A 100-million-year-old time capsule
On the western edge of the Byeonsan-bando peninsula, the sea has carved cliffs that look like a colossal stack of books. This location, called Chaeseokgang, is a geological site inside Byeonsan-bando National Park. The "pages" of these rock "books" are layers of sedimentary rock deposited during the Cretaceous period, around 100 million years ago when dinosaurs roamed the Korean peninsula. Within these layers, scientists have found a record of the deep past: fossilized ripple marks.
These are not ordinary ripples; they are what geologists call tidal rhythmites. They formed on ancient tidal flats as incoming and outgoing tides deposited thin layers of sediment day after day. The thickness of these layers, or laminae, changed in a predictable pattern based on the Earth-Moon-Sun alignment. Tides are strongest during new and full moons (spring tides), depositing thicker layers of sediment. During quarter moons (neap tides), the tides are weaker, and the sediment layers are thinner. This created a repeating, measurable pattern in the rock, recording the tidal cycles from a time when the Earth spun faster and the Moon was closer.