| UNITED STATES GEOLOGICAL SURVEY, NORTHERN ROCKY MOUNTAIN SCIENCE CENTER: Grinell Glacier Photo Gallery. Photo by Fagre. Note the parabolic or "U-shaped" cross-sectional shape of the valley from which the glacier has retreated. Glacial ice that flows down valleys originally carved by streams changes the cross-sectional shape of the valley from a "V" to a "U" shape. | The floor of the valley out of which the waterfall emerges is elevated well above the floor of the main valley and is separated from it by a steep cliff. The tributary valley (called a "hanging valley") used to descend gently to the main valley floor, but its lower part has been removed by the erosional activity of a large glacier that once flowed along the floor of the main valley. |