This is a late 50's or very early1960 film about a space crew launced from earth that end up back on earth in the future.
They do dont relize this for sometime.
They encounter cavemen who are the last of the earth people and the crew then settles down with them. to rebuild the earth.This is how the film ends.
The film is in color and it is a USA made film.
One scene is when the crew defends themselves from the cavemen and the cavemen call the wepon "fire and thunder" or something like this.
Another scene is when the crew is in a cave you can see the cavemen sneaking up on them from the outside of the cave.
Another scene is when the leader of the space crew decides that the mountains that they are looking at look like "The Rockies in Colorado" .
Please help if you can.
Thanks
Could it be "World Without End"?
http://us.imdb.com/title/tt0049964/