When I first read your question, Harry, I thought the answer seemed straightforward and sat down to post it. But the more I thought about it the more I realised things are not quite as simple as they seem. So Ill use your question as a subject for this weeks blog about lithification, or how rocks, particularly sandstones, get turned into stone. Or not, as the case may be.
To start with the simple answer. The Carboniferous age sandstones such as those around Vindolanda were covered over after they were laid down by more and more layers of sediments, eventually many kilometres thick. As with most cases of lithification its the pressure of these overlying layers which, over millions of years, caused the initially loose sediments to turn to hard stone. So of course you dont see the intermediate stages because they happen while the rocks are deeply buried. Eventually, erosion over many more millions of years has re-exposed at the surface (exhumed, as geologists say) the sandstones we see today.
So how does all this pressure cause the lithification? Well, in the case of the sandstones the mechanism is well understood, if a little complicated, and I can see the evidence of it in all the thin sections Ive been looking at. The silica which makes up most of the sand is normally only very very slightly soluble in water. But under high pressure, particularly at the points where a corner of one grain presses into another, the solubility increases dramatically. Water flowing through the sandstones carries silica away from the points of contact and then deposits it again in the spaces between the grains where it acts as a cement, gluing the grains together. This process is called pressure solution.
But there are circumstances where you can indeed see sandstones which are partially lithified. Here are some examples sorry theyre all from Britain but Im sure similar cases exist widely in the USA and most other countries.
Sometimes we see the very early stages of the lithification process in sands which have not yet been buried. Just a couple of miles from where we live to the south of Derby, sand and gravel deposits from the past 15,000 years or so cover large parts of the valley of the River Trent. Of course, most of these deposits will be washed away, or nowadays dug up and carried away, but in time some will be covered by more deposits and eventually perhaps be buried deep enough to be turned to stone.
Even with sands which have been buried, the extent of lithification can vary considerably. During the first geology summer school of my OU studies, in 2001, we visited the aptly named Quarrington quarry in County Durham. In the yellows sands picture below, behind the three disparately sized individuals (Im the middle sized one), is an apparently normal yellow sandstone with what appears to be a cross-section of a big sand dune in it. However, when the quarrymen start to dig at it, this stone just disintegrates into piles of yellow sand. At exactly the same geological time, in the Permian period about 260 million years ago, only 50 miles to the west of Quarrington, the red Penrith Sandstone was being deposited which has become well lithified to a very durable building stone. Both the yellow and red sands were deposited in desert environments, but the yellow ones were close to the sea shore and the differences in the chemistry of the material surrounding the sand grains has produced this great difference in coherence.
Many sandstones have been buried less deeply and for a shorter time than the Carboniferous ones and in consequence are much softer. The Sherwood Sandstone under Nottingham is easily carved away and over the centuries an extensive network of caves has been created. In England the rocks generally get younger towards the south east and most sandstones from this part of the country are quite soft and rarely make good building stone, which perhaps explains why people from those parts think of limestones, such as Bath or Portland Stone, rather than sandstones as the building stone of choice.
Occasionally chemical conditions allow lithification to happen on the surface without the need for high pressure. Such surface-hardened rock is called duricrust and comes in a number of varieties depending on what type of cement binds the grains together. One version is called calcrete and consists of sand grains cemented by calcium carbonate. Extensive deposits of this were formed about 200,000 years ago along the north coast of Cornwall. These deposits are very friable and are themselves now being eroded but their remains, known locally as sandrock, can still be seen, for example at Godrevy Point which is at the east end of St Ives Bay. In the picture, two blocks of sandrock can be seen lying over the eroded surface of folded Devonian rocks.
When a duricrust consists of sand grains cemented by silica it is called silcrete. Extensive deposits of silcrete are thought to have been formed over parts of southern England around 50 million years ago. These are now nearly all gone but in a few places large blocks called sarsens are still found. These are extremely hard and include, as Im sure you archaeologists know, the stones used to build the great trilithons of Stonehenge.
So we usually dont see lithification in progress because it happens at great depth, but various stages can be seen where the sands have never been buried or where the burial was not deep enough and/or long enough to form a good hard stone like our Northumbrian sandstone.
This week Ive joined the ranks of the excavators and am helping to dig away the packing of the 3rd century Via Principalis to expose the floor of a 2nd century barracks. The following two weeks I shall be back in Derby supervising the re-decoration of our house. So it may be a few weeks before I can post another episode of this blog. But fear not, dear reader, I shall return before the end of the season!