Answer
The current shape of the valley in Montana's Glacier National Park, specifically Lake St. Mary, is a result of glaciation and differs significantly from its shape
before glaciation.
Before glaciation, the valley would have likely been a V-shaped river valley formed by the erosive action of the river over time. The valley would have had a narrower width, gentle slopes, and a flat bottom.
During the period of glaciation, massive glaciers flowed through the valley, eroding and reshaping the landscape. The glaciers widened and deepened the valley, transforming it into a U-shaped glacial trough. The erosive power of the glaciers excavated the valley floor, creating a broad and flat bottom. The steep sides of the glacial trough were shaped by the ice movement, which scraped and plowed the surrounding rock, further widening and steepening the valley walls.
After the retreat of the glaciers, the glacial trough was partially flooded by the rising sea level, forming Lake St. Mary. The lake fills the lower portion of the valley, surrounded by the characteristic steep walls and U-shaped profile created by glaciation.
Work Step by Step
As per text in section 18.3.
