500 years to form a beach at the top of Everest?

Having studied geology, this one bothered me. I checked the goofs section to see if anyone else picked up on this but I didn't see anything. Maybe someone can correct me if I'm wrong:

It's been said that the movie takes place around 2500 AD (about 500 years from the day the movie was made)

If the ocean level rose to the top of Everest (let's just assume the massive amount of extra water was introduced from say, a crashed comet, and not from global warming-lol), then Everest would become an island surrounded by cliffs. It would take thousands or probably millions of years to form the beach that is shown at the end of the movie. Not only would the cliffs of Everest need to be eroded to sand size particles, but a sufficient base would need to be formed around Everest in order to hold and support the accumulation of the beach sediments. Millions of years, I think.

"They don't exist:
LOCH NESS MONSTER, FRANKENSTEIN, MAGNUM PI, NON-CATHOLIC GODS, DARTH VADER"