The Blue PoolThe complex of artesian springs which includes the 'Blue Pool' between Stanford Dingley and Bradfield must be one of the most fascinating natural features to have delighted children over generations. I must have first seen it probably back in the 1960's when my parents first took me there. To my tender young eyes it was a captivating sight, a very deep crystal clear pool of radiant blue which bubbled away at the bottom - a magical sight.

It was therefore with great anticipation that I went with my own children to visit it in the summer of 2006. It was a great disappointment to me unfortunately, that contractors of the current owners, despite an amicable request to so, refused access. I believe the last time I remember visiting the pool was sometime back in the 1980's, though I cannot be precise. At the time it was still operating as a functioning watercress farm and access was still allowed. I understand that due to pollution problems that the production of watercress was no longer allowed by the appropriate authorities due to stricter regulations.

I also seem to remember, though again I cannot be precise, that entry used to be charged at 50 pence with which you received a 3 page pamphlet explaining about the 'Blue Pool' by a R. Jessop. Luckily I managed to keep this over the years and the text is hereby reproduced for posterity together with the original handdrawn diagrams from the pamphlet.

The artesian springs, known as the Kimberhead Springs and Blue Pool must be one of the oldest features of the Pang valley. Although they have been used by man for a considerable period of time and modified or adapted for his use their origin is far older than man's occupation of the region.

To understand their origin a simple background to the geology and structure of the region is necessary. Old, hard rocks which form the mountains of Wales and the regions of Devon and Cornwall, are covered in Berkshire and eastern England by newer rocks. The most important from our point of view are the clays beneath the chalk, the chalk itself, and the clays of the Reading Beds and London Clay which overlie the chalk

Diagram 1
The chalk, deposits commenced about 130 million years ago in a sea in which little silt or mud was being deposited, accumulated to probably one thousand feet thick. When the area emerged from the sea a long period of erosion set in during which about 400 feet of chalk were removed.

Diagram 2
These rocks were then warped into a shallow trough and the sea once more covered part of the land approximately from the North Sea and London in the east to Savernake Forest and the Marlborough area in the west. Rivers such as the predecessor of the River Kennet and River Thames filled much of the shallow sea in the trough with sands and clays, beds we now know as the Reading Beds and London Clay. A complicated period of the development of the modern rivers including the River Pang was accompanied by erosion. Our present day river valleys owe most of their shape to erosion during and since the beginning of the last glacial period about 2½ million years ago. No glaciers covered the Downs or Pang valley but certainly snowcaps would have been present with periods of heavy melting alternating with semi-permanent snow cover, rather like the tundra conditions of northern Finland, Russia or Canada. During much of this time even the chalk would have been impervious, that is water would not run through it.


Today, as it is no longer frozen, water is absorbed by the chalk like a sponge or percolates through joints in it dissolving some of the chalk as it does so. Chalk is in fact almost insoluble in pure water, but rain water contains dissolved carbon dioxide from the air and becomes a weak carbonic acid which changes the natural chalk from insoluble calcium carbonate into soluble calcium bi-carbonate which is then washed away in solution. Joints in the chalk are enlarged in this way. We may all have seen hollows, in fields on the chalk which are shallow depressions with well marked joints below resulting from solution into which surface water has disappeared enlarging the hollow in turn over a long period of time

Diagram 4

These features are shown on the diagrams which follow with brief explanatory notes. The high "water table", the level below the surface at which the rocks are saturated, at the end of the glacial melt period when the snowcaps disappeared became steadily lower. Some of the melt streams cut deep valleys which have since become dry, such as ones tributary to the Pang valley, as the water table was steadily lowered.

In the main valley however conditions were ideal for artesian springs to develop, springs that come to the surface from a great depth in the 600' of the chalk below the Blue Pool, entirely under their own pressure. They depend on the layers of clay below and above the chalk and the way in which chalk holds water like a sponge so that the downward pressure of water from the higher parts of the water table over the Downs forces water out of fissures in the valley as in the Blue Pool and Kimberhead Springs.

Diagram 5a

Again this is explained in the diagrams below. Constant bubbling out of spring water and the greater capacity of cool water to hold dissolved carbon dioxide which would actively dissolve chalk must be important features in explaining the formation and depth of the Blue Pool. The blue colour is almost certainly derived from the fine particles of a mineral called glauconite which is blue-green in colour and transmits or reflects light reaching it in the fine sands on the bed of the pool. Chalk in common with other rocks of marine origin contains particles of glauconite which would be insoluble and left on the bed of the pool when soluble elements of the chalk were removed in the water flowing away.

Diagram 5b

How far has man interfered with the natural form of the pool must be a matter of speculation. Saxon settlements in the Pang valley sought sites offering water supply, meadowland, a mill where the stream ran quickly enough, well drained slopes for arable crops and woodland above for pasturage of pigs, a valuable asset; fuel and timber as well as game. The Domesday book records Stanford Dingley as having eight villagers and two small holders with 3 ploughs and a mill worth 12 shillings (taxable value) and a fishery. But from the eighteenth century at least water meadows became a valuable asset and springs such as the Kimberhead springs and water of the River Pang controlled by sluice gates gave early pasturage and several hay crops per year.

Enlargement of the Blue Pool may well have taken place to give greater water supplies or for trout breeding. Early records point to fish breeding as important in the county. Certainly one or two pipes were sunk to improve water supplies for water cress beds in the last hundred years, at least two being visible now and giving exceptionally free flowing springs. The shapes of the deeper sections of the Blue Pool, forming separate basins round particularly vigorous springs where the glauconitic sands can be seen thrown up from the bed as the water bubbles vigorously out, must owe a great deal of their shape and origin to the natural action of the artesian springs as outlined above.

R. Jessop
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