THE SANDSTONES
THE ORIGIN OF SANDS
The essential characteristic of Sandstone is that it consists mainly
of detrital grains of quartz, or occasionally of grains of chalcedonic
silica (flint); these are found to scratch the steel blade of a knife,
and are not affected by boiling in ordinary acids. The grains usually
become cleaner in the boiling process, since the cement that has
bound them together is liable to be destroyed. This cement may cause
effervescence, being often formed of chemically deposited calcium
carbonate.
When we consider the distribution of quartz in nature, we look
to igneous and metamorphic rocks for the origin of the grains in
sandstone. Quartz is one of the commonest minerals; but in granite and
quartz-diorite it rarely forms more than half the bulk of the rock,
felspar and mica and hornblende being its associates. Veins of quartz
(quartz-rock) traverse many rocks, and become broken up into granular
forms on weathering; but they are inconsiderable in comparison with
the bulk of the slates or schists in which they lie. Mica-schists
contribute a good deal of quartz-sand when they decay; but this is
mixed with ferruginous clayey matter, and the soils produced are yellow
loams.
We are easily impressed, then, by the enormous amount of denudation
that was requisite to produce our existing sandstones. Though nowadays
sandstones can be built up by the decay of older rocks of the same
kind, the quartz must have come originally from igneous or metamorphic
sources. Even in the metamorphic rocks, a large part of the quartz is
probably detrital.
The microscopic characters of the quartz in sandstone commonly attest
its origin. The minute liquid inclusions, with moving bubbles, that
arise in the quartz of igneous and metamorphosed rocks, are easily seen
in sections of sandstone. In some quartzites, these inclusions run in
continuous bands from grain to grain, and have clearly arisen since the
detritus was cemented. But in ordinary sandstones the inclusions in
one grain have no relation to those in its neighbours. The felspars,
moreover, of igneous rocks are commonly found, as rolled fragments, in
sandstone. Their grains are usually whiter and duller than those of
quartz, and may easily be distinguished by the naked eye.
Small gleaming plates of mica from the parent rock may accumulate with
the quartz grains. The dark micas of decaying rocks, rich in iron and
magnesium, together with mineral silicates of calcium, magnesium, and
iron, such as the amphiboles and pyroxenes, form on hydration soft
green chlorite. This mineral, in films and easily deformed flakes,
at times occurs as a sort of groundwork to the coarser grains in
sandstone, and colours the rock a delicate grey-green. Fine-grained
sandstones of this type are difficult to distinguish from altered
"greenstones," such as basaltic andesites. When the quartz grains,
however, are large, as in the grits quaintly styled in old days
"greywacke," they form a ready clue to the origin of the rock.
Nature sifts the products of decay so thoroughly, on any slope exposed
to wind or rain, that the finest materials are carried far away, and
the undecomposable quartz remains predominant. The alluvium in the
upper reaches of streams is thus far more sandy than the mixed material
supplied at the outset from the surrounding rocks. The more rapid flow
of the water on the steeper upland slopes naturally removes the mud
into the lowland.
When the detritus, still somewhat mixed, reaches a sea-shore,
wave-action is rapidly effective. Before the continual wash and
pounding of the water, any residual clay, and the finely comminuted
portion of the quartz, are carried down the coastal slope. The colour
of the sea after storms is sufficient evidence of the work that it
performs. Beaches, then, arrive at a great similarity of type. The
inviting yellow sands, formed of comparatively coarse material, occur
alike off shores formed of chalk, slate, granite, or boulder-clay.
[Illustration: Fig. 5. Sand Developing from Sandstone, in semi-arid
climate. Near Laingsburg, Cape of Good Hope.]
