This clay, together with the Bracklesham beds,
presently to be described, has been termed Middle Bagshot by the Survey. In
Barton Cliff, where it attains a thickness of about 300 feet, it is rich in
marine fossils.
It was formerly confounded with the London Clay, an older Eocene deposit of very
similar mineral character, to be mentioned below, which contains many shells in
common, but not more than one-fourth of the whole. In other words, there are
known at present 247 species in the London Clay and 321 in that of Barton, and
only 70 common to the two formations. Fifty-six of these have been found in the
intermediate Bracklesham beds, and the reappearance of the other 14 may imply a
return of similar conditions, whether of temperature or depth or of a muddy
argillaceous bottom, common to the two periods of the London and Barton Clays.
According to M. Hebert, the most characteristic Barton Clay fossils correspond
to those of the Gres de Beauchamp, or Sables Moyens, of the Paris Basin, but it
also contains many common to the older Calcaire Grossier.
SHELLS OF THE BARTON CLAY.
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