If you're talking about drywasher tailings here in Arizona, the last stuff they ran, which would be at the top of the pile, was the deepest and richest dirt in the hole. If you're hunting a well-known area like Greaterville, back in the old VLF days the tailings piles were some of the first places the "old time" detectorists hit. Yes, heavier material (like nuggets) might have trickled down deeper into the piles. I've still managed to find nuggets in them but it's rare. It might also help to have a newer detector like a GPZ 7000 that finds deep wiry or specimen nuggets better than the old VLFs