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.
New Deal
At the beginning of the Great Depression, the economy was destabilized by bank failures followed by credit crunches. The initial reasons were substantial losses in investment banking, followed by bank runs. Bank runs occurred when a large number of customers withdrew their deposits because they believed the bank might become insolvent. As the bank run progressed, it generated a self-fulfilling prophecy: as more people withdrew their deposits, the likelihood of default increased and this encouraged further withdrawals.