Hard rock and hard water

All of these podcasts are geological but this episode is three-quarters pure rock. First the plan is to look closely at the rock that provided the mortar for the wall – limestone – did the Romans use it to sweeten these northern soils too – they can be pretty acid. Next its more whin Sill – I am starting to wonder if there’s too much on this rock already, but it does play a huge role in the landscape and on Roman plans and they say you can’t get enough of a good thing. The Whin has a part to play in a trip to Coventinas Well too, but a subtle part. And finally we are off in search of a very modern rock – one that starts soft and goes hard and one the Romans had a very special job for. 

Transcript

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Episode five Hard Rock and Hard Water All of these podcasts are geological, but this episode is three quarters pure rock. First the plan is to look closely at the rock that provided the mortar for the wall, limestone. Did the Romans also use it to sweeten those northern soils too? They can be pretty acid. Next it's more windsill. I'm starting to wonder if there's too much of this rock already, but it does play a huge part in the landscape and on Roman plans, and they say you can't get enough of a good thing. The wind has a part to play in a trip to Coventine as well, too, but a subtle part. And finally, we're off in search of a very modern rock, one that starts soft and goes hard, and one the Romans had a very special job for. As you journey along the central sector of the wall, you will see many eighteenth and nineteenth century stone built lime kilns. Each one is evidence of the presence nearby of limestone close to the surface. Limestone layers run east west. With the other rocks they impart the grain of this country. There were once limey mud and shells in coral seas around three hundred and thirty million years ago. The pressure of deep burial under thousands of meters of other sediments turned them into hard stone, essentially solid calcium carbonate, CACO three. Millions of years of mountain building episodes and erosion eventually brought them back to the surface. These rocks have provided a source of lime for centuries, for agriculture, mortar, cement and render, but also as a flux for the iron and steel industry, as an ingredient to medicines and toothpaste, and for tanning and paper industries. To minimize the effort needed to transport stone from the quarry, the kilns are almost always sighted close to the limestone outcrops. The chamber within the kiln was loaded with layers of limestone blocks and coal. This was burnt at a temperature of around one thousand degrees centigrade for perhaps five days, and then quick lime, calcium oxide, was raked out of the hole, called the eye at the bottom. The kilns you see are mostly from the eighteenth and nineteenth centuries. They are called pot or running kilns, and they were designed to work with the higher temperatures of coal fires and to be kept burning continuously. The Romans used an enormous amount of lime mortar in the construction of the wall and its related buildings, and they also used lime render in the fort buildings and walls, so we know they were quarrying and burning limestone. However, evidence for Roman kilns along the frontier is rare. One was reported from Vindelander and another very recently at Rochester, but only a single definitive kiln has ever been documented. This was in the little valley of the Nagburn, adjacent to a limestone outcrop a few hundred meters southeast of Housted's Fort. The kiln was exposed by the archaeologist FG Simpson in nineteen oh nine. It had been creating by cutting into the shale bedrock and had a flue four point five meters long connected to an oval chamber three meters across. A ledge within the chamber suggests it was wood fired. Roman pottery of the late third and early fourth centuries was found overlying it and within it, although a recent archaeological study has suggested otherwise. From a geological and quarrying perspective, it seems logical that the Romans would, just as they did with sandstone, extract and process limestone as close to their construction sites as possible. This suggests many kilns along the wall. Perhaps these were ephemeral clamps, hollows in the ground filled with layers of broken limestone and firewood, and covered with turf. They would have left little trace. Exactly where and how the Romans produced lime, a critical raw material for their frontier, will, without more research and clearer evidence, remain a mystery. A few ups and downs and a couple of kilometers later we have reached the trig point on the top of Sioux's Crags. How's the leg muscles doing? The view is dramatic and stretches away to the west to the high point of the wall on Windshield's Crag and beyond. But the feature that dominates the landscape is not the wall. It's the escarpment of the rock that underpins Hadrian's monument, the windsill. This thirty meter slab of molten magma was injected at around twelve hundred degrees centigrade into older carboniferous sedimentary rocks two hundred and ninety five million years ago. The escarpment you see is just the tip of the windsill. It extends for over four thousand five hundred square kilometres beneath the pennines of Cumbria, Durham, and beyond. It is England's largest igneous intrusion, a rock called dolerite, made up of mineral crystals just visible to the naked eye. Zoom out to a rock face and it shows the characteristic vertical columns caused by the magma cooling and then contracting. At landscape scale, its hardness and resistance to erosion creates the prominent escarpment. One hundred and fifty years ago, an anarch draper and postmaster, George Tate, aided by scientists of the Geological Survey, proved definitively it was not a volcanic lava extruded over the surface of the ground, but an injection of molten rock deep underground, and exposed now because of million millions of years of erosion. If you'd like to see the evidence, there is limestone altered by its heat, that's metamorphosed, above the Dollarite at Sewinshields, and further west, barely twenty meters north of Hotbank Farm, the footpath crosses the mudstone directly beneath the sill, which has been baked to a slatey rock. Current thinking on Hadrian's decision where to site his wall is that he had the choice of three locations to separate his empire from the barbarians, the Clyde Fourth, the Solway Tyne, or the Mersey Mersey Humber. If he had designs on the mineral wealth of north of England, he had just two. He chose the Solway Tyne. Was that because of the pre existence of the Stainegate Road, or was it the strategic value of another bit of geology, the windsill? According to recent research, it was the twenty eight kilometre section of the