Giant stair steps
The topographic map to the left shows two excellent examples of cirques; a solid red line identifies their headwalls.
Both cirques still contain glaciers, which are symbolized by blue contour lines on a white background with a dashed blue line marking
the edge of the glaciers. Both glaciers were larger in the past than they were at the time of map creation. Clues that indicate their
former size include moraine deposits, shown as brown speckles, that extend well beyond the glacier termini, particularly with the northern
of the two glaciers. In addition, U-shaped valleys extend down-valley from the glaciers and only glaciers, not rivers, carve out
U-shaped valleys at high altitudes.
Clues that indicate these are cirques include the approximately semi-circular shape of the contour lines, the close spacing of the
contour lines on the cirque headwalls, the presence of glaciers in the cirques, and the fact that the top of the cirque headwall is at
the top of a ridge - there is no place higher to go once you reach the top of the cirque headwall.
Can you find another example of a cirque on this map?
This second topographic map contains multiple cirques identified by a solid red line. Some of these cirques still contain remnants
of the glaciers that carved the cirques, while other cirques have no ice left, just tarns (glacial lakes). Note that the cirques are
not all the same size or the same shape, but they are all approximately semi-circular and they all have steep headwalls. Tulainyo Lake
sits at the bottom of a cirque although the headwall of that cirque is not marked with a solid red line as the other cirques are.
Other glacial landforms in addition to cirques and tarns include horns, aretes, and moraines. The horns are pyramid-shaped mountain
peaks with three or four glaciers eroding away on different sides of the horn. The moraines are symbolized as brown speckles. The aretes
are not labeled but they are the ridges that define the borders of the cirques. For example, the ridge separating the cirque containing
Iceberg Lake from the neighboring cirque to the west is an arete. Likewise the ridge top that separates the cirque containing Tulainyo
Lake from the cirques to the east is also a tarn.
The bedrock defining the cirque in the photo below has a classic semi-circular shape.
The semi-circular shape of the cirque in the photo below is quite obvious (red line). The glacier is flowing out of the cirque
toward the bottom of the photo.
Above Photo: J.A. Hyatt. Used with permission.
|Left Photo: A. Post. 1966. USGS Photographic Library, photo post00008. < http://libraryphoto.cr.usgs.gov/> Accessed June 2010.
Open this link to see a photo of the landscape diagrammed below.
There are three excellent examples of cirques separated by aretes in this photo. The fourth and left-most cirque is partly cut off. The
cirques are the bedrock features that hold the glaciers. Once again note that the cirques are not all the same shape but they are all
approximately semi-circular in shape with steep headwalls, and they are all located at the head of the valleys that were once filled
with glacial ice.