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2. Geology
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2.3 Jurassic
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The
Jurassic System in England and Wales
The Jurassic beds
of England form an almost continuous belt stretching from the
coast of Yorkshire south of the Tees to the Dorset coast about Lyme Regis and
Weymouth. The breadth of the belt varies considerably. It is widest towards the
middle, where it spreads from the neighbourhood of Leicester to King's Lynn in
Norfolk ; but in this region it is partly covered by the alluvial deposits of the Fens.
It is narrowest in South Yorkshire, where, indeed, for a short distance it is
completely overlapped by the Cretaceous beds.
Lithologically, the
system consists chiefly of clays and limestones. Most of these
are marine in origin, but some are estuarine or fluviatile, and occasionally there
are even remains of terrestrial soils. Some of the subdivisions pass almost
unaltered throughout the length of England, but others show considerable
variations. |
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Since the system consists
chiefly of marine deposits, the greater number of the
fossils are marine, but estuarine and terrestrial forms occur. The period has
been called' the Age of Cycads,' * the Age of Ammonites,' and ' the Age of
Reptiles,' according to the point of view of the writer ; and these three names
indicate the most striking features of the fauna and flora.
Plants are found at
several horizons, and especially in the estuarine and
terrestrial deposits. Undoubtedly the special characteristic of the period is the
predominance of cycads, such as Williamsonia and Nilssonia, and ferns.
Conifers are also well represented.
Corals are abundant
in the limestones, but rare in the clays. The rugose corals
of the Palaeozoic period have disappeared, and in their place are aporose and
perforate types. Amongst the common genera are Montlivaltia, Isastrea,
Thecosmilia and Tliamnastrcea.
Of the Echinoderma
the crinoids are relatively much less abundant than in the
Pateozoic rocks, while the echinoids have become far more important. The
crinoids include Pentacrinus and Apiocrinus. The echinoids, which occur chiefly
in the limestones, include Cidaris, Plemicidaris, Acrosalenia, Nucleolites
(=Echinobrissus), Holectypus and Pygaster.
Brachiopods are still
common, but nevertheless they no longer preponderate as
in the Palaeozoic era. The period is characterised by the large number of
species and individuals belonging to the genera Terebratula, Magellania and
Rhynchonella.
Of the Mollusca, lamellibranchs,
gastropods and cephalopods are all abundant.
The most striking feature of the period is the great development of the
ammonites, which occur both in the clays and in the limestones. So abundant
and so widespread are they, and so varied in form, that they serve to divide the
system into zones which can be recognised over a large part of the globe.
Belemnites also appear, and attain their maximum in this period, They are found
chiefly in the clays.
So far as vertebrates
are concerned, the period was certainly the age of
reptiles. They played the dominant part in the sea
The earliest known
birds occur in the Jurassic system; two skeletons having
been found at Solenhofen, in Germany. They are known as Archceopteryx. The
tail was composed of a number of separate vertebrae, with a pair of feathers to
each.
Mammalia are found
in England in the Stonesfield Slate (Bathonian) and in the
Purbeck beds. Most of the forms appear to have been marsupials, but some
probably belong to a more primitive group (Prototheria). |
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Unlike the preceding
subdivisions of the Jurassic system, the Portland beds
have a very limited distribution, at least upon the surface. This is due, in the
south of England, chiefly to the unconformable overlap of the Upper Cretaceous
; and beneath the surface the Portland series may perhaps be continuous from
Dorsetshire into Buckingham-s lire. But from Bedfordshire to Yorkshire no
Portlandian beds are found, and there is no evidence that they exist below the
Cretaceous covering. Kemanie fossils belonging to the period are, however,
common in some of the succeeding strata; and it is not improbable therefore
that the Portlandian series may have been deposited even over this area, but
was removed by denudation before the Cretaceous beds were laid down.
The rocks of this series
are best displayed in the south of Dorsetshire,
especially in the Isles of Portland and Purbeck. Farther north they are exposed
at intervals along the margin of the Cretaceous outcrop, where the Cretaceous
escarpment has been cut sufficiently far back. This is the case in the Vale of
Wardour, the Vale of Pewsey near Devizes, at Swindon, and near Oxford and
Aylesbury. But beyond Leighton Buzzard the series is unknown.
Lithologically, the
Portlandian consists chiefly of limestones and sands. It is a
marine deposit, and fossils are abundant in the limestones, somewhat less
common in the sands. Although calcareous rocks form so large a part of the
series, neither corals nor echinoderms are particularly common. One species of
coral, however, Isastrea oblonga, is occasionally found in considerable
abundance, almost always in a silicified condition. Brachiopods are rare.
