THE THAMES VALLEY
In the last chapter an attempt was made to give a general idea of
the nature of the deposits; we will now give actual examples of what
has been seen. Unfortunately we cannot say “what can be seen,” for
the lower submerged forests are only visible in dock excavations.
As these works are carried well below the sea-level and have to
be kept dry by pumping, it is impossible for them to remain open
long, and though new excavations are constantly being made, the old
ones are nearly always hidden within a few weeks of their becoming
visible. Of course these remarks do not apply to the highest of
these submerged land-surfaces, which can be examined again and again
between tide-marks, whenever the tide is favourable and the sand of
the foreshore has been swept away.
The most convenient way of dealing with the evidence will perhaps be
to describe first what has been seen in the estuary of the Thames.
Then in later chapters we will take the localities on our east coast
and connected with the North Sea basin. Next we will speak of those
on the Irish Sea and English Channel. Lastly, the numerous exposures
on the west or Atlantic coast will require notice, and with them may
be taken the corresponding deposits on the French coast. Each of
these groups will require a separate chapter.
The Thames near London forms a convenient starting point, for the
numerous dock-excavations, tunnels, deep drains and dredgings have
laid open the structure of this valley and its deposits in an
exceptionally complete way. The published accounts of the excavations
in the Thames Valley are so voluminous that it is impossible here to
deal with them in any detail; we must therefore confine ourselves
to those which best illustrate the points we have in view, choosing
modern excavations which have been carefully watched, noted, and
collected from rather than ancient ones.
We cannot do better than take as an illustration of the mode of
occurrence and levels of the submerged land-surfaces the section
seen in the excavation of Tilbury Docks, for this was most carefully
noted by the engineers, and was visited by two competent observers,
Messrs. W. Whitaker and F. C. J. Spurrell. This excavation is of
great scientific importance, for it led to the discovery of a human
skeleton beneath three distinct layers of submerged peat, and these
remains have been most carefully studied by Owen and Huxley, and more
recently by Professor Keith.
The section communicated to Sir Richard Owen by Mr Donald Baynes, the
engineer superintending the excavation at the time of the discovery,
is shown in the diagram on p. 14. As Mr Baynes himself saw part of
the skeleton in the deposit, his measured section is most important
as showing its exact relation to the submerged forests. It is also
well supplemented by the careful study of the different
[Illustration:
———————————————————————————————————————Trinity High Water Level
Water 6’·77
+————+————————————————————————————+— Marsh Level
| | | ^
| 1 | Clay 6’·04 | |
| | | |
|————+————————————————————————————+ |
| | | |
| | | |
| 2 | Mud 10’·76 | |
| | | |
| | | |
|————+————————————————————————————+ |
| 3 | Mud and Peat 1’·70 | |
|————+————————————————————————————+ |
| 4 | Peat 1’·08 | |
|————+————————————————————————————+ |
| | | |
| 5 | Mud 3’·88 | | } 34’·54
| | | |
|————+————————————————————————————+ |
| 6 | Peat 3’·58 | |
|————+————————————————————————————+ |
| 7 | Mud 1’·76 | |
|————+————————————————————————————+ |
| | | |
| 8 | Mud and Peat 3’·24 | |
| | | |
|————+————————————————————————————+ |
| 9 | Sand & Decomposed Wood ·62 | |
|- - - - - - - - - - - - - - - - -|-v Level at which human
| | | remains were found
| | |
| | |
| 9*| Sand 12’·47 |
| | |
| | |
| | |
|————+————————————————————————————+
| | |
| | |
| 10 | Ballast Gravel |
| | |
Fig. 2. Section at Tilbury Docks.] layers made by Mr Spurrell, for
though his specimens did not come from exactly this part of the
docks, the various beds are traceable over so large an area that
there is no doubt as to their continuity.
Owen thought that this skeleton belonged to a man of the Palaeolithic
period, considering it contemporaneous with the mammoth and
rhinoceros found elsewhere in the neighbourhood. Other geological
writers showed however that these deposits were much more modern,
and some of them spoke somewhat contemptuously of their extremely
recent date. But Huxley saw the importance of this “river-drift
man” as an ancient and peculiar race, and Professor Keith has more
recently drawn especial attention to the well-marked characteristics
of the type. The skeleton is not of Palaeolithic date, but neither is
it truly modern; other examples have turned up in similar deposits
elsewhere.
We will now describe more fully the successive layers met with in
Tilbury Docks, condensing the account from that given by Messrs.
Spurrell and Whitaker, and using where possible the numbers attached
by the engineer to the successive beds.
