Giant stair steps
The topographic map below has a good example of a glacial trough, or U-shaped valley. The bottom of the valley is relatively flat,
shown by the wide spacing of the contour lines. The contour lines displaying the valley walls are much closer together indicating much
steeper slopes. On the bottom of the valley is a stream as well as some paternoster lakes (Snow Lakes).
The photo below shows a classic example of a glacial trough or U-shaped valley. The bottom of the valley is wide and relatively flat
and the river does not appear large enough to have carved out such a large valley. Glaciers not only change the cross-profile of alpine
valleys from V-shaped to U-shaped, they also tend to straighten the valleys. The Jollie River Valley is quite straight. It would be
uncommon for an alpine river to carve out such a straight valley for this long of a distance.
Photo: T. Detwyler. Used with permission.
The photo to the right shows a u-shaped valley/glacial trough and the Noatak River. Although no glaciers fill this portion
of the valley, the broad relatively flat floor of the valley and the relatively straight path of the valley indicate a glacier
shaped the valley, not the Noatak River.
|Photo: U.S. Fish and Wildlife Service National Digital Library. 2008. image Volume1\138B220F-1143-3066-40A8372CE4D026CD.jpg. <http://www.fws.gov/digitalmedia> Accessed June 2010.
The long U-shaped valley/glacial trough shown on the topographic map to the right is outlined in turquoise. The source cirques
for the glacier that carved this valley are partly cut off the bottom of the map.
The wider spacing of the contour lines along the valley floor indicate a relatively flat topography compared to the valley walls.
The river flowing along the bottom of the valley is most likely too small to have carved such a wide valley. In addition, at the
elevations shown on the map (over 10,000 feet above sea level) rivers will carve out V-shaped valleys. Due to the shape of the
valley and its relatively straight path, and the presence of cirques in the region, you can deduce that the valley was most
likely carved by a glacier not a river.