130 RELATIVE AGES OF HIGH AND LOW GRAVELS. CHAP. Till, 



posits should resemble, in geological position, the Menche- 

 coui't beds, they must be raised ten or fifteen feet above their 

 pi'esent level, and be partially eroded. Such erosion they 

 would not fail to suffer duinng the process of upheaval, because 

 the Thames would scour out its bed, and not alter its position 

 relatively to the sea, while the land was gradually rising. 



Before the canal was made at Abbeville, the tide was per- 

 ceptible in the Somme for some distance above that city. It 

 would only require, therefore, a slight subsidence to allow the 

 salt water to reach Menchecourt, as it did in the post-pliocene 

 period. As a stratum containing- exclusively land and fresh- 

 Avater shells usually underlies the fluvio-marine sands at 

 Menchecourt, it seems that the river first prevailed there, after 

 M^hich the land subsided ; and then there was an upheaval 

 which raised the country to a greater height than that at 

 which it now stands, after which there was a second sinking, 

 indicated by the position of the peat, as already explained 

 (p. 111). All these changes happened since man first in- 

 habited this region. 



At several places in the environs of Abbeville there are 

 fluviatile deposits at a higher level by fifty feet than those 

 of Menchecourt, resting in like manner on the chalk. One 

 of these occurs in the suburbs of the city at Moulin Quignon, 

 one hundred foet above the Somme and on the same side of 

 the valley as Menchecourt, and containing flint implements 

 of the same antique type and the bones of elephants; but no 

 marine shells have been found there, nor in any gravel or 

 sand at higher elevations than the Menchecourt marine shells. 



It has been a matter of discussion among geologists whether 

 the higher or the lower sands and gravels of the Somme valley 

 are the more ancient. As a general rule, when there are 

 alluvial formations of different ages in the same valley, those 

 which occupy a more elevated position above the I'ivcr-plain are 

 the oldest. In Auvergne and Velay, in Central France, where 



