Creating Terrain from solids - survey contours already extruded

Hi everyone, 

this is a non grasshopper request but my request to join the mcneel forum is pending and i'm in a bit of a rush. 

I've got some survey information for a site. I initially thought the survey was made up of curves (viewing from the top view in AutoCAD) but when i import this information into Rhino I realised that the curves are in fact surfaces which have been extruded upwards. 

In the past I have simply lofted through contour lines, but i don't know how to do this if the curves are in fact solids. I've been back through the survey to see if one of the layers is the curve data, but it doesn't seem to be there. 

I was thinking about extruding the surfaces horizontally (looking from plan view) however I think there must be a more elegant solution. 

I tried to see if I could extract the edges (something i've never actually tried before) but the surfaces have two edges and therefore I would have to individually delete the top or bottom set of lines. 

Please help wonderful and knowledge people....