Its where lightning has run through sand and fused it together, by the look of it. Happens sometimes, I have heard of it - wasn't there a doco on lightning in the last few years that had some better examples of it in? I don't know that I'd call it 'fossilised lightning', though, but it is a record of a past lightning strike on that bit of ground.
pretty cool, although the picture that is supposed to be ball lightning is out of focus water droplets on the lense. look at the lines in all of them the face the same way.
