Nature’s Sculptures
John Palmer
For years geologists believed that the rock columns, known as tors, had been formed as the bedrock lay under the ground level and underwent a process of chemical weathering. But in the late 1950’s and early 1960’s, John Palmer, researching the rocks at Brimham, developed an alternative theory. This theory proposes that the blocks exposed on the surface during the extreme cold of the periglacial periods during the last ice ages were shaped by the freeze-thaw cycle as water seeped into the joints breaking open surfaces which, exposed to the weather, were shaped by erosion.
The original bedrock develops numerous vertical and horizontall joints as a natural consequence of the sediments being laid down and the subsequent distortion of the rocks as they are subjected to earth movements and the loading and unloading of overlying ice sheets..
During the last Ice Age, water within the joints freezes and forces the rocks apart.
As the ice sheets move across the ground, they scour out the gaps between the remaining rock leaving tors, rock pillars and piles of boulders.
An extraordinary feature found at Brimham are various holes, tubes and arches apparently carved out of the solid rock.
Stephen Lewis
One of a number of arches and holes perforating the solid rock.
Stephen Lewis
The Wishing Rock is hidden away close to the Idol Rock.
A challenge to geologists has been to explain how have these been formed in rocks that have not been exposed to the force of rivers eroding the rock for millennia, ever since the sediments were originally laid down.
Dr Jiri Bruthans
Working in the Czech Republic, Dr Bruthans performed a series of experiments on sandstone blocks from the Strelec quarry and came up with an explantion of how sandstone is shaped by the weather. This results from the remarkable negative feedback effects between stress and erosion.
Initially the weight of the rock bears down fairly evenly across the entire block. In this diagram, the vertical lines depict the forces exerted downwards. They are evenly spread and not distorted.
With a heavy weight on the top of the block lines of stress are directed down through the block. These are concentrated in the area underneath the weight.
The effect of the stress lines is to compress the sand grains in the central region. At the edges the grains are far less compressed together and so free to be eroded from the exposed surface.
This process also explains how these forces can lead to the formation of holes and rock arches.
Before any erosion starts, the weight of the rock is evenly distributed across the entire area of the block.
As the surface of the rock is eroded, a natural area of weakness is affected by the wind which leads to a hole in the rock. The weight of the rock above the hole is now directed around the hole and the vertical lines are distorted.
Where the lines are close together, the force bearing down on the grains making up the rock will be greatest and where the lines are further apart, the force is less. Erosion will continue to remove grains where the stress is least and so the hole continues to enlarge.