THE CRETACEOUS SYSTEM.
_Classification._ The rocks of the Cretaceous system are conveniently
divided into Upper and Lower Cretaceous. The following classification
has been widely used for the British deposits, and is founded on
lithological characters:
{ Upper Chalk with flints }
{ Middle Chalk with few flints } Chalk
Upper { Lower Chalk without flints }
Cretaceous { Chalk Marl }
{ Upper Greensand
{ Gault
{ Lower Greensand
Lower { Wealden
Cretaceous { Hastings sands
As the result of examination of the faunas, a more generally
applicable classification has been established and is now largely
adopted. It is as follows:
Danian }
Senonian } Upper Cretaceous
Turonian }
Cenomanian }
Albian }
Aptian }Lower Cretaceous.
Neocomian }
In this classification the Neocomian practically represents the
Wealden and Hastings beds, the Aptian the Lower Greensand and the
Albian the Gault, placed according to this classification in the Lower
Cretaceous, while the Upper divisions represent the strata above the
Gault, consisting essentially of Chalk in England.
_Description of the Strata._
(i) _The Neocomian and Aptian Beds._ In the south of England the Lower
Cretaceous beds succeed the Jurassic rocks with little or no break,
and the type of the lower beds is similar to that of the beds
deposited during the Purbeck age, consisting of estuarine deposits of
variable characters, chiefly arenaceous below (the Hastings sands) and
argillaceous above (the Wealden series), though impure limestones are
found, largely composed of the shells of the freshwater _Paludina_,
and much ironstone is developed in places. At the close of Neocomian
times, the freshwater conditions in southern England were replaced by
marine conditions and the Lower Greensand strata with their marine
fauna were deposited in the Aptian sea. The Neocomian and Aptian beds
thin out westward, and much more rapidly to the northward, so that
both divisions disappear against the now buried ridge which forms a
continuation of the Mendip axis. North of this they appear in another
form. At first the highest Aptian beds alone are developed as shore
deposits. Passing into Norfolk lower beds come in until in
Lincolnshire we get a complete development of the Neocomian and Aptian
beds with a marine facies, though of fairly shallow water character,
whilst in Yorkshire the two divisions are represented by a deeper
water clay, forming the Upper portion of the Speeton series. There is
a consensus of opinion in favour of the Neocomian beds of southern
Britain having been laid down in an estuary of a river flowing from
the west over a continent now destroyed. To the north of this river
stood the London ridge of the Palæozoic rocks, the northern borders of
which formed the coast line off which were deposited the sediments of
Neocomian and Aptian ages which occur in northern England. Before the
deposition of the Albian beds a considerable upheaval of some parts of
Britain occurred, and an unconformity separates the higher Cretaceous
beds from older strata of Cretaceous and Jurassic ages, thus
complicating the major phases by local changes in the characters of
the strata.
(ii) _The Albian and higher Cretaceous Beds._ The commencement of the
deep-water phase of the third marine period may be said to occur in
Albian times in Britain, reaching its maximum during the deposition of
the chalk. The existence of a deeper sea towards the north of England
is indicated by the characters of the Albian and newer strata. The
Albian beds of gault consist of a stiff clay in southern England,
replaced by coarser mechanical sediments towards the west. As one
passes north from the London ridge (which exerted its influence in
Albian times, after which it was finally buried in sediment) the gault
thins out, and becomes gradually replaced by calcareous deposit when
it is known as the red chalk which replaces the gault in northern
Norfolk, Lincolnshire and Yorkshire.
A local unconformity separating the chalk and gault in parts of East
Anglia points to another local uplift with its accompanying
complications in the characters of the strata. After the uplift had
ceased, general depression must have occurred, and the various
divisions of the chalk were accumulated in a fairly open sea, though,
for reasons to be given presently, this was probably of no great
lateral extent, save when united with the open ocean, probably in a
manner similar to the connexion between the Gulf of Mexico and the
Atlantic.
The general variations in the lithological characters of the various
members of the Cretaceous system will probably be rendered clearer by
reference to the accompanying diagram (fig. 24) representing the
variations when traced across England from south to north[101].
[Footnote 101: For information concerning the British Cretaceous beds,
see Topley and Foster, "Geology of the Weald," _Mem. Geol. Survey_,
1875; Bristow and Strahan, "Geology of the Isle of Wight," _Mem. Geol.
Survey_, 1889; Lamplugh, "On the Speeton Clay," _Q. J. G. S._, vol.
XLV. p. 575, and "The Speeton Series in Yorkshire and Lincolnshire,"
_ibid._, vol. LII. p. 179; Barrois "Recherches sur le Terrain Crétacé
supérieur de l'Angleterre et d'Irlande," Lille, 1876; and various
papers by Messrs Hill and Jukes-Browne, in the _Quarterly Journal of
the Geological Society_ and _Geological Magazine_ of recent years. For
the Scotch deposits consult a paper by Prof. Judd, _Q. J. G. S._, vol.
XXXIV. p. 736, and for those of Ireland, see Hume, _Q. J. G. S._, vol.
LII. p. 540.]
[Illustration: Fig. 24.
Ch. Chalk.
Al. Albian.
Ap. Aptian.
