More worms

We also have dark olive green earthworms that live in a band of 6 inch dark gray earth 2 feet below the surface. The overlying clay is orangey-tan in color. I was amazed to see such a strangely colored worm that far down and only in that strata. This dark soil seems to be very porous and I have seen water dripping from it when I've dug a hole for a tree. I wonder if the worms are colored by ingesting the dark gray earth?

What geological conditions produced this dark band? A mystery I'll never solve.