Gastropods, lamellibranchs and ammonites, the latter of enormous size, are the
principal fossils. In the limestones the shell has often been dissolved away, and
only the mould and cast are left. Oeriihium porilandicum, Trigonia gibbosa &
Perisphinctes giganteus are three of the commonest and best-known forms.
In Dorsetshire the
Portlandian falls naturally into two divisions,
viz.:—
Portland Stone or Upper
Portlandian.
Portland Sand or Lower
Portlandian.
The Portland, Sand,
which rests conformably upon the Kimeridge Clay, consists
of yellow and greenish-grey sands with beds of loam and clay, and occasional
layers of sandy or clayey limestone.
The Portland Stone
consists chiefly of limestone, which may be shelly, oolitic,
compact or chalky, with occasional sandy beds. Layers and nodules of chert are
common in some of the limestones. It is from this series that the famous
Portland building-stone is obtained. |
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From the north-eastern
part of Oxfordshire into Lincolnshire the Corallian series
is represented for the most part by clay, known as the Ampthill Clay. In some
places, for example at St. Ives in Huntingdonshire and Elsworth in
Cambridgeshire, there is, at the base, a hard calcareous band with ferruginous
oolitic grains. This band has now been traced for a considerable distance in
Huntingdonshire and Cambridgeshire, and is generally called the Elsworth Rock.
The Ampthill Clay is
darker in colour than the Oxford Clay, and contains more
carbonaceous material. It seems to represent only the Upper Corallian, and is
slightly unconformable to the
Oxford Clay.
The variable Corallian
series is followed by the Kimeridge Clay, one of the most
constant and uniform formations in England. It is a dark-grey or black shaly clay,
which weathers brown upon its surface and often contains much selenite and
pyrites. As a rule it is distinguished from the Oxford Clay by its darker colour and
more shaly structure ; also the layers of septaria are more frequent and more
persistent. Occasionally the shales are highly bituminous, and sometimes there
are bands of shaly limestone.
From Dorset to the
Yorkshire coast the same lithological characters are
preserved, and only the thickness varies. In the south of England the Kimeridge
Clay is about 1,200 feet thick, in Oxfordshire only about 100 feet.
In England the Kimeridgian
series was evidently deposited at a considerable
distance from the shore. The sediment is always fine and without admixture of
coarse material.
Fossils are everywhere
abundant. The dominant groups are those which flourish
in a sea with a muddy bed. Corals and echinoderms are rare ; gastropods are
not very common. A few species of brachiopods are abundant, but the principal
fossils are lamellibranchs and ammonites. Aptychus (probably the operculum of
Ammonites) is very common. Numerous remains of reptiles have been found,
including Ichthyosaurus, Plesiosaurus, Pliosaurus, various Dinosaurs and other
forms. Fish teeth, spines and scales are not uncommon. In spite of the
carbonaceous character of some of the beds, plants are rare. except in
Scotland.
Owing to the general
uniformity of the deposits no lithological subdivisions can
be traced.
Palaeontologically,
the series may be divided into two main divisions:—
Upper Kimeridge.
Lower Kimeridge.
Amongst the common
and characteristic fossils of the lower division, besides
ammonites, are Ostrea deltoidea, Astarte supracorattina and Rhynchonetta
inconstans.
In the upper division
Orbiculoidea latissima is very abundant in some beds.
Exogyra virgula is also very common, but it extends downwards into the lower
division.
On account of the overlap
of the Cretaceous strata, the Kimeridgian does not
form so continuous a belt as the earlier series of the Jurassic system. In
Bedfordshire, for example, the Cretaceous beds spread over it and pass on to
the Oxford Clay. |
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The Corallian, like
the Lower Oolites, is a variable series, and exhibits two
distinct facies. In the south of England and in Yorkshire it consists chiefly of
calcareous rocks, while from Bedfordshire to Lincolnshire it is formed almost
entirely of clay. But both clay and limestone are marine, and in England there is
no indication of estuarine conditions.
The calcareous facies
consists in general of shelly limestones, current- bedded
oolites, masses of coral, together with occasional layers of sand or clay. Ripple
arks and worm-burrows are found in some of the sands, and lignite is not
uncommon. Such deposits must have been formed in a shallow sea where the
water was clear and free from mud.
The argillaceous facies,
on the other hand, consists of finer sediment and shows
no false-bedding. It was probably laid down in deeper but more muddy water.
In the calcareous beds
corals, echinoderms, gastropods and lamellibranchs are
the most abundant forms. Ammonites and belemnites are also found.
