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Natur Cymru Natur Cymru

Dyma gyfieithiad Saesneg o’r erthygl Gymraeg "Morfa Glaslyn – dau can mlynedd ers adeiladu’r Cob" gan TWM ELIAS

Two centuries have passed since the Cob was constructed by William Maddocks across the Glaslyn Estuary. The influence of this development on the area's landscape, nature, economy and society has been huge and far-reaching. Twm Elias describes the substantial changes that were seen back in the 19th Century and explains the change that is still happening today as a result of dynamic river processes in a confined estuary.


Potatoes instead of cockles

Before constructing the Cob, 200 years ago, it must have been an amazing sight to behold the sea at high tide stretching all the way to Aberglaslyn, almost five miles inland, and then the thousands of acres of mud and sand that came into view when the waters receded. It was a very dangerous estuary which was only safe to cross when the sea ebbed, and in the absence of a flood in the river.


When William Maddocks, who gave his name to Tremadog and Porthmadog, constructed his large sea wall, the Cob, in 1811, the area changed for ever. Here is an English translation of what Dafydd Siôn James said about it in 1813:


Potatoes instead of cockles,

 And ponies instead of seals. (1)


Thousands of acres of farming land were gained and a new road was created to connect the former counties of Caernarfon and Meirionnydd. Before long, a tramroad was built from the quarries of Blaenau Ffestiniog to carry slate to the ships docked at the new harbour in Porthmadog to be transported across the world. A shipbuilding industry soon developed here - Porthmadog's famous small three-mast sailing boats – and that had a major effect on the management of the area's native oak trees. The commercial value and use of the trees increased substantially and several new plantations were planted in the second half of the 19th century, such as Coed y Borth near Borth y Gest. This site is now a local nature reserve containing tall and upright oak trees which are about 120 years old.


The habitats least affected by the construction of the Cob were possibly the oak and birch trees on the rocky ridges which are seen, here and there, above the flat fields. These were islands at one time, as their names testify, 'ynys' being the Welsh word for island: Ynysfor, Ynys Ferlas, Hir Ynys and so forth. They are of interest due to the variety of lichen and bryophytes that grow along the tree branches.


Industrial development

It is difficult to believe today how much economic development was seen in the Glaslyn Marsh area during the 19th century. Copper came from the Nant Gwynant area to be exported through Porthmadog and substantial slate quarries, and their connected railways, provided grey slate from Blaenau Ffestiniog and Cwm Croesor on the Meirionnydd side of the Glaslyn River and from the Stradllyn and Pennant Valleys on the Caernarfonshire side (2, 3).


The naturalist is reminded, unexpectedly so, of the busy nature of the international slate trade. It is said that, towards the middle of the 20th century, a number of exotic plants would have established themselves on Ballast Island near the mouth of the harbour. The evening-primrose, Oenothera (agg.), seen on the Cob and in the Borth y Gest area, is considered to have arrived here from America in the ballast of the sailing ships, as also occurred with Welsh mudwort, Limosella australis, from the north-east coast of the United States, and seen in muddy pools on parts of the marsh (4). Unfortunately, the island is now covered with thick scrub and there is no recent evidence of the survival of other ballast incomers.


Another product of Ballast Island is building stones – some from distant parts of the world. And because people in the olden days begrudged wasting anything useful, some were transported to construct the Britannia Bridge (the harbour bridge) nearby, and were used as ornamental stones on some of the buildings in High Street.


Land use

Agricultural land: Alongside this industrial activity there was substantial agricultural development, as the demand for food increased with the rise in the country's urban and industrial population. Even before the construction of the Cob, parts of the marsh had already been reclaimed – including about 1,000 acres by William Maddocks himself in the Tremadog area at the turn of the 19th century – and other land in the Llanfrothen area and other places which were reclaimed by a number of local landowners some half a century or more earlier (1).


In order to create productive land from the marsh after constructing the Cob, earth banks were built on either side of the Glaslyn River, along with the streams running into it, as well as a series of deep ditches and clay pipes in the fields to drain the land. A great deal of good land was gained, although large parts of it would tend to be wet for long periods over the winter.  As a result, the land of some nearby farms was substantially augmented and one new large farm was established, aptly named Tŷ Newydd Morfa (Marsh New House in English). The ditches continue to be important connecting corridors for wildlife today, although they have suffered from the spread of Himalayan Balsam (Impatiens glandulifera), since the end of the 1990s.


