The darker soil piles on on the left and towards the back are from the existing bed. We tilled everything up and then piled the richer soils to the side. We then added the 'brown' pile to build up the grade of the bed (we were filling in an old pond feature here...) Then, we put back the more organic soil on top, where most of the planting would happen. The 'darker' soils are from compost and mulch being applied over the years.