From the beginning of sedimentation, sands have thus tended to
accumulate, and to become cemented into sandstones. These rocks, in
turn uplifted and exposed, have yielded other sandstones. Since coarse
sand does not travel far from the region where it is washed out of the
parent rock, a thick mass of sandstone extending over many square miles
may waste away, and yet become perpetuated in the district. Sandiness
thus begets sandiness, and the physical conditions due to the presence
of sandstone may prevail through long geological epochs (Fig. 5).
Of course, a submergence beneath the sea may change all this in a brief
time; but wrinklings of the crust, raising the sandstones into severer
atmospheric levels, may only accelerate their decay and render the
surrounding lands more sandy.
THE CEMENTING OF SANDS
The cement of sandstones is very varied. On our modern coasts, springs
draining from a limestone land, or even running through banks of broken
shells, will deposit calcite in the interstices of the beach, until
slabs and shelves of conglomerate and sandstone arise in defiance
of the waves. On coasts where calcium bicarbonate is abundant, it
may be precipitated by any cause that diminishes its solvent. Mere
evaporation, and the escape of carbon dioxide from the water as it
is scattered into spray, lead to the deposition of a cement between
the grains of sand. As Linck[6] shows, calcite is thus laid down in
temperate waters, while aragonite forms fibrous crystals between the
detrital fragments on the flanks of tropic isles. Aragonite may also
arise from the action of ammonium carbonate or sodium carbonate on
calcium sulphate or calcium chloride in sea-water. Sands thus become
cemented by one or other form of calcium carbonate. They include,
moreover, calcareous algæ, foraminifera, and fragments of coral and
sea-shells.
Fossil shells are usually represented in older sandstones by mere
external and internal moulds. The texture of the rock allows of their
being dissolved in percolating waters, while in clays belonging to the
same geological series they may be exquisitely preserved.
In shallows, and especially in lakes, where soluble salts of iron
become readily oxidised, brown iron rust, the mineral limonite, is
continually forming at the surface and sinking to the bottom, where
it firmly cements the sand. A group of bacteria[29] extracts iron in
this form from the water of freshwater lakes and swamps, and greatly
aids in its accumulation. Though a red colour may appear also in marine
deposits, masses of red and purple conglomerates and sandstones may
reasonably be assigned a freshwater origin. Such rocks are usually
found to be devoid of marine fossils, and they often contain traces of
land plants.
Barytes (barium sulphate), which sometimes occurs in veins simulating
those of calcite, is an occasional cement of sandstone, evidently
arising from subterranean waters.
Bands of flint (chert) occur in certain sandstones, such as the Hythe
Beds of the English Lower Greensand Series. These are due to the
cementing of certain layers by chalcedonic silica, and the source
of this silica is seen in the hollow moulds of sponge-spicules, and
the glauconitic casts of their canals, that commonly remain. G. J.
Hinde[30] shows that in the Cretaceous examples, as in so many other
flints, the majority of the spicules are of the tetractinellid type.
Under arid conditions, as in parts of Africa, loose superficial sands
may become cemented by calcium carbonate, or even by silica, brought up
in water rising by capillary action from below.
The sand-dunes of the coast of our own islands, which cannot remain wet
for long, become in places toughened by a deposit of calcite derived
from the abundant shells of land-snails. In the Cape of Good Hope[31]
the dunes, as A. W. Rogers states, are converted by invasions of
calcium carbonate, "into hard rock through a distance of many feet from
the surface, and where repeatedly wetted and dried, as happens where
the sea has encroached upon old dunes, the rock becomes intensely hard
and weathers with a peculiarly jagged surface." The General Post Office
and the South African Museum in Cape Town are mainly constructed of
this recently consolidated rock.
The modern sandstones cemented by silica are still more interesting.
In the Cape of Good Hope, and notably in the Kalahari desert, they
form the intensely hard rock known as _Quartzite_[32]. The cementing
material is true quartz, which sometimes deposits itself in bipyramidal
crystals about the grains of sand. The molecules of such crystals are
arranged in continuity with the grouping of those in the original
detrital grain, as is proved in thin sections under the microscope by
the optical continuity of the quartz of the grain and of its coating.
