get washed away by
streams and rivers into new seas, and gave rise to new sediments on the
floor of these seas. And so the rock particles have for untold ages
been going this perpetual round: they become soil; they are carried
away by the rivers, in time they reach the sea; they lie at the bottom
of the sea while the sediment gradually piles up: then the sea becomes
dry land and the sediments are pressed into rocks again. The eating
away of the land by water is still going on: it is estimated that the
whole of the Thames valley is being lowered at the rate of about one
inch in eight hundred years. This seems very slow, but eight hundred
years is only a short time in geology, the science that deals with
these changes.

[Illustration: 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 {125} 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