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{audio}https://cpa.ds.npr.org/krcb/audio/2017/06/nbr__6-5-17_mammoth_rocks.mp3{/audio}
Since he began looking into this issue, the now-retired archeologist has found several other similarly polished rock surfaces not too far from his first discovery. And after a paper Parkman wrote about his explorations was published, he heard about still more such locations in other states.
{audio}https://cpa.ds.npr.org/krcb/audio/2017/06/other_rocks.mp3{/audio}
