February 1, 2013
When it comes to this wall, you might just need some education.
By Bill Langer
This article is about a famous wall; a wall with such an important history that it has been designated a World Heritage Site. But what wall is it? See if you can figure out the name of ‘the wall’ from the clues in this article. Not to worry…by the end of the article, you will know what in the world you were reading about.
Most, but not all, of the rocks in the area where the wall is located are layered sedimentary rocks — limestone, mudstone, sandstone, and coal. There is one noticeably different rock in that part of the world; a very hard, dark gray, nearly horizontal bed of crystalline igneous rock. Some 295 million years ago, movement of the Earth’s crustal plates caused molten rock deep within the Earth to rise upward. The magma was forced between the layered rocks, and when it cooled and solidified, it formed rock referred to by geologists as dolerite.
After millions of years of erosion, the dolerite, some of it nearly 100-feet thick, was exposed at the land surface. Locally, quarrymen referred to the rock formation as the Whin Sill. The rock is resistant to erosion, and the Whin Sill is marked by spectacular cliffs, long ridges, and north-facing crags.
About 122 A.D., a Roman emperor ordered the 73-mile-long wall and fortifications to be built, which, according to Scriptores Historiae Augustae, was “to separate the Romans from the barbarians.” Other scholars speculate that the the wall was to discourage raiding parties and control trade and immigration. Long stretches of the eastern part of the wall strategically took advantage of the Whin Sill cliff lines. The wall was essentially completed within six years and was the most heavily fortified border in the Roman Empire.
The eastern part of the wall, which was about 10-feet wide and 16- to 20-feet high, was made from squared stone blocks. Rock from the Whin Sill could not easily be dressed into blocks, so the local sandstone became the preferred building material. However, the Whin Sill rock did find a home in the rubble core of the wall. The western part of the wall was made of turf. In total, the wall extended across the entire width of northern Great Britain.
Following the emperor’s death in 138, the new emperor, Antoninus Pius, had a new wall (the Antoninus Wall) constructed about 100 miles to the north and demoted the wall to a support role. The Romans were unable to conquer the northern tribes, so when Marcus Aurelius became emperor, he ordered the abandonment of the Antonine Wall and the reoccupation of the wall, which remained occupied until Roman troops withdrew from northern Britain circa 383 A.D.
Once abandoned, the wall became an easy source of ready-made building blocks. Much was reused in other local buildings and for road construction in the 18th century. Preservation of the wall began during the 19th century, and its rescue is largely credited to John Clayton, who purchased large sections of the land on which the wall stood to protect it from plundering.
During 1987, HADRIAN’S WALL, named for Roman Emperor Hadrian who ordered its construction, was declared a World Heritage Site.
Bill Langer is a research geologist who spent 41 years with the U.S. Geological Survey. He can be reached at Bill-Langer@researchgeologist.com