108 PEAT OF THE VALLEY OF THE SOMME. cuAr. vii. 



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 hun- 

 dred 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 onlj" 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. Under- 

 neath 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 geolo- 

 gical change has taken place, except the growth of the peat, 

 and certain oscillations in the general level of the countiy, to 

 which we shall allude b^'-and-by. A thin layer of impervious 

 clay separates the gravel a from the peat No. 1, and seems to 

 have been a necessarj- preliminarj' 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 Yalley 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 alread}- been stated to be in some places thirty 

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



