Why Is the Desert Floor Covered in Flat Stones? A Surface That Builds Itself
By the BrainSnail editorial team. How these articles are written and checked, and how to tell us when one is wrong.
Large areas of desert are floored with a single tight layer of stones sitting on fine material, looking deliberately laid. How that layer forms was argued about for a century and the answer turned out to be surprising.
What the surface looks like
A desert pavement is a continuous layer of closely packed stones, typically one or two stones thick, resting directly on fine silt and sand with very few stones mixed into the material below. The stones are commonly flat-lying and interlocked, and their upper surfaces frequently carry a dark coating that develops over long periods. The result is a hard, stable surface that resists erosion and looks as if somebody arranged it. Such surfaces cover very large areas in arid regions worldwide and carry local names in several languages. Once formed they are extremely durable, which is why vehicle tracks across them remain visible for decades and why archaeological features built on them survive.
The explanations proposed
Three mechanisms were proposed and the evidence now favours the last:
- •Deflation, where wind removes fine material and leaves the stones behind as a lag
- •Washing, where water removes fines and concentrates the coarse fraction
- •Upward migration, where repeated wetting, drying, freezing and thawing work stones towards the surface
- •Accumulation from below, where windblown dust settles through the stone layer and builds up underneath it
- •The first three predict stones scattered through the underlying material, which is generally not observed
- •The last predicts a clean separation, which is what the deposits actually show
The evidence that settled it
The decisive work used dating methods to establish how long the stones had been at the surface. Cosmic rays produce particular isotopes in rock exposed at the surface and not in buried rock, so measuring those isotopes gives an exposure age, and applied to pavement stones the method showed that stones across a surface had been exposed for essentially the same long period, which rules out mechanisms that would expose them progressively as material was removed. Dating the fine material beneath showed it accumulating over the same interval, consistent with dust settling through and building up below rather than being stripped away. The picture that emerged is of a stone layer born at the surface when a lava flow or gravel deposit weathered, and then kept at the surface while dust accumulated underneath it.
The coating on the stones
The dark shiny film on exposed stones in arid regions is a separate phenomenon worth describing, since it is what gives these surfaces their appearance. Desert varnish is a coating a fraction of a millimetre thick, composed largely of clay minerals with concentrated manganese and iron oxides, and it takes thousands of years to develop. Its origin is disputed, with proposals involving microbial activity concentrating the manganese and purely chemical explanations both current. It matters practically because ancient rock carvings were made by chipping through the varnish to expose lighter rock beneath, and the degree to which a carving has re-varnished gives a rough indication of age. The varnish also traps material from the atmosphere as it forms, so it records something of past environmental conditions.
Why it matters
The mechanism has practical consequences beyond the geomorphology. The fine material beneath is windblown dust, which means these surfaces are a record of dust deposition over tens of thousands of years and can be sampled as an archive of past atmospheric conditions and of where the dust came from. Breaking the pavement releases the fine material underneath to the wind, which is why disturbance by vehicles and construction generates dust problems out of proportion to the area disturbed and why military exercises and development in arid regions have measurable air quality consequences. The surface also controls water, since an intact pavement sheds rainfall rapidly rather than absorbing it, so disturbance changes local hydrology. Restoration is effectively impossible on any human timescale.
The takeaway
The surface is a layer of stones one or two deep sitting cleanly on fine material, with very few stones mixed in below, which is the observation that constrains the explanation. Exposure dating showed the stones have been at the surface for the same long period, so they were not concentrated by removal of fines but kept aloft while windblown dust accumulated underneath.