Jackson, Ian. Northumberland Rocks — 50 extraordinary rocky places that tell the story of the Northumberland landscape. Newcastle upon Tyne : Northern Heritage, 2021. The richly illustrated and accessible book series of Northumberland, Cumbria and Durham Rocks are available to purchase from Northern Heritage.
29 Down Hill
Theme: Climate and landscape change
Location
About 2 kilometres east of the Port Gate (Errington Arms) roundabout on the B6318. Walk back east along Hadrian’s Wall Trail
Description
The B6318, Military Road, is usually a very straight east-west road, which for much of its course follows the line of Hadrian’s Wall and its defences.
At Down Hill, however, the road does a chicane around a small hill, as does the Roman ditch, the Vallum, which diverts round it on the south side.
This is a very unusual small hill because it is a giant erratic, a glacial raft of limestone rock, a megablock, that has been plucked from its original position by an ice sheet and carried at least 2 kilometres to rest where it is now.
The limestone (called the Great Limestone) is Carboniferous and is over 320 million years old but as a slab of rock carried by an ice sheet it is possibly only around 20,000 years old and certainly less than 2.4 million. The ice sheet that carried this rock was moving quite quickly from Scotland and Cumbria across Northumberland and as it did so it not only carried lots of rock and debris, within it and frozen to its base, but sometimes literally plucked large pieces of bedrock and took them downstream.
This is possibly one of the larger glacial rafts/erratics in England and because it is so large it puzzled geologists for a long time. It is big (approximately 600x300x15 metres, that’s over 7 million tonnes) and has been quarried for limestone, including by the Romans. Usually erratic boulders are much smaller — a lot less than a cubic metre.
Growing on the old quarry spoil banks are lime-loving plants like salad burnet, mouse-ear hawkweed and quaking grass.