What caused the Great Depression?

"The excess credit which the Fed pumped into the economy spilled over into the stock market, triggering a fantastic speculative boom.

Belatedly, Federal Reserve officials attempted to sop up the excess reserves and finally succeeded in braking the boom.

But it was too late, by 1929 the speculative imbalances had become so overwhelming that the attempt precipitated a sharp retrenching and a constant demoralizing of business confidence."

Alan Greenspan
Conceded free-market deregulatory ideology was flawed in October, 2008