As to the second, the Thames presents a somewhat rare character.

The upper part of the river, which is in lowland valleys the most
easily inhabited, and the part in which, once the river is navigable,
will be found the largest number of small settlements, is in the case
of the Thames the most marshy. From its source to beyond Cricklade the
river runs entirely over clay; thenceforward the valley is a flat mass
of alluvium, in which the stream swings from one side to the other,
and even where it touches higher soil, touches nothing better than the
continuation of this clay. In spite, therefore, of the shallowness and
narrowness of the upper river there always existed this impediment
which an insecure soil would present to the formation of any
considerable settlements. The loneliness of the stretch below
Kelmscott is due to an original difficulty of this kind, and the one
considerable settlement upon the upper river at Lechlade stands upon
the only place where firm ground approaches the bank of the river.

This formation endures well below Oxford until one reaches the gap at
Sandford, where the stream passes between two beds of gravel which
very nearly approach either bank.

Above this point the Thames is everywhere, upon one side or the other,
guarded by flat river meadows, which must in early times have been
morass; and nowhere were these more difficult of passage than in the
last network of streams between Witham Hill and Sandford, to the west
of the gravel bank upon which Oxford is built.

Below Sandford, and on all the way to London Bridge, the character of
the river in this respect changes. You have everywhere gravel or
flinty chalk, with but a narrow bed of alluvial soil, upon either bank