THE SILURIAN SYSTEM AND THE CHANGES WHICH OCCURRED IN BRITAIN AT THE
CLOSE OF SILURIAN TIMES.
_Classification._ The Silurian system was originally divided by its
founder, Sir R. I. Murchison, into three series, as follows:--
Ludlow Series
Wenlock "
Llandovery "
The term May Hill, proposed by Sedgwick, is sometimes used as
synonymous with Llandovery. This classification omits a somewhat
important set of beds intercalated between those of the Llandovery and
Wenlock series known as the Tarannon shales, and in Britain if we were
to classify afresh, it would be more convenient to include some of the
beds formerly referred to the Ludlow in the Wenlock. I shall, however,
adopt the old and well-established classification, adding the term
Tarannon to Llandovery, and speaking of the Llandovery-Tarannon
series. The nature of the two classifications is shown in the
following table:
Old New Palæontological
Stages. Classification. Classification. Classification.
1 Upper Ludlow } } }
2 Aymestry Limestone } Ludlow } Downtonian }
3 Lower Ludlow } } Fauna
} } with
4 Wenlock Limestone } } } _Encrinurus_
5 Wenlock Shale } Wenlock } Salopian }
6 Woolhope Limestone } } }
7 Tarannon Shales } } Fauna
8 Upper Llandovery } Llandovery } Valentian } with
9 Lower Llandovery } } } _Harpes_
[Illustration: Fig. 18.
L = Ludlow. W = Wenlock. Ll-T = Llandovery-Tarannon.]
_Description of the strata._ Lithologically the Silurian deposits of
Britain form a continuation of those of the Ordovician period, with a
local interruption due to the elevation of portions of Wales and the
Welsh borders at the close of Ordovician times. Elsewhere we find a
predominance of shales passing into grits at the top of the system,
the change indicating the incoming of the shallow-water phase before
the commencement of the second continental period. Particular stress
is laid upon the predominant shaley character of the beds, for, on
account of the richness and variety of the faunas of the calcareous
rocks, greater attention is naturally paid to them in geological
works, and the student may get a false idea of their relative
importance. An attempt is made below (Fig. 18) to give a general idea
of the variations in lithological characters of the Silurian rocks in
different parts of Britain.
The Silurian strata are mostly found in the same localities as those
which furnish exposures of the rocks of Ordovician age.
The development in the typical Silurian region of the Welsh borders is
characterised by the abundance of calcareous matter which is found
there as compared with that which exists in the other British
localities.
The Llandovery strata are sandy, often conglomeratic, with a fair
amount of calcareous matter in places. The arenaceous nature is
undoubtedly due to the proximity of land caused by local upheaval at
the end of Ordovician times, and the Upper Llandovery rocks sometimes
rest unconformably on the Lower ones, at other times on Ordovician,
Cambrian, or even Precambrian rocks. The Tarannon shales are light
green shales with intercalated grits. The Wenlock series consists of a
group of shales separating a lower, very inconstant, earthy limestone
from an upper, more constant, thicker and purer limestone. The latter,
the Wenlock limestone, is composed of fragments and perfect specimens
of various fossils, and the fragmentary nature of many of the shells
indicates the occurrence of wave-action and probable formation in
shallow water, in some places against coral-reefs.
The Lower Ludlow beds consist of sandy shales; they are separated from
the Upper Ludlow beds by an impure limestone, the Aymestry limestone.
The Upper Ludlow beds consist mainly of grits and flags, often
coloured red towards the summit.
In North Wales the Llandovery beds occasionally present the shelly
arenaceous types of deposit as near Llangollen, at other times as near
Conway, Corwen, and in Anglesey, the graptolitic shale type. They
also rest unconformably upon the Ordovician rocks in this area. The
Tarannon shales resemble those of the border county. The Wenlock
series consists essentially of shales, while the Ludlow development
differs from that of the borders in its greater thickness and the
absence of any calcareous band in the centre. In Central Wales the
graptolitic type of the Llandovery-Tarannon series is found, but the
graptolite-bearing shales of the Llandovery epoch are thin beds
occurring between grits and flags no doubt deposited in shallow water,
and this division of the series is of very great thickness.
