The bay around the mount's granite outcrop has been steadily silting up in recent years, so that it's an island only during very high tides.
The outcrop is only an island for a few minutes per day. 
mnli hypothesis: The outcrop is only an island for a few minutes per day.  premise: The bay around the mount's granite outcrop has been steadily silting up in recent years, so that it's an island only during very high tides.