Channels and minerals

Time to descend into and out of one of the classic components of this frontier landscape - one of the "gaps". You have already experienced a few and today there will be a few more. But your legs need a break so we are going to deviate south of the Wall too. To see a ditch, then go find about two mineral resources that were used extensively by the Romans - coal and iron - but what do we know about them.

Transcript

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Episode four Channels and Minerals Like all the king's horses and all the king's men, we're marching back up to the line of the wall again. Pause before you go through the little gate beside Peel Bothy. See the stones the Bothy is built of. Many are recycled Roman stones. How can we tell? Well they tend to be oblong and have more rounded edges. You'll see that in spades when you stride along the top of Peel Crags. The wall there is a hundred percent Roman, and the texture of that stretch is classic. Even if it is a rebuild by Clayton's laborers. But before we continue our easterly tramp, let's go a couple of hundred meters onto Steel Rig. There's some rock sticking out just beside a rock chip path. Stand on the rocks and gaze east. This is one of the iconic viewpoints of the wall. Peel Krags and Cragluff. Get your camera out. This is bold, unmistakable wall country. On a clear day it's stunning, but even on a misty wet day it's moody and magnificent. The landscape rises and falls like waves on a sea. Time to descend into one of the classic components of this landscape Peel Gap. You will now be well aware that walking along the middle bit of Hadrian's wall, with all its ups and downs, can be hard on the leg muscles. Have you ever wondered why those undulations are there? From Sowinchields to Wall Town the wall follows the crest of the windsill, a hard igneous rock called dolorite that has been shaped by ice to produce its formidable escarpment. But the escarpment isn't level. It undulates repeatedly and sometimes steeply. Geologists believe that many of these short little valleys, cutting generally north south, are places where the windsill has been weakened, sometimes by faults, dislocations in the rock caused by earth movements hundreds of millions of years ago. Faults are rarely single planes, they are broken and shattered zones, and they are more easily eroded. Fast forward to the ice age, which began less than two point six million years ago. On more than one occasion several hundred meters of ice covered all Scotland and northern England. When these ice sheets began to melt, the water beneath them tried to escape southwards to lower ground, the valley of the Tyne. The escarment of the windsill was initially a barrier to meltwater, but the weaker zones in the rock provided opportunities for water, under considerable pressure and with lots of debris, and it broke through. That's how we think the gaps formed, as glacial meltwater channels. Some of them may have been subglacial as their profiles are humped. Others may have drained glacial lakes or ponded up meltwater. Like Peel Gap, most of these channels, gaps, nicks, and slacks, have names. One, Sycamore Gap is world famous, sadly now because of an act of vandalism. Some like Rapishore Gap, Thorny Doors, Lodem Slack, are less well known. Others like Busy Gap, said to be a gathering place for reavers and rogues, have romantic stories, but each channel has its own character. By the time you have hiked up and down them, others are Castle Nick, Corgap, Greenslack, Milkengap and Hole Gap, their cumulative impact on your leg missile muscles will be apparent. Take a walk east from Steel Rig Car Park, and soon you will descend into Peel Gap. Its origin is a boggy valley west of Cragluff, and then it turns south to run through the wall. In the gap there are the foundations of an extra wall tower between turrets thirty nine A and thirty nine B. It is simply butted onto the wall, which suggests it was built after the wall, perhaps to defend this unsighted low point, or maybe to provide soldiers with shelter and access to the wall. Going up east and out of Peel Gap can be daunting for some. Take it slowly, it's a good path. Tackle the wall's geological ups and downs, and you will soon appreciate why the Roman army built the military way across less daunting terrain to the south. Have you recovered from the gaps and their virtual up and downs? And recovered from the gap left by the sycamore. That can be a shock. We're going to take a short side trip south through another gap in this tale, a less dramatic one, Milking Gap. We're off to take a look at another channel, but this time one made by the Romans, the Vallum. It's about four hundred meters south of the wall here. As we head towards it, there's a ring of large stones on the right. That's an Iron Age settlement. But it's the Vallum we've come to see, a