GeologyRocks

Introduction

Semington, just east of Bath in Wiltshire has recently had a new by-pass built. One of our Geologyrocks members, John Willmouth, spent some time near by during the excavation period. Here are some notes and photographs taken during this time.

In the 2004 Bath Geological Society Journal, we had a report from Sue Cowley on her finds on this site. One of her claims was that biggest gypsum crystal found was 6" in length. She was not aware that a few days later I found one a massive 9.5" long! (Figure 1)

Figure 1: A 9" gypsum crystal found at the Semington by-pass site.

In addition the last time I was able to get into the site prior to the actual road construction I found, what I like to think of as, the 'mother-load'.

All the gypsum crystals had been found in a narrow band, perhaps 6-8 ft across which ran slightly diagonally across where the road is today, about 100 yards south of the canal bridge. It was identified by a slightly yellow colour to the otherwise grey middle Jurassic clay (Figure 2).

Figure 2: The gypsum crystals can be picked out by discolouration. Some have been highlighted.

Following a downpour, trucks going over this area squelched the clay into piles, and some of these perfect diamond shaped crystals literally popped out.

Many locals were quick to collect some, and the name 'Semington Diamonds' started being banded about. One elderly farmer out walking, saw what I was collecting, and claimed "they must man-made, because the edges were so sharp and clear". I pointed out that I was digging in Middle Jurassic clays, and the answer came back: "Well, it must have been Jurassic Man". How can you argue with such logic?

The site manager was not averse to us going into the area, once any vehicle movement had ceased at lunch-time on Saturdays until 7 am Monday morning. Over several months I collected hundreds of these gypsum crystals from tiny 1/8" to a few of around 5". They seemed to average at 3.5". They have been very popular with collectors at mineral fairs, being clearer and better formed than those found at the M40 road cutting in Wendlebury, in Oxfordshire.

These crystals and the fossils have apparently been seen before. An elderly neighbour told me that during the war a lone German bomber unloaded a single bomb which breached the Kennet and Avon Canal to the west of the road bridge in Semington. He says that the water which flowed out exposed both the crystals and the fossils. A somewhat violent form of hushing, don't you think?

As the summer wore on these gemmy crystals became harder to find, and the clay died out. Initially fingers through the mud was the best way to find them, followed later by a hand trowel, then a fork, spade, and finally a pick axe was the only thing which would break through the hardened clay crust.

My last visit - which was intended to be 2 hours, but in fact lasted 8 - produced a 'lump', studded with crystals.

However, by that time (October) there had been quite a substantial amount of rain and the site was ankle deep in goo to say the least. Additionally this 'lump' was 6" below the surface and then went down another 8". It was about 2.5' long and perhaps 8" wide. Water and liquid clay was oozing in as I was digging, making life more difficult - and I think it rained hard at one point, which I vaguely noticed, but it certainly added to my troubles. Unfortunately, it broke in my attempts to salvage it, but in honesty I don't think I could have lifted and carried it as a complete piece anyway. I staggered home with the smallest of the three sections, and came back with a rucksack and bike. The bike was really just wheels to transport the next two parts of the nodule back - in two journeys needless to say.

I could have simply turned a hose on these three sections to wash the mud off, as I had with all the other crystals I collected, but at that point I was too busy to do anything more for some time. I stored them on a shelving unit outside, so they were partially covered, where they dried out.

Happily, I never did find the time to wash them off. Over the past year the clay has been gradually falling off bit by bit, no doubt helped by a touch of freeze-thaw over the winter, and some lovely crystals are now being exposed (Figure 3).

Figure 3:A large lump of Jurassic clay studded with gypsum crystals.

It is also showing that this 'lumps' centre is in fact a sort of open septarian nodule. Any amount of septarian nodules were found, which on being cracked open (on occasions) displayed calcitic veins running through. This one shows those 'veins' in relief, as it were, now the dried clay is falling off.

Figure 4: A calcite vein in a concretion

Many of the nodules were extremely large, and with every one I cracked open I was hopeful of finding a well preserved ammonite inside, rather like those found in the cliffs of the Dorset and Yorkshire coasts. I have to say that I never did, nor to my knowledge, did anyone else.

The diggers actually dug up to within about 30' of the canal, and since the original ground level sloped up to the canal, this produced a 'cliff' about 20' high with some distinct bedding visible (Figure 5).

Figure 5: The distinct bedding (with close-up) left by the excavation at Semington.

