Nearshore subtidal deposits are those which accumulate in relatively shallow water below low tide. They are characterized by ripples in the sand surface, which are a product of multi-directional water currents associated with wave and tidal action. In the Pleistocene sub-tidal limestones of Bermuda, these ripples are recorded as somewhat chaotic small scale cross-bedding.
Sub-tidal deposits commonly include particle sizes which are larger than typical beach sand. This is expected, as sub-tidal deposits are closer to the source of limestone debris and have consequently undergone less sorting than beach sand.
Where a Pleistocene sub-tidal limestone is found above
present sea level, it is a sure indication of previous higher sea level
- assuming, as we do for Bermuda, that the land has not shifted upwards.