N. Neocomian.
J. Jurassic.
]
The clue to the physical geography of Britain during Cretaceous times
is furnished to a considerable extent by study of the foreign
deposits. In Northern Europe the Cretaceous beds of England are met
with in Northern France, and there the characters are generally
speaking similar to those of our British deposits. In Germany
shallower water conditions prevailed, the lower beds gradually
disappear, and the upper beds are replaced by mechanical sediments of
various degrees of coarseness, becoming on the whole coarser, as one
travels eastward, so that in Saxony the chalk is partly replaced by
arenaceous deposits (the 'Quader' sandstones) which are responsible
for the remarkable scenery of the Elbe district above Dresden. In
passing northwards, indications of similar change are noted in the
deposits of Denmark and Scania, whilst to the south, we get a complete
change in the character of the rocks, after crossing the Loire in
France, and a similar change is observable in districts lying further
east. Furthermore, as will be noted more fully in a subsequent
paragraph, the character of the Upper Cretaceous flora indicates the
existence of a large tract of land lying to the north and north-west
of Europe, so that it would appear that the Cretaceous rocks of
Northern Europe were deposited in a gulf-like expansion of a western
ocean, bounded on the north by Scandinavia, on the west by eastern
Germany, and on the south by a ridge running eastward from the mouth
of the Loire[102]. We may speak of this gulf as the Chalk gulf. To the
south of the presumed ridge the character of the strata alters, and
also that of the included organisms. This southern type of Cretaceous
rocks is one which is very widely spread, being found in Europe south
of the Loire, and of the Alps, and in Greece and Turkey, while it
also occurs in the northern parts of Africa. The beds of this type are
traceable through Asia Minor into India and to the shores of the
Indian Ocean, indicating the existence of a widespread Cretaceous
ocean, which is sometimes spoken of as the Hippurite-limestone sea,
for reasons which will eventually appear. The deposits are largely
formed of hard limestone which is very different in its character from
the soft chalk of the northern gulf.
[Footnote 102: The reader will find the existence of this gulf
maintained and supported by a considerable mass of detail in Mr A. R.
Wallace's _Island Life_.]
The climatic conditions which prevailed during Cretaceous times were
apparently similar in most respects to those of the preceding Jurassic
period, and as already stated the climatic zones which Neumayr defined
for Jurassic times are also maintained by him to have existed during
the Cretaceous period. The existence of cold has sometimes been
inferred from the presence of large foreign blocks in the chalk,
especially at its base, but if these are due to the transport, they
might well be caused by masses of floating ice, which are often found
at considerable distances from the coast in temperate regions after
the break-up of the frost which succeeds an unusually hard winter. The
flora and fauna are not suggestive of severe conditions.
_The Cretaceous flora and fauna._ It has been noted in the last
chapter that the gymnospermous flora of the Jurassic period, in which
cycads form a considerable percentage of the whole flora, was
prevalent in Lower Cretaceous times. In the Upper Cretaceous rocks
this flora is replaced by one which consists to a large extent of
dicotyledonous angiosperms. These are found in the Upper Cretaceous
rocks of Europe and North America, and as the researches of botanists
indicate their origin in circumpolar regions, their arrival in Europe
is an additional argument in favour of the existence of an extensive
northern continent, sending a prolongation to the southward in eastern
Europe.
The invertebrate fauna bears considerable resemblance to that of
Jurassic times, and many of the dominant Jurassic genera are also
found in Cretaceous rocks. A most interesting feature is connected
with the character and geographical distribution of the Ammonites. In
Europe they are almost exclusively confined to the deposits of the
northern gulf, and before their final disappearance they undergo many
changes of form. We find the discoid spiral shells of earlier times,
but these are accompanied by shells which are straight, curved,
boat-shaped, and coiled into various helicoid spirals, sometimes
having the whorls in contact, while at other times they are separate.
In the chalk of Britain gastropods are on the whole rare, and this
fact serves to emphasize the palæontological break which occurs
between the Cretaceous and Tertiary rocks; but when conditions were
favourable, as during the deposition of some of the strata of the
Middle Chalk, gastropods are abundant, and some are related to
Tertiary genera, so that we may assume that the palæontological break
alluded to is exaggerated by the difference of conditions which
prevailed during the deposition of the earliest Tertiary and latest
Cretaceous sediments.
In the Cretaceous deposits of the southern sea, where the Ammonite
tribe is almost unknown, the remarkable family of the lamellibranchs
known as the Hippuritidæ furnish the dominant invertebrates of the
period, and the representatives of this family are exceedingly scarce
amongst the Cretaceous strata of the northern gulf, though they are
found on two or three horizons.
Of vertebrates, the most interesting are the reptiles. The families
which predominate in Jurassic times have many representatives amongst
the Cretaceous strata also, but the order Squamata is represented by
the sub-order Pythonomorpha, which is characteristic of the Cretaceous
rocks. The best known representative is the gigantic _Mosasaurus_.
Lastly, we have the remarkable toothed birds or Odontornithes, now
placed in different orders, the genus _Hesperornis_ being the only
representative of the sub-order Odontolcæ of the Ratitæ, whilst
_Ichthyornis_ and allied forms are placed in the sub-order Odontormæ
of the Carinatæ.