The Portage was formed about 10,000 years ago when the flank of the Cayoosh Range, which is the south flank of the valley, let go and slid into the middle of what had been a single lake. The result is a location similar to Interlaken, Switzerland, with two fjord-style lakes flanking a narrow and very short strip of land between them. Remnants of old lake bottom survive as benchlands lining the north banks of Seton and Anderson Lakes. It may be that the glacial moraine at the foot of Seton Lake, which had been at the foot of the Seton Glacier and, after it melted, dammed the older, larger lake in until the slide and its destructive wave (see megatsunami). The inundation then washed part of it away to open Seton Creek and drain the glacial melt to today's lake level, or close to it (since the lake level is 10-12 feet higher because of the power project completed in 1958).