How glaciers widen a valley into a U

A narrow river winding across a broad mountain valley can look too small for its surroundings. The valley walls belong to a larger story than today’s channel. Where moving ice once occupied a river valley, a glacier could reshape the floor and sides into a much wider trough.

Ice occupies more than the river’s channel

The National Park Service’s valley guide describes glaciers entering earlier V-shaped river valleys. As the ice moves, erosion widens the floor and steepens the walls. After the glacier retreats, the resulting cross-section may resemble a U: a broad lower area between steep sides.

Think of the cross-section as a view across the valley, not the route the river follows along it. A winding stream on the floor does not undo the trough inherited from earlier ice.

Moving ice carries abrasive tools

Ice does not work alone. The National Snow and Ice Data Center’s landform explanation describes glaciers eroding the land and carrying broken rock and soil away. Debris dragged with the ice can leave grooves and scratches in bedrock. The NPS account of Yosemite’s geology also describes sediment in the moving ice polishing rock and producing linear striations.

The polished surface and the valley’s large outline operate at different scales. A close view may show marks left by dragged material; a wider view shows the reshaped landscape. Neither supplies an erosion rate simply by looking dramatic.

Today’s flat floor may include later deposits

Yosemite is a useful documented example. NPS describes repeated glacial advances reoccupying stream-cut valleys, deepening and widening them, followed by sediment partly filling those valleys. The visible floor therefore need not be bare rock in the exact shape left when the ice departed.

That is why the small-river puzzle has more than one layer: earlier ice reshaped a valley, later sediment covered parts of it, and water continues to use a channel through the present landscape. The cover illustrates the general arrangement without identifying a real valley.

Hanging valleys and fjords reveal different settings

According to the NPS guide, a smaller tributary glacier may erode less deeply than the main valley glacier. After retreat, its valley floor can remain high above the main trough—a hanging valley, sometimes crossed by a waterfall.

A fjord has another defining setting. NSIDC explains that seawater floods a coastal trough after the glacier withdraws. A hanging branch, a broad inland valley and an ocean-filled trough share a glacial connection without being interchangeable landforms.

Read the whole setting

Across the valley: look for a wide floor and steep walls.
At the tributaries: ask whether side valleys join high above the main floor.
On exposed rock: check local interpretation of polishing and striations.
Under the floor: remember that sediment can hide the bedrock profile.

This is a guide to questions, not a way to date a valley from a picture. Yosemite’s account also describes rock fractures and continuing rockfall. Local geological evidence is needed to untangle the processes. Water can leave smaller traces too: river potholes record abrasion in a rocky channel.

References

National Park Service. (2018, February 9). U-Shaped Valleys, Fjords, and Hanging Valleys. https://www.nps.gov/articles/ushapedvalleysfjordshangingvalleys.htm
National Park Service. (2024, November 22). Geology. https://www.nps.gov/yose/learn/nature/geology.htm/index.htm
National Snow and Ice Data Center. (n.d.). Why Glaciers Matter. https://nsidc.org/learn/parts-cryosphere/glaciers/why-glaciers-matter

Reading notes: NPS and NSIDC provide geological interpretation, not fieldwork by this publication. The Yosemite example is kept distinct from the imagined cover. Page-update dates are used where displayed; linked reports and unrelated climate statistics are outside this article’s scope.