Strange Cliff Face Appears To "Lay Eggs" Every 30 Years

Concretion in the Dakota Sandstone in Colorado, US. James St. John/Flickr (CC BY 2.0)

An egg-laying rockface in the misty mountains of China sounds like some old mythological legend. But, lo and behold, this geological phenomenon does actually occur, albeit with egg-shaped rocks and not actual eggs. 

One of these mysterious cliffs can be found in a remote corner of Qiannan Buyei and Miao Autonomous Region in China’s southwestern Guizhou province. The nearest village contains just 20 households, but has become well-known for its strange egg-laying rockface (video below). So far, they have collected around 68 of these odd rocks, as Chinese media reports.

Every so often, about once every 30 years, the 6-meter (19 foot 8 inch) high cliff appears to “shed” an egg and it falls to the floor. The eggs are no more than 20 to 40 centimeters (8 to 16 inches) in diameter. According to the locals, there are other smaller "stone eggs" nearby.

So what could explain the strange cliff? Scientists who have been to the site are divided, although the main consensus is that it has something to do with erosion and different hardnesses of rock.

The cliff is made of a common type of calcareous rock that was formed about 500 million years ago in the Cambrian period. The eggs, on the other hand, are concretions made of tougher, heavier sediment deposits. This means that the cliff face tends to erode away quicker than the eggs during prolonged rainfall or sudden landslides. The roundness of the “eggs” is likely due to running water, however it isn’t clear how or why so many became embedded in the rocks.

You can find these round rocks popping out of cliff faces elsewhere in the world, such as in Colorado (image above), although it isn’t known whether the egg-shaped rocks also fall to the ground as they do in this small Chinese village.

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