Subsurface black edge I have a surface with a Subdivision Surface modifier, and when I render it in Cycles, there is a black edge at the edge of the surface:



If I render it without Subdivision Surface it looks like this:



What could be the reason and how can I stop this?

EDIT:

After adding the Edge Split modifier as suggested by Shady Puck, the result looks as expected:
That's because you have smooth normals.  Either set those faces to flat shading or enable Auto Smooth under Mesh Data.