Articles	Projects	Resume	Cartoons	Windsurfing	Paintings	Album

British Columbia

by

Waterose

• Stop One:The Canyon

• Stop Two:Canyon Base & Creek Flood Plain

• Stop Three:Under the Boardwalk

• Stop Four:Lynn Creek Channel

• Stop Five:Lynn Creek Braided Channel

• Stop Six:Erratic Boulder

• Stop Seven:Cliff

• Geologic History of Lynn Valley

• Appendix I Bibliography - Works Cited

• Appendix II Water Colour Gallery

• Appendix III Sketch Gallery

      The objective of the field trip was to view evidence of glacial and fluvial activity and analyse how the Lynn Valley Canyon was formed.

      The base of the canyon is the Coast Mountain Range formed by volcanic activity approximately one hundred million years ago. Isostatic adjustment raised the volcanic range above sea level completing the orogenic process. Evidence includes an exposed igneous dike.

      Subsequent to the formation of the Coast Range, three glacial and inter- glacial periods followed eroding the Coast Range to the present formation. Evidence of glacial activity include the formation of the U-shaped valley and the deposition of erratics, glacial till and tongue silt. Inter-glacial fluvial activity eroded the topography forming the Lynn Valley box canyon and rounded fluvial boulders. Deposition during these periods include peat layers, and alluvium.

      At the time of viewing, the water level was very low, but there is evidence of a higher watermark level during the period of spring run-off as evidenced by boulder deposition and river debris in the form of large logs. The watermark level can be viewed by the undercut of soil below tree roots and the level of moss and fern growth on both the cut bank and point bar deposits.

      Lynn Valley Canyon is a beautiful treasure trove of geologic discovery.

Stop One:

The Canyon

Pre-Stop:

Estimate the approximate dimensions of the canyon, particularly the width and depth of the stream channel.

      This section features a deep rock canyon approximately nine meters high and approximately twelve meters across at the uppermost water level marked by vegetation growth including ferns and the edge of the roots from the trees. There is evidence of the high water levels by the large boulders mixed in with the topsoil and interspersed among the tree roots and along the sides and on the narrow path leading down from the edge of the upper precipice. The walls of the canyon are marked by angular fractures of the wall rock, possibly caused by frost shattering and abrasion. Ledges in the rock wall have shallow potholes. The west side of the canyon features a vertical dike that protrudes slightly, light grey in colour, which is resistant to erosion and protects the granitic canyon wall adjacent to the dike downstream.

      At the time of viewing, the water level in the channel was relatively low with a depth of approximately half a meter. At this level, the base of the canyon gorge is narrow, approximately one and a half meters across: The narrowing of the channel causes increased velocity and turbulence. The erosion of the bedrock has formed a waterfall.

      Evidence of erosion include boulders of large, medium and small size with pebbles at the sides of the channel. The types of erosion include hydraulic action as evidenced by the rounded shape of the channels. Abrasive erosion is evidenced by the rounded shape of the rocks and boulders in and around the stream bed. It is reasonable to assume that there is also some degree of solution erosion where dissolved chemicals are transported in the stream water.

Stop Two:

      The trail down from the canyon precipice to the flood plain has evidence of the different levels of the creek bed by the large number of big boulders and mixture of different size alluvium. In addition there is evidence of undercutting of the banks by water erosion of the soil at the tree root level. At the base of the canyon, the flood plain widens and is relatively flat ground. The vegetation is comprised of relatively young stands of alder. There are a few cedar trees that are approximately fifty to sixty years old.


Note where Lynn Creek abruptly narrows as it enters the canyon.

What is the reason for the narrowing of the channel here? Cite evidence for this.

      Looking downstream, the canyon walls climb and narrow. The most significant geophysical evidence to account for the narrowing of the channel at this point is the thick vertical dike in the granitic wall. The dike protrudes slightly from the wall, and protects the granitic wall downstream and adjacent to the dike. The dike is comprised of igneous intrusive magma that cooled slowly forming a smaller crystal structure that is stronger and more resistant than the larger crystalline structure of the adjacent granitic plutons.

