The Yeliu rocks are layers of the soft, yellow, erodible sandstone, topped by the the harder, black/gray limestone. Being both sedimentary rocks they can be dated by the fossils in each. Especially the limestone.
Taiwan started as a volcanic island maybe 20 million years ago. Dinosaurs died out about 70 million years ago. So the dinosaur fossils for sale in Taiwan usually are from Mainland China. From the fossils in the Yeliu, the formation can be dated. The fossils are all over the rocks, you have to be careful not to step on them.
Notice how the presence of the fossil has shaped the erosion around it, the softer sandstone erodes faster than the fossil, so causes the local uplift. At Yeliu, they called these fossils, "sand pipes", I think in the US they are called "sand dollars".
The details is quite explicit. But then again the fossils are only less than 20 million years old!
I though this one was the carapace of a crab.
It is actually the presence of organic material in the limestone that causes the erosion to proceed from very small depression to a uniform distribution of big depressions.
Once the seawater gets a start it grows the hole.
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