Page images

water origin of the loess (loam, or brick-earth) which so often covers the gravel, it follows that as we find this deposit on ground, which is fifty if not 100 feet above the highest beds of the valley-gravels (which fix approximately the position of the main channels of the old rivers), it gives a measure of the floods of that period. It shows them to have exceeded the floods of the Arctic rivers of the present day, for the waters of these Arctic rivers seldom rise more than forty or fifty feet above their low summer level. This fact furnishes, therefore, strong corroborative evidence of the power of the early [Thames and its contemporary) rivers, when taken in conjunction with agencies before described, to excavate the large valleys through which they now flow in such low and dwarfed volumes."*

The rivers in parts of Russia at the present day, at certain seasons of the year, afford an illustration of this earlier period in the history of the Thames basin which we are now considering.

To the eye

“ The enormous volumes of water by which large portions of the surface [in Russia) are still covered at every annual melting of the snows can scarcely be imagined except by those who have travelled (we may say sailed) over some of the central and southern countries in the spring season. of the geologist, the land seems emerging like islands and promontories on all sides from beneath the waters. It is then that each broad valley is for six weeks or more in the condition similar to that which we can imagine to have been the state of England, France, and other countries, when their streams, instead of occupying their present beds, were lake rivers or estuaries, wherein many of the old gravel and sand-banks of the geologist were accumulated, and in which the bones of extinct animals are found.”'t


It should be added that while in such a climate as is revealed to us by the phenomena of the Thames Valley gravels, the excavating powers of the rivers, owing to the torrential character imparted by the summer thaws, would be far greater than at present, even were the rainfall the same, it is by no means improbable that in the neighbourhood of the sea the greater cold would be accompanied by a greater precipitation of rain or snow. I

The Thames Valley, it seems, was excavated at a time when the winters of England were of an Arctic severity. In the Spring the thaws of the accumulated snows filled the valley with waters

Professor RUPERT Jones.


of a torrential volume and velocity. Thus the slopes were wasted, and the channel deepened year after year,

As when impetuous from the snow-heaped Alps,
To vernal suns relenting pours the Rhine,
Wbile undivided oft with wasteful sweep
He foams along.

We find how different in degree have since become the functions of the Thames Valley in the landscapes around us; how feeble and dwarfed the volume of the modern river. In our future landscape studies of this great remnant of ancient physical conditions around London, we may test by our own observation the evidence which thus accounts for those great and disproportionate dimensions of the Thames Valley which we see from Shooter's Hill, Hampstead, and Norwood ; and for the diminished waters which the channel conveys in these modern days from the basin of the Thames to the sea.

The Disruption of the Southern Slope of the Thames Valley.

The common conception of a valley as a V-shaped trench, with a slope on each side more or less steep, is but little exemplified in the present condition of the valley of the Thames. On the north of the river, for a distance of forty miles east of London, the valley still shows the original slope, cut down from the Glacial Clay, but on the southern side the corresponding half has been broken up and destroyed, so that there is no persistent terrace formation left.

The prevailing features of the present southern shore of the Thames are those of an upcast. The land has been subject to a great upheaval, and has been broken in the process. The old alluvia, dislocated and strewn in strange places, stand at heights far above the bed of their original deposit. The hills of Richmond and Wimbledon, in the west, owe their upheaval to this period of the valley's history. In the East of London more violent convulsions have left their traces to tell of the destructive forces which once rent the ground in the environs of our city. Here, Shooter's Hill alone remains to represent the southern slope of the original valley. At Cliffe, on the Kentish shore, has occurred a great dislocation in the earth, which may have entirely altered the course of the Thames.

The Cliffe fault indicates an upthrow on this side of the valley of at least three or four hundred feet.*

The Submerged Forest in the Bed of the Thames.

At Plumstead we find a notable example of the great geographical changes which have marked the history of the valley of the Thames. In constructing the Southern Outfall Sewer, in Plumstead Marshes, an old forest bed, several miles in extent, was discovered by the excavators. It was found lying at a depth of twenty feet below high-water mark. It passes under the bed of the river, and it prevails generally under the marshes between the river Lea on the north shore and Erith on the south. The treetrunks are found lying flat in immense numbers, and the peat is almost exclusively composed of the twigs and leaves of trees. At the base of the peat the stools of trees—of yew and oak and pinewere found rooted, in some cases, in the gravel.

