Hi Cathryn,

Where you live, it is likely the shells are 550' above sea level mostly due to uplift of the land, more so than subsequent drop in sea level. Your five acres may well be a marine terrace, or wave-cut bench.

And, though I'm no expert at all, it's my understanding that such uplift is likely due to either or both the compressional component of the plate boundary/San Andreas Fault System (including offshore strands) motion, and the even more ancient pre-San Andreas "bulldozing" of materials carried on the then-subducting oceanic plate by the overriding North American plate.

You may well have known this.

Mike W.