THE MODES OF ACCUMULATION OF CLAYS
From whatever sources clays may have been originally derived, the manner
in which they have been accumulated in their present positions is a
factor of great importance both in regard to their chemical and physical
characters and their suitability for various purposes.
As explained in Chapter III, the china clays or kaolins may usually be
regarded as primary clays derived from granitic or other felspathic or
felsitic rocks by chemical decomposition. Such clays are found near to
their place of origin, are usually obtainable in a comparatively pure
state and are generally deficient in plasticity. They may occur in beds
of small or great depth, but these are not 'accumulations' in the
ordinary meaning of that term.
Residual clays (p. 70) also form a distinct class, as unlike the
majority of argillaceous materials they are left behind when other
substances are removed, usually by some process of solution. In many
cases, however, the residual clays are really secondary in character,
having been transported from their place of origin, together with
limestone or other minerals, the mixture deposited and subjected to
pressure and possibly to heat, whereby a rock-like mass is formed. This
mass has then been subjected to the solvent action of water containing
carbon dioxide or other substances which dissolve out the bulk of the
associated minerals and leave the residual clay behind.
The chief agents in the transport and accumulation of clays are the
_air_, in the form of wind; _water_, in the form of rain, streams,
rivers, floods, lakes and seas, or in the form of ice and snow as in
glaciers and avalanches; _earth-movements_ such as the changes wrought
by volcanoes, earthquakes and the less clearly marked rising and falling
of various portions of the earth's crust which result in folded,
twisted, sheared, cracked, inclined, laminated and other strata.
These agents have first moved the clay from its original site and have
later deposited it with other materials in the form of strata of widely
varying area and thickness, some 'clay' beds being several hundred feet
in depth and occupying many square miles in area, whilst others are in
the form of thin 'veins' only a few inches thick or in 'pockets' of
small area and depth. These deposits have in many places been displaced
by subsequent earth-movements and have been overlain by other deposits
so as to render them quite inaccessible. Others have been covered by
deposits several miles thick; but the greater part of the covering has
since been removed by glacial or other forces, so that clays of
practically all geological ages may be found within the relatively small
area of Great Britain.
The Transportation of Clays.
By the action of wind or rain, or of rain following frost, the finer of
particles clay are removed from their primary site and as the rain drops
collect into streamlets, these unite to form streams and rivers and the
clay with its associated minerals is carried along by the water. As it
travels over other rocks or through valleys composed of sandstone,
limestone and other materials, some of these substances are dislodged,
broken into fragments of various sizes and with the clay are carried
still further. In their journey these materials rub against each other
and against the banks and bed of the stream, thereby undergoing a
prolonged process of grinding whereby the softer rocks are reduced to
very fine sand and silt which becomes, in time, very intimately mixed
with the clay. If the velocity of the stream were sufficiently great,
the mixed materials--derived from as many sources as there are rocks of
the districts through which they have passed--would be discharged into a
lake or into the sea. Here the velocity of the water would be so greatly
reduced that the materials would gradually settle, the largest and
heaviest fragments being first deposited and the finer ones at a greater
distance.
With most streams and rivers, however, the velocity of the water is very
variable, and a certain amount of deposition therefore occurs along the
course, the heavier particles only travelling a short distance, whilst
the finer ones are readily transported. If the velocity of the stream
increases, these finer particles (which include the clay) may become
mixed with other particles of various sizes and the materials thus
undergo a series of mixings and partial sortings until they are
discharged at the river mouth or are left along its sides by a gradual
sinking of the water level. The clay will be carried the whole course of
the river, unless it is deposited at some place where the velocity of
the water is reduced sufficiently to permit it to settle.
If floods arise, the area affected by the water will be increased. The
_alluvial clays_ have, apparently, been formed by overflowing streams
and rivers, the material in suspension in the water being deposited as
the rate of flow diminished. Such alluvial deposits contain a variety of
minerals--usually in a very finely divided state--clay, limestone-dust
or chalk, and sand being those most usually found.
_River-deposited clays_, _i.e._, those which have accumulated along the
banks, are characterized by their irregularity in thickness, their
variable composition and the extent to which various materials are mixed
together. This renders them difficult to work and greatly increases the
risks of manufacture as the whole character of a fluviatile clay may
change completely in the course of a few yards.
According to the districts traversed by the water, the extent to which
the materials have been deposited and re-transported and the fresh
materials introduced by earth-movements, river-deposited clays may be
(_a_) _plastic_ and sufficiently pure to be classified as 'clays,' (_b_)
_marls_ or clays containing limestone-dust or chalk thoroughly mixed
with the clay, and (_c_) _loams_ or clays containing so much sand that
they may be distinguished by the touch from the clays in class (_a_).
