As it gets lighter, you realize you can’t be on the right track. The canyon seems shallower than it was. The river bed is strewn with boulders that were never there before. The cold wind chills you to the bone; yet it’s the middle of summer. As you climb to higher ground to get a better view, you notice patches of snow. From the top of a ridge you survey a barren plain, frozen lakes, and, in the distance, a massive range of snow-covered moun- tains. You begin to realize you are not merely lost-you are lost in time, and you have somehow been transported to an Ice Age that occurred many thousands of years ago.

You walk toward one of the cliffs that borders the canyon, seeking shelter from the wind, and notice an entrance to another cave. You are tempted to go inside, but you feel you should keep moving in hopes of somehow reaching familiar country.