Photo of glacier © Dr. Kirk Johnson, Denver Museum of Nature & Science

How do we know that ice ages occurred?

The idea that an ice age had occurred in the past was first proposed by Louis Agassiz in the early 1800s. Agassiz noted that the slow action of mountain glaciers in Switzerland produced certain kinds of features in the surrounding landscape. These include glacial till and erratics, rock striations and loess. He also noted that these features sometimes occurred in areas far from the mountains, where no glaciers existed. Based on these observations, Agassiz came up with the idea that extensive glaciers had existed in the past.

Scientific progress since the time of Agassiz has not only confirmed his idea, but has also begun to reveal a much more detailed picture of what conditions were like during the last ice age. To obtain a more detailed picture of the last ice age, scientists study natural recorders of climate change, such as fossil pollen, ancient coral reefs, ocean sediments and ice cores.