continues them down to the mammoth-period in the history of the river. It is to this prolonged excavation of the channel of the Thames by the erosion of running waters and to the deposits the stream has left in its bed, that we gain a knowledge of the physical conditions of the early Thames-country, and can trace in some measure the history of climate from the Glacial period of England's history. The subject of the excavation of the great Thames Valley in the freshwater period of its annals may well belong to a separate chapter. CHAPTER III. THE EXCAVATION OF THE THAMES VALLEY AND ITS DISRUPTION.—THE LANDSCAPES OF THE THAMES VALLEY: A STUDY IN HYDROGRAPHY.— NATURAL ORIGIN OF THE Valley.-DIMENSIONS. PERIOD OF THE EXCAVATION. CLIMATE.-GREATER ATMOSPHERIC FORCES.-ARCTIC WINTERS. -SPRING THAWS.-RIVERS IN RUSSIA TO-DAY.-PRESENT SHAPE OF THE THAMES VALLEY.-DISRUPTION OF ITS SOUTHERN SLOPE.-CHANGES IN THE COURSE OF THE THAMES.-THE SUBMERGED FOREST AT PLUMSTEAD. HERE is no feature of the scenery around London which presents Nature's landscape economy in such picturesque and legible aspects as the great valley of the Thames. Here we have always with us an instructive hydrographical system to entertain us in our suburban rambles. Familiar as we may be with our beautiful Thames and its scenery, how many of us Londoners have ever paused to think of this great valley as a trench which once had no existence in the ground, and which Nature had once to begin, in a definite period of geological time, to excavate in the solid earth! How many of us, as we have glided on the river, or surveyed it from some favourite hill, have thought to estimate the years it has taken to excavate the Thames Valley by the slow agency of the waters that course through it! Yet these familiar landscapes bear on their face the nature of their history and origin. In ranging with our eyes the horizon around us, and in scanning the contours of the ground, we may learn the plan of the land-surface to which the Thames Valley is subservient, and to which it will give us the clue. We may discern the way in which never-resting Nature has gradually fashioned the features of the ground, and get an idea of the time she has taken to accomplish her leisurely processes in the environs of London. 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 debris 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. |