Robin

Very good points.

Your post reminds me of the time my wife and I had a tour of a cave. The tour guide told us of how certain stalactites were over two thousand years old. He said they know this because of the rate the water and calcium flowed to form it.
I didn't say anything at the time, but I couldn't help but think that they were making a lot of assumptions. I.e. how do they know that the rate of growth is always the same year after year? Couldn't the shift in the mountain above and other things affect the rate of growth?

Tom