Hi John, up through the 1930's milk bottles were round with embossed labels. In the 40's square bottles started getting popular. Go to www.antiquebottles.com and you'll find a whole section devoted to milk bottles; history, values etc. Also check the search menu up top of the forum, I'm sure there's been discussions in the past.
Thanks Norm- Im trying to figure out if this spot practically right behind my house is worth digging- theres quite a few round pyro milks in there and maybe a green one or two if the dump is early 30s..