Photographs of this field trip are in the Photo Gallery

Map Reference: 073 972

The day after the Oxford and Cambridge boat race with showery weather forecast a group of 24 were lead on an adventure by Neville Hollingworth to rise through the stratigraphic column on a visit around five quarries near the village of Ashton Keynes.

This quarry area had originally been laid down in the Middle Jurassic in a shallow subtropical sea but millions of years later braided river systems reworked the floodplains leaving gravels laid down between 350,000 to 45,000 year ago. Latton Quarry has only been operational since 1998 but is an extremely fossilliferous formation named the Upper Cornbrash. This bed revealed brachiopods, Microthyridina lagenalis and Digonella siddingtonenis, together with echinoids Nucleolites clunicularis. Deeper down this was a very sticky pit in areas!! Namely the Kellaways Clay. Here my boots got stuck and I lost my balance but worst of all I lost a prized fossil ammonite ‘Cadoceras’. However, misadventure soon turned to excitement when an interesting discovery was broadcast from one corner. We had been warned of the possibility of finding mammoth remains and one couple had been digging in the clay and had found a mammoth’s atlas bone which was huge. Nearby a tooth was also found, this looked rather segmented and measured some 18cm x 10cm. These mammoth remains would have been some 350,000 years old. It was believed that mammoths would have grazed in the area and Neville confirmed that flint implements had been found so this suggested that hunters had been around too.

The second quarry had only been evacuated some 2 months earlier and lay within feet of a tributary of the River Thames. Here the Kellaway Sand Formation is capped by calcareous sandstone and is again very fossiliferous. Numerous Gryphaea and oyster beds were found revealing good quality fossils. Geological hammers pounded away from spoil piles around the new pit and from within where the concretions revealed a wealth of ammonites including Kepplerites and Cadoceras. Most of the group were able to go away with some prized specimens from this new pit. My ammonite had been fossilised so that the internal chambers had been preserved. Lignite was also found within the Formation. This was a particularly interesting pit to visit, perhaps it was because it was new and shallow, but a wealth of fossils could be found so easily.

Although this was only a short walk away from Freeth Wood the scale of the excavation was just enormous. There were island beds with sand martins nesting in the banks for summer. This is the base of the Jurassic Kellaways Sand Formation which underlie the Pleistocene flood plain gravels. Since this quarry was so extensive most activity was centred around the spoil heaps where numerous ammonites were freed from small concretions and small ammonites could be picked from the ground as you walked.

Digging had been started in January 1999 and within the 15 months to March 2000 extensive excavations had been made. In the distance clear banks looked very similar to aolian sand dunes with slip areas, however, a closer inspection showed the fine rounded gravels of Jurassic Cotswold rocks deposited during the Ice Ages. In the drainage channels of the underlying Lower Oxford Clay of Jurassic age, there were areas with blue grey mudstone nodules which were again highly fossilliferous with the ammonite Kosmoceras jason quite abundant. Belemnites, bivalves, brachiopods and fossilised wood were amongst other finds. I found part of a large ammonite that was quite rare and would have had diameter of around 30cm (sorry I don’t have a name for this). Unfortunately, this part of the trip was cut short by a quick hail shower when some of the party decided to make a quick exit, myself included.

Rixon Gate was only yards away from Kent End with the floor of the pit only slightly higher up the Oxford Clay Formation. With time running short there was only chance of a quick glimpse of this pit, that was once you had navigated the sticky route in. Gryphaea and ammonites were to be found but time just ran out for most people.

Photographs of this field trip are in the Photo Gallery

Twenty nine people and two dogs gathered at the West Wycombe car park on a clear and sunny Sunday morning. The group was made up from OU members and some very welcome visitors from the U3A (students of Di Clements) and WEA from Beaconsfield, tutored by Diana Smith. Brian had worked his usual magic and the sun was shining, we could hardly believe our luck as the weather all week had been very unpleasant with a lot of rain and un-summery cold temperatures. We were well equipped with waterproofs but not quite the silk thermals.

Brian gave a brief introduction to the geology and history of the area and added a plea that we did not get ourselves damaged on the main road nor the railway, which we had to cross twice; I am sure that he was thinking of us and not the insurance policy.

Brian explained that our walk was to take us across Middle Chalk, Chalk Rock, Upper Chalk and its capping of Clay-with-flints, but not necessarily in that order!! We would also see some drift deposits of alluvium and dry valley deposits, and some other mysterious stones to give us something else to think about. Our task was to work out just what we were walking on and where the lithology changed, the ‘geo’ bit of our geowalk. For the sharp eyed among us we were told that we may see some Red Kites, now reintroduced into the area; these are the large red raptors with a distinctive forked tail which can be seen floating above the chalk cutting on the M40. We would also look at some of the notable local buildings.

With the geological information at our fingertips we would be able, during the day, to work out the geological history of the past 100 Million years or so. We would also be able to analyse how the geology had influenced the buildings and industry of the area. If we were good we were promised a visit to the Hell-fire caves with the opportunity for a cream tea. Who could resist such an offer?

