Picton Point is the south-facing headland that separates the western Cleddau and the Eastern Cleddau, in the tidal inner reaches of the Milford Haven waterway. It's a delightful spot, with wide vistas to west, south and east. My wife and I went over there for a walk the other day, and I was struck -- not for the first time -- by the frequency of erratic boulders and cobbles scattered along the shoreline. These have not been carried here by longshore drift or by the tides -- there is very little wave action here. So the erratics have dropped out of pre-existing glacial and fluvioglacial deposits as the low cliffs are nibbled away.
The main processes here (as at Mill Bay, the famous place from which the Altar Stone was supposed to have come) are biological and mechanical. Tree roots are exposed as the Coal Measures sandstones and shales are sapped or undercut; then the trees tip over more and more until they fall down onto the beach; when that happens, loose rock debris on the cliffline ( already broken up to some degree by vast and expanding root systems) is dislodged and comes crashing down too, resulting in coastal retreat by a few more feet. And so the process continues. On the land surface above the cliffline there are glacial and fluvioglacial deposits, and these are dropping down onto the foreshore bit by bit as the coast retreats. The process is slow, but inexorable, and is essentially one of sudden or catastrophic cliff collapses in different locations as one big tree after another comes down-- usually during an extreme storm event like Storm Ophelia or Storm Brian in recent weeks. A huge oak tree came down close to Picton Ferry during one or the other of those storms.