Causes of the Great Depression

Reading Michael Barone about the milk-protectionism-big government problem, I was surprised to find this partial sentence: “we realize, thanks in large part to the late Milton Friedman, that the Depression resulted not from the inevitable defects of free markets but from certain specific policy mistakes that we can, unless we take leave of our senses, refuse to remake.”

Wow. I wonder what they were. I’d be interested to know. And did his “specific policy mistakes” also explain the worldwide aspect of the Depression?