From reading the first few lines it's probably from the context: it's a well that's filled up with sediments in which objects from over a thousand years ago wer

From reading the first few lines it's probably from the context: it's a well that's filled up with sediments in which objects from over a thousand years ago were found, so the assumption is that if the site remained undisturbed all objects that share a layer will share an approximate age, too.