As silica continues to be deposited, the coatings interlock, and the
rock passes into true quartzite. It is now often difficult to detect
the outline of the original grains. Such superficial quartzites may be
ten feet thick at most, with uncemented sand below. Rogers suggests
that the cementing process may have originated in shallow pools; but it
has obvious analogies with that which forms iron-pans and superficial
masses of calcium carbonate in regions where capillary waters are
subject to prolonged evaporation. H. G. Lyons[33] has attributed the
cementing of parts of the Nubian Sandstone in the desert of Lower
Egypt to the silica set free by the alteration of the felspars in the
rock. This change, he suggests, was accelerated by the infiltration of
sodium carbonate of local origin. Fossil trees in these strata have
been replaced by silica. A further example is recorded by Armitage[34]
from Victoria, where friable ferruginous Cainozoic sands have been
converted into quartzite. This type of rock, the hardest known, and
associated in our minds with high antiquity and metamorphic action,
proves, then, to be in process of construction at the surface at the
present day.
The observations of Rogers show that quartz and not mere chalcedony
is deposited on the grains of sand. The "crystalline sandstones" of
Permian and Triassic age in England may, then, have acquired their
remarkable characters at the actual epoch of their accumulation. This
is rendered the more probable by the recognised occurrence of arid
conditions, at any rate seasonally, when the strata in question were
laid down.
These English "crystalline sandstones" were described by H. C.
Sorby[35], who showed that the quartz deposited on the detrital grains
was in optical continuity with that of the grains themselves. J. A.
Phillips[36] regarded this quartz as crystallised out during the
kaolinisation of felspars. The phenomena of laterisation, however,
give us a further suggestion as to the origin of the secondary
silica. It is now well known that tropical processes of weathering,
with alternations of wet and dry seasons, allow alumina to be set
free from combination with silica, "lateritic" crusts thus arising
on a great variety of rocks. The felspars of a sandstone may, under
such conditions, become laterised rather than kaolinised, aluminium
hydrate being left, and the silica passing into solution and appearing
again in certain layers as cementing quartz. The almost complete
disappearance of silica from the more advanced laterites shows that
it has been carried away elsewhere, and the cement of quartzite may
thus be derived from rocks at a considerable distance. Just, however,
as the destruction of siliceous sponge-spicules implies the formation
of flint, so laterisation implies silicification as a complementary
process.
The fact that secondary quartz in quartzite often arises in the rock
itself is shown by the frequency of quartz-veins in quartzites, while
they are almost absent from associated slates or schists. Hence it
appears that a removal of silica goes on at some points, leading to an
infilling of all the cracks and interstices at another.
It is clear, then, that sandstones, according to the mode in which they
have been affected by percolating waters, may vary from the crumbling
uncemented condition, known as _Sand-rock_, to that hardest and
most resisting of rocks, quartzite. The permeability of sandstone is
responsible for a wide variety of types.
THE SAND-GRAINS OF SANDSTONE
Sandstones are originally permeable by water, not because they possess
a high percentage of pore-space, or "porosity," but because the pores
between the grains are large. Water can thus move easily by gravitation
through the mass. The capillary rise or spread of water is greatest in
materials of very fine grain, though in these it may be extremely slow.
For the most effective rise of water against gravity by capillary pull,
a large proportion of particles about ·02 mm. in diameter should be
present. Sand-grains, however, often measure ·5 mm. in diameter, and
the fine mud or highly comminuted sand between the coarser matter is
the cause of the spread of water through the mass when the supply comes
from a subterranean water-table. Rain, however, is of course readily
absorbed. It disappears so rapidly on some barren sandstone areas,
coated as they are by loose sandy soils, that vegetation cannot make a
start, even where water is supplied.
Daubrée, Sorby, and others have studied the characters of sand-grains,
and it has been pointed out, that agitated water buoys apart and
carries forward by flotation grains with a diameter of ·1 mm. or less.
