_a._ Sand of the Landes. _b._ Limestone. _c._ Clay.]
The features of an ancient shore may sometimes be concealed by the growth of trees and shrubs, or by a covering of blown sand, a good example of which occurs a few miles west from Dax, near Bordeaux, in the south of France. About twelve miles inland, a steep bank may be traced running in a direction nearly north-east and south-west, or parallel to the contiguous coast. This sudden fall of about 50 feet conducts us from the higher platform of the Landes to a lower plain which extends to the sea. The outline of the ground suggested to me, as it would do to every geologist, the opinion that the bank in question was once a sea-cliff, when the whole country stood at a lower level. But this is no longer matter of conjecture, for, in making excavations in 1830 for the foundation of a building at Abesse, a quantity of loose sand, which formed the slope _d e_, was removed; and a perpendicular cliff, about 50 feet in height, which had hitherto been protected from the agency of the elements, was exposed. At the bottom appeared the limestone _b_, containing tertiary shells and corals, immediately below it the clay _c_, and above it the usual tertiary sand _a_, of the department of the Landes. At the base of the precipice were seen large partially rounded masses of rock, evidently detached from the stratum _b_. The face of the limestone was hollowed out and weathered into such forms as are seen in the calcareous cliffs of the adjoining coast, especially at Biaritz, near Bayonne. It is evident that, when the country was at a somewhat lower level, the sea advanced along the surface of the argillaceous stratum _c_, which, from its yielding nature, favoured the waste by allowing the more solid superincumbent stone _b_ to be readily undermined. Afterwards, when the country had been elevated, part of the sand, _a_, fell down, or was drifted by the winds, so as to form the talus, _d e_, which masked the inland cliff until it was artificially laid open to view.
When we are considering the various causes which, in the course of ages, may efface the characters of an ancient sea-coast, earthquakes must not be forgotten. During violent shocks, steep and overhanging cliffs are often thrown down and become a heap of ruins. Sometimes unequal movements of upheaval or depression entirely destroy that horizontality of the base-line which constitutes the chief peculiarity of an ancient sea-cliff.