You can read about these cliff growing Thuja here http://www.americanscientist.org/issues/pub/cliffs-as-natural-refuges/1
Elroy
Thanks Elroy - what a fascinating read that was. I have Larson's book, but have not read this article. I feel very fortunate to live not a 30 minute drive from some of these ancient cliff forests. It makes for an inspiring weekend hike.
For anyone interested, this is the book about ancient thuja cliff forests I am referring to.