62 R. H. Yapp. 
up the area to a very uniform plain not more than a foot or two 
above mean-tide level.” 1 
The Fen-deposits consist largely of peat and silt. The former 
is developed chiefly in the south, but is also found along the 
western boundary of the area— i.e., in those parts most remote from 
the sea—while the latter is the chief deposit found bordering on the 
Wash (Text-fig. 9). Boulder clay is often found underlying both 
peat and silt, which proves the post-glacial origin of the Fen- 
deposits proper. 2 
During pre-Roman times the Fens had become for the most 
part a great swampy plain, probably studded with shallow open 
meres, similar to the Broads of Norfolk and Suffolk. Here and 
there, especially towards the south, extensive drier areas, the “ Fen 
islands,” appeared wherever the land was raised a few feet above 
the general level. 
But although the thickness of the peat, 3 or turf as it is called 
by the Fenmen, 4 attests the long continuance of swamp conditions, 
yet these conditions did not remain altogether uniform. Proof of 
the variation of physical conditions is afforded by the alternating 
beds of silt and peat to be found in the northern part of the Fen 
area 5 ; and also by the buried forests, which indicate a recurrence of 
drier conditions, found in the peat of certain of the more southern 
parts of the district. These variations are usually attributed to 
slight oscillations of level, 6 but the cause of the oscillations appears 
uncertain. Probably earth-movement played some part, but other 
factors may also have contributed their share in bringing about 
these changes. Such are on the one hand, the blocking of river 
mouths by sand-banks or silt-deposits, with the consequent damming 
back of the river water, and on the other, the subsequent breaking 
through of such barriers. The raising of the general level by the 
upward growth of the peat, would also assist in the local production 
of drier conditions. 
1 Marr & Fearnsides, The Physiography of Cambridgeshire (in 
The Natural History of Cambridgeshire, edit, by Marr and 
Shipley, 1904), p. 5. 
s Miller & Skertchly, l.c., p. 497. 
3 A thickness of eighteen feet has been measured at Earith, but 
the average thickness is much less, Reed, l.c., p. 225. The 
peat is still used as fuel in some parts of the Fens : cf. the 
photograph showing peat cuttings on Adventurers’ Fen 
(Plate IV., Fig. 3). 
4 As in Ireland and elsewhere, cf. German Torf and French tourbe. 
* Miller & Skertchly, l.c., p. 560. 
' Ibid, p. 561. 
