Google Earth image of Ballums Bay, Caldey Island. The till exposure is at the head of the gulley, top left. The edge of the raised beach platform with cemented sediments coincides approximately with the black line.
Ballums Bay from the east (from near Small Ord Point). The beach, backed by the gully containing till, is right of centre. To left of centre we see remnants of raised beach platforms with cemented beach materials, breccia and sandrock, overlain by a terrace of more recent materials.
Following my note about the interesting QN article by John Hiemstra and others, I have been digging into my records of Caldey Island, and have found a number of photos that might be of interest. I have several pics of the till exposure which has been investigated and now described in the QN piece, but we'll leave those to one side for the moment. Of more interest to the debate about age are the photos of the cemented materials found on the old wave-cut platforms immediately to the south.
The platform is seriously broken up, cut by gullies and other inlets affected by ongoing wave action, and with steps and assorted vertical and overhanging sections. Like the other wave-cut platforms on the limestone coasts of South Pembrokeshire, it appears to be a composite feature, representing erosion at a number of different levels between present HWM and +5m. The raised beach platform is a mess -- appearing very old, with segments of many different ages. Ipswichian? My guess is that it is much older, and that it has been modified across several interglacials.
Raised beach platform remnants on steeply dipping Carboniferous Limestone strata on
the south side of Ballums Bay
Cemented brecciated limestone debris incorporating raised beach boulders and cobbles on one of the highest rock platform remnants, Ballums Bay south.
Comparison with other platform remnants is instructive. See these posts on Broad Haven South and Lydstep:
Let's take a look at the cemented deposits. There can be no doubt that these are all older than the Ballums Bay till. The raised beach, limestone breccia and bedded sandrock are intimately associated, although they do not all appear in all exposures. For example, in the extensive high raised beach platform near Lydstep Point cemented breccia and cemented sandrock are both found, but I have been unable to find any traces of cemented raised beach cobbles.
Key Quaternary locations on Lydstep Headland (note that there is a mistake on this map. The raised beach platform is in the bay to the west, behind Whitesheet Rock. To be corrected.....)
Complex cemented deposits on the limestone raised beach platform on the south side of Ballums Bay. Here we see abundant raised beach cobbles, layers of sandrock, and masses of limestone rockfall debris and slope breccia.
Bedded sandrock resting on slope breccia and raised beach cobbles., south side of Ballums Bay. All cemented with carbonate cement -- with some traces of manganese oxide cement.
The South Pembrokeshire Quaternary regional stratigraphy (based on my own observations and those of Dixon and Leach) is as follows:
8. Sandy loam and blown sand
7. Upper head (uncemented)
6. Fluvioglacial sands and gravels -- traces
5. Till from Dewisland (Devensian) glaciation -- many coastal exposures
4c. Lower head (cemented in some localities)
4b. Cemented sands (sandrock)
4a. Head incorporating raised beach cobbles (cemented)
3. Cemented raised beach
2. Older glacial deposits -- mostly destroyed, but exposed at Lydstep (Black Mixen)
1. Raised beach platform (complex modifications over several interglacials?)
If, as suggested by John Hiemstra and his colleagues, the unconsolidated till at Ballums Bay is of Anglian age (but periglacially redistributed or redeposited), it must be older than the cemented raised beach deposits. So why is it not cemented like deposits 2, 3 and 4 in the regional sedimentary sequence? It does not make sense. The authors seem to be suggesting that the original till is the same age as the ancient cemented till at Black Mixen -- that does not make sense either. The fresh till at Ballums Bay lies in a limestone gully, just like the fresh till at Lydstep, so the two deposits must be related.
In addition, the Ballums Bay till looks like the fresh till exposed in many clifftop locations on the south Pembrokeshire coast -- and it lies in the same stratigraphic position, overlain only by unconsolidated blown sand or sandloess, as in the Isles of Scilly. It must also be the same age as the till exposed near the Caldey landing stage, which is reddish in colour, sandy and gravelly in texture, matrix supported, and contains a similar mix of erratic cobbles.
Till exposure near the Caldey landing stage. At the base, broken Carboniferous Limestone bedrock. Above that, 1.3 m gravelly and sandy fresh unconsolidated till. Above
that, c 1m of sandy loess and soil.
Brecciated limestone slope deposit or "head" above broken Carboniferous Limestone bedrock, near the Caldey landing stage. This is uncemented, and appears to underlie the Devensian till. It is probably of Early Devensian age, and correlated with many other slope deposits in West Wales which underlie till and fluvioglacial materials.
