Sandstone

by Dr Tim Palmer

The surface of the earth may seem permanent when measured against the duration of a human life span, but over geological time the combined action of wind, rain, ice and plant action can reduce even the toughest rocks to dust. None of the rocks exposed in the land around us can resist the inexorable effects of erosion.

Erosion is partly physical in its effect and partly chemical. The action of the freezing and thawing of water in cracks within rock is the most severe physical effect, eventually causing the surface to split and fragments to fall off as particles of sand and gravel.

The pressure of roots forcing the cracks apart may also contribute, and so may expansion under the heat of the sun in some parts of the world.

Chemical effects arise because some of the minerals of which rocks are made up are unstable over long periods of time when exposed to a mixture of water, oxygen and carbon dioxide that falls as rain. They dissolve or react chemically to form new chemical compounds that are soluble and which are carried into the ground in solution. Also, plants can contribute when their decay makes the water in the ground more acidic.

However, not all the minerals found in rocks are equally susceptible to erosion and decay. Above all, crystals of the mineral quartz (a hard and chemically unreactive form of silica dioxide) put up such a resistance that the other minerals in quartz-rich rocks weather away around them to leave a residue of quartz grains.

These grains become sorted by winnowing into a pure sand which becomes the main component of the sediment that is carried along the beds of streams and rivers, eventually to be taken to the sea.

A look at the sediment in the bottom of a stream on Dartmoor illustrates the point. The rock is granite, rich in quartz as well as less resistant minerals such as feldspar and mica.

On top of the moor, the stream sediments are full of large, angular fragments of quartz and partly broken and rotten lumps of feldspar a few millimetres in diameter. Further downstream, the feldspars become less common because they have broken down further and changed to clay and the quartz crystals are worn down and smoothed, becoming smaller and less angular and sorted into grains of similar size. The sediment is said to be becoming more mature.

Just these sorts of 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 York paving.

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 grains together.

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 grain surface.

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 air.

Copyright © Dr Tim Palmer


Dr Tim Palmer (tjp@aber.ac.uk) is a Chartered Geologist and consultant on the petrography of building stones and limes with a particular interest in historic buildings and landscapes. He researches and teaches geology part-time in the Earth Sciences Institute at the University of Wales, Aberystwyth, SY23 3DB. Tel: 01970 627107.