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Sunday, 4 November 2012

Saunton and Croyde

I have written before about the "giant erratics" on the coasts of SW England, and I was reminded of the importance of this evidence when I came across this map made by Prof Nick Stephens many years ago.  It show nine erratics scattered along this coast near Croyde, together with four exposures of till -- clear evidence of glaciation.  The biggest erratic is a 48-tonne monster about which I have written before.


We are not sure how many erratics there may be on the coasts of Devon and Cornwall -- at least 15 are recorded in the literature, and because some of them are sealed beneath periglacial and other deposits dating (we suppose) from the Devensian cold stage, we can be sure that there are many more waiting to be discovered as the soft sediments on the coast are eroded back by wave action at times of high tides and storm waves.

As I have indicated before, I am not terribly bothered whether these boulders were carried by icebergs or sea ice, or by a glacier moving onshore from the Bristol Channel.  Because there is till in a few locations on these coasts, my preference is for direct glacial action.  But I also think that there must have been substantial isostatic depression in this region during the Anglian Glaciation -- and that means a lot of ice over the counties of Devon, Cornwall and Somerset -- and maybe Wiltshire too.

9 comments:

TonyH said...

A bit of lateral thinking: UNISON Union's holiday village is at Croyde, next to the beach. I wonder how many Environmental Department Officers from local government realise there are large erratic boulders just beyond Croyde Bay? Good way to teach their offspring some of the wonders of physical geography whilst holidaying.

Constantinos Ragazas said...

Brian, you write

“I am not terribly bothered whether these boulders were carried by icebergs or sea ice, or by a glacier moving onshore from the Bristol Channel”


What I find most interesting is these boulder erratics are found only along the sea coast (according to the map in your post). If these boulders were carried overland by glaciers wont you expect more such boulders to be found over land and not just along the coast?

Have these been sampled to determine their possible provenance? It would answer the question whether they are iceberg/sea-ice deposits or carried overland. And if it can be determined they were carried overland, wont the absence of similar erratics over land raise some questions these are glacier erratics? In which case we need to think of other mechanisms for their transport. I think we can agree at least these were not carried by humans!

Kostas

BRIAN JOHN said...

Kostas -- there are some erratics inland. I have mentioned some of them in earlier posts. But look at their stratigraphic position. On the coasts they appear to be sealed beneath considerable thicknesses of periglacial "head" or slope deposits -- and are only revealed because of the action of waves on the coast, eroding away these later sediments. The same scenario applies inland. It will therefore be extremely rare for any large erratic boulders to be revealed -- which is why we always have to keep an eye open for new road cuttings, pipeline trenches etc -- as these hold the greatest hope of finding interesting things...

TonyH said...

The new Chairman of the WANHS Archaeological Field Group, Bob Clarke, was on hand checking pipeline trenches when the burials of the so-called Boscombe Bowmen, just outside Amesbury, were uncovered. He was then working for Wessex Archaeology. So pipeline trenches (as mentioned in Brian's comment back to Kostas at 09.01 on 5th November) do reveal some useful stuff at times, But will all monitoring archaeologists take sufficient notice of the geology thus revealed?

Anonymous said...

Thanks Brian for your explanation.

Any thoughts about the provenance of these giant erratics? Interesting that these giant coastal erratics are buried “ beneath considerable thicknesses of periglacial "head" or slope deposits” . This fact clearly sets a relative time-line when what happened first. Could these slope deposits have resulted from meltwater streams?

Kostas

BRIAN JOHN said...

Kostas -- most of the erratics have been described but not properly analysed or provenanced. One of them is reputed to have come from Iceland -- I have my doubts about that!! The "head" deposits are periglacial slope deposits with angular fragments derived from upslope -- they are quite different from meltwater deposits,

Constantinos Ragazas said...

Brian,

How are “periglacial slope deposits” form? Any connection with glacial lake formation?

In previous posts I speculated a glacial lake over this area formed by an encroaching Irish Sea Glacier along the coast.

Eager to learn! But you can ignore my quest if you so wish!

Kostas

BRIAN JOHN said...

No connection at all with lakes, Kostas. Please do some research -- just google in "periglacial", "head deposits" etc and you will discover a great deal. I have dealt with head deposits many times on this blog. See here:
http://brian-mountainman.blogspot.co.uk/2011/01/strata-do-not-lie.html

or just use the search facility.

Constantinos Ragazas said...

Brian,

Seeking to understand periglacial “head” deposits through the meaning of words is like trying blindfolded to identify an elephant, feeling only one part of it at a time.

What I mean by “head” deposits formed by glacial/periglacial lake formation may not be what you understand I mean. But you are the expert. And your flow carries greater “head” deposits. I can only be guided by fixed stars and the irrefutable facts on the ground to find my way around this prehistoric landscape.

Lets just leave it at that!

Kostas