Édouard Lartet,
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(1862)
“ In a chalk country, such as that through which the Somme flows, each cubic foot of flint, gravel or sand, represents the removal of at the very least twenty cubic feet of chalk, all of which, as we have already seen, must have been removed from the present area of drainage. In considering, therefore, the formation of these upper and older gravels, we must not picture to ourselves the original valley as it now is, but must, in imagination, restore all that immense mass of chalk which has been destroyed in the formation of the lower level gravels and sands. ”

