Are you saying that they dug all the way to the rock floor, or that they were stopped by a layer of rocks from the walls and roof? If they did not dig all the way to the bottom, perhaps the layer of rocks was caused by a major catastrophe/earthquake, and there might be some very interesting stuff underneath all those rocks that fell from walls and roof.
My take on this is that roof fall occurred at some time in the past, isolating layers below from layers above. A good rock layer would reduce intermixing by rodents and subsequent habitation. This is what they meant by "lowest sublevel" I would think.
Those kinds of isolated layers or features are particularly valuable. If you have a well-defined layer, and you can date either that layer or what isolates it from above, you have much better temporal control.
This find sounds pretty interesting. But I wish the article specified which of the ancient mtDNA types were found!
The DNA testing indicated that the feces belonged to Native Americans in haplogroups A2 and B2, haplogroups common in Siberia and east Asia....
Exactly who these people living in the Oregon caves were is not known, Jenkins said. In their conclusion, the authors wrote: "The Paisley Caves lack lithic tool assemblages, thus the cultural and technological association of the early site occupants, and their relationship to the later Clovis technology are uncertain."
"All we're doing in this paper is identifying the haplogroups," Jenkins said in an interview. "We are not saying that these people were of a particular ethnic group. At this point, we know they most likely came from Siberia or Eastern Asia, and we know something about what they were eating, which is something we can learn from coprolites. We're talking about human signature.
"If our DNA evidence and radiocarbon dating hold up on additional coprolites that are now undergoing testing at multiple labs, then we have broken the Clovis sound barrier, if you will," he said. "If you are looking for the first people in North America, you are going to have to step back more than 1,000 years beyond Clovis to find them."
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During the two summers of fieldwork, Jenkins, colleagues and students, working in four of the caves, retrieved manufactured threads of sinew and plant fibers, hide, basketry, cordage, rope, wooden pegs, animal bones, two forms of projectile point fragments and diverse kinds of feces. These items were found "in an unbroken stratigraphic sequence spanning the late Pleistocene and Holocene," the researchers wrote in the study. Some of the thread is narrower than that holding buttons on many shirts today and date back 12,750 years, Jenkins said.
"To find these threads was just incredible," said Jenkins, who directs the Northern Great Basin Archaeological Field School. "We found a little pit in the bottom of a cave. It was full of camel, horse and mountain sheep bones, and in there we found a human coprolite. We radiocarbon-dated the camel and mountain sheep bones, as well as the coprolite, to 14,300 years ago."
With radiocarbon dating adjusted to calendar years, the materials date back to about 14,400 years ago, he added. Such a dating puts the Oregon site into about the same time period as Chile's Monte Verde site.