Glitchy shadow lines: Toon Lighted (screenshot)

Hi, I'm using the Toon Lighted shader on a rock mesh which is lit from above, but getting some strange lines on the near-vertical surfaces. I don't really know what I'm looking at (light rays? Mesh resolution/aliasing?) I took a screenshot, hopefully you can see it here:

They change when moving closer/further away. Changing between High shadow resolution and Very High shadow resolution made no change. Any thoughts?

Thanks