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Tuesday, 30 September 2014

Flat Holm Erratic Hunt (5): the Pleistocene context

Oblique aerial photo of Flat Holm. Lighthouse Point, at the southern tip of the island, is in the distance.  West is to the right. East is to the left. The famous pink erratic is located at the edge of the beach at bottom right (West Beach).

It's worth reminding ourselves -- in the light of the erratic hunting going on on Flat Holm and Steep Holm -- what the Pleistocene context is, according to other authors.  I'll stay out of it for the moment.......

On this map you can pick out Lavernock Point to the south of Cardiff, and you can see the location of Flatholm and Steep Holm in the mouth of the Severn Estuary, where it broadens out into the Bristol Channel proper.  Click to enlarge.  Note that there is a deep channel (over 20m deep) between the two islands.

There is a complex sea floor here, with a great deal of sediment movement, given the vast tidal range and the fierce currents that sweep in and out twice a day. 

You can see from this Landsat image just how stained the water is within the estuary -- sediment is carried well out into the Bristol Channel on every falling tide.

How many times might Flat Holm and Steep Holm have been glaciated?   The current consensus (not that we should pay too much attention to it) is that this part of the Severn Estuary has been glaciated just once -- during the Anglian Glaciation (coinciding with Marine Isotope stage 12) -- about 450,000 years ago.  This is a reconstruction of the Anglian ice limit by Pawley et al -- and in view of the fact that glacial deposits are now known from well within Somerset this may be taken as a very conservative line drawn on a map:

The authors have effectively taken the coastline of Cornwall, Devon and Somerset as coinciding with the ice edge.  Note that on this reconstruction, both Flat Holm and Steep Holm will have been affected by glacial ice at this time.

A more extreme reconstruction is that of Kellaway, supported in part by Olwen Williams-Thorpe and her colleagues in 1991.  They considered that the ice reached Stonehenge, as shown on this reconstruction:

Note that these authors have very bravely added erratic trails or erratic transport routes on this map.  I have highlighted them with colour.  As far as Flat Holm is concerned, the researchers have shown an ice stream crossing the outer end of the St David's Peninsula before pushing up the Bristol Channel and reaching the Penarth - Flat Holm - Weston-super-Mare area.

It is widely assumed that the Wolstonian glaciation affected Wales; but there is not enough evidence to enable any sort of reconstruction to be done.

When we come to the Devensian glacial episode, the limit most frequently cited is that shown by Pawley et al as a stippled area on the map above.  That limit as published by the Geological Survey was inaccurate, and we now know that Devensian ice affected both Lundy Island and Caldey Island.  But as shown on the following map, it is unlikely that any Irish Sea ice or Welsh ice affected Flat Holm around 20,000 years ago.

That having been said, the ice edge may not have been very far away, ie just inland of Cardiff -- so there would have been a strong possibility of meltwater streams from the ice edge carrying down onto the floor of the estuary large quantities of debris from the eastern part of the South Wales Coalfield.  According to some reconstructions, the ice front extended beyond the present coastline between Cardiff and Newport.  Remember that the sea was not present at the time -- it was still more than 100m lower than its present level.  So a mechanism existed for the introduction of large quantities of fluvio-glacial and glacial material from the north into the vicinity of the hill mass which we now refer to as Flat Holm.


A note about the sediments on the bed of the estuary:

Today, the estuary is known as a ‘drowned valley’. The bedrock floor of the Severn Estuary upstream of a line from Lavernock Point to Brean Down is covered with areas of gravel, sand and mud. These also cover bedrock in the Bridgewater Bay area, but downstream of the Lavernock-Brean Down line, bedrock is exposed on the floor of the Estuary9. Movement of sediment within the Severn Estuary is complex due to the high tidal range and the strong tidal currents that prevail here. Not only are silt and mud carried seaward into the Estuary by the rivers, they are held in the Estuary, deposited and moved again over considerable periods of time. In addition, sand is carried up the Estuary and landward9. This sand is not derived from erosion of the shoreline rocks, but from the deposits of glacial rivers during the last ice age. The surface layers of the sandbanks are largely unconsolidated and may be moved and redeposited by the strong tides. Some of the near-shore sandbanks provide an element of protection against coastal erosion by reducing wave energy. The dredging of sand for the aggregates industry is therefore closely monitored.

Ref 9:  Kellaway, G.A. & Welch, F.B.A., 1993. Geology of the Bristol district. Memoir for 1:63 360 geological special sheet (England and Wales). London: HMSO, xii+199pp.


Alex Gee said...

Congratulations Brian, your erratic hunt is of much interest and further valuable evidence. The Bathymetry and geology of the bed of the Severn estuary however may prove to be of equal value.

Study of the combined geology and bathymetry of the estuary bed, throws up an interesting question!

If according to the data the -30m contour is as shown, it is cut in solid bedrock.Two main palaeo channels appear to exist on the chart. Both most probably cut when sea levels were upto 120m below current levels.

One: obviously cut by the Palaeo Severn and one; cut by a Palaeo river draining the catchment surrounding the Somerset levels along the south flank of the Mendip Hills.

The question that arises, is that if the maximum possible depth of the channel from the levels; cut into the bedrock is only -20m, why is the bed of this channel;40km upstream to the east, cut to a depth of >-36m? I suggest that this channel is a tunnel valley cut by glacial meltwater!

Unless someone can come up with a reason for the bed of a palaeo channel to be cut 16m below the channel bed of its downstream continuation?

BRIAN JOHN said...

Alex -- I don't know enough about the sea floor in the outer estuary to make any sensible comment. But there could well be something in what you say. See this post about Ramsey Sound:

It is of course more than likely that during Anglian deglaciation sub-glacial channels might have been formed. Of equal interest is the question of "Glacial Lake Maw" -- a huge pro-glacial lake in the Severn estuary proposed many years ago by Maw and Mitchell. If Irosh Sea ice did extend right across the Bristol Channel and press onto the coasts of Cornwall, Devon and Somerset, then a vast lake might well have been impounded at some stage. If so, where are the laminated lake sediments on the floor of the Severn Estuary? One might expect some trace of them to have been picked up in sea-bed research over the years.....