This is great - impressive how you captured as much detail as you have in that sand. The strong graphic on the towel really draws the eye and makes the shot that much more effective.
I agree: a total winner! This is black and white photography at its finest! I think if she had known you were photographing her, it would have ruined it. She would have become awkward, and what's so beautiful about this is how natural and unaware she is of anything but the moment!