Note the very large boulder perched on the ground. How did it get there?

      This boulder is called an erratic. It appears too large to have been transported by hydraulic action because it is significantly larger than the largest sized boulders that are abundant in and around the stream bed. Geologic chemical testing can prove that the composition of this erratic is significantly different from the local rock composition. The erratic was transported by glacial action which occurred several time in this area. This erratic was probably transported to this location during the most recent glaciation period, the Fraser Glaciation. Additional evidence of glacial transport on the boulder is the sharp striated marks on this boulder. A boulder transported by hydraulic lift would be rounded and smooth from abrasive erosion.

Stop Three:

Describe the texture and composition of the material. Speculate as to the origin of this material. How old is it?

      The lowest layer of material at the base of the bank section is comprised of a mixture of sand and small rocks. "Dense brown till containing 5% subrounded stones grading to a massive dense brown silt and sandy gravel which is probably Semiahmoo drift" (Maynard 35). This glacial till was deposited during the Semiahmoo Glaciation period 62,000 years before present (B.P.).

Examine the dark brown woody layer superimposed on the material you described in last question. What is the origin and probable age of this material?

      The dark brown woody layer is approximately half a metre thick and is comprised of peat. This peat layer is dark brown and is organic. It breaks off easily in horizontal type chunks or flakes. It was formed during the Olympia non-glacial interval. The peat layer is part of the Cowichan Head Formation (Maynard 35). During this period anaerobic conditions in swamp bogs did not provide enough oxygen for bacterial decomposition of the organic matter. The building blocks of peat are partially decomposed vegetation and peat is primarily composed of carbon. The peat is dated at 32,000 B.P. (Maynard 35). It is a very low grade fossil fuel and it is impermeable to water.

Note the layer superimposed on the layer in the last question. What is the possible origin of this material?

      Superimposed on the top of the peat layer is a very thin reddish brown layer comprised of iron oxides. The iron is carried in solution and precipitates out in deposits. The iron oxides cannot leach down through the peat because the peat is impermeable to water. The iron oxides must have been transported in solution between 20,000 B.P. and 30,000 B.P., after the formation of the peat and probably before the Fraser Glaciation period.

Describe the historical sequence of deposition at this point.

      The historical sequence of deposition from the base layer to the top of the bank wall is:

1. The first (bottom) layer is glacial till deposited during the Semiahmoo Glaciation period 62,000 B.P.
2. The second layer is organic peat, part of the Cowichan Head Formation deposited during the Olympia non-glacial interval 32,000 B.P.
3. The third layer is a reddish brown precipitate deposit of iron oxides transported in solution between 20,000 to 30,000 B.P.
4. The fourth layer is comprised of small boulders, sand, and fine gravel deposited during the Fraser Glaciation period, 17,000 B.P.
5. The fifth layer is a soil profile that contains evidence that the creek bed used to be at the top level of the bank. There are medium and small boulders below the humus layer of the soil. This alluvium was deposited during the Holocene Epoch 5,000 to 10,000 B.P. when the Fraser River Delta was forming and the sea level rose 10 metres. The increased water level and alluvial transport was due to the melt down of the Fraser Glaciers.
6. The top layer is the humus layer of the soil formed from decaying organic matter in present time.

Stop Four:

      Walking towards Stop Four through the old flood plain, the ground is relatively flat with no noticeable slope. The area is vegetated with young trees and saplings. The oldest trees are estimated at fifty to sixty years old. The area is strewn with alluvial boulders and organic debris deposited by the river. There is a small feeder stream off to one edge of the flood plain area with small rounded rocks and pebbles in the stream bed. The flood plain slopes down towards the main Lynn Creek Channel, Stop Four.

• Estimate the width and depth of the channel in metres.

