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ON MY WAY to work, I pass the floors of ancient seas, petrified hot
springs, and the foundations of volcanoes, all in about half an hour. I
work in the centre of London, yet I have found these exotic landscapes in
the stone that makes the bridges, pavements and office blocks of the concrete
jungle. Cities across the world are full of similar potential if you look
at the rocks. With a sharp eye and a little imagination, you can find the
story behind the rocks on your local high street.

I start my walk to work at Euston railway station. In this unlikely-sounding
setting there is plenty of rock, as tiles on the platforms and cladding
on the outside of the building. The platform tiles are pale in colour, a
mixture of pink, white and grey blotches, which resolve into angular shapes
if you look closely. They are made of granite, one of the commonest types
of city rock. People choose granite for buildings because, as well as its
attractive pale colour, it is tough and takes a good polish. Both these
properties come from the way in which the rock formed.

Granite is an igneous rock, so called because it formed when molten
rock cooled down and solidified in the Earth’s crust . As the melt cools,
crystals begin to grow, in the same way that sugar crystals appear when
you cool a solution of sugar in water. Molten rock is made up of many different
elements, however, so the process of crystallisation is much more complicated.
The sort of molten rock that forms a granite is made up of silicon and oxygen
in the main, together with aluminium, iron, magnesium, potassium, calcium
and so on. So the minerals always contain plenty of silicon and oxygen.

Feldspar, an aluminium silicate mineral, is one of the first to form
as the molten rock cools. Thus it has the chance to grow as rectangular
or parallelogram-shaped crystals that are usually white, pink or red. Granites
always contain quartz (silicon oxide), a grey translucent mineral that looks
like broken glass. It fills in the gaps between feldspars, because it starts
to crystallise at a later stage. There are usually a few black crystals
in a granite; these could be black mica, or tourmaline, for example. The
minerals grow together to make an interlocking network that gives the stone
its strength. Each crystal interlocks with its neighbours, so that cracks
which start at the edges of the mineral grains cannot easily grow big enough
to weaken the rock overall.

Polished slabs of granite are the best place to see these different
minerals. Euston station in the rush hour is probably not the best place
to look, but a light-coloured bank or office block would be a good place
to start. A granite looks mottled from a distance, but close up you can
see that the different colours come from different minerals, and some of
those minerals have the angular shapes typical of crystals.

Because granites (and other igneous rocks) are so tough, you will find
them in another guise, beneath your feet, as kerbs, steps and cobblestones
or setts. Kerbstones need to be especially strong, and if not made from
dull grey moulded cement, they will probably be igneous rocks. At the edges
of many London pavements, you will find granite kerbstones with feldspar
crystals like miniature building bricks beneath a surface dimpled to give
your feet extra grip. Many mews streets are still surfaced with granite
setts, coloured in all shades of pink and grey. Where you see granites with
rough surfaces, look for a sparkle as crystal faces on minerals such as
feldspar catch the light. The advantage of these rocks for roads is that
they are so hard that they keep their rough surface for decades, and the
road does not become slippery.

Other rocks make kerbs and cobbles in London; they are usually igneous
too. Some of the darker grey stones look very nondescript. They do not even
have any crystals that are big enough to see with a hand lens. These rocks
have much less silicon and oxygen in them than granites do. Instead, they
have a higher proportion of elements such as iron and magnesium, which form
darker minerals, making darker grey rocks, such as lavas. The molten rock,
known as magma, percolates through the Earth’s crust, and while some erupts
from a volcano, some coagulates underground, as the foundations of the volcano
itself. This is quite an exciting origin for a rock which, because it is
so tough, is routinely used as chippings on the surface of Britain’s roads.

Not all the stone used to decorate the shops and banks is granite. The
outside of Euston Station is faced with dolerite, a mottled black stone
that contains almost no quartz, and so much less silicon and oxygen than
the granite on the platforms. Another black rock, larvikite, is fairly common
on the fronts of shops and banks. It is unusual because some of the minerals
in it sparkle blue in the sunshine. This mineral is another feldspar, but
the colours come from very fine, yet regular banding in the crystal, on
a scale similar to the wavelength of light. Diffraction between these layers
and lenses splits the white light, giving the blue shine. Larvikite gets
its name from Larvik, a port near Oslo in Norway, where the rocks are found.
Each gleaming shopfront on the high street means a bigger hole in the ground
in the original home of these rocks.

What about the paving stones? Again, if you can get away from the ubiquitous
concrete slabs, you should be able to find some flagstones. At first, I
expected these buff-coloured pavements to be common, but I found none on
my regular walk. They are now a rarity in London, although I tracked some
down around Covent Garden and the British Museum. Flags make a natural nonslip
surface, because as they wear down, they maintain an irregular surface.
This is due to the way that the rock formed.

