Why was the American Depression of 1929 so sudden and so severe?

In what regards the depression, the main reason was 'The Wall Street Crash'. In the 1920s the stock market shares went skyrocketing at an incredible and alarming rate therefore people could speculate their rise and make fortunes out of this. Ironically it was speculation itself what brought 'The Wall Street Crash' upon the American people. In Thursday, October 24, also known as 'Black Thursday' 12,894,650 shares changed hands, many of them at such low prices that destroyed …

…demand, as an example there were too many cars being made, and not too many people who could afford to buy them. In other words all the other reason did take their part, but none were as severe and chaotic as the Wall Street Crash, main reason for the depression, and vanquisher of the dreams of millions of Americans. Side Note: Most of this information was derived from the USA history textbook by John Traynor.