We set off from the car park across the fields towards the main road and the railway line and stopped at the top of the slope to have a look at the general picture. There was a small outcrop of the Chalk Rock running along the line of the hedge. The Chalk in this area has a very low dip to the south giving it a horizontal outcrop. We looked down over the valley across the dry valley alluvium, there is no water now but at the end of the ice age there was a river. The chalk itself is not porous, however, due to fissuring caused by the freezing and thawing cycles of the deep permafrost of the ice age, the rock is permeable and water gathered in the valley bottom. The Anglian ice sheet did not reach this area of England, stopping short North of Aylesbury, but the climate was still cold enough for the permafrost.

Now was the time to look for evidence of the rocks that we were going to be walking across. So what were we looking for? In his introduction Brian told us that the chalk itself is a relatively soft, pure limestone made from up to 98% calcium carbonate (CaCO₃) derived mainly from the skeletons of unicellular algae (coccoliths). The Middle Chalk has very few flints, the Upper Chalk has many more, the Chalk Rock between the two is brittle and hard, it rings when struck and makes a relatively good building stone (we were to see evidence of this later). This Chalk Rock, a hard indurated band as much as 2m thick, was formed as a result of reworking of the middle chalk during a period of non-deposition. The following description of the upper chalk is taken directly from Brian’s excellent field notes which can be seen on the LOUGS website. ‘The Upper chalk is characterised by the development of Flint, a form of translucent silica derived from sponges and radiolaria. Precipitation is thought to be initiated by localised acidic conditions derived from decomposing organic matter, the silica replacing calcium carbonate which goes into solution.’ (I did write similar notes from Brian’s verbal description but his written notes are so much clearer!)

We all safely negotiated the road, and the railway, with Diana and Sue Hay watching for trains (the train drivers are supposed to sound their horn as they come around the bend but do not always do so – the train that did pass sounded his horn). The ground started rising steeply as we walked through the Beech woods in which there were signs of old coppicing. Our second stop was by an uprooted tree (conveniently placed half way up the hill – no of course we did not need to stop to get our breath – the tree was just very convenient!). We had a clear view of the soil and associated flints. We had not seen many up to this point, except those that had rolled down the hill, we had been walking on the Middle Chalk but now there was a change. We had real, in-situ flints to look at – this was evidence that we had reached the Upper Chalk. We had crossed The Chalk Rock without having seen it due to trees and scrub lower down.

The Flints were a variety of shapes and sizes, some looking very much like fossils or parts of burrow structures. Brian explained that a possible trigger for precipitation of silica was a change in the balance between pH/Eh in the environment with the focal point of a skeleton or burrow structure of calcium carbonate. In the appropriate conditions calcium carbonate goes into solution to be replaced by silica – hence fossil shaped flints. One of the party picked up a flint which looked like an echinoid – Brian was willing to be convinced, Diana was not so sure, it was very weathered with no surface features! Most of the flints were weathered with evidence of frost shattering on their surface. These were typical of flints from the Upper Chalk; we had not yet reached the capping of Clay with Flints.

Having rested and listened we went on up the hill. The ground did eventually flatten out and we all noticed a difference underfoot, it was soggy and muddy with water lying in large puddles. Our third stop was next to an old saw pit in the woods from which we were able to look back across the dry valley, the weather was still bright and clear, we could see a long way and we had walked a lot of it. There were more flints of various shapes and sizes, from small black shiny pebbles to large cobbles with shatter marks, some which had been bashed about a bit and become rounded and some which were unworn, quite a mixture. This is characteristic of the capping of Clay-with-flints. ‘Clay-with-flints describes a range of deposits resulting from the dissolution of the Chalk with the addition of remnants of Palaeogene sediments. The deposits comprise reddish brown silty and sandy clays with flint pebbles and flint nodules.’ We had reached the top of the succession in this area and with the expert help of Brian, Diana and Sue we had a clear idea about what we had been walking on and its relationship to the topography of the area. But we had not finished with the Geology yet.

On our way up we had seen some large boulders laying on the surface, with no apparent relationship to underlying strata. When we looked closely at these rocks we could see that they were obviously not chalk nor flint. They were hard, gritty sandstone, with rounded silica grains cemented with a hard siliceous cement. This rock is an orthoquartzite, a very hard sandstone with no porosity; another name for these free standing stones found scattered across the surface of the chalk downs are the sarsen stones. There is now, and has been in the past, much discussion about their origin. They are acknowledged to be younger than the chalk but how they got there and where they came from is still a subject of great debate.

We saw sarsen stones again as we stopped to look at the Parish Church of St Botolph in Bradenham along side Bradenham Manor (the home of Isaac Disraeli – father of Benjamin). It is a church of late Saxon origin; the outer walls are mainly of dressed flint, very expensive on mortar due to the irregular nature of the stone. Its windows and quoins are made from sarsen stone and limestone. The Jurassic limestone probably came from just North of the area and was relatively easy and cheap to transport, both important considerations for a small, local Parish Church. The interior of the church is undressed flint and clunch (the Chalk Rock – not used outside because it does not weather very well). Another famous place to see sarsen stones used for building is in Stonehenge in Wiltshire.

