In Stone Age, when you build a boat you receive a Civilization card. This card gives you an immediate bonus, as well as some form of additional points to be calculated at the end of the game. I've always played that when these cards are purchased, they are placed face down on the player's individual board.
I recently saw a game played where the cards are placed face-up in front of each player. This changed the dynamic of the game as players could immediately see which cards where more beneficial to certain people.
What is the correct placement for these cards and are other players allowed to look at previous cards you've purchased?