The suggested mechanism is that sedimentary rocks form over ice. That puzzles me in itself, because it implies a river with ice at the bottom. But anyway, then some heat source arrives and melts the ice, causing a sort of rug pull that means the rock layer drops down and breaks up. Kaboom.

We almost certainly did at one point, but life and tides can fill-in the gaps. We also truly do not know what terrain is underneath Antarctica, Greenland, etc.