Is there a polite term for... well... the buttcrack?

Mid-email, trying to describe in brief and polite terms to my colleagues why something was perhaps not appropriate for Fluther, I got stumped on the word “buttcrack”. Really, it was only the top of a buttcrack. It’s gone now. Let’s move on. I mean, it just seems a rather crass word to me. I wanted to be a bit more professional than that, but I really could not even think of another word for it. Well, other than “cleft”, but if I wrote that in an email, people would think I was nuts and/or way too into romance novels. I am not. Romance novels and I had a brief, guilty pleasure kind of fling in high school, but that that was long ago and we’ve both since moved on.

IS there a nicer way to say ‘buttcrack’? I would love to know.