Giant wall?

Well, as always, I was normally playing minecraft. I decided to create a new world on super flat and when I look around there was this huge wall. I have no idea how it got there or why it's there, but then suddenly I realized I could build so many cool things on it and I will but the main thing that's troubling me is I choose super flat... that isn't flat. Oh, and the preset didn't include trees but at the top there is a tree.

If this has happened to you or you know why it happens, I would like to know. This wall is a big wall that was never meant to be.

Thank you for your time and thank minecraft for that wall... .-.

~Amtrist o °