The discovery of Ice Ages is one of the most revolutionary advances made in the Earth sciences. In 1824 Danish‐Norwegian geoscientist Jens Esmark published a paper stating that there was indisputable evidence that Norway and other parts of Europe had previously been covered by enormous glaciers carving out valleys and fjords, in a cold climate caused by changes in the eccentricity of Earth's orbit. Esmark and his travel companion Otto Tank arrived at this insight by analogous reasoning: enigmatic landscape features they observed close to sea level along the Norwegian coast strongly resembled features they observed in the front of a retreating glacier during a mountain traverse in the summer of 1823. Which glacier they observed up close has however remained a mystery, and thus an essential piece of information in the story of this discovery has been missing. Based on previously unknown archive sources, supplemented by field study, I here identify the key locality as the glacier Rauddalsbreen. This is the northernmost outlet glacier from Jostedalsbreen, the largest glacier in mainland Europe. Here the foreland exposed by glacier retreat since the Little Ice Age maximum around AD 1750 contains a rich collection of glacial deposits and erosional forms. The point of enlightenment is more precisely identified to be a specific moraine and its distal sandur at 61°53′26″N, 7°26′43″E. In memory of Esmark's travel companion who possibly was the first to realise the analogy, it is proposed to name this moraine Otto Tank's Moraine, a pendant to the already famous Esmark Moraine at Forsand by the sea.
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