Two of these
Crags are referable to the Older Pliocene, viz., the White and
Red Crags,--and one belongs to the Newer Pliocene, viz., the
Norwich Crag.
The _White or Coralline Crag_ of Suffolk is the oldest of the
Pliocene deposits of Britain, and is an exceedingly local formation,
occurring in but a single small area, and having a maximum thickness
of not more than 50 feet. It consists of soft sands, with occasional
intercalations of flaggy limestone. Though of small extent and
thickness, the Coralline Crag is of importance from the number
of fossils which it contains. The name "Coralline" is a misnomer;
since there are few true Corals, and the so-called "Corals" of
the formation are really _Polyzoa_, often of very singular forms.
The shells of the Coralline Crag are mostly such as inhabit the
seas of temperate regions; but there occur some forms usually
looked upon as indicating a warm climate.
The _Upper_ or _Red Crag_ of Suffolk--like the Coralline Crag--has
a limited geographical extent and a small thickness, rarely exceeding
40 feet. It consists of quartzose sands, usually deep red or
brown in colour, and charged with numerous fossils.
Altogether more than 200 species of shells are known from the
Red Crag, of which 60 per cent are referable to existing species.
The shells indicate, upon the whole, a temperate or even cold
climate, decidedly less warm than that indicated by the organic
remains of the Coralline Crag.
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