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Wednesday, 9 June 2010

The ice foot holds the key?



More amazing info from Ian West's site:

Medmerry, near Selsey: At very low tides after exceptional storms more may be revealed. In late Victorian times there was much interest in the discovery here of a two ton mass of Bognor rock with striae like those in glacial deposits.

In 1892 Clement Reid discussed an unusual exposure of erratics at the base of the Ipswichian raised beach deposit at Medmerry. The exposure resulted from a storm of October 24th in 1891.

Reid 1892:
"About a hundred of these pits were examined, and the conclusion seemed irresistible that they afforded clear evidence of the agency of floating ice. Drift-ice grounding on the ancient foreshore dropped its burden of erratics between the tide marks. Here they were pressed deeper and deeper into the clay, for the rise and fall of the tide at high-water piled ice upon any projecting rock, while at low water the rock was pressed down by the weight of the ice till it was flush with the general surface. Often, however, the still-projecting boulder would be firmly frozen into a new ice foot, or accumulated mass of pack-ice, and would then be gently lifted out of the hole at the rise of the spring tides. It is thus that I account for the occurrence of empty pits, for they seem to mark the former sites of blocks which may have shifted their position several times before finally coming to rest. Perhaps some of the basins were produced by the stranding, packing and revolving of masses of ice during a storm, but the general appearance of the section suggests tranquil water in a sheltered bay. No signs of furrows ploughed in the clay were observed, and the ice was probably entirely in the form of flat-bottomed ice-foot, which, at a spot like this, sheltered from the prevalent winds by the Isle of Wight [but note that the prevalent wind-direction then is not known], would ground gently and would tranquilly melt away without being driven violently into the shoals, as on a more exposed coast."

4 comments:

Constantinos Ragazas said...

Brian, concerning the empty pits found at the beach at Medmerry.

… quoting Reid from your post above,

“Often, however, the still-projecting boulder would be firmly frozen into a new ice foot, or accumulated mass of pack-ice, and would then be gently lifted out of the hole at the rise of the spring tides. It is thus that I account for the occurrence of empty pits, for they seem to mark the former sites of blocks which may have shifted their position several times before finally coming to rest.”

Question: If boulders were 'firmly frozen into a new ice foot' wont they also be firmly frozen into the frozen ground too? How could these boulders be then 'gently lifted out of the hole' ? And wont these heavy boulders more likely separate from the frozen ice than from the frozen ground? Certainly the force of gravity would favor separation from the ice that covers them than the ground that supports them.

But I have more compelling comments to make. From the excellent sketch in your post made by Reid (I presume) these separate empty pits (at some distance) have a very well defined outline. These pit outlines do not indicate to me some tidal-water and ice-foot agency that moved these boulders out of the pits and into other pits -- without leaving behind some other indicators of such transport on the soft bedrock. If huge boulders were to be dragged and tumbled by the tidal waves and the ice-foot flow, the pit edges would be greatly disturbed and show signs of such movement. They would not be so crispy and preserved.

There should also be some trails etched into the bedrock of the forced movement of these boulders from a vacating pit to a subsequent occupying pit. None seems to exist.

Brian, what likely made these 'empty pits' at the beach at Medmerry (and likely elsewhere) are blocks of ice that were embedded in the beach by forces of nature, forming the pits and latter melting away leaving the pits empty. This explains all the evidence on the ground, in my humble opinion!

Constantinos

Brian said...

These pits are so strange that I really don't know what to make of them. Reid may have given the pits sharper edges (in his sketch) than they really had -- and I agree that if ice-foot movement and freezing and thawing were crucial processes, there should also be striations and grooves on the platform. But I'm not sure I can go with your idea of blocks of ice embedded in the platform -- remember that we are talking about the inter-tidal zone here -- and that ice is buoyant in water. I don't know just how "soft" these sediments are .... but that might be another factor.

Kostas said...

Brian, quoting Reid again

“Drift-ice grounding on the ancient foreshore dropped its burden of erratics between the tide marks. Here they were pressed deeper and deeper into the clay, for the rise and fall of the tide at high-water piled ice upon any projecting rock, while at low water the rock was pressed down by the weight of the ice till it was flush with the general surface.”

I empathize with your bewilderment of how the empty pits were formed. It's hard to imagine blocks of ice drifting ashore and then pressed into the seabed. Reid's explanation above could (maybe) work for sarsens, but I agree with you it can't work for ice blocks. And we both agree his explanation for the empty pits just doesn't cut it. So what are we to make of these empty pits?

There is another explanation, Brian, if only you can suspend your prejudgment for a moment.

Ice blocks dropping from some significant height hard onto the clay shore would get embedded in the fashion that would create these empty pits (once the ice melted). This would be the same process as with the sarsens found similarly embedded there. Aside from the question of whether or not there was 'local ice' that covered the land, do you not agree that this mechanism would explain not only the embedded sarsens but the empty pits as well?

One further point on this, Brian.

If you look closely at the Atkinson excavation photos showing the exposed bedrock under the Stonehenge Layer you will see a 'pattern of empty pits' that could only be created by many fallen ice blocks in a very confined area. It's not that Neolithic builders were indecisive and dug pit after empty pit to fit the stones!

Constantinos

Brian said...

You won't be surprised that I cannot accept your theory. We can only work by analogy -- and I know of NO mechanism for dropping stones from a great height -- or even from a slight height -- leading to them being embedded in a soft substrate. I have seen plenty of ice edges in my time, on shorelines and inland, and I know of no examples of what you suggest. End of debate on this now please.