We bought a box of this Log Builder Caulking
<-- click here, We used it and it worked excellent for filling gaps in our rustic wood flooring. There are 5 - of the big 30 oz. tubes (woodtone color) leftover. That will do a lot of flooring or several gaps in log walls or whatever. It stays flexible so it won't crack and break with expansion and contraction of joints. Highly recommend it. That said, I'd like to sell the left over if you're interested, $60 plus shipping gives you a pretty big discount over new.