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Tuesday, 24 March 2015

Broad Haven: two generations of erratics?

Here is a little hypothesis, which seems to me to have some merit: 

At Broad Haven, and probably in the embayments to north and south, there are two generations of erratics, namely well-rounded boulders generally less than 1m in diameter and which may date from the Anglian Glaciation, and larger irregularly-shaped "giant erratics" which may date from the Devensian Glaciation.

Here are half a dozen photos of well-rounded boulders of many types, scattered about on the beach and on the rock platform in the vicinity of the Sleek Stone.







There is a huge contrast between these boulders and others including the bits and pieces of the "super-erratics" referred to in earlier posts.  Here are four photos of these "fresh" erratics, which have sub-rounded or sub-angular forms and where the abrasive processes of wave action are gradually smoothing off previously sharp edges.






Obviously I'm generalising a bot here, but the 4 photos above seem to me to show either sub-abgular glacial erratics delivered to this locality more or less as they appear today, with small amounts of recent rounding due to wave action, or else chunks that have recently been broken off one or other of the giant erratics which I have described.  Various lithologies are involved.

This all makes sense to me.  The Devensian erratics in this locality will be "fresh", having only had a few thousand years of tides, storms and wave action to modify them.  On the other hand, erratics emplaced here in the Anglian Glaciation, almost half a million years ago, will have been affected by hundreds of thousands of tides and many thousands of Atlantic storms, leading to breakages and substantial rounding through the abrasive processes at work in this high-energy coastal environment.

The last photo above -- of two of the giant erratics -- shows a complex pattern of fractures.  Both of these huge blocks will eventually be broken down into a multitude of boulders and cobbles as a result of the percussive effects of other large stones being thrown against them during severe storms.

2 comments:

Constantinos Ragazas said...

Brian,

Why can't it be one glaciation, with some erratics carried over long distance and other erratics over shorter distance?

You are the expert here. I am just asking! As always ...

Kostas

BRIAN JOHN said...

Of course that is a possibility, and because the small erratics (under 1 m in diameter) seem to be more varied geologically than the giant erratics some of them at least may have come from further afield. But the erratics shown in the photos are much more rounded than glacial erratics tend to be -- so I am assuming that other processes specifically related to this location have come into play. This is a very high energy storm wave environment in which sub-rounded glacial erratics are rolled around and smashed against one another all the time, turning eventually into rounded or ball-shaped boulders and cobbles. That happens on beaches but not so much under glaciers and in glacial transport. In glacial transport many stones are subjected to intense pressure (if they are in sub-glacial situations) and if there are any weaknesses chunks tend to get sheared off, giving facets. This is all difficult to prove -- my mild hypothesis awaits more detailed field examination!