{"id":803,"date":"2019-02-27T00:08:02","date_gmt":"2019-02-27T00:08:02","guid":{"rendered":"http:\/\/dev2.visitcardigan.com\/crwydro\/geological-landscape\/"},"modified":"2019-04-27T18:34:51","modified_gmt":"2019-04-27T17:34:51","slug":"tirwedd-ddaearegol","status":"publish","type":"page","link":"https:\/\/www.visitcardigan.com\/cy\/crwydro\/tirwedd-ddaearegol\/","title":{"rendered":"Tirwedd Ddaearegol"},"content":{"rendered":"

Mae bryniau, dyffrynnoedd ac arfordir ardal Aberteifi yn arddangos amrywiaeth helaeth o nodweddion ffisegol sy\u2019n datgelu ei hanes daearegol cymhleth.<\/p>\n\"\"<\/a>\n\n\n\n\n
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Photo 1 Poppit Sands: alternation of sandstone and mudstone beds of late Ordovician (Caradoc) age. The sandstones are turbidites, laid down in deep water by gravity-driven density currents about 430 million years ago. Some of the sandstone beds are graded with a basal sections rich in coarse sand to gravel and locally mudstone clasts. The mudstones are cleaved and folded following the effects of the Caledonian Orogeny about 410 million years ago<\/td>\n

Cerrig llaid, gafodd eu dyddodi mewn basn dwfn yn eigion y m\u00f4r tua 450 miliwn o flynyddoedd yn \u00f4l, yw\u2019r rhan fwyaf o greigiau\u2019r ardal.\u00a0 Mae cyswllt lleol rhwng y cerrig llaid \u00e2 haenau o dywodfaen caled (tyrbiditau); yn Nhraeth Poppit (Cyf.Grid: 150489) mae\u2019r haenau hyn hyd at fetr neu ddau o drwch, ond mewn mannau eraill gallant fod yn ddim mwy nag ychydig filimetrau o drwch.\u00a0 Cafodd y creigiau hyn eu dyddodi fel mwd a thywod ar lawr y cefnfor ond fe\u2019u claddwyd yn ddwfn dros amser a thua 140 miliwn o flynyddoedd yn \u00f4l cawsant eu gwasgu a\u2019u plygu gan symudiadau sylweddol yn y ddaear.\u00a0\u00a0Trodd y cerrig llaid yn llechfeini yn sgil y digwyddiad hwn.<\/p>\n

Y lle gorau i weld y creigiau plyg hyn yw ar hyd yr arfordir yn Ceibwr (109461) ac yn\u00a0Nhraeth\u00a0Poppit (147490).<\/td>\n<\/tr>\n

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Photo 2 – Cliffs on the north side of Ceibwr bay showing alternation of lighter grey sandstone beds and darker mudstone beds. The rocks are folded into a series of large folds and are locally faulted<\/td>\n

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Photo 3 – Poppit Sands: Folded Ordovician sandstones and mudstones. The softer mudstones are cleaved and have folded without fracturing. The harder sandstones have folded with a more angular form and are locally fractured<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Yn sgil y plygu\u00a0hwn,\u00a0chwistrellodd\u00a0hylifau drwy\u2019r creigiau ac mae\u2019r llwybrau a\u00a0ddilynwyd gan\u00a0yr hylifau hyn bellach wedi eu mewnlenwi\u2019n\u00a0lleol gan rwydwaith o wythiennau cwarts gwyn.<\/p>\n\n\n\n
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Photo 4 – Poppit Sands: quartz veins cutting through Ordovician cleaved mudstones. The quartz veins were formed when the deeply buried rock was permeated by and superheated fluids. On release of pressure quartz crystallised out of the fluids in tension cracks.<\/td>\n