Intermediate to these well-defined classes there are numerous mixtures
bearing compound names such as sandy loams, sandy marls, argillaceous
limestone, calcareous sands, and calcareous arenaceous clays, to which
no definite characteristics can be assigned.
To some extent a transportation of clays and associated materials occurs
in _lakes_, but the chief processes there are of the nature of
sedimentation accompanied by some amount of separation. On the shores
of lakes, and to a much larger extent on the sea coasts, extensive
erosion followed by transportation occurs continuously, enormous
quantities of land being annually removed and deposited in some portion
of the ocean bed. The erosion of cliffs and the corresponding formation
of sand and pebbles are too well known to need further description. It
should, however, be noticed that the clay particles, being much finer,
are carried so far away from the shore that only pebbles and sand remain
to form the beaches, the finer particles forming 'ocean ooze.'
The action of the _sea_ in the transport of rock-materials is more
intense than that of rivers, the coasts being worn away by repeated
blows from the waves and the pebbles and sand grains the latter contain.
The ocean currents carry the materials dislodged by the waves and
transport them, sometimes to enormous distances, usually allowing a
considerable amount of separation to take place during the transit. In
this way they act in a similar manner to rivers and streams.
_Glaciers_ may be regarded as rivers of ice which erode their banks and
bed in a manner similar to, but more rapidly than, streams of water.
Owing to their much greater viscosity, glaciers are able to carry large
boulders as well as gravel, sand and clay, so that the materials
transported by them are far more complex in composition and size than
are those carried by flowing water.
[Illustration: Fig. 14. Illustrating the successive deposition of
different strata.]
Separation and Sedimentation.
The clay and other particles having been placed in suspension in water
by one or more of the natural forces already mentioned, they soon
undergo a process of sorting or separation, previous to their
deposition. The power of water for carrying matter in suspension depends
largely on its velocity, and when this is reduced, as when a river
discharges into a lake or sea, the larger and denser particles at once
commence to settle, the smaller ones remaining longer in suspension,
though if the velocity of the water is reduced sufficiently all the
particles will be deposited. Hence, the deposits in lakes (_lacustrine_)
and at the mouths of rivers (_estuarine_) increase more or less
regularly in fineness according to their distance from the point at
which the water enters, the gravel and stones being deposited first,
then the coarse sand, next the finer sand and finally the silt and clay
(fig. 14). If cross-currents are present, the deposits will, naturally,
be made more irregular, and in some cases variations in the flow of the
transporting water may cause the coarser particles to be carried further
than usual so that they may cover some of the finer deposits previously
formed; but as the clay and silt particles are so much finer than sand
and gravel they usually travel so far before settling that their
deposits are very uniform if the area over which they are spread is
sufficiently large. Lake-deposited clays are for this reason more
uniform than estuarine beds, whilst beds deposited at considerable
depths in the sea and at a great distance from land are still more
uniform.
A _lacustrine clay_ is usually more persistent and uniform than
fluviatile beds though sometimes difficult to distinguish from the
latter. Some of the most valuable clay deposits are of lacustrine
formation; their comparative purity and great uniformity enabling ware
of excellent colour and texture to be produced without much difficulty.
Thus the Reading mottled clays of the Hampshire basin, on the outskirts
of the London basin and in Northern France are well known for the
admirable red bricks, tiles and terra-cotta produced from them. Still
purer clays deposited at Bovey Heathfield in Devonshire are also of
lacustrine origin, though they differ in many respects from the ordinary
lake-deposited clays and are of unusual thickness for deposits formed
in this manner.
[Illustration: Fig. 15. Lacustrine clay at Skipsea. (_By courtesy of T.
Sheppard Esq._)]
The greater purity of lacustrine clays, as compared with fluviatile
ones, is attributed to the much larger area over which the deposit is
spread,--enabling variations in the deposits to be much less noticeable
than when a smaller area is covered--and to the very small velocity of
the water in lakes, whereby all the coarser particles are deposited a
considerable distance away from the clays and silts.
Ries (6) has pointed out that many (American) lake-clays are of glacial
origin, having been laid in basins or hollows along the margin of the
ice-sheet or in valleys which have been dammed by an accumulation of
drift across them. Such clay beds are usually surface deposits of
variable thickness and frequently impure. Like all lacustrine deposits
they show (though in a more marked degree than in the older and larger
lakes) alternate layers of sand and clay, though the former are usually
too thin to be noticeable except for their action in enabling the
deposited material to be easily split along the lines of bedding.
