Thanks, all!
It's actually a standard, vertical Antelope shot, turned 90 degrees clockwise. As well as LR, the processing was done with my newly-acquired Topaz Clarity, which I suspect may have some potential. I wish it didn't crash quite so often, though.
Actually, "fixing" the light peak is even easier than time travel; I have a rather less-cropped version. I don't like it because I find the large, dark rocks at the top give a cramped, oppressive feel; and the idea of a possible exit from the centre of the shot, following the light upwards, appealed to me (God, I hate trying to describe emotions in photography - it's harder than talking about wine).
Anyway, here it is. I've added a little grain.
Jeremy