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Monday 19 October 2015

Ceibwr connected pot-holes and cemented sediments

The subsidiary Channel (with linked pot-holes) -- view towards the sea.  Note the foxy-red cemented sediments on both flanks.

One of the larger meltwater channels at Ceibwr.  The channel is on the right.  To left of centre is the ridge which separates the channel from the larger Ceibwr Bay channel.  Note the foxy red cemented sediments on the tip of the spur.  The inner end of the subsidiary channel is seen at the right edge of the photo.  In the foreground we see the bedrock floor of the channel.  Photo: Ben Ashton.

Yesterday I was over at Ceibwr, near Moylgrove on the N Pembs coast, during a pleasant walk along the clifftop.  I was struck once again by the rather interesting landforms and sediments there.  There are two big meltwater channels separated by an elongated ridge.  One of the channels is now occupied by an arm of the sea (Ceibwr Bay) and the other is dry and occupied by the roadway and car parking area.  This dry channel is fascinating, because it contains a smaller channel which is cut into its floor at the seawards end -- as seen above.  Really this small channel is a series of connected pot-holes, excavated out at a time of very turbulent meltwater flow, with large erratic boulders rolling about in enclosed hollows and excavating out very soft Ordovician shales.  Similar boulders can be seen in the foreground in the photo above.

It's tempting to think that the main channels here are of Anglian age and that they have been modified during the Devensian at a time of more limited meltwater flow.  But that would be too simple.  There are solidly cemented tills, frost-shattered periglacial accumulations and fluvioglacial deposits plastered on the inside of the "newer" small channel, and then fresh till sitting on top of these red-stained cemented deposits.  The fresh till has to be of Devensian age.  So how old are the cemented deposits?  Might they date from the Anglian glaciation?  If they do, that means the small channel might also date from the Anglian, and it might be even older.

Solidly-cemented deposits are common in the Moylgrove-Ceibwr area, maybe because groundwater here is very rich in iron and manganese.  So the reddish and black cement is made essentially of iron oxide and manganese oxides.  How long does it take for a deposit to be turned into concrete?

I'm increasingly convinced that these cemented, stained and rotten deposits (for example at Llangolman and Ceibwr) are very old, but they do need to be dated through the use of one of the modern techniques.  Must work on that........

The sediments at the head of the subsidiary channel.  Foxy-red stained and cemented till and other materials below, and grey uncemented fresh Devensian till above.  The boulders at bottom left are residuals left behind when finer sediments are washed away during extreme storm wave conditions and by runoff during periods of heavy rain.

Stained and cemented sediments at the roadside, on the hillside above the subsidiary channel.  Broken Ordovician shales below.

The crust of cemented till resting on bedrock at the Witches Cauldron, near Ceibwr.  Here the cement is so hard that it has resisted wave attack while much of the softer black shale bedrock beneath it has been eroded away. There's a lot of manganese oxides here, so in places the sediments are stained black rather than foxy red.

There is a lot of discussion of the channels and the cemented sediments in this paper by Glasser et al (2004):

Glasser, N. F., Etienne, J. L., Hambrey, M. J., Davies, J. R., Waters, R. A. & Wilby, P. R. 2004 (August): Gla- cial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, Vol. 33, pp. 224–237. Oslo. ISSN 0300-9483.

I don't agree with all that the authors say -- they don't sufficiently differentiate the uncemented and cemented deposits, and they refer simply to a "pre-late Devensian" episode of sedimentation and cementation.   If I understand them correctly, they seem to think that the cemented materials are mostly fluvial in origin, and relate to a period of very ancient fluvial valley erosion and sedimentation.  I think the cemented deposits are much more varied than that, with ancient till included.

(Note:  thanks to Myris for correcting me!  I have changed the text to refer to manganese oxides....)


Myris of Alexandria said...

Hate to be picky but magnesium oxide is rare geologically, manganese oxide(s), however, is/are commonly associated with iron oxides.
Manganese oxides/hydroxides are collectively known as wad and are a set of disgusting minerals. I refused to work on them. Manganese carbonate is a pretty mineral macroscopically, rhodochrosite.
I bet it is the bloody spell checker that is to blame.

myris said...

Magnesium oxide is periclase a contact metamorphic mineral of thermally altered dolostones. Not certain I have ever recognised it but have the Durness Limestone contact with the Beinn na Dubhaigh Granite in the back of my mind. Also perhaps the Lizard Serpentinite. One for material scientists I think.
Anyway not a non-anthropogenic Holocene mineral.

BRIAN JOHN said...

Ah of course -- thank you Myris -- my fault entirely. Can't even blame the spell checker, which is something I try not to use.... Careless of me. I mean manganese oxides -- will correct in the text.

Keepoff the Grass said...

Hello, from the Salop borders, to our Alexandrian friend.

A complete aside, but the spelling business triggered a thought.

When in your library, have you come across any mention of magnetite in relation to burial chambers, Preseli, or Stonehenge, please?

Any spelling errors are purely down to the sploody chell blecker.

Go steady

Myris of Alexandria said...

Haematite is prehistoric in usage. Magnetite not so much.
There is a little I think in the Stonehenge landscape but rare.
I don't know of it associated with tombs.
It is present in the dolerite as titanomagnetite but only macroscopical visible and much altered to sphene and titanium dioxide minerals.
I think there are unrecorded occurrences close to the Stonehenge Greater Cursus.

S.K. said...

Hello to Myris,

Many thanks for the guidance, I fear the alabastron will burn way past midnight.

Serapis Karanis

BRIAN JOHN said...

This is a rather interesting topic, and I shall come back to it. Found an old thesis about cemented Pleistocene gravels in New Jersey, with mention of "low-magnesium calcite cement."
Take a look:

Myris said...

I fear setting light to the contents of alabastra, unless a devotional act, is just wanton and destructive.

Serapis nice God if you want something fluffy and modern, but for we more traditionalists Divine Apollo suffices.

Normally the Mg-content of calcites tell something about the origin of the calcium carbonate, calcite is not the only calcium carbonate mineral.
I shall take a look at the reference.


Myris said...

I have just checked the original files.
There is no magnetite close to the SH Greater Cursus or from within the SH circle.
It is botryoidal Haematite.

Keypof the Grass said...

Hello M
Thanks for checking, and and also reminding me of my old friend Apollo, I keep meaning to call him but tempus bloody fugit.
Go steady.

Dave Maynard said...

Interesting article in the Guardian about the effect of National Geographic funding of scientific study that may have a bearing on recent work in our area of interest

TonyH said...

Yes, that's a Guardian article well worth reading, Dave, thanks.