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.
Deflation
The deflation of the Great Depression occurred partly because there was an enormous contraction of credit (money), bankruptcies creating an environment where cash was in frantic demand, and when the Federal Reserve was supposed to accommodate that demand, it instead contracted the money supply by 30% in enforcement of its new real bills doctrine, so banks toppled one-by-one (because they were unable to meet the sudden demand for cash see fractional-reserve banking). From the standpoint of the Fisher equation (see above), there was a concomitant drop both in money supply (credit) and the velocity of money which was so profound that price deflation took hold despite the increases in money supply spurred by the Federal Reserve.