Mr. Wood says there is clear evidence in the universal occurrence of this bed under the marshes, between the mouth of the sea and Erith, and that the entire space which is now the bottom of the Thames Valley in this part, was occupied by a forest, so that the Thames could obtain no access to the North Sea but by passing over it. And this is what it has done. The marsh clay (which represents the deposits of the Thames before its embankment) has covered the forest for a thickness of from three to eight feet. The channel into which by


* The able geologist already quoted, believes that the outlet of the Thames to the North Sea is of comparatively modern origin, and belongs to the date of those convulsions that destroyed the southern slope of the valley. He holds that prior to these convulsions, the valley possessed no outlet towards the North Sea, being divided from it by a range of high land. In lieu of such an outlet, it opened in more than one place over what is now the bare chalk

country forming the northern boundary of the valley of the Weald. East of Gravesend it was barred in from the North Sea by a ridge of lofty land, now cut through by the Thames River. Cliffe was the eastern limit of the old valley, where the channel turned south-easterly. It was the upthrow at Cliffe that brought Sea Reach into existence and opened the Thames valley to the North Sea. -SEARLES V. Wood, F.G.S., Geol. Mag. vol. xxiii.


embankment the river has been confined cuts through it in such a way that the peat and trees come out at low water on either bank.

The following, according to Mr. Wood, is the succession of events subsequent to the complete formation of the original valley, -the upheaval of portions of the valley, the dislocation of its deposits, and the destruction of the southern slope of the valley, east of London, with the exception of a remnant forming Shooter's Hill. After this upheaval, east of London, a land surface is formed on the deposits of the raised valley, which supports a growth of forest. This surface is then cast down and intercepts the drainage, giving rise to a swamp which engendered a peat growth, by the agency of which the portion of the forest so thrown down has been preserved from the destruction that by atmospheric, or else by human, agency has overtaken the remainder. Lastly, we have the introduction of the river over the previous land surface and peatswamp.

Thus again we have exemplified in familiar scenes around London wonderful geographical changes, which show truth to be stranger than fiction,

With our own eyes we may see the evidence of great catastrophes destroying old land surfaces and the inhabitants of the country, upheaving the bed of a great river, and then burying a forest beneath its waters. At Plumstead we may realise in the scene of destruction before us the picture which has been drawn by the poet

The forest trees
So massy, vast and green in their old age,

Are over-topped ;
Their summer blossoms by the surges lopp’d,
Which rise, and rise, and rise.

[ocr errors]


For it is here at Plumstead Marshes that an ancient forest, many miles in extent, has been cast down, so that the waters of the Thames now flow over it.

Thus near to London are some of the most typical and impressive of geological phenomena. It is of changes such as these that the poet sings

Where rolls the deep, there grew the tree.-TENNYSON.

* Geol. Mag. vol. xxiii.




[ocr errors]


HE River Thames of to-day is, then, a grand and wonderful

relic of a physical world which has now become obsolete.

As an agent of Nature in fashioning the landscape, the Thames has long since passed its prime and meridian years. It has outlived those different climatic conditions that enabled it in a distant

age to excavate the huge valley that now lies in ruins about

It no longer wastes its banks and widens its trough with the torrents of an Arctic spring, or spreads its alluvia up the hills and terraces for miles around its channel. What a history we may read in its valley to-day, as we trace the old and deserted bed of the river and the gravel and sediment of its wide-spreading floods !

Here, although restricted to an inland walk, away from steep and crumbling cliffs which tumble their fossils on the beach, we may find a shore-line not wanting in grand atmospheric phenomena, or in the relics of ancient life. It is here we find not only how Nature has gradually modelled the land around London into contours of hill and dale, of valley and river-channel. In this great valley of excavation, Nature has for ages been depositing and embedding the remains of bygone life-eras—remains which would

« PreviousContinue »