Hence coarser grains may become rounded like pebbles, by friction
on the bottom of a stream; but small ones remain angular throughout
geological periods, and even when transferred from one sandstone
to another. When their surfaces have been cleaned by boiling in
hydrochloric acid, the sharpness and irregularity of the quartz grains
is strikingly apparent.
Mingled with these grains, in addition to the minerals previously
mentioned, many interesting crystals appear that have become
concentrated in the natural washing processes. Minute colourless
zircons and brown rutiles, derived from granite, have collected,
owing to their high specific gravity, in certain sands. Magnetite and
ilmenite may darken the mass; monazite and thorite, which are sought
after for their constituents cerium and thorium, become similarly
selected in alluvial hollows, owing to their density of 5. Whatever
gathers thus in sands may become preserved in sandstones, and the study
of thin sections of the latter under the microscope is fruitful in
suggestions as to their origin.
Some sandstones are remarkable for their highly rounded and almost
spherical grains. J. A. Phillips[38] compared these with the wind-worn
grains of deserts, which assume similar forms and a considerable
polish. Large quantities of sand are carried from arid lands into
rivers, into lakes, or into the sea, and hence well rounded grains,
in bedded rocks, and even in marine sandstones, may have had a desert
origin. J. W. Judd, when examining the deposits of Lower Egypt for the
Royal Society, commented on the extreme freshness of the felspathic
particles in sands accumulating in rainless areas, and recent
observations on the soils of semi-arid districts show their comparative
poverty in clay. Enough has been said to indicate the variety of
geographical considerations that may arise from the examination of
beds of sandstone. The grains often prove, especially in the coarser
types, to be fragments of rocks rather than isolated minerals, and thus
furnish a picture of the materials that formed the surface exposed to
denudation.
The sandstones of finest grain may be found in beds deposited almost on
the limits of sedimentation from the land, where they are interlocked
with material of truly pelagic origin. Marine muds often contain a high
percentage of comminuted quartz, and the study of shales and slates of
ancient days shows how this almost indestructible mineral finds its way
into beds that might easily be classified as clays[41].
SOME CHARACTERS OF SANDSTONE
Earth-stresses and shrinkage give rise to joints in sandstone, which
may not be so clean and sheer as those in limestone, but which affect
even the softer forms. Cemented sand-dunes of modern date tend to
break away along vertical planes. Firmer sandstones give rise to
stepped table-lands and "edges," and the resistance of many types to
atmospheric decay renders their stratified structure strongly apparent.
Small intervals in the process of deposition, or slight changes in
the coarseness of the sand brought down by currents, give rise to
laminated and flaggy types. Where a broad shore has been exposed
between tide-marks, the drying and compacting of the surface before
the next layer is laid down enables the latter to take a mould of the
inequalities of that below. Ripple-marks, sun-cracks, rain-prints, and
the footmarks of animals, are often preserved in this manner. Where the
shore is subsiding, they may persist through hundreds of feet of strata.
Naturally, the best examples of these casts, and of the original
structure in the underlying bed, occur where a little mud has been laid
down over the sandy flat. Clay by itself, if damp, does not retain the
impressions sufficiently long, and, when once thoroughly dried, it
crumbles when the next water overflows it. But a foundation of firm
sand with a thin mud-layer on its surface, as may be recognised in
some Triassic deposits, furnishes excellent records of local weather
or of the movements of errant animals. On the flat shores of lakes in
a semi-arid climate, the water may retreat for miles, and return,
perhaps months afterwards, when rains in the hills have given it a new
burden of detritus. Under such conditions, broad sun-cracked flats
may be preserved, with perhaps some plant-remains between successive
layers[938].
The castings and tracks of worms, and the tubes of boring species,
which are sometimes infilled by sand of a different colour, are common
in sandstones of all ages.
