Near the village of Gulu in China's Guizhou province, there's a cliff face that looks like it was bedazzled by some primeval interior decorator. Smooth, colorful, egg-shaped stones jut out from the craggy rock, leading locals to call this cliff Chan Dan Ya, or "egg-producing cliff." What exactly creates these "eggs" is a mystery to visitors, but luckily, it's no challenge for geologists.
What's more perplexing about a cliff face speckled with what look like candy buttons is the fact that those buttons eventually fall off. Each smooth, round stone takes about 30 years to edge its way out and tumble onto the rocks below. Visitors often collect them, considering them to bring luck and fortune.
According to MetroUK, geological tests have shown that the 19-foot (6-meter) high, 65-foot (20-meter) long cliff originally formed 500 million years ago, during the Cambrian period. It's made of calcareous rock, a sedimentary rock that's high in calcium carbonate — also known as chalk. How did hard stones make their home in this chalky mineral substance? Through the wonder of geology.
The Scientific Explanation
The "eggs" that dot Chan Dan Ya are what geologists call concretions. You see, sedimentary rock like the one that makes up this cliff forms when water deposits minerals — the weathered remains of other rocks, usually — in layers, which build up and weigh down on the layers below until they're compacted into solid rock. Concretions form when minerals gather around some sort of impurity, like a leaf, a shell, or even a soon-to-be-fossilized animal, and create a rounded mass of minerals. That often happens after the sedimentary rock has been laid down, so the minerals are usually different than the surrounding sedimentary rock.
If those concretions are harder than the rock around them (as they often are), they'll eventually wear down the surrounding rock and break free. That explains why Chan Dan Ya "lays eggs" — these hard, smooth stones weather out of the surrounding cliff face, and eventually fall to the ground to become a visitor's lucky trinket. They might as well be lucky — it took them millions of years to get there, after all.