From the church it was a short walk to the Village green and onto the pub for a well-earned rest – and still no rain!!!

We resumed our discussion about sarsen stones as we walked along side the road. There were a variety of rocks with root holes – this suggests that cementing took place at the surface when the roots were in place and could not be marine (remember this bit of information for later!) There was one large sarsen stone on the green with a millennium memorial plaque. The plaque suggests that the rock was ice age because there was another rock nearby with ‘striations’ on it suggestive of movement by a glacier. According to BGS glaciers did not come further south than St Albans. Brian’s suggestion is that the striations were caused by countless sharpenings of countless tools belonging farm workers as they went to and from the fields, this was no more than a convenient sharpening stone. There have been no other reports of striations on sarsen stones. So who is right?

One of the problems about deciding the origin of sarsen stones is that nobody has seen them in-situ. Look at the ideas and make your own suggestions.

Two suggestions put forward are that a) they could be formed on an erosion surface when the rainfall, pH and temperature were exactly right and could be very late deposits b) they were once a continuous sheet which fell down and broke up as the rocks beneath them eroded away. This is the art and the science of Geology – we can only use the information that we have and come to reasonable conclusions.

Our last but one visit was to the Church of St Lawrence, which unfortunately was closed, and the Dashwood family Mausoleum. The church was originally a Norman church within a 5th century Iron Age fort. In 1751 the church was remodelled for Sir Francis Dashwood with the tower capped with a golden ball which can seat 6 people. The interior of the church is copied from the Sun Temple at Palmyra near Damascus. The Mausoleum near the church was built in 1756 and is a fine example of construction in dressed flint and Portland Stone. Those are Brian’s words from the field notes, he added a few more gems before we moved on, St Lawrence was the patron saint of prostitutes, the golden ball could be seen from a golden ball in Camberley (does anybody know of this? I have, so far, not been able to find records). The Mausoleum is a bit of a cheat being concrete faced with flints with concrete quoins standing on a base of Portland Stone. The Mausoleum is still being used. It is a fine construction to look at, you can see the golden ball for miles around and it is even more impressive close too. I recommend a visit, access is quite straightforward and you may even be lucky enough to find the church open and be able to climb into the golden ball.

We could now turn our attention to the other bit of the walk – the history and industry of the area and the influence of the Geology. Remember the coppicing in the beech wood and the sawpits in the wood; these are the remains of an old industry which is still important in the area - the making of chairs. Timber grew well on the tops of the hills where the Clay with Flints made a relatively fertile soil. The wood was useful as a fuel and could produce enough wood to fuel London. The wood was also used for the making of Windsor chairs – an early incidence of the mass production of furniture. The construction of the Windsor chair is such that it does not need to be made by a skilled craftsman. The backs and legs were made from thin trunks on a simple pole lathe and then holes were cut and the pieces stuck together. This new breed of chair makers of the Windsor chairs were called ‘bodgers’ – hence the term to bodge something together. Wycombe became an important area in the furniture industry and these woods were probably worked up until the time between the two wars. The importance of the industry has been traditionally celebrated by constructing arches of chairs for special occasions. One of the last arches of chairs was constructed for Jubilee of Queen Victoria in 1877. You can see more information about these arches in the Wycombe museum and on their website at www.wycombe.gov.uk/museum/ - well worth a visit. One of the newest was constructed for the Millennium and was on view until 31st May 2000. You can see pictures of this one on the LOUGS website (www.lougs.freeserve.co.uk).

The final part of our walk was a gentle stroll with only a little bit of a hill back to the caves. These caves were excavated from the chalk rock between 1748 and 1752 during a project initiated by Sir Francis Dashwood to give employment to his farm workers after a succession of harvest failures. The rock was used to build the foundations of the A40 between West Wycombe and High Wycombe. The real claim to fame of these caves is their other use as a meeting place for the Hell fire club. It is said that the club held orgiastic meetings in the depths of the caves; quite a few famous people were known to have been involved, the locals were very suspicious! The facade of the caves is built of brick to look like a ruin of a church. We had been good so we were allowed to buy ourselves a token and descend into the labyrinth. It is quite a maze of damp, dark, tunnels, some with quite low ceilings and only just enough room to pass side by side. There are also some large rooms in which the meetings were said to have taken place. These caves are well worth a real visit to see some of the quaint statues and displays or a virtual visit on the Internet at www.blather.net/archives3/issue3no1photos.html.

This was the end of our day – and still it had not rained! We had our cream tea, and said our thanks to Brian for what had been a really interesting walk. There was just the right amount of interesting Geology on a not too taxing walk in a pleasant country setting, almost without the sound of traffic. We all enjoyed ourselves, even the dogs who had been very well behaved. The final question – did we see any Red Kites – well we think so – perhaps just a glimpse of a forked tail disappearing behind a tree – the eye of faith works with birds just as easily as it does for fossils. Many thanks Brian for a very enjoyable day.