FEATURES OF ALPINE GLACIATION

 

Topographic maps record information about the shape of the earth surface.  By studying these maps we can gain insight into the geologic processes that shape the earth surface.  One set of processes that that is of interest is Alpine Glaciation.  Alpine glaciers are "rivers" of ice that flow in and from mountain valleys.  They change the earth surface by eroding  and depositing sediment.  In some places the importance of glaciers is obvious because they still exist today.  In other places the glaciers no longer exist, but there is still evidence they were present in the past.  Lets look at some topographic maps to find features related to alpine glaciation.

 

The map at right is of a section of the Colorado Rockies which has been eroded by glaciers in the past. This can be determined from the map by recognizing features formed by glacial erosion.  Try to recognize the following erosional features on the map (they may occur more than once)

  • "U" Shaped Valley

  • Cirque

  • Horn

  • ArÍte

Scroll down this page to see how you did.

How did you do?  Examples of these erosional features are labeled at right.  Even though there are almost no glaciers in this areas today, the presence of these glacial features tell us that glaciers played an important role in shaping this landscape.

Lets look in more detail on how we recognize each of these features on topographic maps.

 

"U" Shaped Valleys

 

As we learned earlier, the cross sectional shape of a valley stream valleys in mountainous areas is usually "V" shaped.  This shaped is changed when a glacier flows down the valley.  The bottom of the valley will be rounded while the sides of the valley will be steepened. The result is a U shaped valley with steep sides and a rounded valley.

How do we recognize a "U" shaped valley on a topographic map.  We need to look at how the slope changes as we go across the valley.  Remember closer the contour lines are spaced the steeper the slope. 

Follow the red line from X to X'.  It will take you across a valley.  Near X the contour lines are close together( steep slope).  As we move toward X' the spacing between the contour lines increases (slope is decreasing).  Near the stream in the center of the valley contour lines are widely spaced (gentle slope).  After we cross the stream the spacing of the contour lines decreases (slope increases) as we approach X', where the contour lines are closest together.

The insert cross profile shows how the slope of the valley floor changes between X and X'.  Note the steep slopes near X and X' and the gentle slopes in the middle of the valley.  Also note the broad "U" shape for the cross profile for the Valley.

 

Cirques

 

Cirques are bowl shaped depressions found at the head (upper end) of glacial valleys.  Cirques are the places where snow accumulates, is compressed into ice, and begins its flow down the valley as a glacier. 

Cirques are located at the upper end of the U shaped valleys. Steep slopes curve around the flatter center of the cirque in a semicircle (shown by the curved red line at right). The open part of the circle is the opening that allows ice to flow from the cirque down the "U" shaped valley.

The floor of the cirque itself is often depressed (part of its bowl shape).  This depression often fills with water forming a small lake (called a tarn for trivia buffs).

The easiest way to locate a cirque is to look for the semicircle of closely spaced contour lines, the depressed center (lake), and its position at the upper end of a glacial ("U" shaped valley).  Not all cirques will show all of these features.

 

 

ArÍtes and Horns
Glacial erosion in mountainous regions tends to steepen slopes making the topography more rugged.  The jagged ridges and peaks that stick up between the glaciers are arÍtes and horns.  ArÍtes are the sharp ridges that form between glacial valleys and cirques.  Horns are peaks that are surrounded by cirques. ArÍtes often radiate away from horns.   Both of these features can be observed on topographic maps.

ArÍtes can be recognized as steep sided ridges with narrow crests. They are traced in red on the diagram at right.

Horns are the summits of mountains that have had their sides eroded by glaciers.  They are indicated by the blue "X" on the map at right.

MORAINES

 

In addition to the erosional features discussed above glaciers also deposit large volumes of sediment.  These depositional features also often remain after the glacier has melted and can be recognized from topographic maps.  Lets look at the most common depositional features related to alpine glaciers: moraines.

The map at right shows part of one of the glaciers on the side of Mount Rainier, Washington.  Ice covered areas are the white areas with the blue contour lines.  These blue contour lines indicate the elevation of the ice surface.  The white areas with brown contour lines are areas not covered by ice.

Looking at the elevations of the contour lines on the glacier we can the glacier is flowing from the higher elevations in the northwest (upper left corner of map) to lower elevation in the southeast (lower right corner).

Through out this area are deposits of glacial sediment. These deposits are shown by the brown stippled (dotted) pattern. 

Some of the glacial deposits can be seen covering part of the glacier.  Along the sides of the glacier debris from the valley wall collects on the ice surface and is carried down slope by the moving ice. This debris is referred to as a Lateral Moraine (see map).  

As the ice flows down slope smaller glaciers often join together to form larger glaciers.  We two glaciers merge, the lateral moraines between the two also merge and form a moraine that runs down the middle of the glacier. This is a Medial Moraine (see map). If you follow the medial moraine on this glacier uphill you will see the point where the two glaciers are coming together in the northwest corner of the map. 

Most of the down hill end of the glacier is shown covered with glacial sediment. This is because when the ice reaches the end of the glacier it melts, depositing the sediment it was carrying.  This ridge of sediment across the end of a glacier is called an end moraine.

 

Glacial deposits remain after the glacier has melted.  The map at right shows the area around Willow Lake, just the southeast of the wind River Mountains in Wyoming.  The Valley that Willow Lake sits in is a glacial valley formed by ice flowing out of the Wind River Mountains.  The blue arrow shows the approximent direction the ice flowed.  From the map it looks like the water forming Willow Lake has backed up because of a ridge running across the valley (between the red lines). 

This ridge is a relatively broad, low ridge with an uneven top.  Note the numerous depression contours on this ridge. This is very characteristic of glacial deposits.  This ridge is likely an end Moraine formed when glacial sediment was deposited from melting ice when the end of the glacial sat as this location.

 

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Last Updated: 01/18/2014