Page:Quarterly Journal of the Geological Society of London, Volume 24.djvu/213



I. Introduction.

II. Description of the Longitudinal Section.

III. Descriptions of the Transverse Sections.

IV. Characters of the Chalk, Gravel, and Loess. 1. The Chalk. 2. The Gravel and Loess.

V. Conclusion.

exact position, character, and equivalents of the Quaternary deposits of the valley of the Somme have been frequently discussed. On the authority of certain sections and plans of Amiens and Abbeville, the correctness of which will be examined hereafter, theoretical views concerning the relative ages of different parts of the gravel and of different parts of the valley of the Somme have been promulgated by Mr. Prestwich, and repeated by Sir C. Lyell and others.

These geologists have asserted:—

First, that there were two valley-gravels of distinct age at Amiens and Abbeville, one named by them the upper and the other the lower valley-gravel;

Secondly, that the upper gravel was the older of the two; and

Thirdly, that the valley of the Somme was excavated to a depth of 40 or 50 feet since the deposition of the upper valley-gravel, and previously to the deposition of the lower valley-gravel;

Fourthly, that both gravels were fossiliferous, and contained the remains of man, or rather human implements, and bones of extinct mammalia, the lower gravels having the greater number of species of mollusca, the higher gravels containing the greater number of flint implements;