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Friday, 26 August 2011

Devensian Till at Bullum's Bay, Caldey Island

The fresh (Devensian?) till at Bullum's Bay, near the easternmost tip of Caldey Island.  It is preserved in a limestone gully, just above HWM.


On a trip to Caldey island, to give a couple of lectures to the Brothers, I managed to do a bit of exploring, and -- to my great surprise -- found a deposit of till very close to the eastern tip of the island, in Bullum's Bay.  This was at grid ref 147966.  The location is  a gully or small chasm in the Carboniferous Limestone, where the strata are standing almost vertically.  The gully is only about 4m wide and a similar depth, but somehow or other till has been deposited here and has survived.  The exposure is not far above HWM -- immediately below the exposure, which can be easily examined, there is a pile of flotsam and jetsam flung up in winter storms.  Quite possibly, during extreme gales coming from the SE, storm waves actually affect the exposure itself...... and wash stones out of it, which then collect on the beach below.
 Close-up of the till exposure, showing the reddish matrix of sandy and gravelly material and the frequent stones, many of which are rounded or sub-rounded.  See the daisy for scale!
 Some of the erratic stones and pebbles taken from the till face.  There is a considerable variety of stone types.

The till has a reddish colour, which means that it is composed mostly of ORS materials although it is on white or grey Carboniferous Limestone.  It has a sandy and gravelly matrix, and is packed with stones of all shapes and sizes.  The most frequent stone types are ORS sandstones -- some more red than others --and limestone, but there are also shales, conglomerates and (I am pretty sure) volcanic rocks which are blue and green in colour and which are strongly reminiscent of the rocks found on Skomer Island and in the Marloes Sands area.  Have they come from that far away, or are there other potential source areas closer at hand?  I didn't see any striations on the stones (the biggest are c 12" across) but I didn't have time to sample the till properly, or to clean and examine the contained stones as closely as I would have liked.  But I did notice that some of the stones are very rotten, and are coated with manganese.  One stone which I took from the face of the exposure was very heavy and shiny -- and reminiscent of the "ironstone" nodules found in the Coal Meaures and Millstone Grit in Pembrokeshire.

There is no doubt at all in my mind that this is till -- and that it is fresh.  In this location, ancient tills (maybe dating from the Anglian Glaciation) would certainly be solidly cemented or concreted by now as a result of highly-charged groundwater percolation over 450,000 years or so.  I know of one such till, not far away on the headland at Lydstep.  The till also reminds me of the till at West Dale Bay and the till found on the south coast of the St David's Peninsula, where the Irish Sea Glacier had picked up coarse-grained "land" sediments rather than sea-floor sediments composed of silts and clays with contained sea shells.  The till isn't cryoturbated or otherwise redistributed by slope processes or frost cracking -- it seems to me to be in situ and fresh.

As far as I could make out, the overall sequence of deposits here is as follows:

6.  Modern soil
5.  Sandy loess or brickearth (c 1m thick)
4.  Reddish till with ORS and other erratics (c 3m thick)
3.  Blocky broken limestone head (up to 2m thick)
2.  Concreted raised beach and included angular limestone fragments (up to 50 cms thick)
1.  eroded raised beach platform up to 10m above MSL.

There is nowhere where you can see this full sequence, but bits of it (including other patches of till) can also be seen in the cliffs to the south of the main exposure.

What was the Devensian Irish Sea Glacier doing here?   I had thought it skidded to a halt quite a few miles to the west, off the mouth of Milford Haven.  Time for another revision of my map.........  it is now incontrovertible that the ice flowed across Caldey Island.  This till could not have been deposited by a glacier coming in from the east.  But the ice might have flowed from the SW towards the NE or from W towards E.  There is ORS exposed in Sandtop Bay, on the west side of the island -- maybe that is where the ice came in from the west and where it picked up its load of ORS debris before carrying it across the island and dumping it in Bullums Bay.

By the way, Daylight Rock, where the bones of many Ice Age beasts were found, is on Small Ord Point, just a couple of hundred metres away.  Did those beasts roam this territory before, or after, the ice flowed across the island?  That's another nice problem that we need to sort out.............

11 comments:

Constantinos Ragazas said...

Brian you write,

“What was the Devensian Irish Sea Glacier doing here?   I had thought it skidded to a halt quite a few miles to the west, off the mouth of Milford Haven.  Time for another revision of my map.........  it is now incontrovertible that the ice flowed across Caldey Island. “

This is very exciting discovery! I have always been very skeptical of that area in your map designated 'ice free'. Your discovery begins to confirm my reasons for skepticism as I recorded in previous posts in your blog.