SILICEOUS CONGLOMERATES
The deposits of wave-swept beaches leave us _Conglomerates_ formed
of various types of pebbles, among which quartz-rock and quartzite
naturally predominate. In some cases the pebbles are ready formed when
they reach their resting-place. They come rolling out from lateral
torrents into the quieter waters of a main valley, as may be seen
in summer in the broad pebble-banks of the north Italian streams.
Thence they are washed by occasional floods into the great confluent
deltas that constitute the upper part of an alluvial plain, or into
lake-basins, where they promptly settle along the shore. But few such
pebbles, except from pre-existing conglomerates or gravels on the
shore-line, actually reach the sea. The rolled stones upon sea-beaches
are mostly the products of marine action on the spot. While the
fine sand-grains go seaward almost unharmed, the detrital stones,
offering far less surface in proportion to their mass, strike on their
neighbours as every wave shifts them on the beach, and soon assume a
rounded form.
The conglomerates ultimately consolidated may reveal stratification
only by the general arrangement of their pebbles. These can rarely
be spheres, since they are not as a rule turned over, but are pushed
this way and that until they acquire a flat ellipsoidal shape. They
lie with their flatter sides in planes parallel to one another.
Generally, however, alternations of coarser and finer beds mark out the
stratification even in conglomerates.
The sands of deserts include abundant stones and blocks of rock, and
the loose material becomes, moreover, sifted by the wind. True desert
sands may accumulate at one point, the very finest loamy material may
be carried away still farther to form fields of fertile _löss_, and
a rock-desert, formed of stones resting on bare surfaces, may remain
in large areas of the arid region. The loose stones here assume a
characteristic shape, and have been known under the German name of
_Dreikanter_. They are fairly flat below, and are cut away above by the
drifting sand into a form resembling a gable roof dipping at both ends.
Their surfaces are characteristically etched.
_Dreikanter_ have been found in beds that were formerly ascribed to
deposition on the shores of lakes, and it must now be borne in mind
that continued attrition by drifting sand affects mixed detritus on
a land surface much as the wash of waters does upon a beach. Certain
materials are cut away more rapidly than others, and the residue
assumes a more and more quartzose type. In this way, sandstones,
and conglomerates in which fragments of quartzite and vein-quartz
predominate over other constituents, may arise as æolian beaches on dry
land.
SANDSTONE AND THE LAND-SURFACE
The permeability of sandstone has already been referred to. The surface
offered by it is typically dry, and the soil, consisting mainly of
grains of siliceous sand, can neither retain the rain that falls nor
draw up water from below. The idea that trees can flourish on sandstone
soils because they require nothing from the soil itself is of course
erroneous. They depend to a large extent upon the materials set free
by the decay of certain grains, or of the cement of the underlying
sandstone. In proportion as the sandstone is impure, that is, the more
its constituents deviate from pure quartz, the more chance there is
that it will provide a fertile soil.
On the whole, however, areas of siliceous conglomerate and sandstone
are given over, even in temperate climates, to forest and heather.
Where the sandstone is still in the sand-rock state, bare patches are
likely to appear even in the heath that has grown across it, and from
these the wind carries away shifting sands.
Everyone familiar with the Carboniferous areas of the English midlands
will realise the influence of hard grit and sandstone in forming
"edges" across the country. The contrast between these escarpments and
the slopes of crumbling shale that often underlie them gives diversity
to the scenery of Yoredale and the Peak. The more yielding sandstones
of Cretaceous age round about the Weald, or at the foot of the Chiltern
Hills near Woburn, form rounded hills, mostly clad with woods of
coniferous trees. In Surrey, unpaved cart-tracks, used for centuries,
have cut gullies in the unconsolidated Folkestone Sands.
