136 SANDSTONE BLOCKS IN GRAVEL OF SOMME. chap. viii. 



A stone hatchet of an oval form, hke tliat represented at 

 fig 9, p. 115, Avas discovered at the same time, about one foot 

 lower down, at c, in densel}- compi-essed gravel. The surface 

 of the fundamental chalk is uneven in this pit, and slojjes 

 towards the valley-plain of the Somme. In a horizontal 

 distance of twenty feet, I found a difference in vertical height 

 of seven feet. In the chalky sand, sometimes occurring in 

 interstices between the separate fragments of flint, consti- 

 tuting the coarse gravel ]!s o. 4, entire as well as broken fresh- 

 water shells are often met with. To some it may appear 

 enigmatical how such fragile objects could have escaped 

 annihilation in a river-bed, when flint tools and much gravel 

 were shoved along the bottom ; but I have seen the dredging- 

 instrument employed in the Thames, above and below London 

 Bridge, to deepen the river, and worked by steam power, 

 scoop up gravel and sand from the bottom, and then pour 

 the contents pell-mell into the boat, and still many specimens 

 of Limnea, Planorbis, Paludina, Cyclas, and other shells might 

 be taken out uninjured from the gravel. 



It will be observed that the gravel No. 4 is obliquely stra- 

 tified, and that its surface had undergone denudation before 

 the white sandj' loam, Xo. 3, was superimposed. The mate- 

 rials of the gravel at d must have been cemented or frozen 

 together into a somewhat coherent mass to allow the project- 

 ing ridge, d, to stand up five feet above the general surface, 

 the sides being in some places perpendicular. In jSTo. 3 we 

 probably behold an example of a passage from river-silt to 

 inundation-mud, or loess. In some parts of it, land shells 

 occur. 



It has been ascertained b}- 3IM. Buteux, Bavin, and other 

 observers conversant with the geology of this part of France, 

 that in none of the alluvial deposits, ancient or modern, are 

 there any fragments of rocks foreign to the basin of the 

 Somrac, — no erratics which could only be explained by sup- 



