When the Marshalls observe themselves from behind on a further mountain with the binoculars, Will exclaims "It's like the river!". He's referencing Downstream (1974), where they follow a river flowing out of the great swamp, only to find that it eventually leads right back to the swamp, indicating that this land is a closed-in universe.
Captain Jackson says "It's bigger on the inside than it is on the outside!" This is a tagline of Doctor Who (1963) which by 1974 had only been sparingly broadcast in the United States.