The Great Depression was caused by the stock market crash of 1929. This was caused by too many people buying too much stock, making said stock seem much more valuable than it actually was. When a few people started to sell, everyone got scared and started to sell. The Great Depression was the result of a small action snowballing out of control.
The main idea of the Great Depression is that the economy because broke basically and people money that was in the bank couldn't get the money out leaving people hopeless and broke.
