Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/110

 Medway drainage having been worn down step by step as the Lower - Greensand escarpment rose above the waters, until it presented the condition now exhibited by the trumpet-mouthed gorge cutting through that escarpment at Yalding ; at which time, or even previously, through the opening in the Lower-Greensand hill to the north of them, the Lower-Tertiary pebbles and angular flints abundantly present in the gravels near that place, shown in the map, were thus brought from the northward through the Lower-Greensand escarpment.

The Lower-Greensand escarpment subsides and disappears near Ashford; so that where the Kennington, Willesboro', and Smeeth gravels containing the flint and pebble admixture occur, there is no such escarpment at all. The mouth by which the drainage through the Stour valley entered the Weald seems therefore not to have much advanced beyond the chalk escarpment ; so that the gravels at these places represent both the gravels above Maidstone, accumulated when the Medway mouth was near the chalk escarpment, and the gravels about Yalding, accumulated either about the same time or else later, when that mouth had become established near the Lower- Greensand escarpment. The highest of the Stour area, viz. those at Kennington, seem to be probably coeval with the gravels above Maidstone ; while those of Willesboro', which are at a lower level, may be synchronous with the gravels about Yalding.

During the later portion of this change the Hastings-sand country formed, it seems to me, a large island, so that what for convenience' sake I have called sea, was really only an inlet receiving freshwater through these several rivers ; and since the width between this island and the chalk escarpment varies only from eight to eleven miles, and between it and the greensand escarpment from five to six miles only at the narrower parts, there would have been a considerable tidal scour exerted under any circumstances, while the far greater volume of the land-drainage of those postglacial times, compared with what now obtains in the east of England, would tend to push the limit of fresh water further out into estuaries than at present. Great freshets too, carrying with them volumes of river-mud with its associated organisms, would be poured into the Weald ; and throughout it is to be remembered that the fresh water must follow the salt water as the latter recedes by the extension of the shore-line, and occupy its place. In this way it seems to me that there would be nothing repugnant to the events I have traced, if the gravels in question should hereafter be found to yield the remains of land or fresh- water organisms, such as do occur in some of the gravels and brick-earths shown in the map under a different shading, and which are most of them due to the rivers flowing as they do now.

The views thus sketched assume the Weald, when the sea had retired within the chalk escarpments, to have been the island-studded head of a still longer inlet formed by the British Channel while this channel was closed to the north by an isthmus between Dover and Calais, which had come into existence by means of that elevation of the chalk country which put an end to the Thames gravel and its coeval