      The channel is a wider U-shaped channel than the box canyon down stream. At time of viewing, the width of the water channel was 5.5 metres. The average depth the water channel was 1.5 metres. The depth varied from 2 metres maximum to .3 metres minimum due to the deposition of the rocks in the creek bed.

Can you determine how much higher the water can get in the channel? The maximum level is known as bankfull. Cite evidence for this.

      The maximum channel width is 20 metres. The maximum channel depth is 3 metres on the cut bank side and 2.5 metres on the depositional or point bar side. The maximum level of the channel is evidenced by the moss line on the rocks, the undercut and erosion of the soil around the tree roots, and the height of the line of ferns growing in the soil.

Are you standing on a cutbank or a point bar? Cite evidence for your answer.

      This location is on a point bar as evidenced by the accumulation of sedimentary bedrock including large, medium, and small sized rounded rocks and an accumulation of fine grained sand deposits.

Streams can transport sediment (clastic and chemical) in three ways. What type of sediment transport is occurring in Lynn Creek? Cite evidence for this. What time of year does most sediment transport take place?

      There are primarily three types of sediment transport occurring in Lynn Creek: suspension, traction and saltation, and solution.

      Suspension carries very fine particles of sediment in the stream as evidenced by the deposits of fine sand on the point bar.

      Traction and saltation occur when the rocks are dragged along the bottom of the stream bed by the velocity of the stream, or are bounced and rolled along the stream bed. Evidence of traction and saltation include the composition of the streambed which is comprised primarily of rounded rocks of assorted sizes on the point bar and in the stream bed.

      In addition, minerals may be carried in solution, a chemical form of transport. Chemical analysis of a water sample is required to identify the mineral types in solution. However, some minerals may precipitate out of solution and stain the rocks along the course of the channel (Christopherson 422-424).

      The majority of sediment transport occurs during the spring season when the snow accumulated at higher elevations on the local orographic formations melts due to increased insolation. During the spring season, the snow melt and spring precipitation drains into the local river basin. The excess water melt saturates the soil past the level of cohesion and mass erosion ensues. Rocks and soil are loosened and swept downstream. As noted earlier in part b the maximum water level of the channel is significantly higher than the present viewing level. The area from the low summer creek bed back across the flood plain to the boardwalk is littered with larger alluvial boulders and large masses of organic river debris.

Stream discharge is the volume of water being transported past a point over a given amount of time (expressed as cubic metres per second) by the formula:

Q=VA, where

Q=stream discharge

V = velocity in metres per second

A = width x depth (square metres)

Velocity: measured by the average time (seconds) for an orange to travel 20 metres on the surface of the creek.

V = 20m ÷[(49s + 39s + 24s)÷3] = 0.54 ms^-1
V = 0.54ms^-1
Area = width x average height
A = 5.5m x 1.5m = 8.3m²

Discharge (Q) = VA
Q = 0.54 ms^-1 x 8.3m² = 4.5 m³s^-1

How would you describe Lynn Creek at this location?

     At this location, the valley is wider and flatter with no noticeable slope gradient. The Lynn Creek forms a braided stream pattern with channels flowing around an island formed by sedimentary deposition. The island is vegetated by coloniser species including small grasses, shrubs, and alder trees. There is also evidence of fluvial deposition of rocks and stream debris in the form of larger boulders and organic debris. The surface of the island is rocks, pebbles and coarse to fine grained sand. The area has several point bars of deposition.

How many channels are there?

      At this point there are three main braiding channels of Lynn Creek. Braiding occurs when there is an equilibrium between the velocity of the water and the deposition of materials (Christopherson 426). In this case deposition is seasonal from the spring melt down, and evidence of significant mass erosion of the cliff banks 30 metres upstream from the area of braiding.

Why do you think the valley is wider here than at Stop Two and Four?

      The valley is wider here primarily due to the softness of the granitic bedrock. Previous glacial activity would have eroded the U-shaped valley with the grinding and abrasive action of the till carried in the tongue. Pursuant to the retreat of the glaciers, the stream bed was further eroded by hydraulic action, abrasion, and saltation.

