I came across this interesting article on a unique method that was followed for dating ancient petroglyphs in North America.

http://earthsky.org/human-world/neva...-north-america

The article also mentions that horses were present in North America some 13000 years ago and then suddenly disappeared. This information made me think about the debate on the introduction of horse to India by the so called aryans while the other camp provides bone evidence (IVC site: Sarkatoda) that horse was present in India during Indus Valley Civilization time.