Den flate terrassen på Hetlefloten i Os, 58 moh., viser hvor havet stod i dette området. (Svein Nord)
At the end of the last Ice Age a glacier came down the Os valley from the Gullfjell mountains and Sveningen. After the meltwater rivers had built up Ulvensletta and the glacier had scraped together a moraine ridge at Moberg, the ice began to recede. A roughly 30-50m deep fjord opened up in towards the valley. The glacier halted at regular intervals and built up small deltas which we now find as sand- and gravel terraces at an altitude of 56-57 metres, at Tøsdalskrysset, Ulvenskiftet, Hetleflåten and the northernmost one at Søfteland. Between the terraces there is a lot of silt (coarse clay) that was deposited at the bottom of the fjord outside of each delta.