Abstract The discovery of Ice Ages is one of the most revolutionary advances ever made in the Earth sciences. In Norway this discovery was made by Danish–Norwegian geoscientist Jens Esmark and his young student Niels Otto Tank, who on a mountain traverse in early September 1823 observed a number of geomorphological features produced by an extant glacier, and compared these to similar features they had previously noted where glaciers today are absent. Seeing a recent moraine pushed up by an extant glacier they suddenly realized that a big ridge of gravel they had earlier seen at sea-level in Southern Norway had to be an ancient moraine, deposited by a big glacier at a time when the climate was substantially colder than today. The brevity of Esmark's account made the precise location of the site of enlightenment remain a mystery for almost two hundred years until it was rediscovered by the author in 2008. This paper describes the crucial site and its lessons.