For instance, if you were to walk along the shores of the Bay of Fundy in eastern Canada you would not want to go far out on the tidal flat for long. If you walked way out on the tidal flat at low tide in the morning, by late morning or early afternoon you could be under as much as 50 feet of water at high tide if you stayed in one spot. Not all coastlines experience that much daily or semidaily rise and fall of tides, but all ocean-facing coastlines have tides. Tides are caused by the gravitational pull of the moon and sun on the earth's oceans. In some cases, the high tides will actually deposit a thin layer of sediment on coastal tidal flats.
Geologists call such deposits tidal rhythmites.
The thickness of each layer of a tidal rhythmite deposit is determined in a general way by how high the tide rises that day. Thicker layers reflect higher tides and thin layers reflect lower tides. In some cases, tidal rhythmites consist of stacked successions of layers in which successive layers gradually thicken and then thin. This progressive thickening and thinning is in response to the moon and sun changing their positions in the sky relative to our coastal tidal flat. To understand these changes it is often useful to think in terms of purely astronomical tides and equilibrium tidal theory.