D.I.E.D, your post was helpful to me. It had a little too much Greek in it but helpful.
I can really see the differences where I'm digging. Some places it is almost orange and some places almost white, and some places it is marbled with yellow and white. It varies in gravel/sand too. One dozer blade over can be a whole different soil. It drives me crazy. I've run into about six different subsoils in 3/4 acre. With my dirt, only one area had too much sand in it to hold water well.