You also write,

“By the way, Daylight Rock, where the bones of many Ice Age beasts were found, is on Small Ord Point, just a couple of hundred metres away.  Did those beasts roam this territory before, or after, the ice flowed across the island?  That's another nice problem that we need to sort out.............”

A few points to consider:

1)You assume that this area was initially dry and glacier ice flowed into it depositing such till. But could it be that this area was covered by 'local ice' with Caldey Island rising above the surface and becoming a 'dump point' for deposits? If so, then the till you found could have been carried there by meltwater streams on the surface of the ice. It is interesting and very revealing that all of the places on Caldey Island you have mentioned are on the N and NE shoreline of Caldey Island. That is relevant because that's the direction you would expect the meltwater flow to be coming from.

2)If my hypothesis is true that 'local ice' covered this area (possibly the vast frozen meltwater bodies from the earlier Great Melt of glaciers) than the entire area in your current map designated as 'ice free' would be completely covered by a 'local ice sheet' that formed and later melted 'in place'. That would provide an 'ice route' for all the bluestones from West Wales to reach Salisbury Plain.

3)The “bones of many Ice Age beasts” found very close to the till also confirm this hypothesis. Certainly animals to not have 'burial sites'. And most certainly they do not have communal burial sites where bones from many different Ice Age animals are found. These bones where deposited at Caldey Island in the same manner as the till you found. And they all came from the mountains of Wales by the same meltwater streams. These Ice Age animals did not roam in the confines of Caldey Island. Rather, they roamed free in the mountains of Wales.

What do you think?

Kostas

Anonymous said...

I think the Carb Lime in the picture is dipping about 30 degrees from the RHS to the LHS.
I think what you took to be bedding is vertical jointing.

GCU In two minds.
Still it finding the till that is important not the nature of the bedrock.

BRIAN JOHN said...

No -- I think that's an optical illusion -- the strata here are indeed standing almost vertically right through this area. But there are masses of joints and fractures everywhere, making the picture very confusing. The gully is cut through the strata, presumably bounded by joints.

Yes, the till is a surprise -- will put up further posts.

BRIAN JOHN said...

Yes, Kostas, this is a very exciting discovery -- for the last 50 years or more everybody (including me) has assumed that the whole of South Pembs remained ice-free during the Devensian -- or, to be more correct, free of glacier ice. Now I have to change my mind -- but that's OK. That's what happens when we take our eye off the ball, or fail to notice what some of the old geologists wrote into their surveys back in the 1920's and 1930's.

Kostas, till is never carried by meltwater streams -- meltwater deposits have quite different physical characteristics.

The reason why the sites mentioned are on the north side of the island is because that is where the limestone is, and that is where the caves are. Nothing more complex than that.

Your idea that the bones all came from North Wales, carried by meltwater streams, is totally unnecessary. Most animal bones -- and sometime even complete carcasses -- are carried into caves by predators, including human hunters. That is their instinct. That is how predators behave even today, so that they can consume the meat in peace and quiet. Many of the bones have hyena and other toothmarks on them.

Constantinos Ragazas said...

Brian,

My reasoning often gets ahead of my knowledge! That's why I need you to set me straight! Though I may get some of the finer details wrong and abuse more technical terminology, I believe I have the bigger picture right! You know I am a serious thinker. That's why you tolerate me!

Are there other such islands to look for glacier till? My guess is that if there are, the till will be found along the N and NE side, depending on the location of the island. And not from the W or SW sides as would be the case if these tills are Devensian Irish Sea Glacier deposits.

Perhaps Dr Ixer can test the provenance of the till rocks you have found. And if they trace to the same general area as those found in the Stonehenge Layer, wont that suggest the same transport mechanism?

With a very important difference. The stones found in the Stonehenge Layer you claim to have been deposited there some 450,000 years ago. While the stones in the till you discovered at Caldey Island you have concluded are from the Devensian!

The rounded and sub-rounded shapes of many of these stones, as well as the many sizes and types of stones all found together suggests to me, Brian, that these where carried by water rather than by glacier flow. This is consistent with the N and NE direction to Wales where these stones probably came from. It's not likely that such till deposits could have been made by a sea glacier flowing from the W and SW direction carrying stones from the mountains of Wales (if these stones can be traced to Wales). But you are the expert! What do you think?

Kostas

BRIAN JOHN said...

