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
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).
(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.
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.
take it from the top
Back to VFT Lethbridge
Theories of Coulee Development
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
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.
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
Note: Prevailing south west wind direction and the position of
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.
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)
Take it from the Top
to VFT Lethbridge
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