This is a fabulous image taken by Paul Davies and published on the Pembrokeshire Geology Group Facebook page. Click to enlarge.
In the background we can see the Milford Haven waterway. The valley which we can see cutting across the neck of Dale Peninsula is almost certainly a meltwater channel of considerable age. It may well be the same age (Anglian Glaciation?) as the other big subglacial channels of north Pembrokeshire. It is rather a spectacular feature -- I have never seen it so clearly illustrated in a photo. The channel runs along the line of the very important Ritec Fault, which "guides" many major features, including the alignment of Milford Haven itself. The brecciated zone which is often associated with major faulting is easily picket out by fluvial and glacial processes over a very long period of time.
The "plug" of Pleistocene deposits which partly fills the valley is very obvious -- see other posts on this by putting "Westdale"into the search box. I think these deposits are of Devensian age, around 20,000 years old.
Some researchers have suggested that the Dale Peninsula was once an island, and that a narrow strait ran along the course of this valley. That's possible, but I am not sure the valley is deep enough for that. I would like to see some evidence of old sea cliffs and maybe beach deposits well into the valley and beneath the glacial / periglacial sediments. Such deposits could only be found through drilling..............
For the moment, I prefer to think of the valley as a Pleistocene deepening of an old fault-guided river valley. As I have indicated elsewhere, there are other signs of meltwater erosion in this part of Pembrokeshire, and of course the classic kame terrace not far away, at Mullock Bridge.