Some Wall at last

Our journey east continues, we are about one and a half kilometres north east of Lanercost just over the line dividing the red St Bees Sandstone bedrock from grey brown Carboniferous rocks – although there is no bedrock to see here – its covered by a variable thickness of glacial deposits. Those thick stony clays sand and gravels may well explain why the first incarnation of Hadrian’s Wall in the western sector was made of earth and turf and not stone. They also mean its essential to look at the building and wall stones to see get a feel for the bedrock. 

You could abandon the car and walk between the places in the 4 stories in this episode from Hare Hill to Appletree, Harrows Scar and Willowford Bridge. Its only around 8 km one way, pretty flat with great views and some brilliant archaeology including passing Birdoswald Fort. The dedicated Romanists and hikers amongst you could take in the Fort and the Roman inscriptions at Combcrag gorge on the way back - you’ll maybe remember that from Series 2.

Transcript

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Episode two Some Wall at Last Our journey east continues. We're about one and a half kilometers northeast of Lanacost, just over the line dividing the red sandby sandstone bedrock from the grey brown carboniferous rocks. Although there's no bedrock to see here, it's covered by a variable thickness of glacial deposits. Those stony clays, sands and gravels may well explain why the first incarnation of Adrian's wall in the western sector was made of earth and turf and not stone. They also mean it's essential to look at the building and wall stones to see if you can get a feel for the bedrock. You could abandon the car and walk between the places in the four stories in this episode, from Harehill to Apple Tree, Harris Scar and Williford Bridge. It's only about eight kilometers one way, pretty flat and with great views, and some brilliant archaeology, including passing Bird Oswald Fort. The dedicated Romanist and hikers amongst you could take in the fort and the Roman inscriptions at Comcrag Gorge on the way back. You'll maybe remember Comcrag from series two. We are at Hare Hill, and a very short stretch of Hadrian's Wall. While it is pretty tall, almost three metres, much that remains is its rubble interior. Whether you are an expert or a curious visitor, the wall and its fabric can be both an enigma and a paradox. Its remains have been studied for centuries, and reading even a fraction of what has been written is a challenge. Yet the more evidence that is gathered, the more questions are raised. Just how wide and high was this wall, and why does it vary? Who built it and where did they begin? How long did it take them? When did they finish? But the focus of this story is the what? What's it built of? Perhaps the most consistent characteristics of Hadrian stone wall is that it has facing blocks on the north and south sides, and that they are predominantly of sandstone, of Triassic and Permian Age in the west and carboniferous everywhere else. The foundations vary, except where they are bedrock. They may or may not be set in a trench excavated below ground level. Foundation composition changes with location, a mix of blocks, flagstone, cobbles or clay, with edge stones that are laid wider than the wall above. Between the outer facing stones, above the foundation layer, for the full height of the wall, is a core of rubble, broken sandstone and or river cobbles, and near its outcrop winsil dolorite. Because its facing stones and core have often been restored and pointed with mortar, and because the wall was so tall, most visitors assume that mortar was used when the legions first built it. But there is no archaeological consensus. Some believe the evidence shows its outer facing blocks were laid dry, and others that they were mortared in places, but not everywhere. It is even possible that trampled clay was used instead. Bonding with lime mortar did seem to eventually become the norm. Were the initial sections perhaps experiments? The wall rebuilt by the emperor Septimius Severus eighty years later, is assumed to have used hard white mortar. That the bare grey brown or red sandstone facing stones were once covered with white lime render is no longer regarded as likely. But the method of bonding the rubble core, sometimes with a matrix of clay and earth, sometimes with mortar, and sometimes all of the three, is still debated. If there is a pattern in the wall's composition, it seems to point to legionary, even centurial autonomy, and a pragmatic approach of sourcing building materials locally. Nineteen hundred years of weathering and decay, destruction and burial leave an evidence base that is open to many interpretations. At Hare Hill, this short section of the wall remains, the first decent piece of surviving masonry as we head east. Its facing stones were rebuilt in Victorian times, but its rubble core, mostly of angular sandstone, is said to be original, and at three meters tall is one of the tallest fragments left. The western third of Hadrian's Wall that was built first, was a soil and subsoil rampart, called the Turf Wall. Walk towards the apple tree section, a short distance west of Bird Oswald, and you get a fantastic view of these earthworks and their horizontal scale. To appreciate their vertical dimension, you could have visited a full scale reconstruction at Tully Museum in Carlisle, that is until two weeks ago, but that will disappear in its refurbishment this year. So the only way to get a perspective is to go to Vindalander. Turf Wall is its name, and there were sods on the outside of the wall, but the core was more soil, subsoil and clay. It's just one of a series of defensive earthworks, and so I think it does qualify for inclusion in a book and a podcast about Romans and geology. Apple Tree is one of the few places where you can see its line and also easily see all the components of Hadrian's linear frontier, but they can get pretty confusing. The stone wall, ditch and bank are just north of the modern road, and the stone wall is beneath it. The turf wall defences begin a hundred meters to the south. First is a low mound of material that slopes gently north, then the turf wall ditch and the turf wall. Less than twenty meters south of the turf wall itself are the three mounds and ditch of the Vallum. The low mound in front of the turf wall is sixteen point five meters wide, but only oh point five meters high and is made of clay with some stones. It lies directly on clay as the turf and subsoil which were once above it have been struck for building the turf wall. The ditch, almost eleven meters wide and three meters deep, is cut in glacial stony clay. We geologists call this till or diamet. The excavated material will have been used for both the mound and the turf wall. The turf wall was six meters wide and four meters high, but was levelled by the Romans when they built the stone wall. So a broad bank only half a meter high remains now. It had a near vertical north face and a