Subject: how are washboards (ripples) in dirt roads formed

I have always wondered about what we call washboards on dirt roads.  
After cars drive on a dirt road for a while these wash boards 
develop, they are sort of like ripples in the road.  Obviously it is 
because of the wheels turning and some characteristic of the dirt 
(i.e.  it is loose enough to be moved, and seems to be most 
noticeable in hot dry weather), and do the high points get built up 
or the low points worn down?  What every you have to say on this 
would be of interest to me.

Thank you very much

Glen Hickman

Re: how are washboards (ripples) in dirt roads formed