Till will contain all sorts of debris picked up by a glacier -- including water-rounded pebbles that have come from old river deposits and even pebble beaches once formed on the sea shore. Till will also contain rounded pebbles derived from conglomerates such as the Ridgeway conglomerate of the ORS series, or the Cambrian basal conglomerate which occurs in the St Davids area. It can get very confusing....

BRIAN JOHN said...

My dating suggestion for the till is not based on the pebbkles it contains, but on the physical characteristics of the till itself -- and the fact that it is still soft and friable, and is not cemented. (By my reckoning, a till in this locality that dates from the Anglian or one of the other old glacial episodes would be solidly cemented by now, because of the presence of calcium-rich groundwater.)

Constantinos Ragazas said...

Brian you write,

“The reason why the sites mentioned are on the north side of the island is because that is where the limestone is, and that is where the caves are.”

Once again we have important land features pointing to West Wales. Just like the more developed parts of Stonehenge that we discussed in another of your posts. Is that a pure coincidence? Let's keep an open mind on that.

Can you tell me more about these caves? Are they all along the shore with openings towards the sea? How big are they? Do they have a wide or a narrow opening? Are there any caves inland or on the southern side and with openings in different directions say S or W ?

If these caves are like many caves I saw along the coastlines in Greek islands, these may have formed by the action of water and waves. If my hypothesis is correct that the many animal bones were carried to Caldey Island by water streams over an ice sheet, then the location of these caves on the north side would make sense. As well as why there are many diverse animal bones found in these caves.

Caves along a coastline washed constantly by water would make perfect traps for bones and other debris to be caught. We don't need hyenas dragging animal carcases into caves for undisputed consumption. They could as easily do it in the open and in the mountains, as all National Geographic shows depict. It's safer to feast on site, than take the chance dragging the carcass all the way to a cave. But I do appreciate your reasoning.

Of course, determining the provenance of the stones in the till you discovered could put an end to all my wild speculation! Or add more fuel to my fire!

Kostas

BRIAN JOHN said...

There are no preferred orientations to these caves, Kostas. The ones that have been explored are along the coast, but not necessarily opening to the north. Caves and fissures like the bone caves investigated and named occur even on south-facing cliff faces in little inlets and bays.

Remember that the bones found in these caves are not in great piles -- they are often very small fragments (and even teeth and bits of tusks) scattered here and there in the cave sediments.

Sid Howells said...

re: volcanic rocks in ?till (or soliflucted till - on basis of tabular clast alignment evident in photo) at Bullum's Bay, Caldey

There are numerous pebble-sized clasts of rocks from the Skomer Volcanic Group on South Pembrokeshire beaches (eg. Swanlake Bay) presumably re-worked from till and transported as intertidal shingle by post-glacial rise in sea level.

Other erratic streams (eg. distinctive ragged-pumice rhyolite tuffs of Ramsey Island) can be distinguished (these are common on Freshwater West. There are also many far-travelled rock types (eg. rare agates from ?Scotland, jasper from ?Anglesey - but perhaps Strumble Peninsula). I have assumed that these were all from pre-Devensian ice sheets, and perhaps this may be the case for the volcanics in the Bullums Till (or soliflucted material including till).

Let me know if you'd like a tour of South Pembs beaches to study the erratic pebbles.

Good luck with your endeavours.

BRIAN JOHN said...

Thanks Sid --

re the Bullums Bay till, I don't think it's soliflucted. I know that DQB went around seeing soliflucted tills everywhere, but I have always been profoundly sceptical about some of his interpretations, which have, I think, knocked parts of the Welsh Pleistocene chronology cockeyed. What interests me about this Caldey till (and the till near the landing stage hut, and along the cliffs from there to Eel Point) is that it is uncemented and reddish. It is sitting on limestone, but appears to have come from the ORS area. If it was old (pre-Devensian) I think it would have been cemented, like the raised beach at Bullum's Bay.

Conglomerate pebbles in till? Yes, that's an interesting point. We could have pebbles in Pembs tills from the Cambrian conglomerate near St David's, from the Ridgeway Conglomerate (ORS) and I suppose from other conglomerates as well -- but in these old conglomerates I reckon that the matrix is probably as hard as the contained pebbles. So it would be quite exceptional for glacial action to remove the matrix and leave the pebbles.

On balance, I prefer to think that the pebbles found in till and on our beaches are genuine erratics, picked up, shaped (in some cases) and then dumped -- and maybe shifted by tidal streams and longshore drift.

Yes indeed -- it would be good to take a look at some of the beaches and their pebbles -- let's see if we can find a time.