In the course of this digging many fossils were uncovered, but regrettably because of the digging, it is all but impossible to say which horizon many of they actually came from. Additionally the diggers destroyed a large number of the larger fossils. However amongst the smaller fossils were a large number of gryphaea ('devils toenails') in situ, and surprisingly they tended to be buff to creamy in colour. I have only seen these grey prior to this, so perhaps these were the more natural colours. Belemnites were present - but not prolific - up to 5".

I thought that only two distinct species of ammonite had been found, which were the tightly balled, solid looking fellows - these seemed fairly rare, and the larger flatter 'normal' ammonites (Figure 6).

Figure 6: An ammonite produced from the Semington excavation.

I found two of these at 8" diameter and one at 11" diameter. I also collected many pieces of ammonites. One small section of (presumably) the outer rim found I extrapolated with a piece of string, and this creature could possibly have been 36" across (Figure 7)!

Figure 7: A possible giant ammonite!

Some of these ammonites were partially pyritised, and others infilled with calcite. One small section of one shows iridescence much like the Planorbis from the Watchet/Donniford area. Some, when lifted, were the outer spiral or possible outer two spirals, with the inner parts of the spiral soft. I cannot believe that this was soft tissue, but in fact composed of mud/clay, but it was too delicate to take away.

Now if I were to simply write down the names of these species you would think I was incredibly clever - at least those of you who are not aware that my knowledge of fossils could be written on a postage stamp in capital letters. So I photographed four of them - plan and end view of each - and Isobel Geddes put me on to Alan Bentley who had answers back within the hour. My thanks to Alan.

So I thought the best way to approach this was to put in the photographs I e-mailed, and copy and paste Alan's comments.

Specimens 1 and 4 are Proplanulites koenigi (J Sowerby 1820) from the lower to middle part of the Kellaways formation. Specimen 1 is a macroconch (female) which was entombed with its body chamber missing - this would have added a further half whorl to the complete beast, so a big one!

Specimen 2 is a very fine Harpoceras falciferum (again J Sowerby 1820) from the Upper Lias - a complete specimen with only the mouth border damaged. It is from the Barrington facies - pale cream mudstone - of the Junction Bed (now called the Beacon Limestone) and is therefore likely to have originated in the Ilminster or Glastonbury area.

Specimen 3 requires further cleaning up to be unequivocal. Unless I am way off beam it is a Vicininodoceras, or closely related coarse Liparoceratid from the top portion of the Lower Lias. I do not quite recognise the matrix unless it is the Red Band of the Dorset Coast east of Charmouth, but that would seem too late as its appearance is more Ibex Zone than Davoei. There are similar lithologies in Skye. I would need a bit more background on that one to be sure. Cheloniceras martinoides from the Cretaceous Lower Greensand looks very similar! Whatever it is, it is not one of the commoner beasts.

Specimen 4 probably a large microconch (male); most of what you see is body chamber, well preserved back as far as the penultimate suture. Both preserved in concretionary mudstone with some subsequent pyritisation. These would be typical Semington fauna.

I assure readers that these all did come from the digging for the Semington by pass. I am adding one of Alan's e-mails more or less in full:

Hi John, article received - very interesting. The 9.5 inch selenite crystal is a stonker! The yellowing of course is due to accelerated decay of pyrite in the clay, which freed up the sulphate ions to set off the gypsum reaction. Being a mineralogist you will probably know of the well-known disused Oxford Clay pit just outside Weymouth which yielded selenite and gypsum under precisely similar conditions, along with basaluminite and allophane. See also, Journal of the Russell Society Vol 6 Pt 1 1995 - 'Sulphide, sulphate, phosphate and carbonate mineral assemblages in the Oxford Clay, Cassington - Yarnton area, Oxfordshire, England'.

Once you publish you will be open to all sorts of flak, as I know, so in that context can I raise just one concern? Some of the ammonites I identified for you definitely did not originate from the Kellaways outcrop at Semington. It was not clear to me whether they were all found at Semington. If they were, you are bound to get people asking how the Liassic ones got there. The answer has to be, either in road-fill, or fill imported for the canal construction in 1740ish, or in wash ("Head") spread over the landscape from periglacial mudflows in the Ice Age. I am a bit doubtful about the latter because the source of the "Head" is north to north-east, and most of the derived ammonites in it are recognisably from that quarter, whereas your Liassic ones seem to have come from the South.

You consistently refer to a 'tightly balled, solid looking' ammonite. There is a common one of this description in the Kellaways Clay - Cadoceras sublaeve and variants - but it is not one of the ones you showed me, they range from ping-pong ball to the size and shape of a traditional bowling wood.

Best wishes, Alan

Just a couple of boxes full collected...

...and in trays ready for a show.

Some of the miscellaneous fossil finds