Very interesting piece, and I would have picked it up and brought it home too. At first glance it looks like both of the darker ends could have been used for grinding or hammering, especially the larger end. But at second glance, the raised lip on the smaller end seems to indicate that it's current form is not due to use by humans, but rather from erosion of sedimentary material as already noted. If that item had been found on a known prehistoric site, where it would not have been deposited by geologic forces, I would suspect that it was picked up by a prehistoric person and moved there, and possibly used, and thus would be a prehistoric human artifact. Since it was found in a creek, I see no reason to suspect or conclude that it is a prehistoric human artifact.
But, on the bright side, it is an interesting rock, and now that you have handled it and brought it home, it is officially a historic human artifact. HH
