ON a bend in the road above Llanrhystud, a coastal village about five miles south of Aberystwyth, is one of the more curious symbols of Welsh self-determination.

It’s just two words in Welsh, painted on the wall of a ruined cottage. In white capital lettering on a red background, the words urge you to Cofiwch Drywryn – that is to say, Remember Tryweryn.

They refer to the flooding of the Trywern Valley near Bala in north Wales in the late 1950s to create a reservoir to supply water to Liverpool. The destruction of Capel Celyn, an ancient and cultured farming community, was done against the combined opposition of Welsh MPs in Parliament, and quickly became a rallying symbol for Welsh national sentiment.

While it was being built the reservoir was attacked by a bomb which destroyed equipment – the first intervention by a shadowy group known as Mudiad Amddiffyn Cymru, the Movement for the Defence of Wales, that went on to disrupt the investiture of Prince Charles in 1969.

One of the presenters of the 1980s television history series The Dragon Has Two Tongues, the charismatic historian Gwyn Alf Williams had scuba diving lessons so he could be filmed among the remains of Capel Celyn below the surface of the reservoir. In the event a summer drought meant Gwyn could simply walk through the broken-down walls of chapel, cottages and farms.

In February this year the Cofiwch Dryweryn monument was defaced. The word Elvis was daubed across it and some of the stone walling knocked down. Nevertheless, the monument – and it has become a monument – was quickly restored.

This has happened from time to time over the years, but on this occasion the defacement had an unusual effect. Suddenly, all over Wales the same two words began popping up. Cofiwch Dryweryn was painted on many other walls. They were placed on placards, put in windows and appeared on T shirts, mugs, jewellery, postcards. By the end of April, Cofi wch Dreweryn had been daubed on roadsides in more than a hundred places across the country.

It was evident that the damage to a piece of graffiti on a wall had unleashed a widespread sense of wounded national pride.

It also came at a time of growing momentum for the idea that Wales could – and should – be independent. Polls suggest support for the idea is growing as Brexit sees the UK turn in on itself and the tone of political debate becomes ever more toxic.

Up to about a year ago the independence option in polls questioning Wales’s future constitutional status languished at around 10%. Now they’re well above that, and depending on the precise wording of the question, reach beyond 30%. One YouGov poll in June put support for Welsh independence on 41%.

What is interesting, too, is Welsh independence is not being driven solely by Plaid Cymru. For the first time, the idea has been taken on board by a variety of broadly civil society and cultural organisations that seemingly have sprung up overnight.

One is Yes Is More, a collective of Welsh artists from across the musical spectrum. In February an independence gig it organised in Cardiff’s Tramshed venue sold out. It featured zeitgeist acts from Astroid Boys to Boy Azooga, Los Blancos and Gwenno to Charlotte Church and Gruff Rhys.

Yes Cymru is a cross-party pro-independence group whose membership has grown to more than a thousand in the past year. In May it organised the first-ever independence march in Cardiff. It was expected that about 500 people might attend. In the event it attracted more than 3000 people. In Caernarfon in July another march attracted more than 8000. And we’re expecting thousands at a march and rally in Merthyr today.

Then there’s Undod, a republican group campaigning for independence. There’s Welsh Football Fans for Independence, even Labour for an Indy Wales. Former first minister Carwyn Jones took part in an independence debate at the National Eisteddfod in Llanwrst in August. He is not quite yet a convert but said that more and more people were becoming, as he put it, “indy curious”. He said the trend was being driven by the Brexit shambles at Westminster.

All this has persuaded the Western Mail that Welsh independence can no longer be regarded as on the margins of political debate but has become, as it said in a recent editorial, “mainstream”.

Something is happening in Wales. Faced with the prospect of Scotland voting for independence in the next few years, together with the unification of Ireland, whether Brexit happens or not, more and more Welsh people are waking up to the existential choice that is awaiting our nation as well.