slope in south one. There may have been a walkway along the top and a woven wooden hurdle parapet too, but nobody knows. Excavations here in nineteen ninety nine and elsewhere proved the turf wall to be a mix of turf, soil, subsoil and clay, sometimes with wooden stones. At its base they found a black humic layer around four centimetres thick, the old grass surface. The turf had not been stripped. The turf wall provokes almost as much debate as the stone wall. Did the proportions of its building materials change along its length too? The question of why the western sector was built of turf exercises a lot of mines. Was it an expedient solution to the threat of troublesome tribes or the logical use of available raw materials? Or both? On Roman frontiers across the Empire, turf and wood were often used. How long did the turf wall survive before it was replaced with a stone wall? Evidence from Bird Oswald Fort suggest it may have been within Hadrian's reign. Further west, probably later. Walk four hundred meters east from Bird Oswald Fort and a few meters more beyond Harascar's Mile Castle, and you encounter a twenty meter high cliff, a meandascar of the river Earthing. From top to bottom this cliff is made of unconsolidated stony clay, gravel and sand. Carboniferous bedrock is buried deep beneath it. West of the Mile Castle, the original turf wall starts its march to the Solway. To the east, the first wall was sandstone of the Carboniferous variety. The change in building material seems an unlikely coincidence. The position and initial structure of the wall in the vallum were defined by its geology. It is the dominant influence on both the landscape, foundation conditions, and building materials. If you were to roughly divide up the wall corridor in terms of rocks, there are three main chunks, and the surface landscape reflects these. There's an obvious middle third, roughly twenty eight kilometers from Greenhead to Cholliford, defined by the hard dolerite of the windsill. The eastern third to wall's end is Carboniferous sedimentary rocks, covered by a patchy veneer of glacial stony clays. The western chunk is dominated by thick, glacial and more recent river and coastal sediments, which, except in the river valleys, hide the bedrock. It's conventional to stop and start the central and western areas of the wall at something the archaeologists call the Red Rock Fault, a line going approximately north-south through Lanacost, which divides the western red from the eastern grey-brown bedrock. This is, as I've said before, not a fault at all. It's an unconformity. A fault is a tectonic fracture of the rocks, an unconformity is a gap in the rock record. This unconformity is where water depositing Permian and Triassic sediments encroach progressively eastwards on the eroded surface of the Carboniferous rocks around 270 million years ago. The reason archaeologists chose to draw the dividing line here was that this is where the bedrock, the source of the building stone and mortar, changes. However, because that change is buried deep, this is not where geology influenced the initial alignment and construction of the wall in practice. That's around 10 kilometers to the east, at the River Earthing, where we are now, where the Quaternary, or they call superficial deposits, modern geological maps no longer use the word drift. The quaternary deposits thicken considerably. It could be even further east at the Typalt Burn, where the valley of a much larger pre-Ice Age river, the Prototyne, is plugged by tens of meters of glacial sediments. From here to Bonus and Solway in the west, it is on consolidated quaternary deposits, not bedrock that are the dominant influence on the shape and composition of the terrain, and on Roman plans. West of Harriscar, it would no longer have been possible for Roman builders to simply open up multiple snear surface small sandstone and limestone quarries close to the wall. The building materials that are now readily accessible are quaternary deposits, clay, stones, soils, and yes, turf. It's a very short distance to the next stop. These places are pretty much adjacent to each other. The remains of the bridge at Willeford, and the mile of intact wall, mile castle and turrets to the east are one of the highlights of the frontier. There used to be Roman bridges across all the major rivers in the north, the Eden, the North Tyne, the Tyne, and the Tees. Several were washed away and had to be rebuilt multiple times. This is what is thought to have happened at the bridge over the river Earthing at Willoughford, and explains why the remains of its abutment and pier you can see today can be complicated to interpret. But the first and most obvious question is why there's a bridge just here, when the river is a hundred meters to the west. The answer is that since that bridge was built 1,900 years ago, the river has inexorably changed its course, creating a typical meander loop by eroding the thick but soft, unconsolidated glacial sediments of Haras Scar. Rivers are dynamic and channel movement and flooding of their valleys is the rule, not the exception, something our changing climate has reminded us of today. Around AD 122, the Romans initially built a narrow bridge of stone arches, carrying a walkway or road. There was a tower at the east end, either to control the crossing or to provide access to the bridge. Several decades later a flood destroyed all or part of the bridge and its stone arches were replaced in timber. Around the turn of the second and third centuries, the bridge was rebuilt again, this time with a roadway, the military way, and ramps to provide access. There is evidence that the bridge was robbed for stone in the fourth century to repair the fort wall at Bird Oswald, and again in the 19th century to build Willeford Farm. Willeford Bridge abutment is worth more than a passing glance as you stride along the wall trail. Seek out the stone channels interpreted as a sluice for a water mill. There are large carved sockets which carry the timber arches of the second bridge. Butterfly shaped niches are for iron and lead cramps to join the large building blocks, and the narrow slots in some stones are Lewis holes, an ingenious method of lifting heavy stones by slotting in flat iron pins and using cranes. Last but not least, admire the chisel skills of the masons who shaped the huge blocks of carboniferous sandstone. There is one stone, with a large colonial coral fossil in it. Three hundred and thirty million years ago, this animal was living in a warm coral sea on the equator, not a cool fifty five degrees north. See if you can find it. The river Earthing here is a tributary of the river Eden. That was the last stop of episode one, and so there is a nice symmetry in that we end number two beside it. The next episode will start with a side trip to a northern outpost fort, and then change counties to Northumberland, and we start our hike uponto the ridge of once molten rock that was the foundation and defining line of the most famous bit of Hadrian's War.