beds of sand and gravel. In the course of time, as the river
gradually lowers its channel, it will leave behind, at varying
heights along its banks, scattered patches of such beds.
Wherever we find them, no matter how far removed, or how high
above the present river, we are sure that at some time the river
flowed at that height; and standing there, we may try and
imagine how different the country must have looked before the
present deep valley was eroded.

In the case of the river Somme, we have a wide and deep valley,
a large part of which has been excavated in chalk rock, through
which the river now winds its way in a sinuous course to the
English Channel. Yet we feel sure that at some time in the past
it was a mighty stream, and that its waters surged along over a
bed at least two hundred feet higher than now. In proof of this
fact we still find, at different places along the chalky bluff,
stretches of old gravel banks, laid down there by the river,
"reaching sometimes as high as two hundred feet above the
present water level, although their usual elevation does not
exceed forty feet."<32>

The history of the investigation of the ancient gravel beds of
the Somme is briefly this: More than one instance had been noted
of the finding of flint implements, apparently the work of men,
in association with bones of various animals, such as hyenas,
mammoths, musk-sheep, and others, which, as we have just seen,
lived in Europe during the Glacial Age. In a number of cases
such finds had been made in caves. But for a long time no one
attributed any especial value to these discoveries, and various
were the explanations given to account for such commingling.