Page images

Again, then, we leave for a time the subject of the strange zoology of the ancient Thames-the great hairy Siberian mammoth, the fleece-clad rhinoceros and elephant, the hippopotamus, tiger, and bear. We turn to contemplate the separate history of the valley itself, as a feature of Nature which has taken tens of thousands of years to attain the dimensions it exhibits to-day. For the Thames Valley is a valley of denudation and excavation by water, and not a fissure in the ground effected by earthquake agitation; a vast and irregular trench, the work of innumerable years, worn with slow erosion by the channelled waters as they widened and deepened their course.

If we take a survey of the Thames Valley in the environs of London from some commanding station, we are struck with an enormous disproportion between the breadth and capacity of the valley and the narrow stream which courses along its bottom. From Hampstead Hill on the north brow, or Norwood on the south, and from Shooter's Hill in Kent, or the summit of the Essex slope opposite, we may gain some conception of the Thames Valley's dimensions in this part of its course. For although the southern slope of the valley now lies in ruins, the result of earthquake convulsions, we may learn from the northern slope which remains to measure the breadth of the earlier river, and the volume and force of its waters. And not only does the northern slope remain unimpaired, except by the ordinary forces of nature, to contribute to the story of the past; there are remnants and monuments of the southern side of the Thames Valley still standing, like pillars on a plain, to give us more precisely the breadth and mass of the earth which has been eroded and transported to the sea. One such monument and that the most distinguishable, is Shooter's Hill, standing up in isolation out of the earthquake ruins about it, and fronting the corresponding Essex shore. Again, let us look at the old alluvia strewing the slopes of the valley, and rising in terraces to a height, as in Hyde Park and elsewhere, of more than 100 feet above the level of the river, and which to the west of London have still a spread of at least four miles in breadth. Although these deposits fall far short of the extent of surface over which they

originally stretched, they help us to realise the width of the earlier Thames. With a bird's-eye view of the scene thus pictured in the east and west of London we may form some estimate of the capacity of so great a valley for the waters of the Thames basin. We begin to see that the Thames Valley was originally constructed on a scale of magnitude for which nature has no longer any commensurate uses. It has outlived the world of greater physical and atmospheric phenomena, of which it was once both the creature and servant.*

This great valley of the Thames, this vast natural trough, which is now so largely out of proportion to its uses, could never have been excavated by the river which now flows through it. What then were those earlier physical conditions to which we are to look for its origin? If river action as it now exists is inadequate to account for the formation of so large a valley, and would be unable to spread out the old alluvia we find in its slopes, what have been the actual agencies in the past which have sufficed for the purpose? Of what kind and degree have been the physical forces which have now left the slopes of the valley deserted by the waters, and have permitted in these modern days a city of human habitations to rise within their borders?

We must revert again to that remote and twilight time in the history of the Thames, that climatic period of a more Arctic temperature than the present, which is found to have prevailed during the earlier years of the river, if we would account for the excavation of so great a valley. We must return to that remote era, incalculably distant in years, but none the less real and historical, when different atmospheric conditions favoured the more rapid erosion of the river channel, and filled this valley with waters of a vast and commensurate volume. We shall see that the Valley of the Thames acquired its present dimensions in a period of greater

*The magnitude of the original Thames Valley is not, however, to be estimated by a bird's-eye view. At Grays, in Essex, the brow of the valley occupied by the Glacial clay stands now at a height of from 250

to 350 feet above the Thames River; but the geologist finds that the valley here has been cut down for a depth of six hundred feet.-S. V. WOOD, jun., Quar. Jour. Geol. Soc. vol. xxiii. p. 397.

atmospheric energy and waste than the present, an era of rivererosion of greater intensity, of periodical floods imparting a torrential character to the waters of the basin.*

At this more Arctic period, during which the Thames elephant, rhinoceros, and hippopotamus flourished, the melting of winter snows, combined possibly with a larger rainfall, must have afforded to the rivers of south-eastern England and Northern France a volume of water far exceeding their present supply, and giving them the character of a torrent.

"That the rivers of this period were larger and more rapid than now is evident from the great quantity of débris, the prevalence of gravels, the coarseness of the sands, and the general absence of mud sediments.

"But other forces beside an increase in the water-power seem required to account for the excavation of these great valleys. Another great agent, co-operating with the last, is the freezing of the ground, and the rending of the rocks by the frosts of these Arctic winters."+

The only adequate cause to account for the valley deposits of this early period is river-ice, the transporting power of which is well known. The gravel of the Thames consists essentially of the débris of rocks transported northwards, probably from the hills of Surrey and Sussex, and over distances of not less than from six to twenty miles. Still another condition must be supposed. The land, at the period when these broad alluvia were deposited, had a much greater elevation above the sea. This condition is applicable

to every considerable river-course in Britain, along every one of which we find indications of the larger dimensions of the former rivers. The Thames and the Severn are striking examples.

The valley-gravels by themselves give no adequate evidence of the volume of water there must have been in the Thames at this period, or of the extent of the floods which followed the great winter accumulation of ice and snow, and which, as a consequence of rapid thawing in the spring or summer months, would cause immense inundations.

"They merely show, in the quantity of debris, the coarse shingle and the worn blocks, the results of torrential action. If, however, we admit the flood

† Ibid.

*PRESTWICH. Phil. Trans. 1864.
GODWIN-AUSTEN. Jour. Geol. Soc. vol. vi. p. 93.

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.

"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. To the eye 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."+

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.‡

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




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,
While 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

« PreviousContinue »