| My theory is that Valley Girl patois at any given time is keyed to the speech habits of 14 year old girls in the San Fernando Valley, and that the speech habits of those 14 year old girls are keyed to the recreational drugs that are currently in vogue.
For example, I was 14 (early 70's), the fashionable drugs were slow drugs - quaaludes and marijuana. In fact, Valley Girl patois pretty much consisted of the expression, "Oh, wow, man."
When Moon Unit Zappa raised the nation's awareness of Valley Girl culture (early 80's), the fashionable drugs were fast - speed and cocaine. Thus, in the early 70's, Valley Girl speech patterns were considerably slower and less variable than in the early 80's. | 
