Daeareg a Thirwedd | Geology and Landscape
Daeareg a Thirwedd | Geology and Landscape
Mae plwyf Tremarchog, gyda’i bentref a’i ffermydd, yn gorwedd ar ochr orllewinol y penrhyn ar arfordir gogledd Sir Benfro o’r enw Pen Caer (Strumble Head yn Saesneg). Amgylchynnir Pen Caer ar dair ochr gan ffin ddeheuol Sianel San Sior rhwng Cymru ac Iwerddon. I’r gorllewin mae plwyf Treopert ac ymhellach i’r gorllewin plwyf Mathri. Mae’r rhan fwyaf o benrhyn Pen Caer o fewn ffiniau plwyf Llanwnda i’r dwyrain.
Mae gan yr ardal gyfan hanes hir ac amrywiol. rhai nodweddion daearegol eithriadol. tystiolaeth o gyfaneddu cynnar a hanes crefyddol, amaethyddol a diwylliannol cyfoethog.
Daeareg
Yn ddaearegol mae’r ardal i gyd yn dyddio o’r cyfnod Ordofisaidd rhwng 470 a 440 miliwn o flynyddoedd yn ol; a’r rhan fwyaf o’r ardal yn gorwedd o fewn ardal folcanig Pen Strwmbwl a Gwdig o greigiau gwenithfaen igneaidd a pheth sial carreglaid.
Daw’r cyfnod daearegol Ordofisaidd rhwng y cyfnod Cambriaidd a’r cyfnod Silwraidd. Roedd y cyfnod Cambriaidd yn adeg o amodau morol, pan welwyd cerrig llaid, tywodfaen a gweddillion ffosiliedig creaduriaid morwrol yn casglu ar waelod y moroedd bas. Yn yr un modd ymgasglodd tywodfaen sylweddol dros rannau eang o Orllewin Cymru yn y cyfnod Silwraidd rhwng 440 a 400 miliwn o flynyddoedd yn ol. Ar ddiwedd y cyfnod hwn a dechrau cyfnod y creigiau ‘Devonian’ iau caed adegau o adeiladu mynyddig grymus – yr Oregenedd Caledonaidd. Creodd hyn newidiadau anferthol gyda thymheredd a gwasgedd yn symud nifer o’r creigiau o’u safleoedd llorweddol i safeoedd goleddfol a fertigal ac i uchder o filoedd o droedfeddi.
Roedd y cyfnod Ordofisaidd yn gymysg o weithgaredd folcanig mawr o ffrwydriadau a mewnwthiadau. Crewyd cerrigllaid o’r llaid a osodwyd yn y dyfroedd dwfn. Yn sgil y gwres a’r pwysedd gwelir haenau o lechi heddiw. Digwyddodd y symudiadau folcanig islaw’r moroedd ac uwchben. Yn wreiddiol roedd yr ardal gyfan yn rhan o fas cyfandirol Avalonia ac ardaloedd cyfagos. Holltodd yr ardaloedd hyn a symud dros filiynau o flynyddoedd drwy gyfrwng platiau tectonig i’w safleoedd presennol ar arfordir gorllewinol Prydain. Mae’r bryniau a welir heddiw ym Mhen Caer yn weddillion o’r mynyddoedd enfawr a grewyd yng nghyfnod yr Orogenedd Caledonaidd yn sgil y masau cyfandirol hynafol yn gwasgu yn erbyn ei gilydd. Yn ystod y cannoedd o filiynau o flynyddoedd ers hynny mae’r broses o erydu naturiol wedi’n gadael gyda gweddillion ysgithrog yn unig o’r mynyddoedd mawr a fu.
Ymysg nodweddion daearegol hynod yr ardal mae’r canlynol:
- Y rhes o fryniau o darddiad folcanig sy’n hollti’r penrhyn – Garn Fawr, Garn Fechan, Garn Folch a Garn Wnda. Gorweddant mewn llinell o’r gorllewn i’r dwyrain gyda’r uchaf ohonynt, Garn Fawr, yn 213 medr (703 troedfedd).
- Traethau Aber Mawr ac Aber Bach sy’n ffinio’i gilydd gyda’u nodweddion daearegol a llysieuol amlwg.
- Traeth diarffordd Pwll Crochan gyda’i greigiau plyg hynod.
- Clogwyni Pwll Deri yn ffinio Dinas Mawr a’i greigiau igneaidd yn gyfoethog o fwynau.
- Traethell Porth Maen Melyn yn arddangos lafa clustog y clogwyni gerllaw.
- Ardal Pen Anglas gyda’r creigiau dolerit mewnwrthiol a phileri carreg.
