Another Intriguing Cloud Formation

Interesting weather having finally returned, I've been taking a lot of photographs of clouds lately. And last week, I noticed this strangely well-delineated polygon in the clouds. The first photograph is the original, followed by enhancements.

Weird.

EDIT:  Wow!  I just now noticed (January 25, 2012) that there are what almost look like runes or pictographs on the upper part of the polygon!  I see an I, a possible A (or a 4?), an E lying on its side, an L, an M, and a possible H (or K), a C and another L!  How strange that I only now noticed it, after 15 months!