Page:Quarterly Journal of the Geological Society of London, vol. 28.djvu/559

1872.] an implement-bearing bed, which may turn out to be more productive hereafter.

It may perhaps be expected that, in concluding this paper, I should offer some conjectures of my own upon the unsettled question of the age and mode of deposition of the river-gravels. I confess, however, that the evidence which I have been able to collect does not appear to me to warrant any fresh hypothesis. There are, however, one or two fragments of archæological evidence, proving the great antiquity of the present bed of the Thames, which appear to me to have some bearing on the question of the erosion of the Thames valley. Near the town of Dorchester, at the junction of the Isis with the Thame stream, are the ancient earthworks known as the Dorchester Dykes, attention to which has unfortunately been drawn by their recent destruction. These works consist of a nearly straight line of entrenchment in the bottom of the valley, running from the Isis on one side to the Thame stream on the other, and enclosing the flat salient promontory formed by the bend of the river. The flanks of the entrenchment rest upon the stream on both sides. The requirements of defence demanded that they should rest upon the stream at the time they were constructed; there is evidence, therefore, that the river must have run in its present course through the flat bottom of the valley at that time. Nor could the conditions of its flow have been materially different from what they are at present; for the river still floods a considerable portion of the enclosed space, and, if it had habitually risen only a few feet higher, it would have rendered the spot unsuitable as the site of an encampment. Now the associated relics prove that this entrenchment is the work of pre-Roman times. All the works of art discovered within the area of the entrenchment are of flint or bronze; and notwithstanding the existence of a Roman station at Dorchester close by, nothing Roman is found in this place. We have evidence, therefore that the Thames ran in its present course, and under nearly the same conditions as at present, ever since the bronze period of England; and how much longer, we cannot tell. But we have facts of the same kind within the district represented upon our map. From Ham to Petersham the river runs northward, by Richmond to Kew, where it bends to the south, running to East Sheen, and then turns again northward, running by Barnes to Hammersmith; again turning to the south, it flows by Putney and Wandsworth, turning north again, in the direction of Battersea and Chelsea, thus making four bends between Ham and Chelsea in the comparatively flat bottom of the valley. Throughout the whole extent of this winding course, almost wherever the dredging machine is put into operation, relics of the bronze and stone age are turned up at various depths, extending to 10 feet below the existing bed of the river. Mr. T. Layton, F.S.A., of Kew, to whom I am indebted for information on this subject, has collected nearly one hundred specimens of the prehistoric age, many of which are bronze leaf-shaped swords, together with a number of stone and bronze celts from different parts of the river.