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Rh been kept in repair and utilized down to a comparatively modern period; while at Blackheath the positions of old shafts, with presumably chambers at their bases, have hitherto been revealed only when the falling in of the long disused and neglected shafts, through the action of the water concentrated at the base of the Blackheath pebble beds, has at once both choked up shaft and chamber, and at the same time indicated their position by the subsidence at the surface resulting from their destruction.

Before leaving Blackheath it seems to be worth mentioning that Mr. Spurrell alludes to, but does not quote, the following account of a subsidence at Blackheath in 1798. It appears in the Gentleman's Magazine for 1798, p. 1,078: 'A singular accident happened last week at Blackheath. As a farmer and his son were conversing together in a field where a horse was feeding, on a sudden the animal sunk into the earth (hind feet first) to the depth of 15 ft., out of which he was dug, crushed to death. The cavity was only just sufficient to admit his body, the surrounding soil remaining firm.' This account certainly suggests a subsidence similar to the two deeper ones on Blackheath. But it seems to have taken place in some field near the open common known by that name, not on the common itself.

Much careful exploration will be necessary before any definite knowledge can be obtained as to the comparative antiquity of deneholes, and the periods not only of their construction but also of their utilization. We have seen that the pair of shallow deneholes near Crayford, described by Mr. Spurrell, dated from the Neolithic period. Then the exploration of the deneholes of Hangman's Wood by the Essex Field Club in 1884 and 1887 made it probable that they originated in post-Neolithic but pre-Roman times, and were in use throughout the Roman occupation and possibly later. On the other hand, the remark of the young labourer's father at Billericay in 1871 that an excavation like a gravel-pit was 'a denehole which had caved in,' decidedly suggests that, in some form, they must have been made and used, in districts where they had once been in demand, down to a comparatively recent period. Probably the circumstance that they were secret storehouses, etc., tended to a reticence as to their existence on the part of the agricultural population using them, which may explain the absence of any modern antiquarian allusions to them as not only once used, but as still found useful locally.



The embankments of the Thames below London as seen in their entirety present an appearance of completeness which somewhat exaggerates their importance. They are the result of the slow and creeping work of centuries. When the country was occupied by the Romans the low lands of the Thames were dry, that is not invaded with salt water. The river was fresh and very shallow, with meandering streams from the uplands adjacent. Large trees hundreds of years old, of such kinds as we have now growing, covered the bottoms and spread over the area of the present marshland, and everywhere are found Roman remains, pottery, and flint and chalk used in building. This level, which is a little below the Ordnance datum, may be called the Roman level. It is scarcely probable that any banks were needed here and none have been found of the Roman period ; nor can there be found any places indicating the least connexion between a Roman site and an embankment of any date. At the termination of the Roman period or soon after there came an irruption of the sea, which overthrew the trees and buildings and deposited over all grey tidal clay with salt water shells. The river became an estuary and has remained so ever since. The invasion of the sea was sudden, probably in the nature of a catastrophe, and accomplished the destruction of extensive settlements on the low shores and numerous islands eastward of the Medway mouth. This change was probably caused by a small subsidence of the land accompanied by so-called tidal waves. There are no banks for keeping out the tide known to be of Saxon date, except those of Littlebrook and Sittingbourne, and these were hythes of small size placed to haul up ships in winter and guard off storm floods and foes. To two of these can be assigned something of a date, viz., Littlebrook, which is mentioned as a celebrated place in a charter of  454