The Tides are caused by the action of the heavenly bodies, principally the sun and the moon. When they are in line with the earth (in syzygy) the attraction is multiplied, announcing spring tides, this corresponds with the period of highest tides ; on the contrary, if the sun and the moon form a right angle (in quadrature), the attraction is reduced and we are in neap tides, the period of lowest tidal movement.
These phenomena are accentuated in Mont Saint-Michel bay, because with the rock being in the back of the bay, the sea doesn’t reach the Mount during neap tides. On the other hand during spring tides, the sea reaches the Mount but only after 4 hours and 30 minutes after the start of the rising tide, about every fortnight.
The highest tides take place 36 to 48 hours after the full and new moons. These indications are a rough guide only and can be affected to acertain extent by atmospheric conditions. The highest tides in continental Europe take place at Mont Saint-Michel, up to 15 metres difference between low and hight water. During top of the spring tides, the sea goes out 15 kilometres from the coast and comes in again very quickly.
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Sig Nordal 