What did rocks ever do for the Romans and their northern frontier? The short answer is: pretty much everything. As a geologist, I admit I’m biased, but judge me when you have finished reading. My thesis is twofold. Part 1: geology defined the northern edge of Hadrian’s empire and supplied the materials to build and sustain it. Coal ensured Roman warmth and fuelled their industry; groundwater fed their wells and bathhouses. Ores were available for weapons, ornaments, and jewellery, paint pigments, and even cosmetics. Minerals shrouded and coffined their dead. Without stone, there would be no Roman Inscriptions of Britain, and far fewer voices from the past. The bottom line is: Roman knowledge of the area’s geological assets and how to put them to use was comprehensive and expert. Part 2, however, concedes that, here in Wall Country, where the Roman military imprint on the landscape is arguably more profound than anywhere else in Britain, we know so little about the origin of the natural resources they used. This article is intended as an introduction to what we do know – and what we don’t. There is a knowledge gap that can only be filled by archaeologists and geologists working together, and filling it will add richness and credibility to so many social, economic, political, and military narratives from the Roman frontier.
But first, a little geological scene-setting, just to provide some essential context. The geology of northern England is incredibly diverse and spans almost 500 million years. I could divide and describe that enormous period of time using soulless stratigraphy – Ordovician, Carboniferous, Permian, Triassic, and so on – but I’d rather you kept reading. What you need to know is that the reason the northern landscape is so rich in natural resources is precisely because of its varied and dramatic past. Deep oceans and evaporating seas; cataclysmic volcanic eruptions and magmatic injections; coral reefs and vast rivers; tropical swamps and red-hot deserts; and, less than 20,000 years ago, frozen wastes. These very different environments bequeathed to the north terrain that may have been challenging to control but offered a bounty of materials to those willing to try.
Why build the frontier here?
It is not geologists but archaeologists with long experience of the Wall who credit the Whin Sill – a 295-million-year-old, 30-plus-metre-thick (100-plus feet) slab of once-molten dolerite – for the choice of the line of Hadrian’s Wall. The emperor simply had to add his own 4.5m-high augmentation to this natural north-facing barrier, then complete the subservient limbs to the east and west coasts. The same Whin Sill was also the chosen foundation for medieval castles and priories. It is hard to overstate the significance of this landscape-defining, iconic rock: without it, Northumberland would be culturally (and, as the millions of tourists who visit annually underscore, financially) much poorer.
