Bluff Lines?

Before you go back and not even bother reading the question. I would just like the check whether you know about Bluff Lines? What on earth are Bluff Lines? No it's not a park or a tourist attraction it is geographical (this is the part where no doubt all of you will go back and not answer!). I just want to know what they are. I have a huge project on them and there is nothing about them I can find! Help Please! Thanks!