There have never been any remains of Elephant or Rhinoceros found in the Turbiferous series.
Absence of Peat in Areniferous Series.
It is not easy to realise what the conditions were during the formation of the later Terrace Gravels (Barnwell type), and, if it is a fact, why there was not then, as in later times, a marshy peat-bearing area here and there between the torrential deposits of the upper streams near the foot of the hills and the region where the tide met the upland waters. A few plants have been found in the Barnwell gravel but they are very rare in this series. The older Terrace Gravel (Barrington type) might be expected to furnish evidence of the existence of abundant vegetation if we are right in assigning it to about the age of the peaty deposits overlying the Weybourn Crag. But at present we have no evidence of any such deposit in the Cambridge gravels.
Although there are great masses of vegetable matter formed in the swamps of tropical regions, peat is essentially a product of northern climes. Pliny[4] evidently refers to peat as used in Friesland but not as a thing with which he was familiar.
Fen Beds not all Peat.
It must not, however, be imagined that the Fen Beds consist wholly or even chiefly of peat. As we travel north from Cambridge the surface of the alluvium is brown earth for miles and only here and there shows the black surface of peat. The numerous ditches for draining the land confirm this observation, and when we have the opportunity of examining excavations carried down to great depths into the alluvium we usually find only a little peat on the surface or in thin beds alternating with silt and clay and marl. Sometimes, but only sometimes, we have evidence of the growth of peat for a long time, then of the incoming of turbid water leaving beds of clay, then again of the tranquil growth of peat. All this points to changes of local conditions and shifting channels during a gradual sinking of the area, for some of the peat is below sea level.
I believe that the volume of clay is much greater than that of peat, although from the common occurrence of peat on the surface and clay in the depth the area over which peat is seen is greater. We have not, however, the data for estimating the proportion of each.
In embayed corners along the river even above Cambridge we find little patches of peat, while on the other hand in deep excavations near the middle of the valley we find only thin streaks of peat or peaty silt. In the trial boreholes at the Backs of the Colleges there was only this kind of record of former swamp vegetation.
Sections in Alluvium.
In digging the foundations for the chimney of the Electric Lighting Works opposite Magdalene College the following section was seen (Fig. 1, p. 8).