Tidalites are rhythmically laminated sedimentary rocks that record monthly tidal depositional cycles.  They can occur in siliciclastic or carbonate rocks.  Well-preserved tidalites will readily show neap tides & spring tides and can allow days-per-month counts and days-per-year counts.  This information can tell us hours-per-day in the ancient past.  Early in Earth's history, the number of days per year was much higher than the current 365.  The number of hours per day was also significantly less than 24.  How come?  Earth's rotation rate has slowed over time.


Tidalites (9.8 cm worth of section) showing spring tides & neap tides from the uppermost Salem Limestone (middle Middle Mississippian) from a roadcut along Rt. 37, 2 miles north of Harrodsburg, southern Monroe County, south-central Indiana, USA.



Tidalites from the Big Cottonwood Formation (Neoproterozoic) of Utah.  Cleveland Museum of Natural History public display (Cleveland, Ohio, USA).




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