1. Bedrock Geology
The earliest record events contributing to the modern landscape in the region of the Oldman River, and the great plains in general is when tectonic activity, the shifting of the earths plates, produced the ancient Rocky Mountains (60-70 million years ago) during the tertiary era (See this page for in depth time line). The effect of such uplift, was the draining of the interior, continental sea. Previous to this, on the great plains, were millions and millions and of years of deposition of marine sediment still visible in parts of river valley's in Southern Alberta. The rise of the Rocky Mountains, caused the marine sediments to be somewhat evenly blanketed over Southern Alberta. Over millions of years, approximately 30-35 million years (30 million to 35 million years ago), is a period of great accumulation of terrestrial sediment of from the newly formed rivers of the Rocky Mountains. These rivers deposited vast amounts of their erosionary products over the great plains. About 25 million years ago, is a process called "down cutting." In the process of down cutting, the broad scale uplift of the Rockies and the great plains, caused the river channels to seek out new more stable paths. This process of down cutting over the plains began to level off the landscape in southern Alberta. According to Chip Beaty in, The Landscapes of Southern Alberta, information about what landscape looked like in the Tertiary area can be deduced from gas and logs, even though most of the bedrock is buried deep under glacial sediments. The tertiary landscape is believed to have been a rolling one, which would later be flattened by the Laurentide ice sheets some 1 million to 10,000 years ago; where numerous glacial advances advances and retreats laid down up to 100 meters of various till and lacustrine deposits, and flatened the prarrie landscape. Meltwater at the end of this glaciation, known as the Wisconsin Glaciation, would forge out the coulee landscape which have become the typical Southern Alberta landscape in so many minds.(b) The Stratigraphy along the Oldman River
|If you ever travel along the Oldman River, and especially in places where banks have been exposed by various types of erosion, you can see the past 1.2 million years of history of deposition along the Oldman River. Strata, or layers, have been forming here in great abundance since the Quaternary period of the earths history. However, to generalize the stratigraphy over the entire region of the Oldman River would be difficult. Geologists have combed through the strata to make some generalizations about the formation, and types of strata in this region|
The glacial geology, evident in the river bottoms is very complex. It begins with the arrival of ice and deposition of glacial materials. While the numbers of glaciation in this region is unknown, it is known there were at least two times the ice sheets of the Pleistoscene era covered the region of Southern Alberta. These left behind a sequence of tills, sands and gravels. For a closer look at a diagram from several sections showing the complex glacial sequences along the Oldman River, click here.
(Click Image for closer view)
|Pictures of more strata near Lethbridge. Can you see the erosion of the bedrock material at the base of the river?|
2. Land forms along the Oldman River
(a) Flood Plains
Flood Plains are the portion of valley floors, made by streams that flow into them. They consist largely of eroded bedrock, till, and alluvium in southern Alberta. Briefly, a flood plain is a strip of relatively smooth land on a valley floor bordering a stream, made by erosion and/or deposition by that stream. River meander are the bends in the course of a stream. (See picture of flood plain) If you look closely at the picture in the flood plain (in above link), you will notice that there is large flat surface between the coulees that mark both sides of the river. This is the flood plain. Farmers in this area often farm flood plains as seasonal floods bring fresh silt and clay to their fields.b. River Meander and their landforms
Straight channels seldomly persist for long in a river. In the middle section of a river, typically between the turbulent headwater streams and its estuary where it reaches base level, its channels begin to form meanders. River channels quickly form into sinuous patterns. Geomorphologists state that perfect laminar flow seldom persists in a channel, and that channels have natural tendencies away from laminar flow. laminar flow is the smooth flow in which the flow of the river (direction of flow) is parallel to the banks.Simple helical flow initiates meandering, an interesting set of geological features develop called point bars and form on the inside of meanders. Here river flow is decreased, and sediments, rocks and sand are deposited. On the outside of a meander, erosion is taking place from the increased flow of water, carrying with it a great deal of momentum, which in turns erodes the bank of a river- in what can be termed, cut banks.(c) Oxbow Lakes
Can you see the point bar near this Lethbridge road bridge? Looking from the same point bar, the opposite side of the bank show the erosion of the river bank. These are called cut banks.