sill's north facing crags that dictated the route of the wall corridor along the isthmus. The wall's east and west limbs to the respective coasts were subservient. They merely stayed on higher, drier ground, and avoided the rivers and streams as much as possible. Enjoy your walk along the top of Suinchil's Crags, and the rest of the Windsill Ridge. It is rocks that are the foundation of the views you're having. They define the wall country, and they defined its wall. We humans are just latecomers. You can enjoy a gentle descent and six kilometers of flat walking now. We're almost at Broccolicia Fort in a small valley, just west of it, Comantine as well. Water is an essential human need, but it's also played a powerful role in the spiritual lives of the Romans. Water and its associated engineering get a lot of archaeological attention in the drier parts of the empire, but less so in Britain because of our high annual rainfall. That is even more true here in the north, where our wetter climate means that surface water resources, streams, rivers and lakes are abundant. Many northern Roman sites have water courses nearby, and rainwater is also channelled from roofs into storage systems, but the extent to which Romans depended on groundwater and how much they revered it is often overlooked. Coventina's well is just west of Caribra Fort, known to the Romans as Brocolicia. An eighteen seventy six excavation revealed water feeding in a rectangular basin in the centre of a walled enclosure or temple. The well, more a spring, really, emerges on the northern limits of the outcrop of the windsill dolerite, and as this rock is impermeable, it is probable that that's why the groundwater comes to surface here. Almost fourteen thousand coins and many stone altars and inscribed slabs were recovered from Coventine as well. At Haussteads, a spring emerges naturally at the top of a sloping bed of limestone, two hundred meters south of the fort. It flows into a stone lined tank within a structure believed to be a shrine to Mars Thinxus and the El Sej, I hope I pronounced that properly. German war deities worshipped by Frisian soldiers. There are wells recorded at many other sites, for example at Benwell, known as Condicum, and at Vinderlander and Magna. Votive objects and inscriptions are not the only thing that have been found in war Roman water sources. Prior to the Lane's redevelopment in Carlisle around 1980, excavation of a stone-lined third century well exposed the skeleton of a man who was probably murdered and then disposed of. You can read this story and even see his facial reconstruction at Tullyhouse Museum in Carlisle. The same excavation also revealed a second century barrel lined well and a water wheel, and in the 1920s, in the Bluebell Inn in Scotch Street, another well more than twelve meters deep was found. Based on recent boreholes, these wells are likely to have struck water at about five and a half meters down in glacial clay and stones, before hitting the 250 million year old Triassic mudstones at around seven metres. Those are some nice geological details. Most often it's the artifacts that infill the wells that have been studied, but the basics of Roman water and groundwater supply, the use and drainage, also deserve scrutiny. This includes everything, from how they prospected, dug and constructed their wells, to how they dealt with the challenges of ventilation, stability and contamination. Then we would better understand what the Romans knew of water resources, how they supplied enough of it to satisfy the essential physical needs of thousands of occupants of their frontier settlements, and maybe more about their spiritual desires too. Like to try a walk that us locals sometimes do? Take the footpath south to Newbruch, just east of Broccolicia Fort. It's only six kilometres, and there's a lovely little locals pub in the village, the Red Lion. But first we are off to a church, Saint Peter's Church, in Newbrugh. It is thought to be the site of a Roman Stainate fort. In the field next to the church is a spring, known as Saint Mary's Well. Close by is a large block of a porous looking rock called Tufa. The word tufa is easily confused with tuf. That's a rock made of volcanic ash. Both can be light in weight and were used by Roman engineers to construct parts of buildings. But tufa is freshwater limestone formed by physical and biochemical processes in streams and springs which are rich in carbon dioxide. Most tufa in Britain is very young, less than ten thousand years old. It is precipitated on bacteria, algae, mosses and plants, and even on man-made things. It has an open texture and can look like a rock sponge. Tufer deposits are sometimes recorded on the most detailed geological maps, but because they are usually less than ten meters across, they rarely appear on smaller scale published geological maps. The detailed maps show tufa at Bucastle, at Kirkwelpington, and in a few other locations in Roman Wall Country. The Romans were masters at grand design long before modern architects and engineers, and military infrastructure on the northern frontier was usually based on tried and tested specifications. There was even a specification for bathhouse ceilings, which required nature's equivalent of lightweight thermal blocks. Tufa. Next to the bathhouse at Chester's fort are several tufa blocks. Some have notches cut top and bottom. They're called armchair boussoirs. Now there's two words you've probably never heard before. The blocks of tufa were stacked together to form a vaulted roof. Flat kit clay tiles were then fitted into the notches to create a cavity so warm air could circulate in the roof of the bathhouse. Tufa blocks and fragments have been found in other Roman excavations too, in Corbridge, in South Shields Fort, at Howstead's, and at Catrick, where pieces were reused as grave covers. There will undoubtedly be other tufa pieces lying in excavated stone rubble, but the rock type and its significance is not always appreciated by archaeologists. Where the Romans sourced their tufa found along the northern frontier is a difficult question to answer. This very young rock occurs in much larger volumes on the continent, where there are many examples of Roman use. Actually, blocks occur in Roman baths in Bath, and thick deposits are known in Derbyshire, Worcestershire and Yorkshire. It is possible that the Romans exploited our smaller local deposits, like the one at Newb, but to prove that we need to gather the evidence from possible sources and excavations. A few archaeologists could find bits, perhaps astrologists could help you. It's the penultimate episode next. We will be crossing the Tyne, the North Tyne and the Unified Tyne to visit Hexham, and then head into a Roman town, one with a very desirable current town next to it, Corbridge.