In my office we have a little project to take some old artifact collections from a naval facility on the Potomac River below Washington and prepare them for permanent curation. Glancing through the report on an excavation done in 1992, I found the above very faded photo of a cache of stone tools. The cache was found in a large shell midden site along the river, with some pottery of a type called Pope's Creek net-impressed that dates to about AD 1 to 500. Intrigued, I dug the artifacts out of the box and took some pictures.
This little pile of quartz tools is important because of how most archaeology in this part of the world is done. Most of our sites are shallow. Dig on any bluff overlooking a river and you will find prehistoric artifacts, sometimes a lot of prehistoric artifacts. But how do you know how old they are? On an upland site where the surface is stable or eroding, artifacts from the past 13,000 years all end up mixed together in the topsoil. This is why archaeologists love sites that are, as we say, stratified, or layered. You find such sites on river floodplains, where the river dumps sand or silt after ever flood, gradually burying artifacts deeper beneath the ground. On a stratified site, everything from the same layer is about the same age, and it is older than what is above it and younger than what is below it.
Alas, in this part of the world we don't have many stratified sites. To figure out how old what we find in unstratified, near surface sites is, we depend on datable artifacts. Some kinds of stone tools were only made at one time in the past, so we can tell just by looking at them approximately when they were made. The interesting thing about the tools in this collection, which must have all been made within a few years, is that most archaeologists would probably date them to two different periods. Specimens 97-3, 97-6, and 97-10 are good examples of the type called Calvert, which dates to the same time period as the Pope's Creek pottery found nearby; 97-1 could be put into the same type, although if it were found along most people would probably call it something else. Specimens 97-2 and 97-4, though, are not Calvert points. I suspect that almost every Middle Atlantic archaeologist who looks at them will think the same thing I thought, that they are Bare Island points dating to before 2500 BC.
The moral of the story is that the identification of stone tools is not an exact science. The dates we put on sites by looking at a few spear points or potsherds are guesses. For some point types, or some combinations of artifacts, our guesses can be very good, and it is very likely that we are right. But it is never certain. Ancient Indians were perfectly capable of making a stone spearpoint or knife in an unusual way if they felt like it. Our knowledge of the distant past is not really very good, and we should treat all of it as provisional, subject to revision when we get better results from better sites.