An OUGS (Severnside) day trip to look at Jurassic limestone in the Cotswolds
The trip was in two sections, the southern exposure of Middle Jurassic Great Oolite in Stony Furlong quarry near Denfurlong Farm Shop and the northern exposure of older (still Middle Jurassic) Inferior Oolite near Chedworth Roman Villa.
An early start (for a Sunday morning at least!) from Hereford got us to the Farm Shop where we parked while spending the morning looking at Stony furlong quarry, by the southern end of the cutting which is just a short walk away across their campsite. The campsite is on a filled in quarry area from which stone was provided for Chedworth airfield in the mid-20th century
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| Only right that, since we were using the Farm Shop parking, we should patronise them before starting off. Sausage breakfast baps recommended! |
Railway cutting I thought.... level... won't need a walking pole. But I'd forgotten about getting down into it! And then down into the quarry on the far side! Sliding works as well now as it always did though, even if it is a bit hard on the trousers.
Once down in Stony Furlong quarry we could have a look at the exposed face: there is quite a lot of vegetation, and the lower part covered in fallen debris too. A fallen block from the upper section was useful as otherwise that wouldn't have been get-at-able.
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| Stony Furlong Quarry, Chedworth, with a fallen block from the upper section on the left hand side (south) The quarry face is very stable nowadays. We couldn't get very close (perhaps better in winter when plants have died back! |
Paul pointed out the stratigraphy to us and, for a change, there was plenty of time to sketch the exposure!
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| Not sure how many marks it would have got in a summer school assessment! |
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| More or less the bit I sketched |
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The quarry exposes the White Limestone Formation at the base of the Great Oolite Group. The upper part is the Ardley Member was deposited in fully marine conditions and apparently exhibits more biodiversity (lots of brachiopods plus some echinoids, ammonites and crustaceans). The lower Shipton Member was deposited in a more restricted environment (estuarine/lagoonal?) and has less diverse fauna (bivalves and gastropods).
During the Middle Jurassic this area lay around 30 degrees north, in an arm of the closing Tethys Ocean. Global climate was warm, with no glaciation. The Atlantic was opening, and there was also plate tectonic movement in the Indian ocean. Tectonic plate break up created more ocean ridge length and consequently higher sea levels, flooding extensive continental shelf areas. Generally it was a time of rising sea level, with occasional still-stands and only very brief periods of falling sea level. What's now the Cotswolds was an area of shallow water tropical sea with shifting ooid shoals on a stable shelf. This has resulted in a complex, interfingering stratigraphy!
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| Lots of fossil hunting and hammering went on but I didn't see anyone find anything particularly impressive! |
At midday we broke for lunch, agreeing to meet at Chedworth Roman Villa to begin the afternoon part of the excursion. The overflow car park is beautifully shady and, not wanting lunch after that sausage bap, Jan and I took Libby to find the others at the National Trust cafe which is conveniently outside the actual villa area (no need to pay to go in, though must go back sometime to take a proper look at the villa itself!)
Paul had suggested restarting at 2pm but as we were all ready to go (apart from a few stragglers who had temporarily converted into archaeologists and who had to be rounded up), we headed up the hill behind the cafe, via some convenient (but steep!) steps to the disused railway track.
A mile of track runs from a blocked off tunnel at the southern end, to a road (blocked off) at the northern end, and allows access to rocks of the Inferior Oolite Group.
During the Middle Jurassic blocks were being actively uplifted which resulted in gentle warping and folding of all but the uppermost Inferior Oolite strata.
Heading south to the tunnel we looked first at the Clypeus Grit, named after an irregular echinoid (rather like a sand dollar). Grit in this instance isn't a sandstone but a rubbly, fossiliferous limestone and, once again, the fossil hunters spent some time happily hammering away in the hopes of finds!
From here we retraced our steps to a spring and waterfall which is depositing tufa (calcium carbonate from the limestone through which the water passes) on the moss and liverwort covered surface. This is similar to the "petrifying wells" found in Yorkshire and Derbyshire.
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| The tufa waterfall |
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| And a closer view of the calcium carbonate coated mosses and liverworts of the Tufa Waterfall |
Heading back northward, we came to exposures of the Upper Trigonia Grit, stratigraphically below the Clypeus Grit, and which sits unconformably here on the white, oolitic, Notgrove Freestone.
The unconformity is hard to see as there is little change in attitude between the strata above and below, but it is marked out but a layer which has been eroded back more than the other strata.
The Notgrove Member (previously Notgrove Freestone) is the youngest pre-unconformity rock and sits in the centre of a syncline here.
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| The unconformity below the Upper Trigonia Grit is marked by the layer midway up that has been eroded back |
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| A cleaner section enabled a better view of the rocks! |
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| We found Trigonia fossils (left) and also an internal mould of Trigonia |
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Another view of the interior mould (and if you want to know how I know what it is, it comes of drawing one for my A level practical notebook some 50+ years ago)
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Once drawn, never forgotten! Just a shame we weren't encouraged to include a scale and labels :(
Wouldn't have done too well if I'd been marking it for an assignment!
(We used to call bivalves "lamellibranchs" then by the way) |
A normal fault downthrows the Clypeus Grit against the oldest rocks seen. Just before the high embankment at the northern end, we came to a section through the oldest rocks in the section, the Birdlip Limestone (pale grey), Gryphite Grit and Lower Trigonia Grit (shelly, orangey-brown). These have been upfaulted into this position as a small horst.
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| The oldest strata in the section |
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| The elongate Jurassic bivalve, Gervillella. A "boring piddock" |
There was a discussion about the "boring piddocks" which I missed!
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A sample of the hardground from below the unconformity.
Vertical lines are burrows down into the rock from the upper surface. |
Proof of an otherwise hard-to-find unconformity were samples of a hardground with a bored upper surface.
The walk along the railway line had been pleasantly level through shady woodland, though a bit midgy. We headed back to the steps, and down to the cafe where people variously enjoyed coffee, tea, cake, and icecreams, and Norman, our Branch Organiser, thanked Paul with a bottle of wine for guiding us through the section.
It was interesting to contrast this section with the Carboniferous ramp we saw at Three Cliffs Bay a few weeks ago. There there was a continuous stretch of almost vertical beds so we could travel back in time simply by walking down the beach. There were distinct rises and falls in sea level attributed to glacials and interglacials during the Carboniferous and non-depositional surfaces were karstified by subaerial exposure. At Chedworth the strata were pretty much horizontal and non-depositional surfaces were subaqueous hardgrounds. The rising sea level was the result of tectono-eustasy rather than glacioeustasy. Nice to see the two very different carbonate sections in a short space of time! Thanks to Jan for organising yet another good trip.
