In these beds are the records of two drift periods, marked by 2 and 3. The two are separated by a layer of fresh-water deposits, which contains river shells and is sometimes as much as sixteen feet thick. The lower, or gray diluvium, (No. 2), marks the glacial epoch, as distinct from the glaciers of the reindeer epoch. In the lower gravel, lying immediately upon the tertiary formation, were found the flint hatchets, together with the bones of the mammoth and fossil rhinoceros. In order to understand the deposits still more clearly, the following figure is given. [Illustration: FIG. 4. SECTION OF A GRAVEL-PIT AT ST. ACHEUL. 1. Vegetable and made soil from two to three feet thick. 2. Brown loam from four to five feet thick, containing a few angular flints. 3. Bed of sandy marl from five to six feet thick, with land and fresh-water shells, covered with a thin layer of angular gravel from one to two feet thick. 4. A bed of partially rounded gravel containing well-rolled tertiary pebbles. In this bed the flint implements are chiefly found--ten to fourteen feet thick. 5. Formation of chalk. _a._ Part of elephant's molar, eleven feet from surface. _b._ Entire molar of mammoth (_E primigenius_), seventeen feet from surface. _c._ Position of flint hatchet, eighteen feet from surface. _d._ Gravel projecting five feet.] At St. Acheul, in bed No. 4, were found large numbers of flint implements. Some of them have the shape of a spear-head, and are over seven inches in length. The oval-shaped hatchets are so rude in some instances as to require a practised eye to decide their human origin. In the same bed are found small round bodies having a tubular cavity in the centre. Dr. Rigollot has suggested that these perforated stones or gravel were used as ornaments, possibly strung together as beads. In this bed, No. 4, seventeen feet from the surface, was found a mammoth's tooth. About one foot below the tooth, in densely compressed gravel, was found a stone hatchet of an oval form. [Illustration: FIG. 5. FLINT IMPLEMENT FROM ST. ACHEUL. Half the size of the original, which is seven and a half inches long. _a._ Side view. _b._ Same seen edgewise. "These spear-headed implements have been found in greater number, proportionally to the oval ones, in the upper level gravel at St. Acheul, than in any of the lower gravels in the valley of the Somme. In these last, the oval form predominates, especially at Abbeville."--_Antiquity of Man_, p. 114.]