Mud settles with the tides, leaving thin stripes that harden into rock. Count the pattern and you can read a kind of clock: how often Earth turned while the layers grew.
Scientists call these striped records rhythmites (rith-mites). Strong and weak tides leave different marks. Those marks, checked against other rocks, help estimate the length of ancient days.
One study points to a long pause near a 19-hour day, shorter than the day you live through now. The Moon's pull on ocean tides slows Earth's spin. But sunlight warms the air and raises another tide that can push the other way. The pulls could have balanced.
Scientists still argue about that pause. The rock estimates differ, so 19 hours is a proposed fit, not a reading from a clock left on a beach.
The chapter also follows a long stretch with little oxygen and less mountain building. As mountains wear down, they feed minerals to the sea. Fewer fresh mountains may have meant less food for some ocean life. That link is debated too.
Life kept changing. Complex cells existed, and fossils include some bodies made of many cells. The record has gaps, but it has living things in it, beside the stripes left by tides.
