... is from Wikipedia (duh).

The Great Depression was not a sudden total collapse. The stock market turned upward in early 1930, returning to early 1929 levels by April, though still almost 30 percent below of peak in September 1929. Together government and business actually spent more in the first half of 1930 than in the corresponding period of the previous year. But consumers, many of whom had suffered severe losses in the stock market the prior year, cut back their expenditures by ten percent, and a severe drought ravaged the agricultural heartland of the USA beginning in the northern summer of 1930.

And from the related entry:

The crash followed a speculative boom that had taken hold in the late 1920s, which had led hundred of thousands of Americans to invest heavily in the stock market, a significant number even borrowing money to buy more stock. By August 1929, brokers were routinely lending small investors more than 2/3 of the face value of the stocks they were buying. Over $8.5 billion was out on loan, more than the entire amount of currency circulating in the U.S. The rising share prices encouraged more people to invest; people hoped the share prices would rise further. Speculation thus fueled further rises and created an economic bubble. The average P/E (price to earnings) ratio of S&P Composite stocks was 32.6 in September 1929, clearly above historical norms. Most economists view this event as the most dramatic in modern economic history.

Fascinating.