[decimon]

Pardon my seeming ignorance, but if the “valley” filled with rock and disappeared from the landscape, it isn’t really a valley anymore, is it?

I've heard tell of a California valley filled with girls. Still a valley.

[IronJack]

But those girls lack substance. In effect, the valley is filled with nothing. So it still qualifies as a valley.

A silicone valley, perhaps ...