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Castle Mountain

Castle MountainAs you travel west from Banff to Lake Louise, there is an abrupt and dramatic change in the appearance of the mountains as you go from the Front Ranges into Main Ranges.  The castle-like peaks and thick, relatively flat-lying beds of the Main Ranges, such as are seen on Castle Mountain, stand in sharp contrast to the tilted, folded, and faulted beds of the Front Ranges, seen immediately to the east in the Sawback Range.

At first glance, Castle Mountain resembles a medieval fortress with its rock layers turned into cliffs, terraces, and towers by the forces of erosion.  This mountain type is best developed where there are alternating beds of resistant rocks, such as limestone, and easily eroded rocks, such as shale.  As the softer layers erode, the harder rocks above are undermined and break off, slowly forming a mountain of vertical walls separated by sloping ledges.

Castle Mountain Archive PictureThe Main Ranges are composed of older and more colourful rocks than the grey rocks of the Front ranges.  Castle Mountain consists of purple, green, and pink Precambrian and Cambrian limestones, shales, and quartzites (600 million to 400 million years old) that were thrust eastward, via the Castle Mountain Fault, over the younger sediments at the base of the mountain which are now tree-covered.  This fault is geologically significant as it is the feature that separates the Front Ranges from the Main Ranges at this latitude.  Looking at Castle Mountain, you can see that the rocks have been folded into a broad, shallow U-shape, called a syncline.  Before millions of years of erosion, this syncline was once part of the same long syncline that extended north to, and included, Mount Kerkeslin.

Although the Main Ranges have the same southeast to northwest alignment as the Front Ranges, they lack the symmetry because of irregular drainage caused by extensive glaciation and horizontal layers.  Main Range mountains are also generally higher than Front Range mountains because erosion, particularly by glaciers, wears away thick near-horizontal layers more slowly than the tilted folded layers.

On the east side of Castle Mountain is Rockbound Lake.  The large depression in which the water sits was carved out by glaciers during the last Ice Age.  this sort of depression, which is common in the Rockies, is called a cirque.

Reprinted from A Traveler's Guide to Geological Wonders in Alberta  by Ron Mussieux and Marilyn Nelson.  With permission of the authors and the Provincial Museum of Alberta.

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