In South Wales the Silurian rocks are very similar to those of the
Welsh borders, save that the calcareous deposits are fewer and
thinner.
The Lake District Silurian strata generally resemble those of North
Wales. The Llandovery-Tarannon rocks are of the graptolite-shale type,
intercalated with fine grits in the case of the beds of Tarannon age.
The Wenlock beds consist of shales, and the Ludlow beds of gritty
shales beneath, and massive flags and grits at the summit. These
Ludlow beds are here of great thickness (certainly not less than 7000
feet) and were obviously accumulated for the most part in shallow
water.
The Llandovery-Tarannon rocks of Southern Scotland show the two types
which prevailed in the Moffat and Girvan areas in later Ordovician
times. The Llandovery beds of Moffat are known as the Birkhill shales,
and are very thin. The representatives of the Tarannon shales,
however, the Gala beds, consist mainly of grits, and attain a great
thickness. In the Girvan area, the Llandovery beds are of the shelly
type. Here as at Moffat and in the Lake District there is perfect
conformity between the beds of Ordovician and those of Silurian age,
and accordingly it is instructive to note the completeness of the
palæontological break, especially in the Moffat district. The higher
Silurian beds of Southern Scotland present a general resemblance to
those of North Wales and the Lake District[80].
[Footnote 80: For descriptions of the Silurian beds of the typical
region see Lapworth and Watts, _Proc. Geol. Assoc._, vol. XIII. p.
297, those of Wales are described by Lake and Groom, _Quart. Journ.
Geol. Soc._, vol. XLIX. p. 426, and Lake, _ibid._ vol. LI. p. 9. A
description of those of Lakeland will be found in the Memoir of the
Geological Survey "The Geology of the Country around Kendal, etc."
while the Scotch Rocks are described in Lapworth's papers on Moffat
and Girvan.]
On the European continent we find indications of conditions similar to
those which prevailed during the Ordovician period; the strata become
much thinner and more calcareous in Scandinavia, and still thinner in
the Baltic provinces of Russia, where they consist very largely of
calcareous matter. In central Europe the greater abundance of
calcareous matter, compared with that which is found in the Ordovician
strata of that region, points to a change in physical conditions which
became still more marked after Silurian times.
In North America, the succession is very similar to that of Britain,
the calcareous development of the Silurian rocks being found around
Niagara, but towards the close of Silurian times the shallow-water
phase became marked in places by the deposition of chemical
precipitates which indicate the separation of a portion of the late
Silurian ocean from the main mass during the period of formation of
these abnormal deposits.
The conditions of Silurian times, until the advent of the
shallow-water phase, recall those of Ordovician times and point to a
wide expanse of ocean at some distance from the land, though the
earliest deposits become arenaceous where they were deposited against
an old land surface formed by the elevation of the Welsh Ordovician
rocks, which were denuded to supply this material. One marked
difference existed between the physical conditions of our area during
Ordovician and Silurian times, for the volcanic activity which was
rife during Ordovician times almost ceased during Silurian times,
except in the region now occupied by the extreme south-west of
Ireland, and accordingly volcanic material does not appreciably
contribute to the formation of the Silurian deposits. The shallowness
of the sea-floor at times is marked by the occurrence of masses of
reef-building corals in the limestones, and these probably indicate
the prevalence of a fairly warm climate, an inference supported by the
nature of the Gastropod fauna of Gothland, as noticed in Chap. IX.
The shallow-water phase commences fairly simultaneously over the whole
area at the beginning of the deposition of the Lower Ludlow rocks, and
becomes more marked in the Upper Ludlow rocks, being most noticeable
at their extreme summit, when a change occurred which will be
considered at the conclusion of this chapter.
