How can I force only the bottom points on each mesh to be deleted? (Geometry nodes)

How can I force only the bottom points on each mesh to be deleted?
Objects will be at different heights and positions
As I understand it, I need to somehow compare them along the Z axis, but I still couldn’t find a solution.
Test_Node.blend (952.7 KB)

try comparing to the z face normal.

hope that helps.

To be honest, I’m not particularly good at geometry nodes, if you can give an example, I’d be very grateful

not at a computer atm.

A day later I was able to solve this problem in the following way.
Test_Node.blend (925.4 KB)