From "Vaksinen". (Svein Nord)
During the climatic colder period known as the "Younger Dryas" (13,000-11,500 years ago), the ice sheet that had been melting back for a thousand years began to advance again. The glacier advanced from the north, passed Søfteland, and ended up butting against Liafjellet. There it remained for 300 years. The sea level was 57-58 metres higher than today and a small arm of the fjord went into Ulven on the northwest side of Liafjell. Ahead of the glacier a meltwater lake with large amounts of glacial flour (silt and clay), sand and gravel got washed out into the fjord. The sand and gravel formed a delta which now comprises the Ulvensletta plain (the word Ulven is thought to come from alfrvin where the root alfr suggests "gravel, sand", and "sletta" is the Norwegian word for "plain"). The finer glacial flour got transported with the meltwater out the narrow sound between Ulvenfjellet and Liafjellet and was deposited where the sound widens between Hjorthaugen and Grindavoll.
Seismic measurements show that the sediments at Ulvensletta are up to 34 m thick, but this varies a lot. The steep slope down toward Lake Ulvenvatnet shows the old contact surface between the glacier and the delta. The hill called "Vaksinen" in the western part of the plain is a remnant of the moraine that was pushed up by the glacier when it made a little advance before melting back again, to mark the end of the Ice Age at Os.