There must have been a time when Ffynnon Asa, one of several wells in the area dedicated to St Asaph, was as spectacular as other local wells such as St Winefride’s at Holywell and Ffynnon Fair at Cefn Meiriadog. Lhuyd recorded that Richard Perry of Pwll-Alog “had sett neat pillars” aound the well. According to Dr Johnson it had been at sometime “covered with a building that has now disappeared“. Pennant wrote that it was “inclosed with stone, in a polygonal form and had formerly its votaries, like that of St Wenefrede“, and even at the end of the 19th century Thomas records that there were still indications of “five angles or porches”. [1]
By the time the Royal Commission on Ancient Monuments visited in 1910 the spring was “seen to rise in a modern roughly octagonal stone reservoir from 9 to 10 feet across yielding from four to five million gallons per day and supplying the parishes of Prestatyn, Dyserth and Melidan.”
The spring was first diverted to supply these communities in around 1881.
Now, at the start of the 21st century Ffynnon Asa is surrounded by tall metal 
spikes, fences, keep out and danger signs. The well, that was once resorted to for cures for “nervous disorders” and rheumatism is now part of a reservoir, polluted and unsafe for bathing.
Ffynnon Asa’s attraction has been the sheer volume of water it provides. Up to four million gallons per day pours from the well, at one time it flowed down the hillside in a torrent, and until the late 18th century it formed a dramatic and much visited waterfall, “in the deep and rounded hollow of a rock, finely darkened with ivy” (Pennant) at Dyserth a mile or so away. [see postscript]
This power was readily harnessed for water mills, the streams were modified and diverted. Below the well at Felin Fawr the remains of a massive wheel can still be seen, with the aqueduct that carried the wheel water now dry and bypassed by a new channel. A little downstream the remains of further, smaller mills can be seen.
It isn’t clear where the well actually rises today. Most sources seem to suggest that there is a locked brick building beyond the reservoir that houses the outlet from the hillside. Although the water still flows vigorously downhill it is possible that more is drawn off and piped away for use elsewhere.
Ffynnon Asa is about a mile to the north east of Cwm. We reached it by following the footpath running up the side of the churchyard and then joining up with the Offa’s Dyke Long Distance Footpath which runs beside the well. Despite the changes it remains a dramatic sight in the landscape overlooking the sea above Prestatyn, and seeing it and its setting demonstrates how the use of the well has continued and changed over the centuries.
[1] D R Thomas (1870) History of the Diocese of St Asaph. Vol 2
postscript
Apparently during the late eighteenth and much of the nineteenth century the rivers were diverted upstream of Dyserth and for much of this time the water fall was dry. It was dry at the time of Johnson’s visit, although the stream was diverted back so that he could see the spectacle. Towards the end of the Victorian era it was reinstated and has been a tourist attraction in the area since then.
Well Hopper 
Just to add that the Dyserth waterfall is still there and can be visited. Google ‘Dyserth waterfall’ to see some good photographs of it.
I’ve driven down Waterfall Road in Dyserth often enough, and never got round to stopping off to see it.
I think the books suggest it is much less powerful than it was before the mills diverted the river, whether this is the case or not it does still look an impressive sight.
Hi. So why is Ffynnon Asa spring closed off from the public today? What is it contaminated with? Where does the pipe go to from the the spring? And I want to be able to freely go and drink and bottle up as much as I desire from this spring as God intended. It seems strange that once people drank from it but we are told it is now contaminated and whilst it is piped off somewhere for what purpose. It seems so unfair that a minority take from the majority instead of sharing, and I hope it isn’t the case here, but why is this information hard to find regarding what I’ve asked here? Kind regards, Iestyn.
i suspect it has been closed off for many years, back in the 16th century it seemed to be more important for industry, driving water mills downstream than as a drinking source, if it ever was. I have never delved into where the water travels from underground, if it comes through old mine workings etc. which would potentially pollute it. Would be interesting to know whether it is piped away just to dispose of the overflow or whether it is used somewhere these days.