1864] 



of the Valley of the Somme, 



137 



trict, in a slightly inclined and continuous plane rising on either side under 

 the adjacent hills with a slope varying from 10 to 40 feet in the mile, the 

 latter being an extreme case. If we take a mean of 20 feet, as the gravel- 

 pit is not above one-third of a mile from the valley, the rise in the water 

 underneath would not probably exceed 10 feet above the level of the stream. 

 The chalk formation is so generally fissured and permeable that I know of 

 no instance of a hue of water-level or of springs occurring above the ge- 

 neral line dependent upon the level of the adjacent rivers. It is also well 

 known that strong springs are common at the foot of the hills along many 

 of our chalk valleys, as, for instance, that at Amwell, those at Carshalton, 

 and many along the valley of the Thames. These springs are more or less 

 calcareous, often highly so. 



It is evident that the travertin at Drucat has been formed by a deposit 

 from a spring of considerable volume ; and it further appears that it flowed 

 while the loess was in the course of formation. For the tufa could only 

 have been formed at or near the level of the spring ; so that its continued 

 deposit down the slope of the hill shows the spring to have been gradually 

 lowered as the valley became deeper, and while subject to the continued 

 inundations which deposited the loess. The line of present water-level in 

 the chalk here is about 90 feet below the summit of the hill, as proved by 

 a well in an adjacent farmhouse, and at the gravel-pit they have gone down 

 CO feet without reaching water. But the level of the upper part of the 

 tufa shows the line of water-level or of springs to have been at one time 

 70 feet above the valley, which could only have happened when the bottom 

 of the valley was on a level 60 to 70 feet higher than it now is. The gradual 

 deepening of the valley is indicated by the gradual lowering of the spring 

 until it reached to within from 20 to 30 feet of the present valley-level, when 

 it became extinct. Further, we have in the adjacent bed of high-level gravel 

 evidence of the origin of this important spring ; for the sands and gravel- 

 beds are not only very thick, but they are also perfectly free from calca- 

 reous matter and very permeable, and they show in their numerous gravel- 

 pipes how great must have been the volume and solvent power of the rain- 

 water which at one time percolated through them. The water, after pass- 

 ing through the gravel and acting upon the underlying chalk to form these 

 large vertical cavities, would, upon reaching the original line of water-level, 

 have flowed off horizontally and escaped in a strong spring at the base of 

 the adjacent slope. It there parted with its excess of the carbonate of lime, 

 and so formed the calcareous tufa. This case furnishes therefore new and 

 good evidence on two points : — first, on the connexion of the sand- and 

 gravel-pipes with the percolation of fresh water through calcareous rocks ; 

 and secondly, on the condition of the former land surface and of the springs, 

 only possible on the hypothesis of former higher levels of the bottom of the 

 valley and of its gradual excavation. 



VOL. XIII. 



M 



