Blewbury is one of several ancient settlements sited around natural springs that rise at the foot of the Berkshire Downs section of the North Wessex Downs. Filtered through many layers of chalk, the water from these springs has in the past provided a steady living from mills and cress beds and today the Mill Brook, the Cleve (a large pond) and a number of small, privately owned lakes still form the main geographic features of the village. From here and elsewhere, tributaries feed the Mill Brook which carries the water to the river Thames at Wallingford. The A417 road runs along below the escarpment, above the springs and through the south of the village.
Blewbury can trace its geological history back 350 million years. However, most of the visible features of the village and its surrounding area are the product of weathering and erosion since the last glacial period, some 12,000 years ago. There is evidence that people have been living here for about 4,000 years, supported by the local springs and streams. The downs above the village are made of porous chalk rock and so there are no ponds or streams. This geology has had a very significant impact upon the landscape and the siting of the village. Rainfall falling on the chalk percolates slowly down to the water table, dissolving carbonate minerals, to create the chalk aquifer which is the main source of the local drinking water. The chalk groundwater is hard water of excellent quality, naturally rich in calcium. The long residence time of the water in the chalk provides some protection against surface contaminants and the water requires little treatment. The springs originate from this rainfall on the downs and their flows can be influenced by the water table as far away as 3 miles.
When a hole is dug in permeable rocks, at a particular depth water begins to flow in. The surface of the water that accumulates in the hole is the water table and the water in the ground below the water table is groundwater. The variations in the shape of the water table reflect the topography in a subdued form. The water table is near the ground in valleys, actually intersecting the ground surface where rivers, lakes and marshes occur, but it is at much greater depths below hills. The pore spaces of rocks are saturated with water below the water table and groundwater is said to occur in the saturated zone. Immediately above the water table, water is drawn up into pore spaces by capillary forces into a thin zone called the capillary fringe. Rocks above the water table, including the capillary fringe, form the unsaturated zone; although they do contain water, they are generally not completely saturated and the water cannot be abstracted.
Groundwater comes from rain. The average annual rainfall over the UK is about 1100 millimetres, ranging from more than 2500 millimetres over highland Britain to less than 600 millimetres on the lowlands of eastern England. A significant part, almost 500 millimetres in lowland areas, evaporates, mainly in the summer. The remainder is available to infiltrate permeable rocks although where the rocks have low permeability or where they are overlain by layers of relatively impermeable clay, part will flow over the ground as surface runoff. Water infiltrates the ground mainly in the winter and slowly moves down through the unsaturated zone, eventually reaching the water table and becoming groundwater. After temporary storage in the ground, groundwater drains from springs and seepages into streams and rivers. Maximum discharges occur at the end of the winter when groundwater levels are high following the seasonal infiltration. They steadily decline throughout the summer into the autumn. The contribution that groundwater makes to the flow of rivers is called base flow and it is responsible for maintaining the flow of rivers during extended periods of dry weather, when surface runoff virtually ceases.
The direction of groundwater flow follows a curved path through an aquifer from areas of high water levels to areas where water levels are low; that is from below high ground, which are recharge areas, to groundwater discharge points in valleys or the sea.
An aquifer that crops out (i.e. is exposed at the surface) is said to be unconfined. Because of earth movements in the past, many aquifers dip below younger strata of impermeable clay. As the thickness of the clay increases, the aquifer becomes saturated throughout its entire thickness and the pressure of the water it contains also increases. The water rises above the top of the aquifer and may overflow at the surface from a borehole that penetrates into the aquifer; it is said to be under artesian pressure. Such aquifers are called confined aquifers and examples have been used as earthcaches elsewhere.
Water flows through confined aquifers to discharge points some distance down-gradient, at a spring or possibly offshore into the sea. Isolated oases in deserts exist because groundwater is issuing from a confined aquifer which may have locally intersected the ground surface, or where the water is rising up through a fracture in the overlying, confining rocks. Where such outlets do not exist, the water discharges from confined aquifers by slow upward seepage through the overlying clays. The velocity of flow under confined conditions is much slower than that in unconfined aquifers.
When groundwater is pumped from a borehole, the water level is lowered in the surrounding area. An hydraulic gradient is created in the aquifer which allows water to flow towards the borehole. The difference between the original water level and the pumping level is the drawdown, which is equivalent to the head of water necessary to produce a flow through the aquifer to the borehole - the greater the yield from the borehole, the greater the drawdown.
The beauty of this site is that the geology that leads to the presence of this aquifer and its abundance of clean water is plainly visible from the posted coordinates. The type of rock and the shape of the landscape have led directly to the village being formed just here, instead of a few miles away.
From the 1950's until 2007 Blewbury obtained its water from boreholes located at Lids Bottom, just south of the village and close to the site of this earthcache, derived from aquifers in the chalk and Upper Greensand. Drinking water is now supplied from deep boreholes at Gatehampton, near Goring, still taking water from the chalk but blended with water from the Thames. These boreholes are among the most productive in Britain and supply other towns, both in Oxfordshire and as far away as Swindon.
A common, modern day assumption in regard to the water supply of a village like Blewbury is that each house had a well. In fact, this is not the case. An 1875 map of the village shows the houses and locations of wells and the existence of six pumps.The pumps are of real interest as they would have been used to draw drinking water from underground sources and the houses that had them were those of some importance within the village.
(Acknowledgements: pictures from the UK Groundwater Forum, http://www.groundwateruk.org.)
The footpath between the two reference points passes through an area of Access Land called Lids Bottom. The site is a County Wildlife Site, of particular importance for its lowland chalk downland and associated flora. The land is farmed to encourage this diversity so please control your dog and do not leave litter. Dogs are to be kept on a lead when livestock are grazing and during the bird nesting season (from April to August). Please ensure that you close gates securely behind you and follow the Countryside Code at all times.
In order to log this cache, send me a message or e-mail with your answers to the following questions (do not put the answers in your log):
1) visit reference points 1 and 2 and, by use of your GPS device or other means, measure and tell me the difference in elevation between the two points (I recommend walking from south to north as the views are outstanding).
2) look at the shape of the slope and tell me whether this is the scarp or dip slope of the Downs, justifying your answer with the evidence you can see.
3) at the location of the former borehole, was the aquifer confined or unconfined?
4) at reference point 2, look at the ground near the gateway and in the fields, where there are many scattered pieces of rock. What type of rock is this? Would this rock confine the aquifer and is this answer consistent with your answer to Q3? If not, why not?
5) at reference point 1, you will find a large, round gatepost with a metal letter nailed to it. just to make sure you are in the correct location, tell me which letter this is.
6) take a photograph of the view looking north from reference point 2 and post with your log (optional).