After the Earth formed four and a half billion years ago, its first atmosphere was much denser than now and was made up mainly of carbon dioxide. At that time the sun was only about 70% of its current brightness, but the greenhouse effect of all that carbon dioxide meant the surface temperature was much the same as at present. Over the eons which have passed between then and now, as the sun has got brighter, the amount of carbon dioxide has decreased in such a way that the surface temperature has always been amenable for life. And it is life itself which has caused this decrease, in a way which some people see as evidence for the idea that the Earth itself can be thought of as an organism. Most scientists agree that there is indeed such a feedback mechanism involving life, but dont think this effect is comparable with the feedbacks within organic life forms. Whichever view you take, its clear that all that carbon has gone somewhere, so where is it now? Two of the rock types we find around Vindolanda provide the answer.
About 20% of the carbon has gone into the coal, oil and gas which collectively we call fossil fuels. Such deposits are the organic remains of living things land plants in the case of coal and microscopic sea creatures in the case of oil. Most fossil carbon is in such low concentrations that it will never be economic to extract it but concentrated deposits have been, and still are, the basis of industrial society. Coal was extensively mined in the area around Vindolanda well into the 20th century and was also an important resource for the Romans. Lumps of it regularly turn up during excavations.
The remaining 80% or so of the carbon became locked up in the carbonate rocks classified as limestones. Although there are some special circumstances in which limestones are produced by chemical precipitation from water, the vast majority of such rocks are derived from the hard parts of once-living organisms. The oldest limestones known are at least three and a half billion years old and are part of the evidence that life, albeit just in the form of bacteria, got started very early in the Earths history. But the story of limestone really got under way with the so-called Cambrian Explosion around 550 million years ago when big creatures with calcium carbonate shells and skeletons first evolved.
Limestones of various types are amongst the most characteristic rocks of England and include (getting older down the list):-
The Chalk, as in the White Cliffs of Dover, formed in the Cretaceous period from sub-microscopic platelets produced by single-celled algae
Jurassic limestones from which many of our best-known buildings are constructed, for example in the city of Bath
Permian dolostones, so-called because they are formed of a mineral called dolomite which is calcium magnesium carbonate
Carboniferous limestones, for example in the Mendips, in the Peak District and in the Pennines from the Yorkshire Dales to Northumberland.
The Carboniferous limestones of Northumberland are composed of the remains of shelly sea creatures, mostly ground into a fine powder by the action of the sea but sometimes as identifiable fossil remains. The commonest and most characteristic fossils are those of crinoids. There are still some of these remarkable creatures around today, but much reduced in number from their heyday in the Carboniferous. You can find many examples of picture of crinoids and crinoid fossils on the web, for example at http://palaeo.gly.bris.ac.uk/Palaeofile ... /Crinoidea
. Usually the fossils take the form of a short, thick-walled cylinder, anything from a millimetre to a centimetre or more in diameter. Each cylinder is just one segment of a crinoid stem or arm. Corals, in a wide variety of forms, are also common and shells of a great variety of molluscs can be found as well as fossil burrows.
Limestone outcrops can be seen at many places in the area. They can often be distinguished from sandstone outcrops because, in the limestones, horizontal joints between the beds are often wavy, on a scale of a few centimetres, as a result of the way the rock slowly dissolves as water runs through it. Each of the limestones was formed at a time of high global sea level, so they can be correlated over a wide area by the details of the fossils in them and by their position in the sequence of rocks. Often, the quarrymen gave the limestones names which reflect their typical thickness. The five key ones in the Vindolanda area are, from bottom to top, the Five Yard Limestone, the Three Yard Limestone, the Four Fathom Limestone, the Great Limestone and the Little Limestone. The Vindolanda Museum is sited on the Four Fathom Limestone. A very good quarry exposure of the Great Limestone can be seen next to the restored lime kiln at Crindledykes which is visible from Vindolanda on the hillside to the north east. The most extensively worked coal seam is just below the Little Limestone.
These limestones rarely make good building materials. They are hard to quarry and shape, they weather quite rapidly and they are often dark grey as they contain amounts of mud and/or organic material. The Romans were well aware of this and very few limestone blocks come up in the excavations. However, limestone is, and was to the Romans, an essential material for making lime, which is used for mortar and for agriculture. Heating the calcium carbonate in a kiln drives off carbon dioxide to leave calcium oxide (quick lime). This reacts vigorously with water to form calcium hydroxide (slaked lime) which sets very hard and makes a very serviceable mortar if mixed with sand.
Even stronger than lime mortar is cement, which the Romans pioneered, but this also involves shale which Ill write a bit about next week.
Of course, odd bits of limestone do get excavated. The first picture is something a digger thought was bone; only the tiny crinoid fragments told Malise when she cleaned it this afternoon that it was limestone. The second picture shows the glacially eroded top of the Four Fathom Limestone where the contractors briefly uncovered it below many metres of boulder clay during the building work on the new study centre.