Over millions of years, weathering and erosion of sandstone have produced unique landforms, such as arches, alcoves, pedestals and pillars. Until now, the natural process remained a mystery. It was difficult to study, because of the huge time-scales involved in the erosion of natural slabs of sandstone. Gravity-induced stresses had been assumed not to play any role in landform preservation. Instead gravity was thought to increase the rates of weathering and natural erosion…
Geologists have now shown that increased stress within a landform, as a result of vertical loading, actually reduces the weathering and erosion rates. Dr Jiří Bruthans from Charles University in Prague, said the new study revealed the mystery sculptor behind some of the world’s most famous natural rocky landmarks – Gravity.
Forged by Gravity
Understanding the stress fields within those rocks allow a prediction to be made of where the erosion will take place. Using laboratory experiments on sand cubes and numerical modelling, the geologists showed that areas squeezed by vertical stress are protected from erosion, while others wash away. The process had proved difficult to study, because natural slabs of sandstone take over millions of years to erode.
The breakthrough, published in Nature Geoscience, came from an unusual “locked sand” dug from a Czech quarry.
It seems erosion by wind and/or water is merely the chisel that sculpts such gravity-defying structures. The remarkable shapes are controlled by internal stresses and strains within the rock, applied by the pull of gravity.
As well as showing they could predict the shapes with mathematical models of that stress field, Dr Bruthans and his team sandwiched small cubes of sand under various weights and submerged them in water.
The sides of the cube started to fall away within minutes, leaving fewer and fewer grains of sand to bear the weight. As that process continued, eventually the pressure on the remaining column caused the grains to lock together and resist further erosion.
The scientists were able to reproduce successfully a gallery of the sort of shapes seen in natural sandstone landforms. Adding faults and distortions to the sandstone cube shifted the direction of the pressure applied. They were able to watch all this happen because of the strange and sticky quality of the sand they used in their experiments.
Strelec Locked Sand
The team collected samples of sand with particular interlocking properties from a quarry in the north of the Czech Republic. For many years, the “Strelec locked sand” was soft enough to be mined from the Strelec Quarry using water hoses. When it dried out, explosives had to be used.
10-centimetre blocks of this ‘locked sand’, dried out in an oven, were used for the experiments. To demonstrate that the same principle applies to other types of sandstone, Dr Bruthans and his team also took small cubes of normal sandstone from the same quarry, and submitted them to cycles of heat, cold and salt, in order to simulate the natural erosion process.
One of the experiments showed that a brick of Strelec sand, straddling a small gap and left outside in the rain for 15 months, would naturally form an arch.
Dr Simon Mudd, a lecturer in landscape dynamics from the University of Edinburgh, said: “It was very clever to find this rock out of a quarry that would behave in an accelerated way. They’ve really demonstrated convincingly that as you erode this material, it begins to concentrate stress. It’s a very compelling combination of experimental and numerical work.”
Solving a Natural Mystery
All of these processes can be predicted by modelling the stress field. Dr Bruthans showed it was pressure that determined the emerging shapes: “You select the pillar direction by choosing the points where you apply the compression. The stress controls the shape – nothing else.”
For decades, those natural formations have enchanted and puzzled countless tourists and visitors, many of whom have stopped and paused to wonder how they were formed. Today, Buthrans and his team have the answer.