With the success of Gilligan's Island (1964) under his belt, Sherwood Schwartz forged ahead with this closely tied concept. Two astronauts travel around the earth close the speed of light and upon landing, find they've traveled back in time. They're "marooned on the desert island" of the paleolithic era and taken in by caveman, Gronk (Joe E. Ross) and his wife, Shag (later Shad when network censors learned the meaning of the word in the UK) (Coca). Will they get back to the 20th century? Well, yes. They found their way back as soon as the ratings started to flag, and they brought Gronk, Shag(d) and their kids Mlor and Breer with them. The last seven episodes of the series were all about the cave family assimilating into contemporary America, so the series went from Gilligan's Island to The Beverly Hillbillies (1962).
This series has never rated a commercial DVD release, though unofficial copies are available. You can also find complete episodes on YouTube. Above is the promo reel for the show, uploaded to YouTube by RetroGoop.
Obscurity factor: 7 (not on DVD, has small cult following, episodes available on YouTube)