Geomorphology I (GEOG 213)
LABORATORY 3
LANDFORMS OF MASS WASTING
PART A: Airphoto interpretation of landslide features
This exercise is devoted to the interpretation of landforms produced by slope failures. The photographs
to be used in the laboratory are: BC7721:208 and 209 (Churn Creek, Fraser Valley). There is also a
topographic map available for you to examine the local topography. Churn Creek is a placemark in the
GEOG 213 Google Earth file available at http://www.sfu.ca/~jvenditt/geog213.html.
The field site is located in central British Columbia where the Fraser River flows in a valley formed in
poorly consolidated Tertiary sediments and late Tertiary lava flows. The valley has been filled with
several hundred meters of unconsolidated material, including till, lacustrine, fluvial and colluvial
deposits, during and immediately after glaciation. Since deglaciation, Fraser River and its tributaries
have become incised into these deposits, producing widespread gullying and in many locations, large
slope failures of the over-steepened valley walls.
A large landslide can be seen on the right (west) bank of the river. It involves the terrace deposits, which
are mostly glacio-lacustrine sands, silts and clays. The base of the rupture surface could well be a weak
plane in Tertiary bedrock, beneath the glacio-lacustrine sediments. The landslide area is identified by a
pair of sharply defined flank scarps. The crown scarp is also sharply defined, but relatively low. A
number of internal scarps and cracks are visible. The toe of the slide is in the river.
The irregular, hummocky area directly behind the crown of the slide is the toe lobe of an earthflow,
originating in weathered sedimentary rocks higher up the valley side.
Part A Exercise (12 marks):
1) Outline the limits of the landslide. Estimate its volume, assuming that the scale of the photos is 1:
15,000 and that the slide material is 20m thick on average (2 marks).
2) Outline the margins of the earth flow lobe and the landslide (2 marks).
3) Using lines and arrows, show the pattern of main surface drainage paths on top of the terrace. Write a
brief description of the drainage pattern. Do you see any signs of water ponding? How is the drainage
influenced by the presence of the earth flow? (3 marks)
4) Sketch a cross-section of the slide area along Line A-A. Use a scale (approximate) about 2x larger
than the scale of the photos. Show schematically the position and attitude of all the minor internal scarps
(3 marks).
5) With reference to the surface morphology of the cross-section, sketch the most likely shape of the
rupture surface in Cross-section A-A. Is the slide rotational, translational or compound? (2 marks)
SLIDE TERMINOLOGY
PART B: Exploring the causes of landslides
In this exercise, we will examine some data from Donald Tubbs’ PhD dissertation (Tubbs, 1975) that
allow us to explore the causes of landslides. During the early part of 1972, the Puget Sound area
suffered numerous landslides, which
resulted in damages totaling millions
of dollars (1972 dollars!). The Seattle
area (Fig. 1) was particularly hard hit,
owing partly to extensive urbanization
and partly to geologic factors. Tubbs
examined the climatic and geologic
factors that contributed to landsliding
in 1972. The Seattle area is
susceptible to landsliding due to its
glacial history. In particular, there is
surficial material (Esperance Sand)
overlying more impermeable deposits
(either the Lawton Clay or pre-Lawton
sediments). Here, we will examine
the influence of topography and
climatic conditions on landsliding.
In order to complete this laboratory
exercise, you’ll need to download the
Figure 1: Location map (from Tubbs, 1974)
following files from the course
website (http://www.sfu.ca/~jvenditt/geog213/):
1) Daily_Precipitation_Landslides_Seattle_1972.txt
2) Annual_precipitation_Landslides_1932_to_1972.txt
Part B Exercise (15 marks):
1) Examine the airphoto of landslides that occurred in 1972 (Figure 2) and the map of topographic slope
in the Seattle area (Figure 3). What topographic characteristics do the locations of most of these
landslides have in common? (2 marks)
2) Using the data presented in the file “Daily_Precipitation_Landslides_Seattle_1972.txt” generate two
bar graphs of daily precipitation and landslides between February and June 1972 (3 marks).
3) What is the relation between daily precipitation and landslide occurrence? (2 mark)
4) Using the data presented in the file “Annual_precipitation_Landslides_1932_to_1972.txt” plot annual
rainfall against the number of landslides. (2 marks).
5) Is there a relation between annual rainfall and the number of landslides? What appears to be more
important for landslide generation, the amount of rainfall in a given year or individual storms? Why? (3
mark)
6) What conditions are likely to increase the risk of landslides in the Seattle area? (3 marks)
References
Tubbs, D. W., 1974a, Landslides in Seattle: Wash. Div. of Geol. and Earth Resources Information
Circular 52, 15 p.
Tubbs, D.W., 1974b, Landslides and associated damage during early 1972 in part of west-central King
County, Washington: U.S. Geol. Survey Misc. Geol. Inv. Map I-852B.
Tubbs, D. W., 1975, Causes, mechanisms and prediction of landsliding in Seattle: Ph.D. thesis, Univ.
Washington, 88 p.
Figure 2:
Airphoto
showing the
locations of
landslides in
1972 (from
Tubbs, 1974b)
Figure 3: Map
showing
topographic
slope of the
Seattle area
(from Tubbs,
1974b).