Removing all points below an objects surface I have an object that is cubical in form, but the top surface is roughened. The point density on all sides of the cube (including top roughened surface and flat bottom) is pretty high.  For illustrative purposes, it's similar to the following image:



Is there an efficient way to remove all points under the top surface (shown highlighted in orange)? This includes all 4 sides, the bottom, and any interior points. I've read about Blender's Circle Select, Lasso select, etc., but wasn't sure if these were effective options for high-density meshes.
Blender has a built in tool for this called "Select side of Active". To use it you have to switch to vertex select mode and select exactly one vertex. This vertex marks the boundary, that means that either all vertices e.g. to its left or right will be selected after you run the command. See my screenshot for the result, the white vertex was the active one before i ran "Side of Active".

Using the last operator panel (Shortcut F6) you can tell blender which axis and which direction to look for vertices to select. Once done, simply delete them.