\"\"Photo 5 – Fossils are rare in the local rocks. Graptolites are used to date the rocks but these are poorly preserved. Animals that crawled along the ancient sea bed locally left tracks which are termed \u201ctrace fossils\u201d. This example comes from Ordovician mudstones exposed on the beach north of Llangranog<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Prin iawn yw\u2019r ffosilau sydd i\u2019w gweld yn y creigiau oherwydd bod y gwaddodion wedi\u2019u dyddodi mewn d\u0175r dwfn.\u00a0\u00a0\u00a0Mae\u2019r ychydig graptolitau (organebau rhydd-nofio sy\u2019n edrych fel coesau pryfyn ac sydd tua\u2019r un maint \u00e2\u2019r rheiny)\u00a0a ddarganfuwyd yn dangos bod y creigiau yn perthyn i\u2019r System Ordoficaidd.\u00a0 Yn y 19eg ganrif, roedd\u00a0nifer o chwareli\u00a0llechi\u00a0yng ngheunant Afon Teifi,\u00a0rhwng y Ganolfan Bywyd Gwyllt (191449) a Chilgerran (206430).\u00a0\u00a0C\u00e2i\u2019r\u00a0llechfaen ei thorri\u2019n slabiau, yn hytrach nag yn llechi toi, ac roedd\u00a0y\u00a0rhain\u00a0yn cael eu defnyddio\u00a0i wneud cafnau, lloriau a llefydd t\u00e2n.\u00a0\u00a0Dympiwyd\u00a0rhywfaint o\u2019r gwastraff o\u2019r gwaith llechi yn yr afon\u00a0a\u00a0newidiodd\u00a0hyn batrwm\u00a0ei llif gan arwain at lifogydd mawr i fyny\u2019r afon a siltio i lawr yr afon.<\/p>\n

Ni\u00a0cheir\u00a0unrhyw dystiolaeth o greigiau sy\u2019n iau na\u2019r cyfnod Ordoficaidd yn ardal Aberteifi, er bod cryn dipyn o Fae Ceredigion wedi ei fewnlenwi gan\u00a0olyniaeth\u00a0drwchus o greigiau\u00a0Jwrasig,\u00a0tebyg i\u2019r rhai yn ne Lloegr.\u00a0 Mae\u2019r creigiau hyn yn cyrraedd\u00a0bron\u00a0hyd at wely\u2019r m\u00f4r o fewn 5 cilometr\u00a0i\u2019r gogledd orllewin\u00a0o Benrhyn Cemaes (131502).<\/p>\n

Adnewidiwyd tirwedd ardal Aberteifi gan Oes yr I\u00e2.\u00a0 Rhyw ddau gan mil o flynyddoedd yn \u00f4l, gorchuddiwyd yr ardal gan y llen cyntaf o i\u00e2.\u00a0 Dinistriwyd y rhan fwyaf o\u2019r dystiolaeth dros y digwyddiad hwn gan y llen i\u00e2 diwethaf i ymestyn dros yr ardal a gyrhaeddodd ei stent mwyaf rhyw 18 000 o flynyddoedd yn \u00f4l.\u00a0 Ar y tir, roedd ffin ddeheuol y llen i\u00e2 hwn yn gorwedd i\u2019r de o Aberteifi, yn agos i\u00a0gopa\u00a0Mynyddoedd y Preselau, ond roedd yn llifo ymhellach i\u2019r de yn Sianel San Si\u00f4r.\u00a0 Cr\u00ebwyd nifer o arweddion gan y prosesau oedd yn gysylltiedig \u00e2\u2019r llen i\u00e2:<\/p>\n\n\n\n\n\n\n\n\n
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Photo 6 – View looking east from Carn Bica on the crest of the Preseli hills. The crags on the right are Carn Menyn, the site of the stones used to construct Stonehenge. Foeldrygarn, a hill fort, is on the left in the distance.<\/td>\n

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Photo 7 – Close up of Carn Menyn. Many of these jointed blocks of dolerite were transported to Salisbury Plain and used to construct Stonehenge. There is still a debate whether man or glacial processes transported the blocks.<\/td>\n<\/tr>\n

