Up on the Down

we're on to high chalk-land. This is the Western edge of the Marlborough Downs.

There may be some confusion about these things called "Downs", when they are, unmistakably, up. Well, the origin of the word is the Anglo-Saxon dun - meaning hill. Lots of duns make Downs.

The landscape is green - but with fairly uniform grassland. There aren't many trees. Here and there, the blinding white rock peeps through - mostly because of some human incision into the sod. This invokes an odd feeling; these hills have a monumental solidity - apparently the same seam of white rock going on for miles - but at the same time, this whiteness suggests some airy, insubstantial substance. It's as if we're on some gigantic film set, the moody backdrop fashioned out of polystyrene.

In fact, this chalk is relatively recent - in geological terms - no older than 65-100 million years. It's formed from the calcareous bits of tiny marine organisms - corals and the like - collected over many millennia at the bottom of a tropical ocean. We are talking about a lot of marine organisms; around here, the chalk is over 700 feet thick.

There are very few impurities in this calcium carbonate - just a few particles of clay. There are three fairly distinct layers laid down. Each has a slightly different percentage of clay.

In the bottom layer, clay makes up 10-15% of the volume. That doesn't sound much, but the fine clay particles clog up the tiny gaps in the chalk - making it fairly impervious to water. In this bottom layer, the clay has settled (mostly) to the bottom. So, any water which does seep through, gathers there. Where this rock opens out to the surface (at the edge of the Downs), springs flow out.

The second and third layers have decreasing quantities of clay in them. This is soft, friable rock, but is surprisingly long-lasting. The action of water doesn't erode it, because the water flows right through. Admittedly, some of the calcium carbonate dissolves in the water - which makes for particularly "hard" water in the homes served from these hills.

Again, rain water falling here percolates down through the porous rock, until it meets the less-porous layer - or until it meets a body of water which has already collected in the rock. When this happens, great reservoirs of water may be contained.

Of course, if the underlying (waterproof) layer has a tilt to it, this water will continue to flow, down and along - an underground river, flowing through solid rock. In some cases, such rivers will stay underground until the chalk peters out, and a spring emerges. But, sometimes, the surface of the chalk has weathered away, until the flowing water is exposed. Then, a common-or-garden river flows in the open air. Sometimes.

The River Kennet is one such river. It rises somewhere a little to the West of this road - the precise spot is difficult to identify, as we shall see. In the winter, when these hills are full of water (i.e. the "water table" is high), the River Kennet flows along an easily identifiable course, Southwards from near Wroughton.

But in summer, the water table goes down until it no longer reaches the surface, and the River Kennet disappears. It hasn't dried up, exactly; it still flows underneath the surface. The course of the river is still easily identifiable by the lush growth which burgeons there. So summertime at Winterbourne Monckton (and for another fifteen miles - down almost to Marlborough) finds a verdant channel, while winter reveals a healthy stream beneath pretty bridges.

A Winterbourne is a spring (a "bourne") which only flows in winter.

Reader Pete Glastonbury has managed to capture the first flow of surface water of the River Kennet, and has put an animation Youtube. Excellent work.
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