At the height they were flying and with all that cloud cover, it would be highly improbable to land giant boulders accurately on the beach in strategic enough positions to be effective.

And suppose the soldiers do find cover behind the rocks, of which you will need tons of them for the tons of soldiers.
Then what?
Wait for the Germans to run out of ammo?
They will have to move close enough to take out the bunkers and when they move from behind their cover that is when they will get hit.
"Please vote to preserve the unique character of Warren…" - Robert Duvall

You have to find rocks big enough then somehow have the equipment to load them onto trucks then drive to the airport or runway and load them onto the plane.
I dunno if planes could carry loads that heavy back then and they would have the risk of getting shot down or attacked by German planes.
Also if the Germans saw this they would know something is up and prepare for it.
D-Day was a sneak attack and this would take away the surprise.
Overall it just seems to be too much trouble than it is worth.
"Please vote to preserve the unique character of Warren…" - Robert Duvall

I was speaking hypothetically!!

Now you suggest this?
Too late for that idea.
"Please vote to preserve the unique character of Warren…" - Robert Duvall

Too late
My password is password