Leader Charles Jackson

This was an unusual trip in that we were basically a working party assisting the owners of the site, the charity Plantlife, to survey the Sarsen stones in the field. I had been before to the site in February 1999 with the Dorset Geological Association Group (DGAG) and members of the local Dorset Regionally Important Geological Sites Group (DIGS) to survey another field and Charles Jackson asked OUGS Wessex to help later in the year. The weather forecast was dreadful but the actual weather on the day was excellent with bright skies and sunshine.

In comparison to our efforts in the year the OUGS members were amazingly efficient. I think it made a difference having Charles as our director of operations and Alf who mentioned as an aside that he has done surveys across the desert in the Army using three poles and a compass. I my astigmatism for me having no sense of direction of clue about what I was supposed to be doing!

The reserve has been designated as a Site of Special Scientific Interest because of the botany as well as the geology. The farm has been managed using the old methods and no fertilisers. There is therefore a wealth of different plants, grasses and flowers, which is a rarity on intensively farmed land nowadays. The heifers were very benign and contented throughout our survey and agreed to move quite calmly from our centre line when I asked them nicely.

 The reserve is in an area characterised by landslippage during the last Ice /age. The underlying Fullers Earth Clay and the Oxford Clay cause slippage where the gradient are steep. The reserve is situated on an outcrop of the Kellaways Formation. This is a potential aquifer as is the underlying Cornbrash. The northern, lower end of the field is extremely boggy and ends in a steam flowing west to east. The stream cuts through the Oxford Clay and is fascinating as there are examples of all sorts of local rock such as Cornbrash and Forest Marble. Above is a diagram which outlines the older geology which is Jurassic. The Sarsen stones are much younger.

The Sarsen stones are extremely hard and resistant rocks, which are 98% or more silica or quartz. These can range from a coarse conglomerate with pebbles to a very fine grained and smooth quartzite. The origin of the Sarsen stones is thought to be the Tertiary sands, which were deposited on top of the Cretaceous chalks. There are debates as to why Sarsen stones formed as there are endless debates about the formation of cherts and flints. There is no evidence of Cretaceous or Tertiary in this area as it has all been eroded away. But because of the highly resistant nature of the Sarsen stones they remain. Sarsen stones have moved from their place of origin by periglacial action and landslides. They are still moving due to soil creep. I have observed this in my visits over a 30 year time period to a neighbouring farm just inside Somerset. Local farmers curse them as they prevent the use of heavy machinery. In an argument with a mower or a plough, the Sarsen stone hiding under a thin layer of soil will win! But of course this means the botany has been preserved.

We recognised towards the end of our survey that a modern GPS system would have helped us to do the same job in a fraction of the time. If we are able to borrow one we may return to do a comparison, as a ‘scientific’ exercise. Jo Thomas is going to do a description of the 15 Sarsen stones we surveyed as well as the 27 in the adjoining field. I have asked Charles to do a write up about the survey technicalities.

We were invited to carry out a survey of the fields in question in order to provide the biologists with an accurate map of the area which would show individual stones so that they could log accurate data concerning the rare lichens which were growing on many of the

stones. Eventually they would then be able to identify the species present and then survey the changes taking place with time, and relate these to the land use and all the other factors affecting the flora.

For our purposes therefore, a sarsen is a large stone exposed at the surface of the grassland, and we needed to prepare a plan showing the accurate position of the stones relative to the field boundaries. The method adopted was the ‘Straight Line Traverse with Offsets’, where a baseline is located on a plan of the field and then marked in on the actual ground surface as accurately as we could (We were fortunate in that Plantlife’s Reserves Officer, Joe Costley, had already given us a plan of the field boundaries, otherwise we would have had to produce one before we laid out the baseline). It was a fortunate coincidence that one of the major National Grid lines, Easting 51, shown on the I to 50 000 maps, runs north-south right through the middle of our field, so it was decided to survey that line in by measuring the distance from the field boundaries, and then to use that as our baseline. This would mean that we could measure the northings directly along our baseline and the eastings directly by measuring the offsets at right angles to the baseline, thus avoiding having to carry out any trig calculations. While one team pegged out the baseline the other carried out a field-walk to locate and mark the stones; with these preliminaries complete we could then devote ourselves to one of the major tasks of the day - LUNCH, sitting in bright warm sunshine, with hardly a breath of wind.

Once lunch was over we could get down to the accurate stuff; with each stone being given an (arbitrary) number, brief details of its size and appearance being recorded, its northing being measured directly along the baseline and its easting being given by the offset distance at right angles to the baseline. Simple!

A final word on our use of this technique: alternative: methods might have been to use a Plane Table, or even a GPS satellite receiver. However, in considering the GPS, the civilian versions are less accurate than surveying (say, worse than plus or minus five metres against plus or minus less than one metre) and we needed the greater accuracy. Also it would not be so very much quicker, since it takes time for the receiver to lock on to the satellites and more time to compute the position and then convert from latitude and longitude to grid references. There was another disadvantage which effectively decided us against UPS - we did not have a receiver....

IF at some later date we could borrow a receiver, PREFERABLY from the armed forces (since military systems are an order of magnitude more accurate than civilian ones) we could well be persuaded to go back and recheck our measurements.

That left plane-tabling to be considered. Here a drawing board (the plane table) is set up at a known point and the direction of each stone surveyed is sighted in and marked directly on the paper. At the same time another known point is sighted in on the paper, since the two points are known the distance and bearing between them is known, and can be plotted on the paper to form the baseline. The table is then set up at the other known point, correctly orientated with respect to the baseline, and the directions of all the stones from that point drawn in. Thus the only distance and compass beating needed is that of the baseline between the two known points, and all the stones can be sighted in without moving from the two known points. Ideal - except that you need a clear sight of all the stones from the two points, and the field was too full of hollows and bumps to permit this. And there is a further problem once you get more than four or five stones surveyed in; there are so many lines criss-crossing the plan that it is only too easy to mix up which lines belong to which stone, and mistakes are frequent. So we adopted the ‘traverse and offsets’ method, especially since this would give more people a chance to share in the action than the other two methods.

It only remains to thank ‘Plantlife’ for giving us the chance to exercise these skills and gain experience in carrying out a useful survey, and to thank both English Nature and the Dorset Wildlife Trust for their financial support for the project, and the volunteers who gave up their Sunday so willingly in order to increase our knowledge of the Dorset flora, and our ability to preserve it for the future.

CHARLES JACKSON