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Erosion and stream-transport events, happening
at times in the far geological past, wore away long-vanished
granites (and other types of quartz-rich rock) and eventually
dropped the quartz-rich load to form sandstones. Close to
the source, the immature sediments gave rise to sandstones
in which the grains were easily visible to the naked eye and
in which some feldspar was also present. Such coarse sandstones
are often known as grits - the famous Carboniferous Millstone
Grit of Derbyshire is an example.
Further downstream, the more mature sediments
formed the sand banks that were destined to become pure sandstones
which, by definition (to a geologist), are made up of grains
ranging from 1/16mm to 2mm in diameter.
Not all the sand carried by rivers made
it as far as the sea. Generally speaking, rivers cut down
and erode the underlying rock in their fast flowing upper
reaches, but slow down and start to meander across so-called
alluvial plains as they approach their mouths. The sand bars
on the inside of meandering river channels and the deltas
at river mouths are two of the sites where sandstone may start
to form, first as sand-banks but later, when buried, becoming
hardened into rock.
River sandstones of this sort form beds
which may reach several metres in thickness, but which may
rapidly taper out along a quarry face, as is seen in many
of the Yorkshire sandstones. Often they show an internal lamination
that is oblique to the natural bed, showing that they were
deposed as small, moving underwater dunes.
Occasionally, the rivers flooded and broke
their banks, spilling out a fast-moving torrent of water and
suspended sand onto the surrounding flood plain. These sands
settled quickly and show thin, horizontal laminations internally,
along which the stone splits easily. This is the origin of
Much of the sandstone in Britain comes from
the Carboniferous period (some 300 million years ago) when
the climate was wet and large rivers flowed across the northern
counties into shallow seas.
Locally, the same conditions existed at
other times: the North York Moors are made of much younger
Jurassic sandstones, a mere 150 million years old.
At yet other times, land erosion took place
in a dry desert climate and the quartz grains were blown by
strong desert winds, accumulating and moving as large dunes.
These desert sandstones are easily recognisable. They have
large-scale, sweeping, oblique internal laminations, just
like the modern sand dunes of the Sahara, and the individual
quartz grains are all much the same size and spherical because
they have been rubbing together in the driving desert winds.
The New Red Sandstone of Cheshire and the west Midlands, formed
in Permian times following the drying u p of the Carboniferous
rivers and coal swamps, and the geologically older Old Red
Sandstone of Scotland, are examples.
The sandstone story doesn't finish until
the soft sediments become hardened into a usable rock, and
this happened deep in the earth, after the banks or dunes
had been buried. Minerals of natural cement, carried in solution
in the ground water that fill the pore spaces in all sedimentary
rocks at depth, precipitated as crystals and glued neighbouring
The type of cement reflects the chemical
composition of the pore water. Lime or clay minerals give
rise to softer sandstones. The hardest sandstones, sometimes
referred to as quartzites, have a natural cement that has
the same quartz composition as the original grains. Under
the microscope, it is difficult to see where the original
sedimentary grain stops and the cement begins, though sometimes
a thin strain of iron minerals marks the site of the original
Many desert sandstones are pink or red,
because the cement (or one of the cementing minerals) is the
red iron oxide, haematite. Some older books show pictures
of what these ancient deserts are thought to have looked like,
with bright pink sand glowing in the Permian sunshine. The
truth, alas, is less spectacular. The sand was sand-coloured.
The red haematite only formed as the water table moved upward,
bringing dissolved iron into contact with the oxygen in the