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Monday, 6 March 2017

West Angle Bay -- the classic coastal section -- going, going, and almost gone.......

I went down to West Angle Bay today, to see how it might have changed since my last visit.  Sadly (since this is one of the classic British Pleistocene localities) marine erosion has been doing its work, and I suspect that within the next five years the last remnants of the "rampart" between the coast and the old clay-pit will be whittled away, leaving a mess of brambles, scrub and made ground.  Once that happens, bits and pieces of the sediment sequence will still be visible, but it will be very hard indeed to work out what the Pleistocene history actually is.  I estimate that the coast has retreated about 10m since I first researched this site in 1965.

Parts of the old clay-pit are already visible, in a 30m stretch at the southern end of the section, and in the middle of the section to the north of a prominent "plug" of reddish Irish Sea till (seen in the photo above with a "cap" of dark green vegetation).

The "plug" of dark red gravelly Irish Sea till, approximately in the centre of the current exposure.  To the left we can see the breach which has exposed the floor of the old clay-pit.  At this point the silt and clay series of Ipswichian sediments have been cut out by overriding and eroding ice flowing in from the north-west.

The left edge of the above photo is approx at position D on my original surveyed profile of the cliff exposure of the 1960's.   For the next 20m northwards, the cliff exposure has been completely transformed, and there has been large-scale sediment removal.    What we now see is a "melange" of dark red till and exposures of black, blue-grey and orange-coloured silts and clays.  As glacier ice has come in from the north-west it is clear that it has over-ridden the fine-grained interglacial sediments and incorporated plugs or rafts of it into the dark red gravelly till. This is one of the smaller rafts, about 1m long, with dark red till above and beneath it:

About 10m further north, we see an exposure where interglacial silts and clays apparently overlie the dark red till, as in this photo:

The dark red till is full of Devonian sandstone erratics which have probably not travelled very far.  The interglacial clay is in the top right corner of the photo.

A further 5 m north we see this -- an almost vertical contact between dark red or black clay to the north (left) and dark red Irish Sea till to the south (right):

There has clearly been shearing, thrusting and sediment incorporation into basal ice layers on a substantial scale, and it appears that some "rafts" of over-ridden sediment are in excess of 10m long and 4m thick.

Then again we see this -- a layer-cake effect, with two slabs of blackish clay separated by layers of gravelly dark red till:

Again, till is seen beneath members of the silt and clay series.  Note that the till colour in this photo has been masked by a brownish-grey layer of sediment-rich water flowing down the cliff face.

There is now no doubt in my mind that glacier tectonics have played a vary significant role in determining the appearance of this cliff face of 2017 --  and probably all earlier exposures, all the way back to 1916.  The presence of large rafts of interglacial silt and clay sediments have confused some researchers (including Leach, Dixon, Bowen and Morey) who observed silts and clays ABOVE dark red till and who erroneously concluded that the till was of pre-Ipswichian age.  A mistake easily made.............

The northernmost part of the drift cliff of 2017 shows an exposure of a low rock platform and a cliff face about 30 m long and 3 m high exposing severely rotted Lower Limestone Shales  (occasionally reduced to a clay-like consistency) capped by patchy dark red till and silty soil and made ground:

The northern part of the West Angle coastal section, showing the low raised beach platform cut across Lower Limestone Shales, rotten bedrock in the cliff face and a thin cap of dark red till.

 I cannot for the life of me understand why anybody should doubt that the dark red till at West Angle is actually a till, and actually Devensian in age. There are many exposures of it in this cliff face.  Some are shown above, and here is another:

To the north of the main beach at West Angle, the same red till is seen in the three coves greatly beloved of geologists looking at structures in the Carboniferous Limestone.  Dixon refers to this one in Cove No 2 as a "pipe of drift":

In the above photo a "plug" of dark red Devensian till is exposed in the cliff face.  It is about 12 m long and 4m thick, and rests on rotted bedrock, silts and clays.  The till is capped by colluvium which has moved downslope from the higher land near West Pill Farm.  This colluvium occupies the same position as the "Upper Head" elsewhere around the Pembrokeshire coast.


My observations today have confirmed the essential correctness of my conclusions in the three earlier posts, namely that:

1.  The interglacial silts and clays (of various colours) are all older than the dark red till exposed in the cliff face.

2.  The till is quite fresh, containing striated pebbles of Old Red Sandstone and also igneous erratics presumably from the St David's area.  Some of these erratics are over 50 cms in diameter.

3.  In the middle part of the section there are clear erosional contacts / unconformities which show that substantial amounts of older fine-grained sediments have been sheared away from their places of origin and incorporated into the glacial deposits as slabs or rafts.  The scale of glacial tectonic activity is clearer now than it has been in the past.

4.  There is no reason to believe that any till exists here beneath the Ipswichian interglacial sediments.


Here is an oblique aerial shot of the sediment cliff, taken in summertime.  Maybe a year or two old?  The main features as seen today are all present. Click to enlarge.


Dave Maynard said...

Hm,... Your choice of trowel might flavour some of your relations with archaeologists!

Been looking at the raised beach at Poppit recently. How is is that there are what I think of as beach deposits sitting directly on wave cut platform?

I'm assuming that horizontal layered deposits of rounded and sorted material are the beach material. Much of it is also cemented in a dark brown iron pan type formation.


BRIAN JOHN said...

It's a very nice trowel -- used a multitude of times for building and repairing stone walls. Should greatly enhance my archaeological reputation, methinks.

Yes, Poppit is a classic location. I think I was the first to describe it. It's a common scenario -- raised beach cobbles (often cemented) resting on a raised beach platform. Sometimes around HWM and sometimes up to 2 or 3 m higher. Altitude depends in part on tidal range and partly on exposure. Most experts think that the cobbles and the platform are NOT of the same age -- the assumption is that the platform is formed over multiple interglacials, with the sea coming back to its "interglacial normal" every time. The beach is generally thought to be Ipswichian. That makes sense, since it ties in to the overlying Devensian deposits, dated downwards from the top and matching known climatic oscillations. Accumulating radiocarbon dates now confirm the sequence that I first proposed back in 1965........

Why are there no sediments between the platform and the beach? In some cases there are -- especially angular fragments from periglacial accumulations. But not at Poppit -- here wave action seems to have stripped out all older deposits prior to the accumulation of the raised beach cobbles.