The western stretch of the frontier, from the river Irthing to Bowness-on-Solway, was originally constructed from turf rather than stone. Even the milecastles were built of turf and timber, although the turrets were constructed of stone. It seems as though Hadrian’s men cut turf blocks from land to the south and north of the Wall’s line. They then laid these on a foundation of cobbles or turf blocks.
When the stone Wall was built the turf was removed in most areas. However, west of Birdoswald, the line of the Wall was altered, with the stone replacement section positioned some distance from the turf construction. Because of this, a stretch of the turf version can still be seen to the south of the later stone Wall’s line.
Archaeologists have various explanations as to why the the frontier’s western section was originally built in turf. For example, it may simply have been that turf and wood were more readily available in this area, while stone was scarce. A Dutch archaeologist called Eric Graafstal has argued convincingly that the western turf section of Hadrian’s Wall was the first part to be constructed, perhaps as early as 119AD in response to military threat from native tribes in the north west. Building turf and timber ramparts was standard practice for the Roman army, which constructed turf ramparts every night for protection when it was on campaign.
At the famous siege of the hill fort of Alesia in Burgundy, France, Caesar’s troops constructed around 15 miles of turf and timber fortifications four metres high in about three weeks. A turf wall would have been a quick and practical response to a real or perceived threat in the north west.