_The Silurian Faunas[81]._ The Silurian period has been termed the
period of Crinoids, and this group of creatures certainly contained a
great variety of very remarkable forms, which are specially numerous
in the Wenlock Limestone of the Welsh borders, Gothland, and North
America, but many of the rocks of the system display few traces of
these organisms. The trilobites and graptolites still contribute
largely to the fauna, the latter becoming very scarce at the summit of
the system, though a few specimens have been detected in the rocks of
the succeeding system. The trilobites belong to few genera though
these are mostly more highly organised than those of the Ordovician
period. The genus _Harpes_ has been taken as fairly characteristic of
the lower part of the system in Sweden, and it occurs there abundantly
in places in Britain, whilst _Encrinurus_ is more abundant in the
upper series, but both of these genera range from higher Ordovician
beds into the Devonian. Mention has already been made of the corals.
Brachiopods are very abundant, and Mollusca appear with considerable
frequency. The appearance of true insects is of importance,
cockroaches have been recorded from Silurian rocks and a number of
other insects have lately been recorded from Canada[82]. Eurypterids
occur in considerable abundance in the higher parts of the system, as
do also the remains of fish.
[Footnote 81: For an account of the Silurian faunas the student may
consult Sir R. I. Murchison's _Silurian System_ or the shorter
_Siluria_ and Lapworth's paper on the Geological Distribution of the
Rhabdophora.]
[Footnote 82: See an article by Dr G. F. Matthew, "Description of an
extinct Palæozoic Insect and a review of the Fauna with which it
occurs," _Bulletin_ XV. _of the Natural History Society of New
Brunswick_. The Silurian Rocks of the Little River Group of St John,
New Brunswick, have yielded species of land snails, two doubtful
saw-bugs, several arachnids, and myriopods, two insects of the order
Thysanura (Spring-tails), and eight Palæodictyoptera.]
The close of Silurian times ushered in the second continental period
in Britain when a large part of our area and the adjoining areas to
the north and north-east were uplifted to form land, which in the case
of our area was interpenetrated by watery tracts, whose exact nature
is still a subject of dispute. Accordingly the deposits which were
formed during this period are local and in some cases abnormal, but
they will be considered in the next chapter. Simultaneously with the
formation of these deposits, uplift of the sea-floor converted wider
and wider areas into land, and this land underwent considerable
denudation, so that the tops of the anticlines were worn away. The
general trend of the anticlines was east-north-east and
west-south-west, and accordingly a series of mountain chains possessed
that direction, for the epeirogenic movements were accompanied by
orogenic ones. Between the regions of uplifts were depressions in
which sediments accumulated. The principal axes of uplift in our area
range through the North of Scotland towards Scandinavia, across the
Southern Scotch Uplands to the North of Ireland, through the Lake
District and through Wales. As the result of lateral pressure, a
cleavage structure was impressed on many of the Lower Palæozoic rocks,
the strike of the rocks extended in the direction of the ridges and
depressions, and the rocks as a whole became considerably compacted
and hardened, thus producing one of the most important portions of the
framework of our island, for although the ancient mountain chains were
largely denuded during their elevation, and their stumps were
afterwards covered by later deposits, upon the removal of these, the
ancient stumps were once more exposed as fairly rigid masses which do
not yield greatly to denuding influences, and accordingly stand out as
the most important upland regions of Britain at the present day.
It is interesting to notice, as an illustration of the now well
established fact that successive earth movements often occur in the
same direction, that the axes of the folds produced during this second
continental (Devonian) period, run parallel with the lines separating
tracts of different lithological characters. It has been seen that
the Ordovician and Silurian rocks of the Southern Uplands continue
into Ireland, and that the beds of similar characters run in belts
having a general east-north-east and west-south-west trend, which
accordingly must have been the direction of the coast-line parallel to
which they were deposited, and as that coast-line was due to uplift,
the movement which produced it would naturally produce foldings with
east-north-east and west-south-west trend. This is one of many cases
where the lines separating belts of rock having different lithological
characters run parallel to axial lines of folds which have been
produced in the rocks at a later period.
As the result of the existence of land over parts of north-west Europe
in Devonian times, it is comparatively rare to find a passage from
normal Silurian rocks into normal Devonian ones; there is often an
unconformity above the Silurian strata. As we proceed southwards
towards central Europe, where the epeirogenic and orogenic movements
died out, this is not the case, and we get complete conformity between
marine sediments of the Silurian and Devonian periods.