Erydiad
Tros gannoedd o filiynau o flynyddoedd mae’r ardal gyfan wedi’i heffeithio gan erydiad gan ddwr, gwynt, ia a gwres. Achoswyd cryn dipyn o’r erydiad gan y cyfnodau ia niferus a chyfnodau twymach, yn ymestyn o 150,000 o flynyddoedd yn ol i 10,000 o flynyddoedd nol pan doddodd yr haenau olaf o ia a orchuddiai’r Iwerydd.
Arferai’r ardal wreiddiol oedd yn ffinio clogwyni Gorllewin Cymru a Bae Aberteifi ymestyn gryn dipyn ymhellach i’r gorllewin. Mae bellach dan y dwr yn sgil toddi’r ia a lefelau uwch. Gellir gweld nodwedd arbennig yr erydiad mewn gwahanol fannau ar hyd arfordir Gogledd Sir Benfro pan fo’r llanw’n isel, ac yn arbennig felly yn Aber Mawr lle mae gweddillion ffosiliedig fforest hynafol yn gorwedd dan y tywod. Mae gan ochr orllewinol Pen Caer gwaddodion sylweddol o dywod a adawyd gan y rhewlifoedd.
The parish of St. Nicholas, together with its village and surrounding farms, lies on the western side of the headland on the north Pembrokeshire coast known in Welsh as “Pencaer” and in English “Stumble Head”. The Pencaer headland is surrounded on three sides by the sea on the southern edge of St. George’s Channel between Wales and Ireland. To the west is the parish of Granston and further west the parish of Mathry and most of the Pencaer headland lies within the adjoining parish to the east of Llanwnda.
The whole area has a long and varied history, some exceptional features of geological formations, evidence of early habitation and a rich religious, agricultural and cultural history.
Geology
The whole peninsula and adjoining areas to the south are part of the Ordovician rock formations, approximately from the time period of 470 to 440 million years ago. Most of the area lies within the Strumble Head and Goodwick volcanic areas being mostly igneous granite rocks, together with some mudstone shales.
The Ordovician geological timescale, lies between the Cambrian and Silurian geological periods. The Cambrian period was an era of marine conditions, and shallow seas in which much mudstone and sandstone materials were laid down, together with the fossilised remains of sea creatures. The Silurian period from approximately 440 to 400 million years ago, also produced much sandstone material and covers large areas of west Wales. At the end of the Silurian period and running into the younger Devonian rocks very strong forces of mountain building episodes occurred known as the Caledonian Orogeny. This produced huge metamorphic changes through temperature and pressure moving many of the rocks from a horizontal to inclined and vertical positions and substantial elevations to heights of many thousands of feet.
The Ordovician period was a mixture of major volcanic activity of eruptions, and intrusions. There were also mud deposits laid down in deep water which have become mudstones. As a result of the heat and pressure, these are today deposits of slate. The volcanic activity occurred both underwater and above ground. The whole area was originally part of the early continental masses, of Avalonia and surrounding areas. These areas split and moved over millions of years by plate tectonics to the present positions on the western coast of Britain. The hills of Pencaer which we see today are the remains of the mountain ranges created during the Caledonian Orogeny as a result of the ancient continental masses pressing against each other. In the course of the hundreds of millions of years since that time natural erosion has left us with the jagged core material of these former mountains.
Among the outstanding geological features of the area are the following.
- The line of hills which split the peninsular and are all of volcanic origin comprising, Garn Fawr, Garn Fechan, Garn Folch, and Garn Wnda – they lie in an east to west direction with a maximum height on Garn Fawr of 213 metres (703 feet).
- The two adjoining bays and beaches at Aber Mawr, and Aber Bach with their notable geological and botanical features.
- The secluded beach of Pwll Crochan with its extraordinary folded rocks.
- The Pwll Deri cliffs and adjoining Dinas Mawr mineral rich igneous rocks.
- Porth Maen Melyn a small beach inlet showing the extensive pillow lava.in the adjoining cliffs.
- The Pen Anglas area with the intrusive dolerite rocks and stone pillars.
Erosion
Over many hundreds of millions of years the whole area has been affected by erosion of water, wind, ice and heat. A great deal of the erosion was caused by the numerous ice ages and intervening warmer periods, ranging from 150,000 years ago to 10,000 years ago, when the last ice sheets covering the Irish Sea and Britain melted.
The original land area adjoining the present day cliffs of west Wales and Cardigan Bay extended much further westward and is currently under water as a result of the melting ice and rising sea levels. A particular feature of this erosion can be seen at low tide in various places along the north Pembrokeshire coast, particularly at Abermawr where the fossilised remains of the ancient forests lie covered by the sand. The western area of Pencaer has substantial sand deposits left by the glaciers.