It will be noticed that the marsh-level lies several feet below
Trinity high water. Below the sod of the marsh came a bed of fine
grey tidal clay (1), in which at a depth of seven feet below
the surface Mr Spurrell noted, in one part of the docks, an old
grass-grown surface strewn with Roman refuse, such as tiles,
pottery, and oyster shells. This fixes the date of the layer above as
post-Roman; but the low position of the Roman land-surface, now at
about mean-tide level, is due in great part to shrinkage since the
marsh was embanked and drained—it is unconnected with any general
post-Roman subsidence of the land.
Beneath the Roman layer occurs more marsh-clay and silt (2), resting
on a thin peat (4) which according to Mr Whitaker is sometimes
absent. Then follows another bed of marsh-clay (5), shown by the
engineer as four feet thick, but which in places thickens to six
or seven feet. Below this is a thick mass (6) of reedy peat (the
“main peat” of Mr Spurrell), which is described as consisting mainly
of _Phragmites_ and _Sparganium_, with layers of moss and fronds
of fern. The other plants observed in this peat were the elder,
white-birch, alder and oak. Associated with them were found several
species of freshwater snails and a few land forms; but the only
animal or plant showing any trace of the influence of salt water was
_Hydrobia ventrosa_, a shell that requires slightly brackish water.
The main peat rests on another bed of estuarine silt (7 and 8), which
seems to vary considerably in thickness, from 5 to 12 feet. It is
not quite clear from the descriptions whether the “thin woody peat”
of Mr Whitaker and the “sand with decayed wood” (9) of the engineer
represent a true growth in place, like the main peat; it is somewhat
irregular and tends to abut against banks of sand (9*) rising from
below. In one of these banks, according to Mr Spurrell, the human
skeleton was found.
The contents of the sand (other than the skeleton) included
_Bythinia_ and _Succinea_; and as Mr Spurrell calls it a “river
deposit,” it apparently did not yield estuarine shells, like the
silts above. The sub-angular flint gravel (10) below has all the
appearance of a river gravel; it may be from 10 to 20 feet thick, and
rests on chalk only reached in borings.
The floor of chalk beneath these alluvial deposits lies about 60 or
70 feet below the Ordnance datum in the neighbourhood of Tilbury and
Gravesend, and in the middle of the ancient channel of the Thames
it may be 10 feet lower; but there is no evidence of a greater
depth than this. We may take it therefore that here the Thames once
cut a channel about 60 feet below its modern bed. We cannot say,
however, from this evidence alone that the sea-level then was only 60
feet below Ordnance datum, for it is obvious that it may have been
considerably lower. If, as we believe, the southern part of the North
Sea was then a wide marsh, the Thames may have followed a winding
course of many miles before reaching the sea, then probably far away,
in the latitude of the Dogger Bank. This must be borne in mind: we
know the minimum extent of the change of level; but its full amount
has to be ascertained from other localities.
This difficulty has seemed of far greater importance than it really
is, and some geologists have suggested that at this period of maximum
elevation, England stood several hundred feet higher above the sea
than it does now. I doubt if such can have been the case. Granting
that the sea may have been some 300 miles away from Tilbury, measured
along the course of the winding river, this 300 miles would need a
very small fall per mile, probably not more than an inch or two. The
Thames was rapidly growing in volume, from the access of tributaries,
and was therefore flowing in a deeper and wider channel, which was
cut through soft alluvial strata; it therefore required less and less
fall per mile. Long before it reached the Dogger it probably flowed
into the Rhine, then containing an enormous volume of water and
draining twice its present catchment area.
The clean gravel and sand which occupy the lower part of the ancient
channel at Tilbury require to be more closely examined, for it is
not clear that they are, as supposed, of fluviatile origin; they may
quite well be estuarine. In the sand Mr Spurrell found the freshwater
shells _Bythinia_ and _Succinea_, and in it was also found the
human skeleton described by Owen; but, according to Mr Spurrell, on
the surface of this sand lay a few stray valves of the estuarine
_Scrobicularia_ and of _Tellina_. The bottom deposits were probably
laid down in a tidal river; but whether within the influence of the
salt water is doubtful.
As far as the Tilbury evidence goes it suggests a maximum elevation
of the land of about 80 feet above its present level; but we will
return to this question when we have dealt with the other rivers
flowing more directly into deep sea. The animals and plants found at
Tilbury were all living species.
It is unnecessary here to discuss more fully the submerged forests
seen in dock and other excavations in the Thames flats, for they
occupy a good many pages in the _Geology of London_ published by the
Geological Survey. Even 250 years ago, the hazel trees were noticed
by the inquisitive Pepys during one of his official visits to the
dockyards, and later writers are full of remarks on the ancient yew
trees and oaks found well below the sea-level. Most of these early
accounts are, however, of little scientific value.