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Date: |
Friday 4th November 2005 |
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Location: |
Bedfordshire and Buckinghamshire |
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Leader: |
Dr Jill Eyres |
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Author: |
Fred Tomsett |
After returning to the hotel for the lunch which we had taken out with us that morning, it was off to Munday's Hill Quarry, cunningly hidden from view about five minutes drive from where we were staying. This is a quarry in the Lower Cretaceous Greensand, covering the period from about 115 Ma to 95 Ma. There are three layers to the greensand as seen and they reflect the problems that old-style nomenclature creates. From the bottom up, there is the ‘Brown Sands’, then the ‘Silver Sands’ and on top of this the ‘Red Sands’, although these were absent from the quarry. The problem of course is that the sands as we saw them are not those colours. The ‘Brown Sands’ were silver-white though they became redder as you went up the succession and the ‘Silver Sands’ were of course red-brown. Jill had written a paper to try to have the names changed but was currently having problems getting the thing published – she was very coy in not telling us what she wanted the sands renamed as.
Our doc’s PhD had actually been about these sands and what do you know – she overturned a good few ideas on them. It had always been assumed that they were the products of shelf seas, but when she began working with the sands, she noticed clay drapes, indicative of slow currents and bi-directional cross-stratification, below an area of asymmetric ripple marks indicative of currents. The only theory to fit was that the sands were laid down in some sort of estuary. When the micro-paleological results came through, with specimens taken from the clay, they indicated beasties from brackish surroundings which supported the estuarine theory.
The sands at the bottom of the sequence were mostly silver-white, like the sort of thing you would expect to find in the Caribbean, and there was quite a thick sequence of them. As we moved across the quarry and up-sequence, we were all jolly glad the rain had held off, but even so the ‘topsoil’ was pretty boggy and by the end of the day, my nice grey boots were a sandy brown (Sunday’s rain would soon return them to their right colour). As we traipsed over the quarry, we crossed a nice braided river sequence with delta fan which the rain had carved out of one of the banks and maybe Don will put in his photo of it for you.
The ‘silver sands’ were of course not silver at all, but much redder with iron. Deposition was further out to sea – no clay drapes and ripple marks – but it could well have been a sandbar outside the estuary. At last we began to find some fossils, bits of wood and lignite as well as some belemnites, including a rare species, though sadly not a complete specimen (Neohibolites minimus var. attenuatus ? Rings a bell, Ed.).
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Munday’s Pit, the Greensands overlain by Gault Clay. |
This is in the Lower, estuarine sequence, bi-directional cross bedding. |
Further up the sequence, |
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Photos © Don Cameron |
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The light was beginning to go, but we were all still very impressed with the vivianite that was lying about in large blocks that the quarrymen had hauled out as they had no use for it. Fortunately some of the specimens were still fairly fresh so the oily lustre and rainbow sheen of the mineral was still visible – it would soon disappear in daylight conditions. It did look like the sort of effect you get by spilling petrol onto water (don’t try this at home) – quite remarkable.
The Gault Clay overlay the sands in an unconformity, but sadly time and weather conditions were rather against us scrabbling up onto it and trying to unearth various tasty fossils, such as ammonites and corals which was a shame. Instead we wended our weary way back to the Cock Hotel for our dinner and lashings of free wine (well, I suppose we had paid for it, but we certainly didn’t leave any for the chef!).
Two web sites that may be of interest to readers. Both have downloadable leaflets that you might like to use for walks if you are in the area as well as covering the geology and extractive industry; www.sandpitproject.co.uk and www.bedsrigs.org.uk.