I found a grey stone with an image of a white bird on a beach in north Donegal, Ireland. How would this structure have been formed? How did the white rock get into the grey rock?
鈥� The 鈥渄uck stone鈥� is an excellent find. Knowing it comes from Donegal doesn鈥檛 help much with identification of the rock type, but it looks like a fine-grained, homogeneous rock that has been broken by a series of small-scale earth movements producing cracks with different, intersecting alignments.
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Further ground movements widened the cracks and created a space that has been filled by mineral solutions seeping through the rock and depositing pale-coloured crystalline material such as quartz or calcite. At the time, this rock would have been deep underground where pressures are high, so the cementing of the rock types has been almost perfect.
Glaciers covered this area later on, when the rock was nearer the surface. As the glaciers moved, they broke off a chunk of rock that was the precursor to this pebble. Its movement by ice and water has since given it a more rounded form.
The circular patterns we can now see are caused by the intersection of the crack features on the curved surface of the cobble as it is today.
Jonathan Wilkins
Haulfryn, Conwy, UK
鈥� The white rock is probably a common mineral, quartz, and the grey around it is probably limestone, although it could be clay, mudstone, sandstone or siltstone 鈥� the grain size can鈥檛 be seen clearly, so it is hard to say.
鈥淵ou see a white bird in a rock on a beach 鈥� is this like an inkblot test for geologists?鈥�
It is most likely that this rock was weathered in the sea or a river, as it has fairly smooth surfaces. Over time, this would reveal more of the quartz, but it is sheer chance that it looks like a duck right now, controlled by the shape of the spaces in the rock that the quartz filled in.
It is worth noting that the host rock might be softer than the quartz. Eventually that 鈥渋mage鈥�, if the stone is allowed to weather further, may well reveal a different shape, and, before that happens, the quartz will stand out from the rock around it.
Limestone, in particular, is susceptible to weathering by rainwater, which is a weak solution of carbonic acid. Massively scaled up, this is how caves form. Other rock types are harder and take longer to weather.
Helen Gould
Peterborough, Cambridgeshire, UK
鈥� The pebble could be granite, a rock type typical of coastal Donegal, veined with quartz. Veins, which have branching sheet-like appearances, form when minerals carried in an aqueous solution within the rock are precipitated in cracks and voids.
Gerald Legg
Hurstpierpoint, West Sussex, UK
鈥� Your grey pebble is probably a siltstone about 1 billion years old, which was metamorphosed 鈥� changed and warped by heat or pressure 鈥� about 700 million years ago. When this happened, it distorted cracks in the rock filled with veins of milky quartz.
Tumbling of this pebble on the beach has sculpted it into an intriguing shape, but basically there were two planes of intersecting quartz veins. Any resemblance to a duck is purely coincidental 鈥� an example of pareidolia, which is the tendency to see faces or animals in inanimate objects.
Geoff Townson
Charmouth, Dorset, UK
鈥� I have seen many similar stones, ranging from pebbles to boulders, in the Whanganui river valley in New Zealand. This river runs through rock that started out as sediment deposited at the bottom of a shallow sea.
The white parts of the stone mark spots where shells were buried in the sediment, revealed in cross-section on the water-worn surfaces. The most common are bivalves similar to oysters, which often appear as arcs or ellipses, but conical shells are also common.
Tony Ellis
Titahi Bay, New Zealand
鈥� They see a white bird in a rock on a beach in Donegal鈥� is this like an inkblot test for geologists?
Holland Oates,
Cleveland, Ohio, US
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