I would suggest, to look for the parts of the piles that have the broken rock, as this is an indication that they were mining the bedrock. Round rock is from higher up. As for size, the larger cobbles in a pile were off the grizzly. The smaller stuff went thru the plant. Either may have gold, depending how well they washed the large cobbles (especially in areas with clay) or if there was larger gold that didn't get thru the grizzly. The smaller stuff is also good in spots, especially when they had a problem with their sluice (overloading, blockages, or just ineffective design).