Dig to solid bedrock as that's where gold is in concentrated paystreaks. Excessively hard layers of clay(false bedrock) will also trap some gold on ocassion but gold miners make the bedrock to prosper-John
I agree with John but be sure to check the top six inches too. This is especially true if you have found a good spot (behind boulders, low spots, narrow spots) and you're in an area that gold has been found in the past. GL HH
Go to bedrock in your area since the gold was mostly laid down by glaciers if I remember right. Dig/move out any boulders that are sitting on bedrock but do so in a safe way if working a moraine where the rocks are stacked up.
If I'm just prospecting a new area I'll pan in layers going down, looking for the streak. Then move laterally in the richest layer. But picking the spot to start is important. Behind a big rock is always a good place to start.