<P> Depending on the topography, the rock types and the climate, a lot of transitional forms between V -, U - and plain valleys exist . Their bottoms can be broad or narrow, but characteristic is also the type of valley shoulder . The broader a mountain valley, the lower its shoulders are located in most cases . An important exception are canyons where the shoulder almost is near the top of the valley's slope . In the Alps--e.g. the Tyrolean Inn valley--the shoulders are quite low (100--200 meters above the bottom). Many villages are located here (esp . at the sunny side) because the climate is very mild: even in winter when the valley's floor is completely filled with fog, these villages are in sunshine . </P> <P> In some stress - tectonic regions of the Rockies or the Alps (e.g. Salzburg) the side valleys are parallel to each other, and additionally they are hanging . The brooks flow into the river in form of deep canyons or waterfalls . Usually this fact is the result of a violent erosion of the former valley shoulders, a special genesis found also at arêtes and glacial cirques, at every Scottish glen, or in a northern fjord . </P> <P> A hanging valley is a tributary valley that is higher than the main valley . They are most commonly associated with U-shaped valleys when a tributary glacier flows into a glacier of larger volume . The main glacier erodes a deep U-shaped valley with nearly vertical sides while the tributary glacier, with a smaller volume of ice, makes a shallower U-shaped valley . Since the surfaces of the glaciers were originally at the same elevation, the shallower valley appears to be' hanging' above the main valley . Often, waterfalls form at or near the outlet of the upper valley . </P> <P> Hanging valleys also occur in fjord systems under water . The branches of Sognefjord are for instance much shallower than the main fjord . The mouth of Fjærlandsfjord is about 400 meters deep while the main fjord is 1200 meters nearby . The mouth of Ikjefjord is only 50 meters deep while the main fjord is around 1300 meters at the same point . </P>

Name the agent of erosion that produce hanging valley