Why does a mountain’s shadow appear triangular?

MOUNTAINEERING — Even experienced mountaineers have been stopped in their tracks occasionally by the rare sight of a triangular shadow darkening the landscape in the distance away from a major peak.

The triangle shadow phenomena is fairly commonly seen from the tops of large mountains and volcanoes, but not well understood.

Check out this astronomy website for a good explaination.  Navigate to the Astronomy picture of the day for July 5, 2011.