Another way to hunt is to clear the vegetation off a little area (for instance, with Roundup and then come back later and rake/tear the dead stuff out) and let the wind and rain work on the bare dirt for a while. If there's a spot with a good view above the spring thats where I'd start.
I found the camp I've been digging just by seeing enough flakes on top to be suspicious. Digging turns up lots of flake, bones, big clam shell, fire rock, and some nice artifacts. The highest concentration is about a foot down. About 2-2.5 feet down, the black topsoil (mixed with rock/gravel) gives way to only clay/gravel/rock/sand and thats where the flake and everything else stops. I think depth totally depends on the location, because some areas lose soil, some build up soil, and in others the soil moves around (sand drifts etc). Also, every time an animal digs a hole or a tree goes down with root ball intact, some of the dirt a couple feet down gets put back on top -- and over thousands of years that can lead to a lot of turnover/mixing in certain areas.