136 Notes on the Geological Position of the Human
account of the nature and formation of alluvial deposits will
probably form a useful preface to my other remarks.
If we glance at the map of the Geological Survey, showing
the valley of the Thames between London and Gravesend, we
see that the Chalk and Tertiary rocks of Kent are separated
from those of Essex by a broad band of alluvium or marsh-
land, which occupies the ground between the bends of the
river. And outside this tract of marsh-land, but still in the
neighbourhood of the Thames, we see less continuous patches
(of other colours) of gravel and brick-earth, which rest
indifferently on Chalk, Thanet Sand, Woolwich Beds, or
Loudon Clay, and whose nature, distribution, and fossil
contents all testify to their formation by the river at former
periods of its existence. These last-named beds are all,
however, to be found at various heights above the level of the
marsh-laud, though the old river-gravel also underlies the
marsh alluvium. When a tract of land emerges from the
sea, the rain falling upon it tends to form brooks and rivers
where slight hollows exist. These hollows gradually become
deeper and deeper, from the erosive action of the stream and
its tributaries, and should a slow continuous upheaval of the
land accompany the deepening of the river-valley by the.
stream, so as to preserve its angle of slope towards the sea,
a river-valley as deep as that of the Grand Canon of the
Colorado may in time be formed. On the other hand, should
a subsidence of the area take place, the river, having its fall
lessened, soon no longer tends to deepen its valley, but to
meander in bold curves and to widen it, forming, as it does
so, broad alluvial flats with the material brought down in
suspension. And, apart from any subsidence, the natural
effect of the deepening of a river-valley by the action of rain
and rivers is to bring about, sooner or later, a state of things
in which the river can no longer deepen its bed, though it can
shift it laterally. Then, as the current impinges on and
erodes the bank on one side, deposition of the sand and gravel
brought down in the channel of the stream takes place on the
opposite shore, where the motion of the water is slight. And
in times of flood the surface of the sand and gravel thus