Alternator Overcharge

I had been in and out of my 73 Valiant 225 all day yesterday running errands and on my way home i start my car and notice my dash lights are VERY bright and my alternator guage is maxed out above 40+ if I have any kind of acceleration going on. If its idleing, the guage reads straight up, but any increase in RPM and the guge shoots all the way up. What could be the cause?