Where does it say the newer layers weren't contaminated?

Hi Masondees, The passage doesn't explicitly say that the the upper layers were not contaminated, but we are told that each of the layers form a consistent series of human activity dating all the way to the present. The proposed contamination mechanism involves older carbon filtering through, and leaving artificially old carbon at lower layers. However, we know that the carbon at higher layers dated to newer time periods, and if those higher layers had been contaminated, we could expect the dates from them to be similar to the dates from the contaminated lower layers. Thus we can deduce that it would not be consistent with the passage to say that the upper layers were also contaminated.