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 {126} 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.

[Illustration: 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]

[Illustration: Fig. 58. Ford and Coldharbour, near Harpenden]

{128}

APPENDIX

The teacher is advised to procure, both for his own information and in
order to read passages to the scholars:

Gilbert White, _Natural History of Selborne_.
Charles Darwin, _Earthworms and Vegetable Mould_ (Murray).
A. D. Hall, _The Soil_ (Murray).

Mr Hugh Richardson has supplied me with the following list of
questions, through many of wh