Brachiopods and polyzoa are comparatively rare. Of the corals, Isaslrea
explanata, Thecosmilia annularis and Thamnastrea amalmoidea are the
species most often found. Nucleolites scutatus is the commonest of the
echinoids; Hemicidaris intermedia, Cidaris florigemma and Pygaster umbrella
are also frequently met with. The gastropods include Nerincea, Pseudomelania,
&c. Trigonia clavellata is one of the characteristic lamellibranchs. Amongst the
principal ammonites are Cardioceras vertebrata and Perisphinctes martelli.
The argillaceous facies
contains few corals or echinoderms. Lamellibranchs,
ammonites and belemnites are the characteristic fossils. The fauna is, to a large
extent, a mixture of Oxfordian and Kimeridgian forms, including, for example, the
common Oxford Clay species Gryphasa dilatata, and the Kimeridge Clay
species Ostrea deltoidea. But it includes also a certain number of the
characteristic species of the calcareous Corallian. The commonest fossil is
Ostrea discoidea.
Owing to the great
variability of the deposits, the lithological subdivisions are
only of local value.
Palaeontologically,
the series may generally be divided into:—
Upper Corallian.
Perisphinctes
martelli.
Lower Corallian.
Cardioceras
vertebrata.
In the south-west of
England the Corallian consists of calcareous grits, rubbly,
shelly and oolitic lime-stones, with occasional beds of clay. They may be
grouped as follows:—
Upper Calcareous Grit
and Upper Coral Rag. Coral Bag and Coralline Oolite
Lower Calcareous Grit
The deposits are very
variable. Generally, however, the Lower Corallian
consists of calcareous sandstone, forming the 'Lower Calcareous Grit.'
The Upper Corallian
consists chiefly of shelly limestones (the Coral Rag), oolitic
limestones (the Coralline Oolite), and sometimes the top beds are arenaceous,
forming an ' Upper Calcareous
Grit.'
At Upware, about nine
miles north of Cambridge, there is a small isolated mass
of coral rag and coralline oolite similar to the calcareous Corallian of the south
of England, and containing a similar series of fossils. A small boring has shown
that at the base there is a hard band of calcareous ferruginous oolite like the
Elsworth Rock. |
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The variable group
which has just been described is followed conformably by a
much more uniform series known as the Oxfordian. It consists for the most part
of bluish, greenish, or lead-coloured clay, which usually becomes brown or
yellowish at the surface. Generally the lower part is more or less laminated and
shaly, but the upper part often shows little signs of bedding. Calcareous bands
and irregular lines of septaria occur, and, as in most of the great clay
formations, there is often much pyrites and selenite.
Towards the base of
the clay there are inconstant beds of sand and calcareous
sandstone, often with a more or less marked concretionary structure. In some
areas these sandy beds become so prominent that they have received a
special name, and the Oxfordian is divided lithologically into two divisions—
Oxford
Clay;
Kellaways
Beds.
But the development
of the sandy deposits varies considerably, and accordingly
the line between the Kellaways Rock and the Oxford Clay does not run along a
constant horizon.
The general uniformity
of the Oxford Clay, the fineness of the material and the
absence of false-bedding indicate a deeper sea than that in which the Lower
Oolites were deposited—a sea so deep or tranquil that the sediments lay
beneath the limits of action of waves and currents.
The fossils of the
Oxfordian are naturally such as lived in a muddy sea. Corals
and echinoderms are rare. Neither brachiopods nor
gastropods are particularly
common, although some species are occasionally
found in abundance. It is the lamellibranchs, especially 'he oyster-like forms, and
the cephalopods, both ammonites and beleranites, that constitute the most
characteristic part of the Oxfordian fauna. Gryphaea dilatata is one of the
commonest fossils in the upper beds, while O. bilobata is found in the Kellaways
Rock. Of the belemnites, B. Oioeni is the species which is most widely known.
The ammonites, as in the other clay formations of the Jurassic system, serve as
zone-fossils.
Macrurous Crustacea
are fairly common, but brachyurous forms are rare.
Fish are not very abundant.
Remains of reptiles, such as Mcgalo- saurus,
Ichthyosaurus, Pliosaurus, &c., are found.
Fragments of plants
are sometimes numerous, but the species are
unrecognisable.
Palaeontologically
the Oxfordian may be divided into a number of ammonite
zones, but they have not yet been traced across the country. In the south and
centre of England the following broader divisions may be recognised:—
Upper Oxford Clay.
Qryphcea dilatata,
ammonites of the genus Cardioceras.
Middle Oxford Clay.
Quenstedticeras lamberti,
Peltoceras athleta.
Lower Oxford Clay.
Cosmoceras jason,
Cosmoceras duncani.
Kellaways Beds.
Sigaloceras calloviensis.
Owing to the general
uniformity of the series, it is unnecessary to give any
separate account of its development in different parts of England. Over most of
the country it varies only in thickness and in the amount of arenaceous material
at the base. |
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