448 PREHISTORIC E UR OPE. 



to the glacial series. Both by their geological position and their 

 organic contents they are proved to be of postglacial age. 



The beds above the gravels are composed of peat, shell-marl, 

 and marine silt and clay, with Scrdbicularia piperata. The gravel- 

 paved basin in which these accumulations rest has been gradually 

 conquered from the sea chiefly by the deposition of silt brought 

 in, says Mr. Skertchly, by the sea itself, and laid down only at 

 the slack of high- water upon the coast ; and this process has 

 been going on apparently with little interruption since the Fen 

 peat and silt began to form. We have evidence, however, in 

 the presence of buried forests at some depths below the present 

 sea-level that the land was formerly more extensive than it is 

 now. Mr. Skertchly gives a table showing the depths from the 

 surface at which buried forests have been found. They occur, 

 he says, on at least four horizons, and in one place he observed 

 no fewer than five buried forests one above another, with separ- 

 ating beds of peat. The lowest forest rests upon the floor-gravels, 

 which form the basement beds of the Fen-series, and its presence 

 clearly indicates that, after the deposition of the gravels, the sea 

 retreated, and a well-drained land-surface existed in its stead. 

 " The next horizon includes the trees which occur at about 10 feet 

 from the surface ; the third those about 5 feet from the surface ; 

 the fourth those nearer the surface than the third." It must 

 not be supposed that those various buried forests occur constantly 

 throughout the Fenland. On the contrary, some sections may 

 show only three or two, or even again only one. The most con- 

 stant of all the forests appears to be the lowest — that namely 

 which rests upon the basement gravel-beds. The trees and 

 shrubs comprise oak (Quercus robur), pine (probably Pinus 

 sylvestris), elm, yew, birch, hazel, alder, and willow (several 

 species), all of which, with the exception of Pinus sylvestris, are 

 still natives of the Fenland. 



Each forest-bed is generally covered by a less or greater 

 thickness of peat, but it must be understood that the peat of 

 the Fens is not necessarily always associated with buried trees. 

 In many places thick and extensive beds of this material occur 



