I have a large plateau, and roughly in the center are mile-high cliffs surrounding a valley.
Inside the valley is a green sandy desert. The sand is composed of olivine. Unfortunately for the inhabitants of the green desert, a hostile power has resolved to remove all the sand out of the valley, off of the plateau, and into the lowlands below it via a huge ramp/shaft that they dug and some vague (and very classified) magic trickery.
What does this do to the local climate? Is a desert in a closed-off valley even entirely possible?
Of equal interest: what happens to the lowlands? This particular area of them where the sand is being drained is a region of badlands. Keep in mind that it isn't all of the sand being dumped at once, but rather a steady (albeit massive) flow of the stuff.
And what would happen to the flora and fauna on both ends?