I have found 3 of the 4 Aetherium Shards. How do I find the last one?

When I go to click 'Show On Map' it is faded out and I cannot see a white arrow on the map anywhere.

From looking at Katira's (sp?) journal I believe the location is somewhere NE of Markarth. I have spent two hours searching but am getting bored now.

I assume there is something in the game that I have missed that lets you discover the location other than simply wandering around? Have I missed something?

That would be at Deep Folk Crossing, usually the first one I go to.
It's just a little north of Bthardamz and shown as a diamond 'point of interest' icon on the map.

Ok thanks I'll have a look