Battery Gauge

Me and a buddy went off roading yesterday and the trail we were on was pretty tough and our jeeps hit some good bumps along the way, no damage aside from a scratch on a fender and we were surprised to drive away with just that. Well on the way home (30 minute drive at about 60-70 mph) my battery gauge dropped to about 10-12 rather than the 13-14 it usually sits at. Then after a little bit it dropped to the bottom of the gauge. I got home and she started right up and it didn't seem to be an issue other than the gauge. Well today I was looking at my amp for my sub and it was reading 11.6v instead of 14.6 so I believe something is causing me to have a lower output from my battery. Any idea on what it could be? I haven't done any searching yet but I plan to tomorrow afternoon

A real classic is getting mud and water in the alternator and making it not work

So take out the alt and clean the brushes?

That would be a great place to start

I'll give it a look, thanks!