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
A large contribution to the recession was the closure and suspension of thousands of banks across the country. Financial institutions failed for several reasons, including unregulated lending procedures, confidence in the Gold standard, consumer confidence in future economics, and agricultural defaults on outstanding loans. With these compounding issues the banking system struggled to keep up with the public's increasing demand for cash withdrawals. This overall decreased the money supply and forced the banks to result to short sale (real estate) and liquidation of existing loans. In the race to liquidate assets the banking system began to fail on a wide scale. In November 1930 the first major banking crisis began with over 800 banks closing their doors by January 1931. By October 1931 over 2100 banks were suspended with the highest suspension rate recorded in the St. Louis Federal Reserve District, with 2 out of every 5 banks suspended. The economy as a whole experienced a massive reduction in banking footholds across the country amounting to more than nine thousand closed banks by 1933. 
The closures resulted in a massive withdrawal of deposits by millions of Americans estimated at near $6.8 billion (equivalent to around $60 billion in today's dollars). During this time the Federal Deposit Insurance Corporation (FDIC) was not in place resulting in a loss of roughly $1.36 billion (or 20%) of the total $6.8 billion accounted for within the failed banks. These losses came directly from everyday individuals' savings, investments and bank accounts. As a result, GDP fell from the high seven-hundreds in 1929 to the low to mid six-hundreds in 1933 before seeing any recovery for the first time in nearly 4 years. Federal leadership intervention is highly debated on its effectiveness and overall participation. The Federal Reserve Act could not effectively tackle the banking crisis as state bank and trust companies were not compelled to be a member, paper eligible discount member banks heavily restricted access to the Federal Reserve, power between the twelve Federal Reserve banks was decentralized and federal level leadership was ineffective, inexperienced, and weak.