What causes these black streaks along certain (concave?) edges? I have a model with an open mesh terrain surface and some closed building meshes – when rendering in Cycles I'm seeing black streaks along some mesh edges.
On the building models it seems to be wherever there's a concave edge connecting two faces, on the terrain it's less clear but I think also at concave edges:

What is causing this??
Edit: Here's what the material nodes look like:

And how it appears in a full render:

Also, after duplicating and separating out a section of the terrain mesh for clarity, and correcting the normals:

Blendfile:
You need to recalculate the normals of your mesh.
go in edit mode with TAB
select all with A 1 time, or 2 times, or 3 times (depending on if something is selected and if you're using 2.79 or 2.8)
- If you're using 2.79 or earlier, use Ctrl+N (recalculate outside) or Shift+Ctrl+N (recalculate inside)
- If you're using 2.8, Shift+N (recalculate outside) or Shift+Ctrl+N (recalculate inside)