yet still rugged, was surrounded by ice originating the landscape of Snowdonia, it is landscape. from the Irish Sea. This led to Series 17 Summer issue Unit 350 Glaciation in Snowdonia © 2006 Nelson Thornes GeoActive Online This page may be photocopied for use within the purchasing institution only. Page 1 of 4 N Frost shattering Mechanical erosion occurring in rocks with crevasses and Llandudno joints. Water enters joints and on freezing expands by 9%; Rhyl daily freeze and thaw widens and weakens joints, causing Bangor pieces of rock to shatter from the main body of rock. Abrasion Rocks become embedded in the ice. Moving under gravity MERIONETH the rock-laden ice acts like sandpaper as it rubs against the & SNOWDONIA floor or side of the valley or its floor. ICE CAP Plucking Ice freezes onto bare rock, and then ice movement pulls away the bedrock. It is likely that this rock has already been Harlech loosened by earlier erosional activity, such as freeze – thaw (see Frost shattering above). Dolgellau Rotational movement Downhill movement of ice pivoting around a point and overdeepening at that point (see cwm in Figure 4). 0 25 km Aberystwyth Figure 3: Types of glacial erosion Key Local Welsh ice movement Irish Sea ice movement Feature Example (+ OS grid reference) Figure 2: North Wales at the ice Pyramidal peak Mount Snowdon (GR 6056) maximum, and conflict between Arête Tryfan (GR 665595) local and Irish ice sheets Cwm (also known as a corrie or cirque) Cwm Idwal (GR 6459) conflict, which meant that some of U-shaped valley/glacial trough Nant Ffrancon (GR 6460) the Snowdonia ice was blocked in Ribbon lake Llyn Ogwen (GR 6560 / 6660) its path to the lowlands while Hanging valley Nant Ffrancon (GR 635625) elsewhere it radiated outwards. It Truncated spurs Nant Ffrancon (GR 630610) flowed eastwards to the Shropshire Roche moutonnée Nant Ffrancon (GR 639627) – Cheshire Plain, north into the Figure 4: Features of a glaciated landscape Irish Sea and westwards into Cardigan Bay. Its maximum depth Features of erosion and see Figure 8), which has steep has been estimated at over 1,000 These four erosional processes led sides and a flat valley floor. Its sides are marked by truncated spurs (GR metres, covering many of the to the formation of a classic 640611) which are the remnants of highest peaks in the Snowdonia glaciated upland landscape, whose the interlocking spurs of the original area. features are listed in Figure 4. To river valley. These were eroded as the help you locate these using the six- ice forced a straighter, more efficient This ice met further sheets of ice figure grid references supplied, you path to the Irish Sea. Along the sides that had accumulated and which will find it useful to refer to a copy of Nant Ffrancon there are also flowed away from the Lake District of the OS 1:50,000 map sheet 115 hanging valleys (eg GR 637627) and southern Scotland, causing of Snowdonia. which mark the old upper courses of major erosion (Figure 2). Later rivers which were either not affected • Cwms (corries or cirques), arêtes and there was deposition in the area of by ice, or only by smaller glaciers pyramidal peaks are all formed at the which did not have the ability to Llandudno on the northern edge of source of the glacier. A cwm is an erode to the depth of the main glacial Snowdonia. This has been verified armchair-shaped hollow that is valley or trough (Figure 7). On the by the presence of erratics in this usually found on sheltered north- valley floor (GR 639627) is a roche area. An erratic is a rock of a facing slopes, at angles between 310 moutonnée which was formed where different geological origin from the and 120 degrees, where ice a hard volcanic dyke protrudes from accumulated and freeze–thaw and area in which it is currently found. the floor of the valley. This was possibly chemical weathering acted at The erratics in this area have been smoothed on the upstream side, but the base of the ice. Rotational identified as originating in the Lake made rugged by plucking and movement and frost shattering aided District and the Firth of Clyde. abrasion of the rock on the this process and scoured out a deeper downstream side. This feature floor. A higher rock lip also formed in The easiest path for the Snowdonia therefore displays the area’s varied front of this hollow, where deposition ice to take was to follow the routes geology and the effects of ice both occurred as the ice flowed away and cut by rivers. However, as ice is plucking and abrading the rock. lost its erosional power (Figure 5). An relatively solid and is far bulkier arête occurs where two corries form, • Llyn Ogwen is a ribbon lake, which than particles of water, as it moves it one on each side of a mountain or occupies a glacial valley floor and can significantly alter the landscape ridge. A pyramidal peak forms where follows the shape of the valley. It over which it flows. Consequently, three corries form on all sides of one extends from the lip of the Ogwen classic glacial features have mountain and are separated by three Falls in the glacial hollow created by developed in Snowdonia. ridges (Figure 6).
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