The Saint Mary River, Glacier National Park, flows in a classic example of a U-shaped valley, also called a glacial trough. Alpine glaciers erode valleys that have broad, relatively flat floors and steep sides, whereas rivers in mountainous regions tend to erode V-shaped valleys. Thus, from the shape of the valley, we know that glaciers eroded this valley, not the river presently existing here. In the photo, we are looking towards the northeast. On the map, north is towards the top. The relatively flat floor of the valley is revealed by the wide spacing of the contour lines on the map. The Saint Mary River is shown in blue.