That was a straight creek, it was straightened out for flood control runoff. When it was straightened, they dug through many different sites, what he is showing is where run off has dumped all the rocks and shells. You can sometimes find the same on gravel bars in normal creeks. Those spots are usually multi cultural, with every thing from paleo to post contact. And, the creek keeps putting stuff in the bars, so if you keep checking them after a rainstorm, you will keep finding stuff. Bill