"Which" is a relative pronoun. The antecedent, presumably, is the absent previous sentence. Punctuating the relative clause as a complete sentence is certainly informal (done here for dramatic emphasis?).
 
Of course, the usual antecedent of a relative pronoun is a noun phrase:

*	
        Elsie bought a red Miata, which is a nice car.

But other phrases and entire clauses can also be antecedents to relative clauses:

*	
        Elsie bought a red Miata, which surprised her mother. (clause antecedent)
*	
        Max stashed the loot under the bed, which is an obvious hiding place. (prepositional phrase antecedent)
*	
        Ferdinand washed his cash in the dishwasher, which is an odd thing to do. (verb phrase antecedent)
*	
        Rhonda flew to London today, which was earlier than we expected. (adverb antecedent)

Dick Veit
UNCW

________________________________

From: Assembly for the Teaching of English Grammar on behalf of Edgar Schuster
Sent: Tue 9/28/2004 9:39 AM
To: [log in to unmask]
Subject: Which as demonstrative pronoun?

I have noticed "which" used as a sort of demonstrative pronoun quite frequently in the New York Times, as in today's "Which is one reason Gov. Tom Vilsack is optimistic about John Kerry's chances . . . ."  Does anyone know how long this usage has been around?


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