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.
Theme: Volcanoes and molten rock
North of the B6318 east of Greenhead
Walltown is a huge, restored old quarry in the Whin Sill. The quarry opened in 1876 and closed in 1976. The Whin Sill (whinstone to quarrymen, dolerite to geologists) is so hard that they had to use explosives to blast it from the rock face.
Its hardness meant it was perfect for roadstone chippings used in tarmac and a century ago, for setts for cobbling streets. The Whin Sill was once molten magma; it was injected from deep in the Earth in between the layers of Carboniferous limestones, sandstones and shales and then it cooled, solidified and contracted forming its prominent vertical cracks and fissures. Millions of years of erosion since left the hard dolerite rock ridge standing proud… until it, and the section of Hadrian’s Wall that ran along it, was quarried away.
The enormous hole that was quarried out was infilled with compacted stony glacial clay as part of its restoration. Slowly nature is returning and there is now a wide assortment of plants on the varied soils and rock faces of the quarry. Look out for common rock-rose on thin whin soils. On the quarry floor are common spotted-orchid and northern marsh-orchid (and their vigorous hybrid), with numerous sedge species.