About 11,000 - 12,000 years ago, the North American climate began to
change. Exactly why and how this happened is still debated, but the change
coincides with the catastrophic extinction of the big-game animals at the
end of the Ice Age. The continental ice sheets were withdrawing and the
melt-water from them filled valleys and basins, while the shorelines of
the continents began to change as the then existing continental shelves
came to be drowned by rising sea levels.