The semi-intensive agricultural land on the marsh, as seen, for example, between Llanfrothen and Pont Traeth (near Prenteg) is of ornithological interest. Over the winter between 40 and 80 whooper swans come to the flats, and during the summer, at the top of the marsh, one of the few pairs of osprey which nest in Wales has successfully bred here on an annual basis since 2004.


Morfa Gwyllt : As the Glaslyn River would often flood the land between the Cob and the Cambrian railway (built in 1867), as well as land higher up the marsh, there was no point managing the river nor treating the land in the way it was done at the top of the marsh. Therefore, the only agricultural control seen in these areas was some grazing by cattle and sheep during the summer. From the outset, it was recognised that this semi-unstable marshy land had another value besides agriculture, namely shooting ducks in winter.


This part of the marsh is still extremely important for ducks and other wetland birds. Over the winter, it will be one of a number of sheltered spots through the Glaslyn and Dwyryd Estuaries as far as Harlech marshes, where birds will roost or shelter. From time to time during the winter, when there is a combination of high tide and stormy weather, even scoters and other birds more linked to the mouth of the estuary come here, including divers and grebes, in order to shelter behind the Cob (5).


Today, wet areas of Morfa Gwyllt are a valuable habitat for rare plants such as the Welsh mudwort (it is only found in three other places in Europe); the dwarf spike-rush (Eleocharis parvula) and the sharp rush (Juncus acutis). On neutral grassland, at the south-eastern end of the marsh, there are a number of other rare plants which are typical of wet meadows. Of special interest here are the whorled caraway (Carum verticillatum); Welsh broad-leaved marsh orchid (Dactylorhiza majalis, ssp. cambrensis) and the small-flowered evening-primrose (Oenothera cambrica) (6).


Another important habitat here is the alder marsh. One of Wales' largest wet alluvial woodlands is found on Glaslyn Marsh and such habitats are of European importance. Mature alder trees are most common here as well as some grey willows and other trees in the drier parts (6). There is very little disturbance here by man and beast and it is a very rich habitat with regard to wetland plants and particularly its entomological interest.


Changing the course of the river

If the course of the River Glaslyn was successfully managed and contained to a channel between substantial earth banks in the upper reaches of the marsh, no effort was made to do this for the wild parts of the marsh near the Cob. Therefore, small occasional changes can still be seen today in the pattern of the river channels near the Cob, and substantial new islands appeared in the widest part at the end of the 1990s.


Before the Cob was constructed, the river flow, in contrast to the sea's ebb and flow, would have been responsible for constant (and unpredictable) changes in the pattern of the river channels and the distribution of sand and mud beds. At the time, the site which Porthmadog occupies today was only sand. But having finished the construction and stabilising the spot where the Glaslyn River flowed through large wooden gates near Ynys Tywyn, the force of the river flow managed to open a deep new channel with the side of the rock – creating a suitable spot for ships. In 1821, William Maddocks obtained the right to build a new harbour and town and it became known as Porthmadog (2).


For 200 years, therefore, the river's escape from the Cob has been limited to one place, except between 1812 and 1814 when the sea wall was breached by storms. This had a major effect on the dynamics of the rest of the estuary, including Traeth Bach, the area through which the River Dwyryd flows to join the Glaslyn. As the river could not wind its way freely between one side of the estuary and the other, after the main channel was limited to the western end of the Cob, the other sections of the estuary were not affected by estuarine processes to the same extent. As a result, the sand started to accumulate gradually in sheltered spots, such as between the Cob and Trwyn y Penrhyn, and on the south-eastern edges of Traeth Bach where the salt marsh on the northern side of Harlech sand dunes is gradually filling up and drying out. The mouth of the estuary, which is the most powerful and dynamic part of it, is now gradually moving out to Tremadog Bay.


TWM ELIAS is a lecturer and course organiser at Plas Tan y Bwlch, Snowdonia National Park Study Centre.



(1) Owen, Bob (1943). Diwidiannau Coll

(2) Davies, Edward, (1913). Hanes Porthmadog

(3) Gwyn, David (2006). Gwynedd: Inheriting a Revolution

(4) Preston, C.D., Pearman, D.A. a Dines, T.D. (2003). New Atlas of the British & Irish Flora (5) Cambrian Ornithological Society Reports

(6) Site Description, SSSI, Glaslyn Marsh, CCW


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