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.
Calculating an ancient day
By meticulously measuring thousands of these fine layers, paleontologists can read the ancient tidal record. They count the number of daily layers between the thickest deposits to find the number of days in a lunar month. They can also identify yearly patterns in the sediment to determine the number of days in a Cretaceous year. The data frozen in the Byeonsanbando rock shows: a year during the Cretaceous had more days than it does today.
This does not mean the Earth's orbit around the sun was longer. days themselves were shorter. Calculations based on tidal rhythmites from various locations show that 620 million years ago, a day was just under 22 hours long. The data from Cretaceous rocks like these indicates a day was roughly 23.5 hours long. This shows that the Earth's rotation is gradually slowing down. The gravitational pull of the Moon creates tidal friction, which slows the spinning Earth, causing the day to lengthen by about 1.7 milliseconds every century. The ripple marks at Byeonsanbando are a silent, solid-rock clock that lets us measure time on a planetary scale.