(Click image to enlarge)
|In the middle section of a river, the valley is characterized by broad flood plains, at least as wide as the amplitude of meanders. The valley sides are visited only infrequently by the slowly shifting channels of the streams. As the flood plain develops, oxbow lakes become numerous. Oxbow lakes are crescent shaped lakes formed in an abandoned meander river bend by a meander cutoff. Oxbow lakes are lakes or ponds found in association with river channels. When a river channel becomes obstructed by silt and debris, the river will often cut a new channel around the obstruction. With time the obstructed area may become completely cut off from the river and begin developing as a lake. Over time an oxbow lake may become filled with organic material and be transformed into a marsh.|
There are quite a few theories for the development of the coulees we see today. In any case, it is known that the coulees were originally formed by the final glaciation of the Pleistocene, known as the Wisconsin glaciation. When the glaciers of the Wisconsin retreated, large amounts of water flooded the Alberta landscape. These meltwaters forged the dry valleys that lead into the Oldman River. Southern Albertan's, among other people, call these dry valleys coulees. Geomorphologists have began to think the coulees we see today aren't entirely the product of the Wisconsin glaciation.
C. Beaty in the Landscapes of Southern Alberta notes that coulees from Lethbridge to the Rocky Mountains are roughly parallel to the wind (see above diagram) These include the coulees of the Oldman River, Castle River, Belly River amongst other rivers in this south western region. They seem to have a mean trend of being oriented 70 degrees east of north. According to Beaty, as seen from low flying aerial photos, is an alignment with the south westerly prevailing wind of this region. This theory is know as coulee alignment. Some geomorphologists refute coulee alignment, they say that sand blasting is observed more on the interfluves where they face west instead of within the coulees. (Visit link for picture and description)
Another theory to the origins of the modern coulee, according to Beaty is that coulees orientation is largely a product of structural features of bedrock below the surface, mainly joints and faults. According to Beaty, while this theory may be true for some surface features, it is quite inadequate for coulee alignment. R.J Rogerson, (personal communication, 2000) feels that structural joints resulting from the westward movement of the North American Plate are important in controlling orientation, as found elsewhere particularily in the Great Plains of the United States.
There seems to be quite a few theories to the coulee formation in Southern Alberta, unfortunately, there is not a universally accepted theory of coulee evolution in Southern Alberta. For more information refer to C. Beaty's Landscapes of Southern Alberta. Also visit this link
(e) Mega Blocks
|A picture of mega block near west Lethbridge Alberta. Look closely at what appears to be a white band in the rocks. Click image for closer view.|
(f) Elements of mass
Creep is the slow, downslope movement of superficial soil or rock debris. Creep, is usually observable in areas like the coulees over long periods of time. Though may appear to be continuous, it is usually the sum of numerous minute, discrete movements of slope material, called colluvium, under the influence of gravity. Creep in the coulees is usually visible as steps like formation called teracettes. (Click on left image to enlarge)
Research at the University of Lethbridge shows a strong correlation between urbanization and water consumption of Lethbridge's residents and the rate of slumping in the coulees. Researchers say that waste water may percolate through the rocky unconsolidated till, and cause increased soil movements in the coulees. This, in turn, leads to increased slumping in our coulees.
Slumping is the downward and outward sliding of a large mass of material along a curved, usually concave upward, shearing plane. A slump block, which is evident as the main block that has slidden often breaks into mini slump blocks. (See diagram below) In the coulees, areas that are often NOT expected to slump, are slumping which is concerning the citizens that live near by. They are concerned about their physical safety and safety of their property. In Lethbridge, the residents of Scenic heights, have seen a large slump appear almost overnight near their back yards. (click on left image for larger view)
|(click on image to see actually coulee slump which have residents in South Lethbridge for wondering for the safety of their homes.)|
|(click on this image to see a diagram of what a typical slump looks like- does the picture above fit this diagram?)|
Date of visit December, 1999
2. "The Palliser Triangle" Barendegt et al.
1988, University of Lethbridge.
3. Beaty, C.B. (1975). The Landscapes of Southern Alberta.
The University of Lethbridge Printing Services.
4. Surface Landforms and processes, Donald J Easterbrook
MacMillan publishing company, 1994.
5. Helen Schuler Coulee Center
Information Guide (1999)
6. M.S Stalker, Stratigraphy of Southern Alberta
Canadian Geological Society, 1982.
Image of slump : Easterbrook 1994
Image of Oxbow Lake: http://creative.smsu.edu/U/ejm893f, 2000
Time Line: Royal Tyrell Museum
Diagram of Coulee alignemnt, C.B. Beaty (see above)