108 PEAT OF THE VALLEY OF THE SOMME. CHAP. vn. 



general structure of the country, so much so, that they might 

 easily be overlooked in a cursory survey of the district, and 

 are usually unnoticed in geological maps not specially devoted 

 to the superficial formations. 



It will be seen by the description given of the section, fig. 7, 

 that No. 2 indicates the lower level gravels, and No. 3 the 

 higher ones, or those rising to elevations of eighty or a hundred 

 feet above the river. Newer than these is the peat No. 1, which 

 is from ten to thirty feet in thickness, and which is not only of 

 later date than the alluvium, 2 and 3, but is also posterior to 

 the denudation of those gravels, or to the time when the valley 

 was excavated through them. Underneath the peat is a bed 

 of gravel, a, from three to fourteen feet thick, which rests on 

 undisturbed chalk. This gravel was probably formed, in part 

 at least, when the valley was scooped out to its present 

 depth, since which time no geological change has taken place, 

 except the growth of the peat, and certain oscillations in the 

 general level of the country, to which we shall allude by and 

 by. A thin layer of impervious clay separates the gravel a from 

 the peat No. 1, and seems to have been a necessary pre 

 liminary to the growth of the peat. 



Peat of the Valley of the Somme. 



As hitherto, in our retrospective survey, we have been 

 obliged, for the sake of proceeding from the known to the 

 less known, to reverse the natural order of history, and to 

 treat of the newer before the older formations, I shall begin 

 my account of the geological monuments of the Valley of the 

 Somme by saying something of the most modern of all of 

 them, the peat. This substance occupies the lower parts of 

 the valley far above Amiens, and below Abbeville as far as 

 the sea. It has already been stated to be in some places thirty 

 feet thick, and is even occasionally more than thirty feet, 