Photo 8 (right) – River Teifi flowing through the gorge at Cilgerran with the castle overlooking the gorge side. The lower course of the River Teifi flows through some narrow gorge sections followed by wider, more open valleys.<\/p>\n

Cyn Oes yr I\u00e2, llifai Afon Teifi ar hyd dyffryn llydan, dolennog, tebyg i\u2019r hyn sydd i\u2019w weld heddiw rhwng Llechryd a Chenarth.<\/p>\n

Gynt, pan oedd i\u00e2 yn gorchuddio\u2019r ardal, torrodd yr afon ar draws\u00a0rhai o\u2019r dolennau gan\u00a0endorri dyffryn cul, tebyg i geunant, fel sydd i\u2019w weld heddiw rhwng Llechryd a\u2019r Ganolfan Bywyd Gwyllt.<\/p>\n

Bellach, mae\u2019r hen ddolennau a dorrwyd i ffwrdd wedi eu\u00a0mewnlenwi ond gellir eu gweld (i\u2019r de o\u2019r Ganolfan Bywyd Gwyllt) drwy edrych yn ofalus ar fapiau ac ar y\u00a0dirwedd.<\/td>\n

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\nPhoto 9 – Looking south from the Cardigan Wildlife park one can see the steep sides of the gorge in which the river flows<\/td>\n		\"\"
\nPhoto 10 – Looking from the same viewpoint as Photo 9 but looking slightly to the west one can see the sloping sides of the older course of the river. This old river course is now infilled by up to 50m of sediment laid down before the last ice sheet covered the valley.<\/td>\n<\/tr>\n
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Photo 11 – Looking northwest from Trwyn Carreg-ddu (Poppit Sands) at the section of coast where a raised beach is exposed.<\/td>\n

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\nPhoto 12 – Raised beach along the coast shown in Photo 11. The beach is cut a metre or two above high water mark, and is overlain by boulder clay deposited about 18 000 years ago during the last ice age.<\/td>\n<\/tr>\n
Cyfordraeth ychydig fetrau uwchlaw\u2019r marc penllanw i\u2019r gorllewin o Draeth Poppit (147489).\u00a0 Cr\u00ebwyd y traeth hwn gan arwaith y tonnau tua 125 000 o flynyddoedd yn \u00f4l ac mae wedi\u2019i orchuddio gan glogfeini llyfngrwn gafodd eu herydu gan y tonnau ac sydd, yn eu tro, wedi\u2019u gorchuddio gan glog-glai cyfoethog, gludiog a ddyddodwyd yn ystod yr oes i\u00e2 ddiwethaf.<\/td>\n\"\"<\/p>\n

Photo 13 – Close up of the raised beach. The folded Ordovician mudstone are truncated by a smooth horizontal surface on which lies a boulder-rich beach deposit. Boulder clay overlies the beach deposit. The raised beach was probably cut during a sea level high stand some 125,000 years ago.<\/td>\n<\/tr>\n

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Photo 14 – Boulder clay deposited during the last ice age about 18 000 years ago infills the coastal valley at Mwnt.<\/td>\n

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Photo 15 – Close up of boulder clay at beach level at Mwnt. The clay is full of clasts ranging in size from sand to boulder grade. Some of these clasts have been transported within the ice sheet from the floor of the Irish Sea, North Wales and Southern Scotland. These glacial erratics are common on the beaches of the area and include a range of igneous and sedimentary rocks.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Tmae\u2019r clai hwn, a ddyddodwyd rhwng tua 18 000 a 15 000 o flynyddoedd yn \u00f4l,\u00a0yn cynnwys llawer o glogfeini a charegos o Ogledd Cymru a de orllewin yr\u00a0Alban (meini crwydr rhewlifol)\u00a0gafodd eu cludo\u00a0tua\u2019r de yn yr i\u00e2.<\/p>\n\n\n\n\n\n
\"\"Photo 16 – Erratic boulder of quartzite \u2013 probably from North Wales – resting on mudstones, at Poppit<\/td>\n\"\"<\/p>\n

