Page:Quarterly Journal of the Geological Society of London, vol. 25.djvu/190

96 at a height of 140 feet, it is only covered by 6 feet of gravel, while in fig. 30 it is covered by about 50 feet of the Quaternary series.

Fig. 31 is a drawing to natural scale, showing the relations of the Thanet sands to the covering bed of gravel, where these sands have fallen into pipes in the chalk. The Thanet sands have not been faulted by any general movement, but have dropped down where they have been undermined by the removal of chalk by chemical action.

It would appear as if this chemical action had been contemporaneous with the denudation of the Thames- valley district by pluvial and fluvial agency, and had preceded the deposition of the covering bed of sand and gravel. The proof of this is that the fissures in the sand are filled by veins of gravel from 1/2 inch to -1/2 inch wide to a great depth.

When the sands fell by gravitation into the great pipe (A), (which penetrates the chalk 25 feet, and the Thanet sands an equal amount), they filled it up to a height of 20 feet. This must have left a large funnel-shaped hole on the surface of the sands. It will be seen that, although the gravel is only 5 or 6 feet thick over the undisturbed sands at B, it has been able to fill a cavity 30 feet deep in the Thanet sands and chalk ; and yet the top of the gravel is so smooth that no one could