Toward the end of prehistoric time there was a general settling down with the coming of agriculture, and all sorts of new things began to be made. Archeologists soon got a general notion of what ought to appear with what. Thus, it would upset a French prehistorian digging at the bottom of a very early cave if he found a fine bronze sword, just as much as it would upset him if he found a beer bottle. The people of his very early cave layer simply could not have made bronze swords, which came later, just as do beer bottles. Some accidental disturbance of the layers of his cave must have happened.According to the passage, why would an archaeologist studying a very early cave be upset at finding a bronze sword?