Flagstones did not crystallise from molten rocks, as granites did, but
come from the floors of ancient rivers and seas. They are sedimentary rocks,
formed at the surface of the Earth. Flagstones are made almost entirely
of grains of sand, the mineral quartz. They break naturally into slabs a
few centimetres thick because the grains of sand form layers separated by
another mineral, mica. Occasionally, before the rocks became solid, thin
blankets of mud covered the sand. As the rock was buried, the clay minerals
in the mud changed to micas. The important thing about micas is their shape;
they are very thin crystals, like sheets of paper. Flagstones contain thin
layers of mica, which form easy surfaces where the rock can split. As the
flags wear, thin layers of sand grains flake away, leaving flakes of mica
on a sandy surface marked by gentle ridges, good-looking and good for walking.

You can recognise sandstone in buildings because it is made of distinct
round sand grains, as you would expect to find on a beach. Builders have
used it for hundreds of years, because it is easy to work, yet fairly strong.
Rocks of this type are made of sand, mud, pebbles, animal remains and what
geologists refer to as cement. (They mean any mineral that holds the assorted
parts of the rock together.) Cement minerals usually grow in the spaces
between sand grains, as the sediments slowly turn to rock. The strength
of the rocks depends on how strongly the cement holds the grains together.

There is more to these city rocks than just their names, or types. The
internal structure of the stone holds clues to the processes that formed
it, millions of years ago. Many of the granites in the city contain occasional
crystals that are much bigger than most in the rock. The rock looks spotted,
or blotchy. The big crystals are usually white or pink feldspars shaped
like rectangles or parallelograms, with straight edges. The size of crystals
depends on how quickly the fluid cools, which in turn depends on how much
hotter the molten rock is than its surroundings. Deep in the Earth, molten
rock cools slowly, and crystals grow large. Closer to the surface, it cools
much more quickly, and crystals are small. A rock with both large and small
crystals started to cool deep in the crust, then moved upward before it
eventually coagulated.

Because a rock such as granite moves after some crystals have formed,
the large feldspar crystals in some kerbstones are lined up parallel to
the flow of this viscous fluid. The rocks look striped, rather than spotted.
Other rocks with the minerals typical of a granite are also striped, but
not because the rock flowed when it was molten. They come from parts of
the world where the crust of the Earth crumpled or stretched millions of
years ago. The intense pressures and temperatures that accompany this deformation
string out the different minerals to make a striped stone which geologists
call a gneiss. If muddy sediments are squeezed in this way, they can form
slate, sometimes seen on city roofs and walls. Slate splits easily because
the minerals that make up the mud change to micas and other flat minerals,
aligned parallel to each other; the rock cleaves easily along these surfaces.

Some slabs have even more information about how these granites came
to be rocks. When they are still molten, and moving upwards through the
Earth, granites can ‘swallow up’ fragments of the surrounding rocks. These
lumps of rock look different enough in the solid granite to be known as
‘zenoliths’ by geologists, and ‘heathen’ according to Cornish mining tradition.

Much of London’s granite comes from Cornwall, where it makes Land’s
End and the high ground of the moors. Cornish granite is pale grey, and
can contain large white crystals of feldspar, sometimes centimetres across.
Other British granites in the city come from Shap, in the Lake District,
and Aberdeen and central Scotland. These are both usually pinkish. Feldspars
in Shap granite form large pink rectangles, whereas in Aberdeen granite
they are nearly square.

Sandstone often has a banded or layered look. In close-up, the layering
is made up of alternating larger and smaller grains. Each different layer
represents the floor of the river or the sea at some time in the past, and
their shapes depend on the flow of water or wind. Straight, parallel layers,
the sort that make flagstones, mean that the floor was flat. This happens
when the currents are either very slow and gentle, or veryfast. In between
these extremes, ripples and dunes form in response to eddies in the flow
of water or air.

Which way was up?

Ripples form with a long side, facing upstream, and a short side facing
downstream. Any slight bumps or hollows in the sediment disrupt the smooth
flow of water. Sand grains gradually move up the back of a bump, then fall
off the steeper front side. As the strength of the current fluctuates, the
ripple builds up layers of lighter and heavier grains parallel to its short
downstream side. Ripples slowly move downstream, and eventually the whole
ripple is made up of layers of sand tilted downstream.

Subtle changes in the flow mean that ripples stop moving, and become
buried, to be frozen into the rock record. The bed of a stream gradually
changes shape, and cuts through the delicate layering within each ripple.
So if you see a herring-bone pattern within the rock in the side of your
office building, you can see which way was up when the sandstone formed.
The more continuous layers always formed last, so they were once on the
top.

