Part 3

This is well seen in the Whittlesea brickpit where an ancient gravel with marine shells rests on the Oxford Clay and over the gravel there creeps the base of the Turbiferous series. It here consists chiefly of white marl which thins out to the left of the section and above becomes full of vegetable matter until it passes up into peat, over which there is a flood-water loam.

About a mile west-north-west of Little Downham near Ely, and within a couple of hundred yards of Hythe, the Fen Beds were seen in a deep cut carried close to the gravel hill which here stretches out north into the Fens.

They consist at the base of material washed down from the spur of gravel and sand of the Areniferous series against which the Fen Beds here abut.

This basement bed is succeeded by beds of silt and peat of no great thickness as they are near the margin of the swamp.

When any considerable thickness of the older Areniferous gravels has been preserved, the base of the Turbiferous series is smooth or only gently undulating. But where only small patches or pot-holes of gravel remain, there the top of the clay has been contorted and over-folded so as often to contain irregularly curved pipes and even isolated nests of sand and gravel. The base of the Areniferous gravel must generally have been thrown down upon clay which had been clean cut to an even surface by denudation without any soaking of the surface or isolated heaps of gravel sinking into the clay under alternation of dry and wet conditions, such as would puddle the surface under the heaps and allow the masses of heavy gravel to sink in pipes and troughs. These small outlying patches of gravel are sometimes so little disturbed that we leave them in the Areniferous, whereas they are sometimes so obviously rearranged that we must include them in the Turbiferous series, taking care not to include derivative bones from the older in our list of fossils from the newer series.

Cf. Archaeol. Journ. Vol. LXIX, No. 274 2nd Ser.; Vol. XIX, No. 2, pp. 205-214.

ABSENCE OF ELEPHANT AND RHINOCEROS IN TURBIFEROUS SERIES.

The basement beds of the Turbiferous or Newer Alluvial Fen Beds are clearly separated by their stratification from the Areniferous or Older Alluvial Terrace Beds down the sloping margin of which they creep, but there is not anywhere, as far as I am aware, any passage or dovetailing of the Fen Beds into the gravel of the river terraces, while the difference in the fauna is very marked.

It is however from such sections as those just described that the erroneous view arose that the Elephant and Rhinoceros occurred in the older Fen Beds. It is true that they have been found under peat in the Fenland, but that is only where the gravel spurs of the Old Alluvial Terraces or Areniferous Series have passed under the newer Fen Beds.

I saw the remains of Rhinoceros tichorhinus in the gravel beds belonging to the older or Areniferous Series at Little Downham, and from the base of the gravel in the Whittlesea brickpit I obtained a fine lower molar of Elephas antiquus. This was, however, not in the Gravel, but squeezed into the soft surface of the underlying Jurassic Clay.

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 evidently refers to peat as used in Friesland but not as a thing with which he was familiar.

Lib. XVI, cap. 1.

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).

Under the new Tennis Courts in Park Parade facing Mid-summer Common the section was somewhat different (Fig. 2, p. 9).

While in the pit dug some years ago by Mr Bullock at the other end of the Parade at the lower end of Portugal Place in the south-east corner of the Common there was a section very similar to the last (Fig. 3, p. 10).

+------ | Made ground | | 7'-8' | +------ | Black silt | | 7'-8' | +------ | 4' Peaty silt | +------ | 4' Gravel | +------ | Gault |

These three sections, immediately north of Cambridge where the valley of the Cam opens out on to the Fens, are important as showing the variations right across the alluvium from side to side and the absence, here at any rate, of any indication of a constant sequence distinctly pointing to important geographical changes. A section seen under Pembroke College Boat House gave 16 feet of clay and peaty silt on the black gravel which here, as in the borings at the Backs of the Colleges, forms the base of the alluvium. About half way down were bones of horse and stag, but I do not believe that these are of any great antiquity, probably not earlier than mediaeval.

Thickness Depth

+------ | Irregular made ground | 5 | Clayey | | Alluvium 5 +------ | 4 | Peat | 9 +------ | | | | | 10-12 | Sand and Gravel | | | | | 21 +------ 2 | +------ Gravel 2 | 25 +------ | 4'6" | Running Sand | 29' 6" +------ Gault |

Scale 8' to 1"