six meter wide, three meter deep, flat bottomed ditch with earth mounds on either side. For much of its course the Vallum stays close to the wall, but sometimes it changes direction, significantly, and deviates southwards. Minor deviations at forts may simply relate to the sequencing of their construction. Elsewhere, because it is a trench, the surveyors have to be even more conscious of the geology underlying the vallum than they did the wall. Choosing the wrong course across the ground would have meant significant delays and even failure of the earthwork. Excavating the incredibly hard windsill would have increased the time to complete the one hundred and thirty kilometer excavation enormously. The vallum may have been cut through this rock at one point south of Limestone Corner, but, perhaps learning lessons, they made no attempt elsewhere, and their unsuccessful efforts to complete the ditch through the dolerite north of the wall at Limestone Corner speaks volumes. Trying to excavate such a large trench through peat bogs and soft lake sediments would have been sisyphan. The sides would have collapsed on the high water table meant it would have filled with water almost immediately. Dollerite and peat appear to have been avoided, if at all possible. On detailed geological maps, the Vallam's navigation of difficult ground from Sewinshields to Gavoran is obvious. It steers clear of the outcrop of the windsill, finding a route in glacial clays just south of the exposed bedrock. The same geological maps show you how its excavators negotiated their way round peat bogs too. South and west of Milken Gap is a classic example. The Vallum takes a double kink to avoid thick, wet peat, an area likely to have been much wetter and perhaps a lake 1,900 years ago. The Vallum takes evasive action, heads towards the military road, and then runs parallel to it. The Vallum ditch in its ramparts would represent a huge task even for modern excavating machines. Its completion by troops equipped with only spades, hammers, chisels, barrows and carts is a staggering achievement. The purpose of all this effort? While the Vallam may mark the southern limit of the military zone, there are only a few places at forts where it could be crossed, and these causeways were closed by a gated arch. While its function is still debated, what we may deduce is that its changing course nicely demonstrates the pragmatism of its planners and diggers, and their geological wisdom. Vinderlander is a justifiably well known place. It's one of the highlights of the frontier. A fort, actually nine successive forts, built approximately on the same site on the Stainegate Line, so Vinderlander is fifty years older than Hadrian's Wall. It's most famous for the amazingly preserved wooden writing tablets that provide a unique insight into life on the frontier. There is so much here, worth a day or more of anyone's time. The ruins of a stone fort, a wonderful museum how in the art housing the artifacts found over several decades, and throughout spring and summer months active excavations you can watch, and if you sign up well enough in advance, you can join. But Vinderlander is also a perfect place to share another puzzles about Romans and their relationship with coal. I'll explain the geological origin of coal later when we get to Benwell, and how many workable seams underlie Northumberland and Durham in the coal fields. But to the west there are other older carboniferous coals. These may not be as numerous or as thick, but they stretch from north of Annock to Brampton and Alston in Cumbria. Their outcrops straddle the lines of the wall and the staingate for many kilometres. One of these older seams is exposed at the surface, a few hundred metres southeast of Vinderlander, and, like most coals, miners and geologists gave it a name, the Little Limestone Coal. It was mined extensively in the nineteenth century at Barkham Colliery. This same seam was also mined at Acom, Forestones, Barden Mill, Henshaw, Holtwistle, and Vlegill, and is still mined today at Isle near Alston. It is to this little coal, once a three hundred and thirty million year old vast tropical swampy forest, that so many Tyne Valley and Pennine settlements owe their existence. It seems inconceivable that the Romans were not aware of this coal seam. After all, they quarried the sandstone directly above it on Barkham Hill, and probably worked the adjacent limestone layers too. It is also possible that unlike southern Britain, trees and thus wood and charcoal were not as abundant as a fuel here. Like many other Roman sites throughout the years of excavation at Vindeland, pieces of coal have been found. It was likely used for heating, but also for metalworking, and perhaps for lime burning. The intuitive conclusion drawn by many is that the coal used in Vindalander, at least for some of the time, was extracted from the nearby