TIDALITES
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).