Causes of the Great Depression
Second, there are the monetarists, who believe that the Great Depression started as an ordinary recession, but that significant policy mistakes by monetary authorities (especially the Federal Reserve) caused a shrinking of the money supply which greatly exacerbated the economic situation, causing a recession to descend into the Great Depression. Related to this explanation are those who point to debt deflation causing those who borrow to owe ever more in real terms.
Great Depression in the United States
The Wall Street Crash of 1929 is often cited as the beginning of the Great Depression. It began on October 24, 1929, and was the most devastating stock market crash in the history of the United States. Much of the stock market crash can be attributed to exuberance and false expectations. In the years leading up to 1929, the rising stock market prices had created vast sums of wealth for those invested, in turn encouraging borrowing to further buy more stock. However, on October 24 (Black Thursday), share prices began to fall and panic selling caused prices to fall sharply. On October 29 (Black Tuesday), share prices fell by $14 billion in a single day, more than $30 billion in the week. The value that evaporated that week was 10x more than the entire federal budget and more than all of what the U.S. spent on World War I. By 1930 the value of shares had fallen by 90%.