Additionally, though the San Diego researchers think that their site is rather pristine and altogether free of intervention or mucking around, others disagree. The silt these bones were stored in, the authors say, experiences a slow and steady of flow of water but is protected from flooding. They take that to mean the rocks were left near the mastodon remains the way they were found, as opposed to being pushed there by strong waters after the fact. But that doesn't mean some other act couldn't have brought these artifacts together. Ultimately, "the authors have not demonstrated that they have an in situ archaeological association," says archaeologist Luis Alberto Borrero of the University of Buenos Aires. Until it's shown that the rocks and bones have existed together since the Pleistocene, skepticism will remain that the rocks found their way there more recently.