THE LAW OF SUPERPOSITION.
In a previous chapter this law was given as follows: "Of any two
strata, the one which was originally the lower is the older;" the
general truth of the law depends upon the fact that except under very
exceptional circumstances the strata are deposited upon the surface of
the lithosphere, and not beneath it. There are occasions where strata
may be deposited beneath the lithosphere, but as a general rule the
geologist will not be misled by such occurrences. In caverns,
accumulations often occur which are newer than the strata over the
cavern roof, and so long as caverns are formed in ordinary sedimentary
rocks, no great difficulty will result from this exception to the law
of superposition. When caverns occur beneath masses of land ice, the
order of superposition may be misleading. A deposit may be formed on
the surface of the ice, and subsequently to this a newer deposit may
be laid down in a sub-glacial or englacial cavern; upon the melting of
the ice the newer deposit would be found with the older one resting
upon its surface.
Apart from these exceptional cases, the law as stated holds good, but
the reader will notice the insertion of the word 'originally' which
requires some comment.
A geologist speaks of one bed lying _upon_ another not only when the
beds are horizontal, but when they are inclined at any angle, until
they become vertical, so that until beds have been turned through an
angle of 90° by earth-movement the test of superposition is
applicable, but when they have been turned more than 90°, the stratum
which was originally lower rests upon that which was originally above
it, and in the case of these _inverted_ strata, the test of
superposition is no longer applicable. It was formerly supposed that
cases of inversion were comparatively rare and local, and that the
test of superposition could therefore be generally applied with
confidence, but it is now known that though this is generally true of
such strata as have been subjected only to those widespread, fairly
uniform movements which are spoken of as _epeirogenic_ or
continent-forming, where the radius of each curve is very long,
inversion is a frequent accompaniment of the more local _orogenic_ or
mountain-forming movements, where the radius of a curve is short.
Though orogenic movements are limited as compared with those of
epeirogenic character, they often affect large tracts of country, in
which case the apparent order of succession of the strata need not be
the true one, and examples of inversion may be frequent[9].
[Footnote 9: For a discussion of the principles of mountain-building
see Heim, A., _Untersuchungen über den Mechanismus der
Gebirgsbildung_, and Lapworth, C. "The Secret of the Highlands,"
_Geological Magazine_, Decade II. vol. x. pp. 120, 193, 337.]
It is not easy to lay down any definite rules for detecting inverted
strata, where the top of an inverted arch is swept off by denudation
or the bottom of an inverted trough concealed beneath the surface,
beyond stating that if an easily recognised set of beds is obviously
repeated in inverse order, inversion must have occurred, though even
then it may not be clear which side of the fold shows the beds in
original and which in inverted sequence. Suggestions are frequently
made that ripple-marks and worm-tracks may be utilised in order to
discover inversion, for the well-formed ripple-marks will appear
convex on the upper surface of a bed which is not inverted, and we may
note concave casts of these ripple-marks on the under surface of the
overlying bed, whilst worm-tracks will appear concave on the upper
surface, and their casts convex on the lower surface of the succeeding
bed under similar conditions. In the case of inversion the occurrences
will be the exact opposite to these. Unfortunately ripple-marks and
worm-tracks may, as will appear in the sequel, be simulated by
structures produced in quite a different way, and unless the observer
is certain that he is confronted with true ripple-marks and
worm-tracks he may be seriously misled. The geologist must take into
account all the evidence at his disposal, when he is dealing with
cases of possible inversion, but oftentimes he will after due
consideration of all the phenomena be left in doubt unless he is able
to supplement his observations on the succession of the strata by
evidence derived from the included fossils.
The test of superposition is most apt to be misleading when the strata
have been affected by the faults known as reversed faults or
thrust-planes.
Reference to text-books will show that a fold consists of two parts,
the arch and the trough, and that the two are connected by a common-,
middle-, or partition-limb. In the case of an inverted fold, an
=S=-shaped or sigmoidal structure is the result (Fig. 1 A).
[Illustration: Fig. 1.
A. A sigmoidal fold, showing a bed _xx_ in an overfold with arch
(_a_), trough (_t_) and common limb _c_.
B. A similar bed _xx_ affected by a thrust-plane _tt_ which replaces
the common limb.]
Here the portions of any bed (_xx_) which occur in the arch or trough
are in normal position, and have not been moved round through an angle
of 90°, whilst the portion which occurs in the common limb c has been
moved round through an angle greater than 90° and is inverted, so that
its former upper surface now faces downwards. In Fig. 1 B the common
limb is replaced by a reversed fault, or thrust-plane, and the
inverted portion of the bed seen in the common limb is therefore
absent. An observer, applying the test of superposition, might suppose
that the position of the bed _x_ on the left-hand side of the figure
was a different bed from the portion which is seen on the right-hand
side, instead of belonging to the same bed, and in this way, if a
number of parallel thrust-planes affected one bed or a set of beds, he
might be led to infer the occurrence of a great thickness of strata
where there was in reality a slight thickness, or even one bed only
repeated again and again by faulting. It is quite certain that
exaggerated estimates of the thickness of deposits have frequently
been made owing to the non-recognition of the occurrence of repetition
as a consequence of the existence of thrust-planes.
