Hanging valleys are fluvial (stream) valleys that flow into
a glacial trough. Fluvial valleys in mountainous areas generally
have steep slopes, so their cross-sections will be V-shaped. The
water in the stream will have dumped out onto the glacial ice. The
glacier had more erosional power than the stream, and so the glacial valley
will be cut deeper than the fluvial valley. So, hanging valleys are
V-shaped (fluvial) valleys which were NEVER glaciated that
join a glacial trough or U-shaped valley. The difference in the elevation
of the base of the two valleys often results in a water fall.
This photo was taken in the glacial trough leading to
Snowbird, Utah near Salt Lake City. The V-shape is outlined in the
right hand copy.