This pattern of patches of sub-parallel layering cross-cutting each
other is called cross-bedding. You can see it on all scales in the blocks
of stone in churches and bridges. The bigger the cross-bedding, the bigger
the ripples or sandwaves, and the bigger the river or current in which they
formed. Dunes from deserts have cross-bedding on the largest scale, because
they are the biggest structures. If you see a thin layer of pebbles caught
up in a sandstone, then you are looking at rock from a river, or from a
shallow sea. It is only in these settings that the currents have enough
energy to move stones this big.

Most of the swirling patterns that you see in sandstones come from the
interplay between currents and the floor of the ancient seas or rivers.
But these same currents did not just flow where the seabed was sandy; coral
islands and their lagoons were shaped by the same processes. Limestone and
marble are the relics of these long-buried seashores. Rocks of this type
make up the best parts of my walk to work, that on the south bank of the
Thames. Most of the off-white buildings along the river are in fact made
from white limestone from Dorset. This Portland stone – from Portland Bill
in Dorset – formed in seas similar to the Bahamas today.

The special feature of limestones is that living creatures provide most
of the rock. Corals, shells and tiny floating protozoans all become part
of the rocks when they die; limestones are made of their fossils. Limestones
consist of the minerals calcite (calcium carbonate) or dolomite (calcium
magnesium carbonate). Stonemasons use the name ‘marble’ for rocks of this
type which take a good polish, but geologists use the word differently.
Geologically speaking, a rock called marble started its life as a limestone,
but has been heated up to several hundred degrees, and put under such pressure
that its crystal structure has changed. The overall chemical composition
of the rock is not very different, but the crystals of, say, calcite, have
reformed. This makes a tougher rock, because the crystals hold together
much more tightly. This process is known as metamorphism. True marbles,
such as the Carrara marble from which Michelangelo carved his David, make
beautiful, shiny and very heavy floor tiles. They are often pure white,
milky grey or cream-coloured, but building stone called marble can be blood-red,
green or black. Limestones tend to be grey, in contrast to the buff, yellow
and red of sandy rocks.

The way to track down limestone is to look for pale-coloured rocks that
contain fragments of shells and corals. The larger pieces may mark cross-bedding,
as in sandstones. Limestones have more information about what sort of ancient
seas the rocks came from. If the shells are unbroken, and still together,
in pairs as you sometimes find them on a beach, there cannot have been waves
or strong current to break them up. If the only shells are in fragments,
and there are broken pieces of coral and other animals, then perhaps you
are looking at the edge of an ancient coral island, with waves forever crashing
onto the shore.

Waterloo Bridge is built of Portland limestone, and is full of fragments
of oyster shells, corals and other animals that lived in tropical seas.
The pieces of fossil form layers that follow the currents in this shallow
sea. The fossils are broken, and the cross-bedding is on a scale of metres,
so the rock formed where there were strong currents, perhaps in a lagoon
near a coral reef, or in shallow tropical seas.

Other varieties of Portland stone look odd because they are full of
holes. If you can see that the holes are in the shapes of shells, then you
are looking at stone known as ‘Roach’. This too is from the south coast,
near to Portland. The holes form when shells that were buried in the rock
dissolve away. The moulds that they leave behind can be clear enough to
identify the original fossils. Another ornamental stone that is valued because
of its holes is travertine, a lacy, striped rock, which comes from hot springs.
Calcite dissolved in the spring waters precipitates out in layers, and traps
tiny bubbles of carbon dioxide as they escape: it is fossilised mineral
water, in fact.

A common structure which confirms that you are looking at a limestone
is a pattern of roughly parallel jagged lines, usually coloured brown. If
you can see around a corner of a single block of rock, these lines continue,
confirming that they are, in fact, planes. They arise when the limestone
is under stress, usually because it has been buried. Calcite dissolves much
more easily when it is compressed, so it starts to dissolve along planes
at right angles to the direction of the greatest pressure. This is the same
process that allows an ice skater to slide forwards on a film of water.

Portland stone dominates parts of London: the quarries have supplied
stone for buildings old and new. Many of the buildings that I dismiss as
ugly concrete blocks, such as Paternoster House, near St Paul’s Cathedral,
are faced with this type of stone. But much as I enjoy seeing stone in our
cities, it is there at a price. Quarrying takes its toll on the landscape,
often in areas that people treasure for their natural beauty.

As I approach the office, there are still more stones to see. The shopfronts
are made of larvikite, and the pub next to the office is faced with granite.
But to start my day, I turn away from the coral seas, and the heat of the
ancient magmas. It’s the office block for me. I cannot help wishing it was
built of warm pink granite and sandstone, rather than concrete, but at least
I can look out onto a relatively rocky city.

Further Reading: London Illustrated Geological Walks, Books One and
Two, Eric Robinson, Scottish Academic Press, 1984 and 1985; Stone on the
South Bank,R. H. Roberts, British Geological Survey (IGS), 1979.