How to visit prehistoric France

I am interested in seeing the cave paintings in the Dordogne region, as well as the megaliths near the area of Carnac. I am relying solely on public transportation. Is this possible and/or reasonable? How much time should I allow? And if it is possible, where can I quickly find more information on how to make it happen? I have several guide books, none of which have very detailed information on this area. Thanks.