First, thank you for your site, your info, and all your help, my learning is regularly enhanced by your hard work.
I copied the question and answer below and I am still confused. Does the original berais know that if there is a little lime in between that this was accidental and would therefore believe that in that case it is Tahor? Thank you!
b) Question: A Beraisa says that the space between two marked rocks is Tamei, even when the space itself is not marked (provided the space has not been plowed over, in which case it is Tahor).
(c) Answer: The Beraisa is talking about a case where a bit of the lime goes beyond the rocks, into the intervening space. If a plow has been there, we may assume that the plow scraped some marking off the rock onto the adjacent space, but the space itself was never actually marked. If there was no plow, this possibility doesn't exist, so we must assume that the extra lime was put there purposely to indicate that the space is Tamei.
Mordechai Litwack, Silver Spring, Maryland - USA
According to Rav Papa, the original Beraisa understands that if there is a little lime in between the rocks and the ground in between them is plowed, one can assume that this is due to the plowing, which caused a little lime to be peeled off of the rocks. If the ground in between the rocks is not plowed, the little lime between the rocks is to indicate that the area in between is Tamei.
All the best,
Yaakov Montrose