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Friday 18 May 2018

Lydstep ancient till site confirmed

National Trust map of Lydstep headland, showing key Pleistocene sites

Further to my earlier note about the suggested ancient till at Lydstep, I have been back there today to check it out -- and it is very impressive indeed.

The first record of an ancient till at this site is from Arthur Leach, in his papers and manuscripts held in Tenby Museum:

The full grid reference for the ancient till site is SS 08802 97473.

I also visited the site and published a brief record of it in 1974.  The key location is a little peninsula that extends southwards from the "heel" of the big peninsula.  Here, by a freak of nature, a vast expanse of cemented limestone breccia has been protected from extensive marine erosion.  An area of at least 30m x 20m supports a most unusual -- and slightly surreal- -- landscape of concreted ridges and hummocks of limestone debris (mostly sharp-edged fragments which are partly frost-shattered and partly natural scree slope accumulations) on an undulating rock platform about 20m above sea-level.  I agree with Leach that this is not a wave-cut platform -- it is too high, and it does not have a wave-cut notch against an old cliffline.  I have struggled a bit to work out where on earth all this shattered scree has come from, and  have come to the view that it cannot all have come from the cliff slope to the north.  I think this is the floor of an old karst dry valley like the one that runs northwards from the western edge of Lydstep Headland today.  The southern side of this valley has, I think, been completely lost to marine erosion.  This is an indication of the great age of the breccia and everything that lies beneath it.

Cemented limestone breccia about 2m thick on the little peninsula at Lydstep, with the karst coast beyond (looking west)

Cemented limestone breccia about 3 m thick, at the foot of the northern valley slope.  There is no till exposure at this location.

Cemented limestone breccia sheet about 2m thick, resting on broken limestone bedrock.  To the left of centre is a chasm that falls straight down to the sea below.  

Looking straight down through another gaping hole in the sheet of limestone breccia.  The breccia now forms the roof of a gigantic cave, and eventually it will all collapse into  the sea........

At the eastern edge of the breccia exposures, there is a spectacular overhang or projection of the breccia sheet, sticking out eastwards by about 2m -- and beneath is is the classic exposure of ancient till.  The grid ref is approx SN 088975.  

The spectacular overhang beneath which the ancient till deposit has been protected.

The till exposure is about 10m long, and the till is about 1 m thick.  It is solidly cemented, like the breccia above it.  It rests directly on an undulating bedrock surface that has the appearance of ice moulding -- but no striations could be seen because of heavy manganese oxide staining. 

Cemented till (here containing mostly broken limestone fragments) resting directly on a stained bedrock surface

Approx 1 m of cemented till exposed beneath the overhang.

Large erratic block and smaller cobbles in a sandy till matrix, beneath the overhang.

Erratics in the cemented till, including ORS, buff sandstones, mudstone and shale fragments, as well as many local limestone fragments.

Here and there above the till is a cemented deposit of sandrock or sandy loam, foxy red in colour and full of interesting holes.  Animal burrows, or root holes, created when the sediment was fresh and soft?

I have a full record of the site, and the Pleistocene sequence seems to be:

4.  Up to 3m of cemented limestone breccia, in places blocky from catastrophic rockfalls, and in places full of smaller (frost-shattered?) fragments.  No clear stratification.

3.  Cemented sandrock containing some till and limestone fragments -- up to 20 cms thick.

2.  Cemented ancient till up to 1 m thick, containing abundant foreign erratics.

1.  Bedrock floor of old valley, apparently smoothed beneath glacial deposits.

There are no raised beach deposits here, and no fluvioglacial deposits either.  The best guess must be that the till dates from the Anglian glaciation, that the brickearth represents a climatic amelioration (interglacial?), and that the thick cap of limestone breccia has accumulated during the whole span of the Devensian glacial episode.  There is no fresh Devensian till here, but it is found a short distance away (about 200m) at the head of the creek leading into the dry valley.

As far as I am aware, this is the most extensive and most accessible ancient till deposit in Wales.

In some texts the site is recorded as follows: The rough looking headland "Black Mixen"  is covered with angular Limestone Fragments , cemented together - a Gash Breccia rock type , formed possibly by a Debris slide in the Triassic period ( 200 - 250 million years ago ) , when a Wadi was infilled.  This is way off the mark.  There is no reason to think this is a gash breccia --although gash breccias are common in the area in fissures, associated with fault movement and with reddish staining.  I like the idea that the dip between the outer peninsula and the main slope of the headland might be all that is left of an old valley floor -- but the fact that there is till beneath the breccia means that it cannot possibly be Triassic, and it cannot possibly have anything to do with a warm and arid climate.

PS.  (added 31st August 2018)    This little peninsula is referred to in the Pembroke and Tenby Geological Memoir (1921) as "Black Mixen" (p 192).   I am not sure whether Dixon or any of the other surveyors actually visited this site -- but they refer to the platform as a part of the raised beach platform (I agree with Leach that it is too high for that) and while they mention the great expanse of cemented limestone head, they make no mention at all of the underlying cemented ancient till.  Strangely, they do mention a deposit of current-bedded sand and gravel in a fault fissure at the back of the cave then runs under the neck of the peninsula "at about the height of the raised beach."  They speculate that this might be glacial........  it needs to be investigated.


Michael said...

hi Brian, there is tons of this stuff between Maros and Pendine. Some seem to have flow down a meltwater valley.
Have a look if you have the time but make sure you can get round the headland from Pendine to Maros.
Good luck

Michael said...

There is loads of this stuff between Pendine and Maros. Some V thick and some seem to have flowed down a meltwater valley. Have a look if you have time.
Good luck, Michael

BRIAN JOHN said...

Thank you Michael -- that's interesting! It's an area I have never examined systematically -- Dai Bowen did some work there many years ago. Can you give me some grid references for sites to look at?

BRIAN JOHN said...

Are we talking about the concreted head or rock breccia? Dai Bowen did not find till in this area, but he described raised beach and solifluxion materials on top of it......

If you have seen till under the head, or on top of it, that would be REALLY interesting.......