Geoscience Reference
In-Depth Information
In 1910 Harmer was invited to contribute chapters on the Pliocene and Pleis-
tocene Epochs in eastern England for the Jubilee Volume of the Geologists
Association, Geology in the Field (1858-1908). The following accounts are based
on these chapters together with other key works.
3.3 The Pliocene Epoch in East Anglia
During the Pliocene Epoch the British Isles was shaped by various geological
forces into a physical pattern which more or less defines the geography of the
region today. However, there were some regional differences. For example, at the
beginning of the period southeastern England was about 200 m (650 ft.) lower than
today and the crest of the Wealden dome would have formed a low island from
which the former surface of chalk had been removed to expose the underlying
rocks; further north in eastern England the shore-line of the Pliocene or Crag Sea
would have extended farther west than that of the present North Sea. The Crag Sea
was essentially a great gulf into which the proto-Rhine flowed to be joined on its
left bank by the proto-Thames; the deposits of this river system contributed to the
formation of the Pliocene Crag. Whilst initially the Crag Sea lingered in the
syncline of what today is the London Basin, it later progressively retreated
northeast to occupy the position of the present North Sea.
3.3.1 Pliocene Crag
In East Anglia, the Pliocene Crag beds, ranging in age from the Coralline to the
Weybourne deposits (oldest to youngest), were of particular interest to Harmer as
they represented successive stages during a period when tectonic changes were
accompanied by a refrigeration of the climate. Coralline Crag, the oldest of these
deposits comprises fragmentary polyzoa, sea-mats and related species, formerly
termed corallines. As the land rose and the Crag Sea retreated northeast, later
deposits are not superimposed on earlier beds but are found progressively further
away from the original Pliocene basin.
Immense numbers of molluscan shells were discovered in the Crag beds with
the older shells generally of warm-water species, indicating that in earlier times a
seaway was probably open through central Europe to warmer waters. However
with the cooling of the climate during the Pliocene epoch there was a gradual