_Estuarine deposits_ partake of the nature of both fluviatile and marine
beds, according to their position relative to the river from which they
originate. They are usually uncertain in character and are often
irregular in composition owing to the variations in the flow of the
water. The Estuarine clays of Great Britain--with the possible exception
of the Jurassic deposits in Northamptonshire and Lincolnshire--are of
minor importance, but in some countries they form a valuable source of
clay.
[Illustration: Fig. 16. Clay at Nostel, showing Marine Band. (_By
courtesy of T. Sheppard Esq._)]
_Marine clays_ are, as their name implies, those deposited from sea
water. They are frequently found at a considerable distance from the
shores of the ocean in which they were laid down, and subsequent risings
and fallings of the surface of the earth have so altered the areas
occupied by sea water, that a large number of marine deposits now form
dry land. Though usually of enormous size and of generally persistent
character, marine clay deposits vary considerably in the composition of
the material at different depths, as well as in different areas. This is
only to be expected from the manner of their deposition, from the varied
sources of the material and from the numerous river- and ocean-currents
taking part in their formation. For this reason it is generally
necessary to mix together portions of the deposit drawn from various
depths in order to secure a greater uniformity than would be obtained if
a larger area were to be worked to a smaller depth.
The Oxford clay which extends from the centre of England to the centre
of France is a typical marine clay.
At the bottom of all oceans at the present day is a deposit, of unknown
thickness, of red calcareous clay or _ooze_ which is steadily increasing
in amount and is thereby forming a fresh marine deposit, though at
present its inaccessibility deprives it of all economic value.
It is important not to overlook the enormous part played by variations
in the level of the land relative to that of the ocean in past ages. For
instance, there is abundant evidence to show that practically the whole
of Great Britain has been repeatedly submerged to great depths and has
been raised to heights far greater than its present average. These
oft-repeated risings and settlings have caused great changes in the
nature of the deposited materials so that in the Coal Measures, for
example, there are deposits of obviously fresh-water origin sandwiched
in between others undoubtedly marine. It can readily be understood, as
stated by Arber (24), that if, at a given period, the dry land during
the formation of the Coal Measures gradually subsided, it would first be
covered with clear water, whilst from those portions of the area which
occupied the higher ground the rivers and streams continued to pour into
their estuary a large amount of fresh-water material. Later, a stage
would be reached when mud of marine origin invaded the area and covered
the previous deposits. When, after an indefinitely long period, the
ground again rose, fresh-water deposits might again form, and this
alternation of marine and fluviatile deposits appears to have been
repeated with great frequency during the Carboniferous period.
In the Lower Coal Measures of Yorkshire and Lancashire, Stopes and
Watson (23) have shown that the shales forming the roof of the Upper
Foot Coal were derived from drifted sediments of marine origin.
Precipitated Clays.
If the plasticity of some clays is really due to the colloidal nature of
their particles, it is obvious that they must have been formed by a
process of coagulation or precipitation at a distance from the site of
the minerals from which they have been derived. According to the
'colloid theory,' felspar and other alumino-silicates are decomposed by
'weathering,' the chief effect of which is the formation (by hydrolysis)
of a colloidal solution of 'clay.' This apparently clear solution flows
along in the form of a small streamlet, joins other streamlets and
continues its journey. So long as it is quite neutral or contains free
alkali the solution will remain practically clear, but as soon as acids
enter the stream, or are formed in it by the decomposition of organic
matter, a coagulation of the colloidal matter will commence and the
amount of 'clay' thus thrown out of solution will depend on the amount
of such free acid.
If the coagulation or precipitation occurs in still water, the 'clay'
will be deposited almost immediately, otherwise it will be carried
forward until it reaches a place where it can be deposited in the
manner already described.
Such precipitated clays need not necessarily be pure, as other
substances may be present in colloidal form and may be coagulated at the
same time as the clay. In addition to these, the admixture of sand and
other minerals present in suspension in the solution may become mixed
with the particles during coagulation and be deposited with them.
Clays formed in this manner are extremely difficult to identify on
account of the highly complex nature of the reactions occurring in their
vicinity both during and subsequent to their formation.
Re-Deposited Clays.