Photo 17 – Pebble of Jurassic ironstone- probably from the floor of Cardigan Bay- picked from the beach at patch.<\/td>\n<\/tr>\n

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Photo 18 – Sand pit west of Penparc, part of a sand unit that extends southwards to also include Banc-y-Waren. This sand body and that at Monington were laid down by deltas in an extensive lake during the early stages of the retreat of the last ice sheet about 18 to 15 000 years ago late.<\/td>\n

Y twyni tywod helaeth sy\u2019n ffurfio Banc-y-Warren (204475) a\u2019r chwareli i\u2019r gogledd.\u00a0 Cafodd y rhain eu dyddodi gan afon oedd yn llifo allan o\u2019r llen i\u00e2 diwethaf wrth i hwnnw doddi.\u00a0 Mae lleoliad presennol y dyddodion hyn yn dangos bod rhaid bod y llen i\u00e2 hwn wedi bod yn gannoedd o fetrau o drwch a\u2019i fod yn ymestyn ar draws y dirwedd gyfan.\u00a0 Mae dyddodion tebyg i\u2019w gweld yn Trefigin (132434).<\/td>\n<\/tr>\n
Dyffrynnoedd lleol tebyg i geunant (Cwm Degwel, Llandudoch \u2013 162453) a sianelau a endorrwyd gan dd\u0175r tawdd yn llifo islaw\u2019r llen i\u00e2.<\/p>\n

Unwaith yr enciliodd y llen i\u00e2, roedd y dirwedd ddiffrwyth yn agored i wyntoedd cryfion a chwythwyd tywod i gopa Towyn Warren (168 490) yn Gwbert, i\u2019r de o Glwb Golff Aberteifi<\/p>\n

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Photo 19 – View looking north from above the Webley Hotel at the Pen yr Ergyd spit that occupies the mouth of the Teifi estuary. The spit is fed by the gravel on the foreshore on its seaward side. Waved blowing from the north move the gravel from the foreshore onto the spit with is slowly extending southwards and forcing the river channel towards the western (near) shore. A sailing vessel is stranded on the outer beach, having missed the river channel on its journey to sea.<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Mae\u2019r dirwedd yn yr aber yn dal i newid.\u00a0 Mae\u2019r tafod (spit<\/em>) ym Mhen yr Ergyd (160486) yn ymledu gan achosi erydiad ar lan de orllewinol yr afon, ac mae\u2019r twyni tywod yn\u00a0ardal\u00a0orllewinol Poppit wedi ymledu cryn dipyn yn ystod y ddau ddegawd diwetha\u2019.<\/p>\n","protected":false},"excerpt":{"rendered":"

Mae bryniau, dyffrynnoedd ac arfordir ardal Aberteifi yn arddangos amrywiaeth helaeth o nodweddion ffisegol sy\u2019n datgelu ei hanes daearegol cymhleth. Photo 1 Poppit Sands: alternation of sandstone and mudstone beds of late Ordovician (Caradoc) age. The sandstones are turbidites, laid down in deep water by gravity-driven density currents about 430 million years ago. Some of […]<\/p>\n","protected":false},"author":1,"featured_media":775,"parent":791,"menu_order":7,"comment_status":"closed","ping_status":"closed","template":"","meta":{"_acf_changed":false,"iawp_total_views":24,"footnotes":""},"class_list":["post-803","page","type-page","status-publish","has-post-thumbnail","hentry"],"acf":[],"_links":{"self":[{"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/pages\/803","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/comments?post=803"}],"version-history":[{"count":8,"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/pages\/803\/revisions"}],"predecessor-version":[{"id":811,"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/pages\/803\/revisions\/811"}],"up":[{"embeddable":true,"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/pages\/791"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/media\/775"}],"wp:attachment":[{"href":"https:\/\/www.visitcardigan.com\/cy\/wp-json\/wp\/v2\/media?parent=803"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}