Tour near glaciers

The following is supposedly a true story relating to an actual guide and his response to questions.

Swiss mountain guides who always do the same trails can get tired answering the same questions over and over. One time an English tourist was giving his guide an especially hard time with silly questions. They were walking through a mountain valley that was strewn with rocks, and the traveler asked, "How did these rocks get here?"

"Sir," said the guide, "they were brought down by a glacier."

The tourist peered up the mountain and said, "But I don't see any glacier."

"Oh, really?" said the guide. "I guess it has gone back for more rocks."