Why can it be historic? Well, what an archaeologist is looking for are the material remains of past behaviors. This means that they may find material remains that don't go back very far in time (dating to when history was being written.) Sometimes these can be important because they provide a means to add to that historic record or they may support or even refute what has been written down. Archaeologists working in the area of the new baseball stadium in downtown Phoenix discovered materials remains from the historic period. These were part of what use to be the "red light district". Historians did not write about this part of Phoenix society because of what it stood for. That is why archaeologists look for any material remains of past behavior.