Where thrust-planes are suspected, it is well to look for some of the
following features:
(_a_) The strata of a country affected by thrust-planes often crop out
as lenticular masses, thinning out rapidly in the direction of the
strike[10]. This is true of beds thrown into sharp folds whether or
not inverted, but the lenticles will be wider in a direction at right
angles to that of the strike as compared with their length when
inversion has not occurred. It is also true of beds which were
originally deposited as lenticles, such as many massive sandstones,
and as almost any kind of deposit may be formed originally as a
lenticle, the test by itself is by no means sufficient as a proof of
thrusting, though it is suggestive.
[Footnote 10: For definitions of the terms dip, strike, outcrop and
allied expressions, the reader is referred to a _Text-Book of
Geology_.]
(_b_) The _surfaces_ of the strata are often affected by the
striations known as slickensides, and the joint-faces of gently
inclined beds are also frequently marked by slickensides which often
run in a nearly horizontal direction.
(_c_) A parallel structure presenting the appearances characteristic
of the mechanically-formed features of a foliated rock is often
developed, and one or more of certain accompanying phenomena will
probably be found, which will be noticed more fully in a later
chapter.
(_d_) Extension or stretching of the rocks will have been frequently
produced, causing rupture, and the resulting fissures are usually
filled with mineral-veins, though this occurrence is by no means
characteristic of rocks which have been affected by thrust-planes.
(_e_) Chemical changes may have occurred which have resulted in the
reconstitution of some of the rock-constituents, which may crystallise
where pressure is least, thus we often find rocks which have undergone
movements of the type we are considering marked by the existence of
sericitic films upon the surfaces.
Another reservation must be made when considering the law of
superposition. The test is only applicable for limited areas. Suppose
we find a deposit of clay _a_ resting upon another deposit of
limestone _b_ in the south of England, and can prove that the apparent
succession is the true one, that is, that there has been no inversion;
it is clear that the test of superposition is applicable in that area.
Now, we may be able to trace the two deposits continuously across the
country, one as a clay, the other as a limestone; so that when we
reach the north of England we find the clay _a_ still reposing upon
the limestone _b_. The test of superposition is applicable in that
area also, the clay of the northern area being newer than the
limestone of the same region. But, for reasons which will ultimately
appear, it by no means follows that the clay of the north is newer
than the limestone of the south, although the two deposits are
continuously traceable with the same lithological characters; it may
have been formed simultaneously with the limestone of the south, or
even before it. Something more, therefore, than the test of
superposition is necessary in order to make out the relative ages of
continuous deposits in a wide region, and this is still truer in the
case of deposits which are discontinuous, whether separated from one
another by the sea, or by outcrops of older or newer rocks.
A few words of warning may be added with reference to the detection of
bedding-planes. A bedding-plane is one which separates two beds, and
its existence is determined during the deposition of the beds. Many
other planes are formed in rocks subsequently to their deposition, and
it is not always easy to distinguish these from true bedding-planes.
That even experienced observers may be led astray is shown by the fact
that, of recent years, it has been proved that great masses of rock
have been claimed as of sedimentary origin, and their apparent order
of succession noted, which are in truth naught but irregular masses of
intrusive igneous rocks affected by divisional planes which simulate
bedding, produced in the rocks subsequently to their consolidation.
Joints, faults, and cleavage-planes may all at times simulate planes
of bedding, and it is frequently very difficult to distinguish them in
the limited exposures with which a geologist has oftentimes to deal.
It is easier to make suggestions for distinguishing bedding-planes
from other planes which simulate them, than to apply the suggestions
in practice, and the student of field geology will find that
experience is the only guide, though after years of experience he may
be confronted with cases where the evidence is insufficient to
convince him that he is dealing with planes of stratification and not
with some other structure.
From what has been remarked, it will be inferred that the test of
superposition though of prime importance to the geologist is
frequently insufficient to enable him to ascertain the true order of
succession of the strata, and he is compelled to supplement this test
by some other. There are several useful physical tests which may
frequently be applied. Thus, if a rock _a_ contains fragments of
another rock _b_, _under such circumstances as to show that the
fragments of_ b _were included in a during its deposition_, it is
clear that _b_ is older than _a_. Here again, it will be found from
what appears in a later chapter that the student is confronted with
difficulties when actually examining rocks, for fragmental rocks of
cataclastic origin, where the fragments have been formed as the result
of fracture produced by earth-movements subsequently to the deposition
of the rock, simulate epiclastic rocks in which the fragments were
introduced during the accumulation of the deposits to so surprising a
degree as sometimes to baffle the most experienced observer. Not only
are the fragments of these cataclastic rocks broken up, but they may
be further rounded so as to imitate in a remarkable manner the
water-worn pebbles of an epiclastic conglomerate. Again, an older
series of rocks may have had structures impressed upon them as the
result of changes subsequent to their formation, and before the
formation of a newer set which the latter therefore do not exhibit.
Jointing, cleavage, and various metamorphic phenomena may thus be
exhibited by the older rocks, but great care is required in applying
this test, especially with a limited thickness of rocks, as one set
may not exhibit the structures not because they were not in existence
when the structures were developed, but because their nature is such
that they were incapable of receiving or retaining the structures. For
instance a mass of grit which is older than a mass of clay-slate may
not be cleaved, because, although subjected to the pressure which
produced the cleavage, it was of a nature not adapted to the
development of cleavage structure.
On the whole, application of tests dependent upon physical features of
rocks, does not often supplement to any great extent the information
supplied by ascertaining the order of superposition, and in all
cases, where possible, every other kind of information should be
supplemented, by that which is acquired after examination of the
included organisms of the strata.