The oldest record of tides
Sandstone in South Africa preserves the rhythm of daily, fortnightly and monthly tides 3.22 billion years ago.
- When
- Known to
- ± 10 million years
- Era
- Paleoarchean, Archean
The Moodies Group holds Earth's oldest well-preserved tidal deposits: sand waves with mud drapes stacked in bundles that record individual tidal cycles. Counting them recovers the length of the Archean day and month, and confirms the Moon was closer and Earth spinning faster. It is a working clock preserved in rock.