Although many clays and other materials have been transported and
accumulated in the manner described, the majority of those now available
have been subjected to repeated transportation and deposition, owing to
the frequent and enormous changes in the relative levels of land and
water during the various geological epochs. So far as can be
ascertained, it is during these changes of position and the recurrent
exposure to air and to water containing various substances in solution,
together with the almost incessant grinding which took place during the
transportation and deposition, that most secondary clays became
plastic. If this is the case, it explains the impossibility of
increasing the plasticity of clay by artificial means, at any rate on a
large scale.
The simplest of the agents of re-deposition are rain-storms and floods
which, forming suddenly, may cause the water of a stream or river to
flow with unwonted velocity and so carry away previously formed deposits
of various kinds. Clays transported in this way are termed by Ries (6)
_colluvial_ clays, the term 'diluvial' is generally employed in this
country. If these are derived from a primary clay which has not
travelled far since it left the original granite from which it was
formed, they will usually be white-burning and of only slight
plasticity, but if the flood affects materials which have already been
re-deposited several times, the colluvial clays may be of almost any
imaginable composition. Floods of a different character--due to the
subsidence of the land so that it is partially covered with lake- or
sea-water, which beats on its shores and erodes it in the manner already
described--are also important factors in the transportation of clays.
So far as clays are concerned, the action of the sea is both erosive and
depository, though the sedimentation in it being that of the pelagic
ooze at great depths the clayey material is quite inaccessible. Under
certain conditions, however, the sea may erode land in one area and may
return the transported material to the land in another area. The
diluvial clay-silt known as _warp_ in the valley of the Humber is of
this character.
Quite apart from the action of water, however, much denudation,
transportation and re-deposition of clays and associated materials has
been due to the action of ice in the form of glaciers, though these do
not appear to have had much effect in increasing the plasticity of the
clays concerned.
_Glacially deposited clays_ are characterised by their heterogeneous
composition, some of them containing far more sand than true clay,
whilst yet retaining a sufficient amount of plasticity to enable them to
be used for rendering embankments impervious and for the manufacture of
common bricks, and, occasionally, of coarse pottery; others contain so
much sand as to be useless for these purposes. Most glacial deposits
contain a considerable proportion of stones and gravel which must be
removed before the clay can be used.
The large proportion of adventitious matter is due in great part to the
much greater erosive force and carrying power of ice as compared with
water, resulting in much larger pieces of material being carried, and as
the whole of the ice-borne material is deposited almost simultaneously
when the glacier melts, only a very small amount of separation of the
material into different grades takes place. The comparative freedom from
coarse sand of some glacial clays shows that some sorting does occur,
but it is very limited in extent as compared with that wrought in
materials which have been exclusively transported by water.
For the manufacture of bricks, tiles and coarse pottery in Yorkshire,
Lancashire and some of the more northern counties of Great Britain,
glacially deposited clays are of great importance in spite of their
irregular composition. They are frequently termed 'boulder clays' or
'drift clays' (p. 65), but in using these or any other terms for clays
transported by glacial action it is important that they should not be
understood to refer to the whole of the deposited matter. Large
'pockets' of coarse sand and gravel frequently occur in deposits of this
character and veins of the same materials are by no means uncommon. The
custom of some geologists of referring to the _whole_ of a glacial
deposit as 'boulder clay' has, in a number of cases, led to serious
financial loss to clayworkers who have erroneously assumed that, because
some 'boulder clays' are used for brick and tile manufacture, all
deposits bearing a similar title would be equally suitable. This
difficulty would largely be avoided if, as is now increasingly the case,
the term 'drift' or 'glacial deposit' were used for the deposits as a
whole, the term 'boulder clay' being restricted to the plastic portions
and not including pockets of sand, gravel and other non-plastic
materials.
_Boulder clays_--using this term in the limited sense just
mentioned--consist of variable quantities of sand and clay, stones and
gravel being generally associated with them. The stones may usually be
removed by careful picking, and the gravel by means of a 'clay cleaner'
which forces the plastic material through apertures too small to allow
the gravel to pass. The plastic material so separated is far from being
a pure clay and may contain almost half its weight of sand, the greater
part of which is readily separated by washing the material.
Boulder clays, when freed from stones and gravel, are sufficiently
plastic to meet the needs of most users, without being so highly plastic
and contractile as to necessitate admixture with sand or similar
material.
Some boulder clays contain limestone in the form of gravel or as a
coarse powder produced by the crushing of larger fragments. These are
less suitable for manufacturing purposes as the lime produced when the
articles are burned in the kilns is liable to swell and to disintegrate
them on exposure.
Owing to their origin and the nature of the impurities they contain,
boulder clays are never pure and when burned are irregular in colour and
somewhat fusible unless subjected to some process of purification.