Eric
that 'charcoal' line is partly shadow that extends fractionally down into the bunker, in that part of the face with some organic material from the bunker lip. Then the sand starts, with the grey fines washed out onto the floor, giving the whiter face (not silica white, just a whitish grey). The bunker floor is darker in apeearance, especially if it has rained, with more of the greyer fines.
James B