A Glacier Carves a U-shaped Valley
Margerie Glacier showing the U-shaped valley being carved.
Glaciers carve some of our planet's most spectaculer scenery, but it's not until
glaciers melt that their rasping handiwork is exposed.
Glaciers and running water sculpt the land in different ways. While streams tend
to cut winding curves and V-shaped valleys, glaciers carve nearly straight valleys
with U-shaped cross-sections. The imposing, sheer rock walls of glacial troughs
(U-shaped valleys) are among the most fundamental and distinctive features of
glaciated landscapes. The U-shaped valleys left behind by valley glaciers are
usually 1 kilometer (1.6 miles) or more in width and typically hundreds of meters
Follow along to see how it's done:
Start with a typical stream
As streams wind their way downstream they tend to cut away the outsides of bends
and deposit sediment on the insides of bends. This gradually makes the stream
valley more sinuous.
Running water gradually cuts a deeper V-shape. The end result is a typical
meandering, V-shaped stream valley.
Glaciers move in
Climate cools and glaciers grow and begin their slow downhill push, usually taking
the easiest path down - the path already cut by streams. Glaciers ooze into stream
channels, but unlike streams, glaciers focus their grinding energy on the insides
of bends. Bit by bit, the glacier eats away at the meandering curves of the
original stream valley, carving a wider, straighter valley.
Work of a glacier revealed
During times with warmer climate like today's (called interglacial periods),
glaciers gradually melt away except at high elevations and latitudes. As glaciers
melt, their effect on the landscape is revealed. Former stream valleys have been
transfomed to broad, very steep-sided troughs and waterfalls cascade from hanging
valleys perched high above the valley floor.
Yosemite National Park
has often been referred to as a "classic" glacial valley.
It's landscape as we see it today strongly reflects the dynamic influence of
flowing ice that long ago covered much of its higher regions. Geologists are
still uncertain how many times ice mantled Yosemite, but at least three major
glaciations have been well documented elsewhere in the Sierra Nevada. In the
higher country, icefields covered extensive areas, except for the higher ridges
and peaks. Lower down the western slope, at middle elevations, glacial tongues
were confined to pre-existing river canyons, such as those of the Merced and
In contrast to the sinuous V-shaped valleys of normal streams in unglaciated
mountainous terrain, glaciated valleys tend to be straighter and have U-shaped
profiles. Whereas a stream erodes the outsides of bends preferentially and makes
its course more sinuous, glacial erosive force is concentrated on the insides of
bends, removing the protruding spurs of the original stream valley and leaving a
wider, straighter valley.
Copyright © 2003-2008 Calvin & Rosanna Hamilton. All rights reserved.