As 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.
The 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.
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.
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.
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.