> In the last ice age, a permanent ice sheet formed over the Alps. Up to one kilometre thick, its tremendous weight ground against the terrain. It thawed and refroze in stages, and with every thaw, meltwater washed out the rubble. Over the course of 100,000 years, this ice sheet tore the top 250 metres of rock and soil from the surface of the Swiss Central Plateau. At its peak, about 24,000 years ago, it extended across all the northern cantons. It did not reach the Jura or Ticino. In 1964, Dr Franz Merke, a Basel surgeon, showed that the extent of the ice sheet ‘corresponded precisely’ with the prevalence of goitre: Switzerland had been stripped of its iodine.