Background Information
The North Saskatchewan River has been flowing through this region for millions of years through a broad, shallow valley known as the Beverly Valley. Around 27,000 years ago, a glacier advanced south over the Edmonton area. Meltwater from that glacier formed Glacial Lake Edmonton. The glacier and the lake deposited thick sediment which completely covered the Beverly Valley. The North Saskatchewan river valley has formed over the past 12,000 years. This "new" river valley followed a different path than the Beverly Valley. The North Saskatchewan River has cut down through the glacial sediments into the older Cretaceous sedimentary rocks.
Keillor Road History
In 1918, Dr. Frederick Keillor bought land between Saskatchewan Drive, Whitemud Drive and the river. Later, he gave some of his lands to the city to build a road down the hill. That road became Keillor Road after his death in 1971.
Landslide History
The landslide began as a minor failure of the toe of the slope in 1989. Further instability in the slope was identified in 1994. The road was closed to vehicles and converted into a multi-use trail. In 1996, a slide occurred and in 1997 the City of Edmonton built a retaining wall to stabilize the roadway. In 2001, cracks began to form behind the retaining wall. In late 2002, a significant collapse occurred.
Initially, investigators believed that the slope failure was a shallow one in the glacial sediments of the upper slope. Following the significant slide in 2002, investigators found that cause of the slide was likely to be a bentonite seam in the Horseshoe Canyon bedrock a few meters above the river. Further movement was monitored in 2003-04.
Geology
Layers
In geology, stratigraphy is the analysis of the order and position of geological layers. The area of the Keillor Road landscape features the following layers:
- Bedrock, the Horseshoe Canyon Formation of the Edmonton group. This is made up of layers of clay shale, bentonitic sandstone, sandstone, coal and bentonite seams. The clay shale layer is often brecciated which means that it is made up of broken, angular fragments.
- These bedrock layers are overlain by up to 6 meters of glacial till deposits. Till can be made up of clay and sand, and small gravel up to the size of small boulders.
- Above the till is a layer of lacustrine clay up to 7 meters thick.
Types of Landslides
The U.S. Geological Survey describes landslides as follows:
The term "landslide" describes a wide variety of processes that result in the downward and outward movement of slope-forming materials including rock, soil, artificial fill, or a combination of these. The materials may move by falling, toppling, sliding, spreading, or flowing.
SLIDES: Although many types of mass movements are included in the general term "landslide," the more restrictive use of the term refers only to mass movements, where there is a distinct zone of weakness that separates the slide material from more stable underlying material. The two major types of slides are rotational slides and translational slides.
Rotational slide: This is a slide in which the surface of rupture is curved concavely upward and the slide movement is roughly rotational about an axis that is parallel to the ground surface and transverse across the slide (A).
Translational slide: In this type of slide, the landslide mass moves along a roughly planar surface with little rotation or backward tilting (B). A block slide is a translational slide in which the moving mass consists of a single unit or a few closely related units that move downslope as a relatively coherent mass (C).
Horsts and Graben
A horst is a raised block of land that has lifted or has remained stationary, while the land on either side has subsided.
A graben is a depression or valley with a distinct escarpment on each side caused by the displacement of a block of land downward.
Below the Keillor Point viewpoint, is a visible horst and graben.
Summary
The landslide began with the erosion of the toe of the slope by the North Saskatchewan River. This led to the whole mass of the hillside to slump. The displaced material moved downwards forming a graben as material slid out to either side lower down the hillside. The main body of the slide translated along a horizontal plane in the bedrock.
Logging Requirements
Send me your answers to the following questions. Please, do not post your answers in your log.
- What type of landslide do you think the Keillor Road landslide was? Was it a rotational slide (A), a translational slide (B), or a block slide (C)? Why do you think so?
- Estimate the height of the escarpment between the bottom of the horst and the top of the graben below you at the posted coordinates. Think about what the horst and graben are before answering - most people answer this incorrectly.
- How stable do you think this area is? Is the landslide finished, or will it resume in the future? Why do you think so?
- Please post a photo of you (and your group) from the viewpoint. If you do not wish to post a photo of your face, you may submit a photo of any other Geocaching-themed item (ie., travel bug, Signal the Frog stuffy, cache container, etc...), as long as it shows the view from Keillor Point.
References