The crags and ridges of the central section of Hadrian's Wall in Northumberland make up some of the most dramatic scenery in northern England.
What impresses today's visitors had the same effect on the Emperor Hadrian, who decided that a place with such natural defences was a grand site for his Wall.
Much is made of the near 2,000-year-old Roman frontier.
But as far as frontiers go, Hadrian's Wall is a mere newcomer.
For beneath the Wall is a 420 million-year-old frontier zone.
This is where two continents collided. Before that truly epic event, the land which was to become England and Scotland lay thousands of miles apart on different continents, separated by the Iapetus Ocean which was probably as wide as today's Atlantic.
The continent of Laurentia included Scotland while England and Wales were part of Avalonia.
The continents drifted towards each other and the Iapetus Ocean disappeared as the land masses collided.
Britain was now joined along a great seam of rocks known as the Iapetus Suture, which runs roughly from beneath Berwick west to the Solway Firth.
The story of this upheaval comes from a joint project between Northumberland National Park and the British Geological Survey.
It is examining the geology of the whole of the national park, which dictates what the landscape looks like, and influences plant and animal life, buildings, and how people have lived and worked over the centuries.
An aim of the geology project is to help visitors make more sense of what they see in the diverse landscapes of the park.
The first fruits of the venture are the book Ancient Frontiers, priced at £8, which explores the countryside around Hadrian's Wall.
It has been put together by geologists Brian Young, from Riding Mill in Northumberland, David Lawrence, who lives in Jesmond in Newcastle, Elizabeth Pickett, based at Stanhope in County Durham, national park ecologist Gill Thompson and archaeologist Rob Young.
Brian says: "The point can be made that the line of the A69 and the Newcastle-Carlisle railway are dictated by the geology derived from the collision of the continents and the closure of the Iapetus Ocean 420 millions years ago.
"Learning more about the geology of the area will give people a new way of looking at the countryside. For visitors it gives the scenery another dimension and for locals a new insight into the ground they walk on every day.
"The more you know about a place, and why that hill is a funny shape, helps you enjoy it so much more. Everything goes back to the rocks underneath you."
Rocks mean quarries for buildings, roads and walls. Quarried rock, mainly sandstone, provides the distinctive appearance of the area's castles, villages, farms and miles of drystone walls.
Prehistoric people erected their stone circles, and even the Romans were defeated at Limestone Corner on Hadrian's Wall by the hardness of the dolerite rock of the Whin Sill outcrop.
The Wall's vallum and ditch were designed to cross a bare stretch of the Whin Sill but the job was left incomplete. Wedge holes in the rock are evidence of the abortive attempts by the Romans to break the rock.
On the Wall, the quarrying of dolerite, or whinstone, at Walltown ended only 30 years ago while Cawfields Quarry ceased production in 1952. Both have been restored as picnic and wildlife areas.
Working roadstone quarries remain at Barrasford, Keepershield, Swinburn and Divetill. Dolerite is well suited for surfacing roads.
Gill Thompson says: "It seeks to link the geology to the landscape, natural heritage and buildings of the park. Geology is about what you see and not about boring rocks in the ground. We want to bring geology to life. There is a lot more to the Hadrian's wall corridor than the Romans."