Selecting Mesh Vertice under certain conditions

Hi All,

I have a mesh surface, that I have attached, the valley and peak vertices of this deformed mesh have been located and then deleted thanks to some assistance from members of this forum - however I would like to exclude the vertices located along all four edges of the mesh from being included in this operation so that they will not be deleted. I'm not sure how to undertake this.

I would then like to locate the vertices surrounding the faces that have been deleted so that I can project them toward a ground plane below and create columns for my mesh to stand upon.

Any help is much appreciated.