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Geology
Natural History
 
    Since this is a virtual field trip of Lethbridge, a tour guide has a duty to fulfill and that is telling everyone on the trip about the area.  If anyone has ever been to Lethbridge (other than on a computer screen), they would soon notice strange looking formations which seem to sweep through the center of the city.  These are called coulees and they have a rich geological history, just as the rest of the area has as well.
 
                                                                                                                photo by: D Collins
Coal
    Lethbridge was built on a bed of coal, and due to adventurers like Nicholas Sheran, it was discovered and utilized as a economic booster and town builder.  The coal the city grew up on was a seam of  medium-carbon, high volatile "c" butuminous coal ranging from 1.2 to 1.9 m in thickness.  The coal came from millions of years of climatic change in the area.  Seventy million years ago, during the Cretaceous period, the area we know as Lethbridge was part of vast coastline which bordered an inland sea.  The climate was humid and the growing season was a full year.  So, sub tropical plants matured and died quickly.  There was a thick vegetative layer covered by mud and gravel.  It was the heat, pressure and lack of oxygen which produced a chemical reaction turning the plant carbon into the coal we still see in Lethbridge.  Each of these eras have there own mark in time due to the coulee profile (see below).


Bedrock Geology
 
   (Berg, 1997, p. 45)
 
    A coulee profile provides a good illustration of the bedrock in the Lethbridge region.  In the above diagram, the drawing shows that the difference between the bottom of the river valley and the top of the coulees in 100 m in elevation.  This gives us a huge profile to examine.  Upon further examination of the diagram, one will soon notice that it gives us a geological timeline of the area.  For example, during the last glacial advance, the glaciers scoured rocks and debris from the Canadian Shield and deposited them along the prairies as unstratified glacial deposits (glacial till).  According to the geological timeline, the upper  layers of bedrock are glacial till.


Landscape Development
 
    Some of the most notable features of the land around Lethbridge are the coulees. The word coulee originates from the French verb "couler" which means to run off.  Physically speaking, coulees are the dry valleys (in the picture above) that at one time channeled large volumes of glacial melt water.
 
    In order to have glacial melt water, glaciers must have present in the area at least once.   Such is the case with Lethbridge, but it not only happened once, but four times in the last one million years.  The coulees one sees today were carved out by the glacial spill water 13 000 years ago during the Wisconsin Advance, thus changing the face of Lethbridge forever.

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Theories of Coulee Development

  1.  Some believe that the coulees developed due to the prevailing winds of the past, and continue to be carved out of the land because of the present wind direction.  The map below illustrates that coulees are oriented in a northeast direction (prevailing southwesterly wind), so a theory, through correlation can be extrapolated.  The coulees were developed due to the wind.  Chester (Chip)
Beatty realized that the Lethbridge Regional Airport airstrip matched the prevailing wind direction and was oriented in the same direction as the coulees themselves.  This extra evidence made the theory much more concrete.
    Sandblasting of the interfluve spurs has proven the above theory otherwise.  If the aeolian theory were to be true, the sandblasting should be occurring at the base of the coulee, rather than at the end.  The aeolian action is occurring at the end of the coulee, proving the wind theory to in fact be wrong.

       Note:  Prevailing south west wind direction and the position of
                    the Airport
 
  2.  An alternative theory of coulee development has to do with tectonic plates and joint fractures in the glacial till.  The orientation of the coulees match up with the joint fractures in the glacial till.  Thus, the shifting of the North American Plate affects the joints and the outcropping of the evidence is present in the orientation of the coulees (Scheidigger, 1983).  Click here for an illustration of the geological event.  From personal communication, Dr. Robert J. Rogerson also noticed this corelation between the joints in the till and the orientation of the coulees.


Coulee Length

    According to the map below, the coulees on the eastern side of the Oldman River are noticably longer than those on the west.  Since we have two theories on their development, the length must have a theory as well.  At one time, glaciers moved through this area causing most of the land formations we see today, when they melted, it caused land transforamtion as well.  If one could imagine a glacier in the middle of the map below (where west Lethbridge is situated) and imagine what would happen when the glacier melted.  Because the land has a natural tilt towards the Hudson's Bay, it would make sense that the glacial mentioned would melt in the north eastern direction, thus causing the coulees on the eastern side to be longer.
     Note: The coulees are longer on the east side of the Oldman
                                                                               River than the west side. (click for bigger image)

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 The Coulees
 
    The coulees are a magnificent landmark in an oasis of the prairie landscape.  they provide a break in the flatness which tends to characterize the Southern Alberta horizon.  The coulees also have other features native to them which have different characteristics on the north facing slopes and the south facing slopes.
                                             photo by: Dr R J. Rogerson
North Facing Slope

South Facing Slope

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