Note 20 — J. E. MARR, M.A., F.R.S. (continued 2)

An unconformity may be simulated or an actual unconformity rendered apparently more important, as the result of underground solution of the underlying strata subsequently to the deposition of the upper set upon them, and any insoluble materials in the underlying strata may be left as an apparent pebble-bed at the base of the upper beds. This is seen at the junction of the Tertiary beds with the chalk near London. Subterranean water has dissolved the upper part of the chalk, increasing the unconformity which naturally exists between chalk and Tertiary beds, and the insoluble flint of the dissolved chalk is left as a layer of 'green-coated flint' at the base of the Tertiary deposits.