Here is the crucial discussion on the age of the till, from the QN article by John Hiemstra and his colleagues:
This was the scenario we set out to test. Several features certainly favour this origin: (i) ‘foreign’ igneous erratics; (ii) dominance of sandstone clasts mostly probably from outcrops on the west of the island lying slightly south of Bullum’ Bay, which would tally with an ENE flow direction of ice as envisaged by Brian John; (iii) clast form characteristics; and (iv) probable loessic sediment overlying the diamicton without obvious hiatus, consistent with a Devensian origin. There are, however, several drawbacks to this scenario: (i) the very low content of any material that does not have a likely relatively local provenance: (ii) no observed far-travelled erratics or glacigenic diamicton in parts of the island we could access, and also very few such erratics on the beaches; (iii) flow structures atypical of till; and (4) no convincing striations. (The grain fabric evidence is equivocal, neither supporting nor ruling out a till origin.)
(2) Redistribution of pre-existing sediment including any residual Anglian glacigenic sediment during Devensian periglacial conditions
An in situ Anglian till origin is not feasible because raised Ipswichian sea level - probably at least c. +5 m OD (Dutton and Lambeck, 2012) - would have removed any existing sediment from the lower part of the gully where the diamicton lies. An origin by redistribution of regolith and soil as well as any Anglian glacial material by mass flow under Devensian periglacial conditions is, however, plausible. The igneous pebbles could have been derived from two possible transport directions and by two possible sequences of events. (In either case, mass flow would have moved sediment downslope from higher ground in the south of the island to the gully.) One alternative is that they could have been deposited on the island during the Anglian glaciation when many argue that ice from the Irish Sea Basin moved eastwards and southwards across this region. Mass flow processes would then have carried these clasts into the gully. The other alternative is that there is a possible source to the south of Bullum’s Bay of the igneous pebbles in conglomerates of the West Angle Formation of the upper ORS, which is said to be ‘rich’ in such clasts as well as those of some other lithologies (Howells, 2007). In this case, sediment containing clasts derived from mainland sources would have been transported to the island by glacial action, and during the Devensian any remnant glacigenic material would have been carried north(east)wards downslope to Bullum’s Bay by mass flow processes together with any locally eroded material including the igneous erratics. The flow structures, lack of striations, clay-rich texture and grain fabric evidence tally with an origin by periglacial slope processes. Clast rounding could be the result of one or a combination of long-term weathering, residual glacial modification or limited modification acquired during mass flow transport. The clast form characteristics of the igneous clasts would favour a glacial rather than fluvial (i.e. ORS conglomerate) origin, although reconstructing transport history from just four clasts must be treated with caution. Lack of angular clasts from the underlying limestone either in the diamicton or forming the main constituent of a discrete single-lithology periglacial diamicton might appear problematic given the frost susceptibility of this rock type. However, the lack of steep surrounding slopes with exposed bedrock that could act as the source of such material is the probable explanation. (Transport to Bullum’s Bay of the igneous erratics by northward-moving longshore drift during raised Ipswichian sea level is conceivable but their incorporation into the exposed diamicton seems unlikely given the lack of contained marine shells.)
On balance, therefore, we prefer the second scenario to explain the Bullum’s Bay diamicton as it provides a better explanation of the evidence.
In my view, as indicated earlier, the discussion by the authors is not at all convincing, and includes a good deal of special pleading. The till is not unusual or unique in any way, and like most of the related deposits in South Pembrokeshire, if it looks like a till it probably is a till. One gets the impression that the authors are rather desperate to demonstrate that the deposit is not an in situ till, and their suggested "drawbacks" are perhaps based on the fact they are not familiar with related deposits, their lithologies and their stratigraphic relationships. The low content of foreign materials is not at all unusual in Pembrokeshire tills, as I demonstrated in my 1965 thesis. There ARE far-travelled erratics and other tills on the island, as I have shown above. There are erratics on the beaches. There are scratches on cobbles which I have interpreted as striations. And they have not demonstrated that the flow structures seen are "atypical of till" -- this is not a lodgement till or a true flow till, but a till laid down in a wasting ice environment, which was mobile during deposition and then subject to a degree of remobilisation or rearrangement following ice wastage. In that respect it is typical of most of the till exposures of West Wales.
The "preferred explanation" of the till -- namely that it is "a discrete single-lithology periglacial diamicton" is so convoluted that it is slightly embarrassing! Is this how the authors would explain the other clifftop diamictons on the south Pembrokeshire coast, or the similar diamictons on the north coasts of the Isles of Scilly? In my view, if there had been Anglian or other ancient tills littering the ground surface of the Caldey limestone plateau, they would have been "locked in" by interglacial carbonate cementation processes and would not have been available for periglacial redistribution.
Unfortunately, the authors appear to have been so concerned about the implications of fresh Devensian till in this part of the Bristol Channel that they have refused to believe the evidence of their own eyes! Occam's Razor, chaps. This looks like a fresh till and it is a fresh till -- and there was Devensian Irish Sea ice in this part of the Bristol Channel around 20,000 years ago.
Finally, this is where some work needs to be done -- in Drinkim Bay, south of Ballums Bay. I was not able to look at it last time because of tidal and time constraints, but it looks as if there are exposed sediments and platform traces there........
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