I hate to be picky, (I guess it is my perfectionism) but the yahoo chart leaves something to be desired in terms of the numbers for the '29 crash and subsequent market action, the following shows the daily close on the dow from 1928 to the  present day.

http://www.generationaldynamics.com/cgi-bin/D.PL?d=ww2010.i.djia

  You, bring up a interesting point about the smoot- hawley bill, in my view, the  bill was a sociological manifestation of american society turning inward and created a negative feedback loop that was impossible to get out of, we only began to come out of it at the start of WW2.

 Protectionism, can take many forms, from currency devaluations  to bills like smoot-hawley, but the most dangerous, is when a society turns inward, and as a result, trade flows are brought to a stand still. 

  What happens, if china(or other creditor nations) turn inward, to fix their own problems and say the hell with putting their hard earned savings into america?