The underlying Hythe Beds, however, stand out between Reigate and
Guildford as a bold escarpment, and it is interesting to reflect that
this fine feature of south-eastern England is probably due to the chert
which the beds contain (see p. 62). The local growth of siliceous
sponges in a Lower Cretaceous sea enables Leith Hill in our days to
dominate even the arch of Ashdown Forest, where another unfilled
sandstone area rises in the centre of the Weald.
The sands of Bagshot Heath, and numerous similar areas in the Paris
Basin, show how impossible it is to cultivate such strata, even near
the best of markets. The flint gravels that cover much of the upland
in the New Forest may also be borne in mind, as presenting the worst
features of highly siliceous lands.
In a semi-arid climate, or one with only seasonal rains, the processes
by which sandstone begets sandstone tend to develop desert wastes. The
soils produced by weathering do not cake together, and are carried
away by wind during the drier months. The bare rock appears over broad
surfaces, just as it does in storm-swept limestone areas, and any
hollow where shelter is afforded tends to become filled with sand (see
Fig. 5).
The hummocky and extremely irregular surface of some of our Silurian
areas, such as parts of the Southern Uplands of Scotland and the
hard-won farmlands of Down and eastern Monaghan, is due to the presence
of resisting sandstones among the shales. These sandstones, passing
into true grits, are repeatedly folded, and their upturned edges have
resisted even the passage of glacier-ice. They jut out along the crests
of ridges, and even the smaller beds furnish angular fragments to the
soils.
[Illustration: Fig. 6. Siliceous Conglomerate. Characteristic
weathering; moraine-blocks at Coumshingaun, Co. Waterford.]
Far wilder scenery is formed by the more continuous sandstone masses of
the Harlech Beds in western Wales, which are grits so firmly cemented
that the rock breaks across the quartz-grains. Much of the Old Red
Sandstone is of equally hard quality (Fig. 6). Its purple or grey
conglomerates, the pebbles of which are quartzite in a quartz cement,
form bare and rugged masses in the Great Glen south-west of Inverness,
and are responsible in Kerry for some of the wildest rock-scenery in
the British Isles. Variations in coarseness allow of the development
of a marked stratification on the weathered mountain sides, and
differential erosion of the beds has taken place where ice has pressed
against them. Even on precipices, grassy ledges may occur, marking
bands of sandstone or shale in the conglomeratic mass.
The red sandstones and conglomerates that form huge outstanding bluffs
from Applecross to the north of Sutherland represent the denudation
of a pre-Cambrian mountain region. These Torridon Sandstones cover a
very irregular surface of old gneiss, with which their almost level
strata are in striking contrast. P. Lake[39] has compared them with
the deposits styled _dasht_ in Baluchistan and Afghanistan, which
similarly fill up valleys and cover hills, as products of extensive
and rapid denudation. There is much, indeed, to suggest that the
Torridon Sandstone, some 10,000 feet in thickness, was accumulated in
a dry country on a continental surface, with the aid of floods during
occasional rainy seasons.
Quartzite, which fractures into small angular blocks under earth
stresses, yields an intractable surface of bare rock and taluses of
shifting stones. The latter sometimes crumble down into white sand,
which provides some basis for the growth of heather. The numerous
joints, independent of the bedding-planes, cause the rock to break up
almost equally on any exposed slope, and the crests of quartzite hills
become typically converted into cones (Fig. 7). Viewed from a distance,
the white taluses, streaming down evenly from the crests, resemble caps
of snow.
[Illustration: Fig. 7. Quartzite Cone. Croagh Patrick, Co. Mayo.]
The absence of soil and the smoothness of weathered surfaces render
quartzite mountains hard to climb. The uniform cementing of the rock
leaves the bedding with little influence on the surface-features, and
rock-ledges and shelves are rare. The traveller ascends over taluses
of angular and obstinate blocks towards slippery and inhospitable
domes. But the wildness of the scenery will be his sure reward. It is
of interest to reflect that the material of these bold outstanding
mountains may in certain cases have originated, in all its hardness, in
the levels of a sun-parched plain.