What would happen to the land under a fantasy-style floating city?
One day, a rare and powerful magic lifts a chunk of land off of the ground and into the air. You can think of the magic as a sort of flat bottomed bowl or circular cake-pan.
It is about 50 meters deep, has a flat bottom (relative to gravity), and a radius of about 2.5km. The magic is invisible and impenetrable (things can anchor to the top edges, but water and soil cannot fall through it).
The magic keeps it in "geosynchronous orbit" about 300 meters above the previous ground level. The city is in European latitudes (~48deg N) and is in a mixed forest biome like southern Germany or central France. They have an early renaissance level technology. Magic is extremely rare and limited to once-in-a-century miraculous events like this floating city. It is extremely durable though. Almost everyone is certain that the city will remain afloat for millennia at least.
I expect that initially, the edges of the hole will collapse into something a little more crater shaped. Water is likely to rush in (either groundwater or from water sources that flowed into the now airborne city), forming a shallow lake.
After that, it's hard for me to guess at things like the erosion pattern, how the shadow of the city would affect things even before the human element gets involved.