An agger is the raised bank, or embankment, on which a Roman road was built. It lifted the carriageway above the surrounding ground and helped water drain away. In British archaeology the word usually means this road ridge.
Key takeaways
- An agger is a raised ridge of earth and stone that carried a Roman road above the surrounding ground.
- Many aggers look larger today than when new, because repairs and resurfacing added material over centuries.
- Lidar can show faint aggers even where ploughing has flattened them.
- Our map shows road lines, not aggers: the Fosse Way is 372 km long in our data, and none of it is classed as certain.
How an agger works
The word agger is Latin for a mound or embankment. It has two meanings: a defensive rampart, and, in modern British archaeology especially, the raised ridge on which a Roman road was built [1].
For a road, builders cleared the topsoil, often dug a shallow trench, and laid a firm base. They then built up the agger from nearby material such as stone, earth, sand and clay, rammed into a solid mass. The material often came from the roadside ditches or from quarry pits. The surface was metalled with small stones in gravel or earth and cambered so water drained off [1]. In Britain the carriageway was commonly built on this raised bank, with shallow ditches either side [2].
The bank does two jobs. It lifts the road clear of wet ground, and it gives a firm, well-drained base. Its line often survives in the landscape, and where farming has erased it, lidar can reveal a faint mound [1].
Not every Roman road had an obvious agger. Roads on firm, well-drained ground may have needed little raising, and size varied from place to place.
Agger as a clue to the route
Because an agger is a long, low ridge, it is one of the easiest signs of a Roman road to follow on foot. It tends to show as a straight raised line through a field or wood, often with a shallow ditch beside it. Archaeologists use such banks, together with aerial photographs and lidar, to trace roads that have no surviving surface. Those traced lines are what datasets like Itiner-e draw, and the less evidence a stretch has, the more likely it is to be classed as conjectured or hypothetical.
An example from our data
Ermine Street is 421 km long in our data and the Fosse Way is 372 km. The map does not show aggers, only the route. Nothing in Itiner-e records whether a section had an agger, how high it was or how wide.
At Tillbridge Lane, on Ermine Street, the original agger was about 6.5 m wide and 50 cm high. Most of the large mound seen there today is post-Roman [1].
The thing people get wrong
A tall, steep-sided agger is not always a sign of a better road. Many famous aggers look enormous today but were much smaller when first built. Repeated resurfacing added material over centuries, and later centuries added more [1]. A ridge you can see on a walk may therefore be partly medieval or later. Also, a missing agger does not mean a missing road: ploughing flattens banks, and many Roman roads were never raised much at all.
Frequently asked questions
What does agger mean in Latin?
It means a mound or embankment. It was also used for the earth rampart of a fortification, which is a different meaning.
Can you still see aggers today?
Yes, in some places, as a low ridge in a field or wood, and lidar can show faint ones.
Sources
- Agger (ancient Rome), Wikipedia.
- Roman roads in Britannia, Wikipedia.
- Every figure from our own map is worked out from the datasets on the data sources page (checked against Itiner-e version 2024 1.3).