How do you destroy the "middle-sized" rocks on Akua?

Hi.

I've crafted 20k concrete blocks, and want to make a "landing zone", as big as possible, on a field.

I've found a nice field I'd like to use, but it seems I can only remove the trees and "small" rocks from the surface, but the bigger rocks seem unaffected both by the drill and the mini-gun.

How do I get rid of them, so I can have a large flat surface to build my landing zone on?