little limestone coal seam. However, the only hard piece of specific evidence currently available says otherwise. Analyses of fossil plant spores and a test of the petrological immaturity of single Vindalanda coal sample points to it coming from West Durham, perhaps near Ebchester. Single samples from the House Deads and Corbridge coals gave the same result. The scientists who did those tests was a very credible world expert on carboniferous plant spores, but in each case they were just single samples of coal from single contexts. And even if he is correct about these samples, it's quite possible that the quartermasters of forts along the wall, over their 320 years of occupation, sometimes ordered coal from Durham and sometimes found it locally. Sounds like another unsolved puzzle, doesn't it? But this one is about to have some light shed on it. I can't give the game away yet, but archaeological and micropaleontological colleagues and I have just finished a little research project to identify Vinderlander's coal using fossil plant spores. The results of that will be published very soon. We haven't moved far. We are beside Brackisburn, a little stream that runs a few hundred meters north of Winderlander. But to make sense of this story, first you'll need some background. We don't know if or where the Romans mined iron here in the north, but we do know they used a great deal of it. Iron slag plus fragments of the fuels that they may have used to heat the ore are often found, but whether this is from smelting, the production of metal from ore, or from smithing, the working and reworking of iron to make things, is always difficult to determine. Costly analysis and expertise are not always available. Establishing whether ancient mining took place is a challenge that is not unique to iron. Older mineral workings almost always get obliterated by the younger ones. But Roman iron mining and production in the weeld, the forest of Dean, and around Northampton is well documented. Were these the sources of iron that were then transported long distances and turned up in so many military and domestic objects all along the frontier? Or did the Romans use local bog ore, a secondary and highly variable iron deposit formed since the last glaciation on ill drained ground by the biogeochemical oxidation of iron rich water? Based on research some distance from the wall, it is often asserted that it was bog ore that Iron Age peoples and Romans used here. But with rare exceptions, that remote evidence is circumstantial, and where primary ore sources are available, as they are along the frontier, this presumption would be unsafe without site specific confirmation. It is equally possible that both societies exploited local geological resources of iron in the rocks. One resource is the extensive deposits of carboniferous iron ores called ciderite, iron carbonate, FECO3, which occurs as clay ironstone bands or nodules, called cats heads and ballstones. They occur in shales. We know they were mined here for many years, most extensively between the eighteenth and twentieth centuries, for example, in Bellingham, Reedsdale, Shotley Bridge, and Consert. There are many other places across the north where workable deposits of ciderite ore are found, and they occur near the wall, at quarries near Chesterwood and Corfields Common. But there is a source in the shale, mudstone, above a rock layer called the three yard limestone beside Brackisburn, less than three hundred meters northwest of Vinderlander. Vinderlander is a fort with fabricae, workshops, and clear evidence of industrial and ironworking activity. The iron ore at Brackisburn is typical, heavy ciderite nodules in the shale. Uneven ground on the north side of the burn is a product of extensive modern workings, probably from the eighteenth and nineteenth centuries. There are obvious natural exposures of the shale and nodules in the meanderscars of the burn, and these would have been clear to the Romans. We know the Roman army worked sandstone, limestone, and possibly coal beside Winterlander. Throughout history miners and quarrymen, with the generic skills and investment in infrastructure, invariably extract all of the worthwhile resources that exist at a location. Did they exploit these local resources of ironstone too, or use bog ore, or rely on southern iron imports, or perhaps over three hundred and twenty years of occupation, used all three? So many natural resources the Romans exploited appear on the frontier, and we know so little about these minerals, and the narratives geological evidence could enrich and constrain. We geologists and archaeologists really do need to become better buddies, and you'll have and you'll have guessed by now that this is a conversation I'd like to start and nurture. Back to the wall in the next episode, and another set of rocks, and some special groundwater. That'll make a change.