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.
History of the United States (1918–1945)
Historians and economists still have not agreed on the causes of the Great Depression, but there is general agreement that it began in the United States in late 1929 and was either started or worsened by "Black Thursday," the stock market crash of Thursday, October 24, 1929. Sectors of the US economy had been showing some signs of distress for months before October 1929. Business inventories of all types were three times as large as they had been a year before (an indication that the public was not buying products as rapidly as in the past), and other signposts of economic health—-freight carloads, industrial production, and wholesale prices—-were slipping downward.