Fig. 56. The winding river Stour. The river winds from the right to the left of the picture, then back again, and then once more to the left, passing under the white bridge and in front of the barn.
Water does more than merely push the rock particles along. It dissolves some of them, and in this way helps to break up the rock. Spring water always contains dissolved matter, derived from the rocks, some of which comes out as "fur" in the kettles when the water is boiled.
Rocks are also broken up by other agents. There is nearly always some lichen living on the rock, and if you peel it off you can see that it has eaten away some of the rock. When the lichen dies it may change into food for other plants.
We have learnt these things about soil formation. First of all the rocks break up into fragments through the splitting action of freezing water, the dissolving action of liquid water, and other causes. This process goes on till the fragments are very small like soil particles. Then plants begin to grow, and as they die and decay they give rise to the black humus that we have seen is so valuable a part of the soil (p. 51). This is how very many of our soils have been made. But the action of water does not stop at breaking the rock up into soil; it goes further and carries the particles away to the lower parts of the river bed, or to the estuary, to form a delta, and mud flats that may be reclaimed, like Romney Marsh in England and many parts of Holland have been. Many of our present soils have been formed in this way. Finally the particles may be carried right away to sea and spread out on the bottom to lie there for many ages, but they may become dry land again and once more be soil.
One thing more we learnt from the river Stour. Why did it flow quickly at the bridge and slowly elsewhere? We knew that the soil round the bridge was gravelly, whilst up and down the stream it was clayey. The river had not been able to make so wide or so deep a bed through the gravel as it had through the clay, and it could therefore be forded here. We knew also that there was a gravel pit at the next village on the river, where also there was a bridge and had been a ford, and so we were able to make a rough map like Fig. 57, showing that fords had occurred at the gravel patches, but not at the clay places. Now it was obvious that an inn, a blacksmith's forge, and a few shops and cottages would soon spring up round the ford, especially as the gravel patch was better to live on than the clay round about, and so we readily understood why our village had been built where it was and not a mile up or down the stream. Almost any river will show the same things: on the Lea near Harpenden we found the river flowed quickly at the ford (Fig. 58), where there was a hard, stony bottom and no mud: whilst above and below the ford the bottom was muddy and the stream flowed more slowly. At the ford there is as usual a small village. The Thames furnishes other examples: below Oxford there are numerous rocky or gravelly patches where fords were possible, and where villages therefore grew up. Above Oxford, however, the possibilities of fording were fewer, because the soil is clay and there is less rock; the roads and therefore the villages grew up away from the river.
Fig 57. Sketch map showing why Godmersham and Wye arose where they did on the Stour. At A, the gravel patch, the river has a hard bed and can be forded. A village therefore grew up here. At B, the clay part, the river has a soft bed and cannot be forded. The land is wet in winter, and the banks of the stream may be washed away. It is therefore not a good site for a village
