paddling. Before the bridge was built
there had been {122} a ford here. But further away, either up or down,
the stream was deeper and wider, flowed more slowly, had a muddy
bottom, and so was not good for paddling. At one place about a mile
away some one had widened out the river to form a lake, but this made
the stream flow so slowly (as it was now so much wider) that the silt
and clay deposited and the lake became silted up, i.e. it became so
shallow that it was little more than a lake of mud. The same facts
were brought out at the bend of the river. On its convex side, Fig.
55, the water has rather further to go in getting round the bend than
on its concave side _B_, it therefore flows more quickly, and carries
away the soil of the bank and mud from the bottom. But on its concave
aide where it flows more slowly it deposits material. There is at the
bend a marked difference in depth at the two sides. On its convex side
the stream is rapid and deep, and scours away the bank; on its concave
side it is slower, shallower, and tends to become silted up. Thus the
bend becomes more and more pronounced unless the bank round _A_ is
protected (the other bank of course needs no protection) and the whole
river winds about just as you see in Fig. 56, and is perpetually
changing its course, carrying away material from one place, mixing it
up with material washed from somewhere else, and then deposits it at a
bend or in a pool where it first becomes a mud flat and then dry land.
Some, however, is carried out to sea. We need not follow the Stour to
the sea; reference to an atlas will show what happens to other rivers.
Some of the clay and silt they carry down is deposited at their mouths,
and becomes a bar, gives rise to shoals and banks, or forms a delta.
The rest is carried away and deposited on the floor of the sea. {124}
Material washed away by the sea from the coast is either deposited on
other parts of the coast, or is carried out and laid on the floor of
the sea. Thus a thick deposit is accumulating, and if the sea were to
become dry this deposit would be soil. This has actually happened in
past ages. The land we live on, now dry land, has had a most wonderful
history; it has more than once lain at the bottom of the sea and has
been covered with a thick layer of sediment carried from other places.
Then the sea became dry land and the sediment became pressed into rock,
which formed new soil, but it at once began to