The answer is a shale. Shale is a very fine grained rock composed of microscopic particles of clay compacted together. Shale often displays fissility or having many closely spaced bedding planes. Where shale and sandstone occur together, such as in the Gulf Islands of British Columbia, the relatively soft thinly bedded shale forms the bays (recessive weathering) and the points or peninsulas are underlain by resistant hard sandstone.
You won't notice much at sea. Maybe a slight change in sea level.
Closer to the land though, water will draw away from the land maybe leaving the sea floor exposed. Then it will surge back in, flooding the land and inundating the coastal area.
