I would guess the upper brown layer here was that loose muck that still has organic matter in it. Is the black/gray stuff below muck that has lost all organic matter and oxygen?
I would imagine that the soil types in every region is going to make the answere vary on this. On my property, there was a alot of sand in the soil but it looks all grey like yours. I was told that the color was because it had never been exposed to oxygen and that when exposed it turnes brown (corosion?). Some of the dirt, but not all, did turn brown after sitting out for a couple years. We also have in our area what they call blue clay. It will always look like this and is excellent clay for ponds.
I am really enjoying your posts on this project. Keep it coming and thanks for sharing.......Jim
