From the essay, WHEN THE EARTH WAS SQUARE ~ 1960-1978, in the book, THE ART OF THE AMERICAN SNAPSHOT ~ 1888-1978:
The effect of the square format, neither portrait nor landscape, on habits of ordinary picture taking is impossible to assess, although its implicit resistance to storytelling makes it a singular choice for snapshots ... [T]he square shape supplants narrative flow with iconic stasis, and it tends to draw attention away from the picture to the object as such.
iMo, an interesting thought on the idea of the square shape as the perfect shape for snaphot pictures.