      Stop Four is narrower because it is a point of deposition on the point bar.

      Stop Two is even narrower due to the protective capping of the dike which protrudes protecting the adjacent granitic bedrock wall.

What is this large rock?

      This large boulder is either an erratic or an exposed batholith. It is difficult to ascertain without a doubt because the base of the boulder is below the surface ground level, thus it is unknown if this if a free form boulder, or a component of the bedrock. It is most likely an erratic because there is an abundance of erratics that are in the flood plain area right back to the top of the box canyon. The boulder has evidence of striations and a sharp carved feature that is more similar to a Roche Montonnee than a fracture caused by frost shattering. Chemical analysis of the component materials would help identify if the rock is similar or dissimilar to the bedrock in the area.

Is this a depositional or erosional feature? How was it formed? How did it get here?

      It is very unlikely that this boulder is an alluvial feature transported by the creek flow because it is significantly larger than the other boulders deposited by fluvial process in the area as observed in the flood plain and along the embankment region.

      An erratic is a depositional feature transported by glacial activity. Material transported by glaciers are a result of orogenesis, erosion and transport. Erratics are usually identified by rough striations or grooves in the rock caused by the abrasive grinding action of the rock over other bedrock carried in the tongue of the glacier. Another identifiable feature of an erratic is that the mineral composition of the rock is different from the adjacent bedrock in the region in which the erratic is deposited (Christopherson 521-522).

Observe and describe the cliff above you.

      Past the erratic and over the bouldered sandy surface of the lightly treed island, the braided stream bends into a cliff face. The cliff bank is undercut. There is a 5 metre pile of tallus deposited at the base of the cliff resting at an angle of 45°. Human intervention placed piles of small and medium sized boulders near the base of the cliff to provide additional stability. There is small shrubbery and small trees growing at the base of the cliff. As the slope increases to vertical there is no vegetation on the undercut cliff or the steep slope adjacent to the exposed cliff. There are mature trees growing on top of the cliff.

Estimate the height of the cliff.

      The estimated height of the cliff is 20 metres.

Sketch the cliff and label the distinctly different layers (formations) of material. Describe the depositional environment for each layer.

      The cliff face is composed of soft sedimentary layers and is poorly stratified. Generally speaking the layers and the depositional environments from the base of the cliff to the top of the cliff are:

1. The base layer is fine clay deposited during the Olympia non-glacial interval.
2. The next layer is fine sand and small rocks deposited as glacial toe silt during the Fraser Glaciation period.
3. The next layer is larger boulders lain down during the post glacial flooding period after the Fraser Glaciation period.
4. The top layer is soil and humus.

What will this cliff look like twenty years from now?

      Twenty years from now there will be increased slumping as the cliff continues to erode at the undercut. If the organic material and soils slump down to the base, there will be increased vegetation at the base in the form of smaller shrubs and other coloniser plants. Boulder debris will accumulate as gravity pulls down the boulders deposited during the flooding period. As the mass wasting accumulates, and the new vegetation takes hold, the cliff will eventually look like a rounded hill.

Geologic History of Lynn Canyon:

      The second glacial period, the Semiahmoo glaciation occurred more than fifty thousand years B.P. during the middle Wisconsin glaciation period (Maynard 10). There is evidence of Semiahmoo till deposits at Lynn Valley at the base of the peat wall discussed in stop three. The Semiahmoo was followed by the Olympia inter-glacial period.

      The Olympia inter-glacial period occurred between thirty to sixty thousand years B.P. during the middle Wisconsin, lasting about 30,000 years (Maynard 10). During this period, the climate was warm and wet as evidenced by the Cowichan Head Formation of the peat bogs (Maynard 10). The peat layers found at stop three formed from peat bogs which form in wet anaerobic conditions where there is not enough oxygen for bacteria to completely decompose organic material. There is evidence of Olympia clay silt deposited at the base of the cliff wall in stop seven. The clay layer is darker in colour because the particles are fine and they retain water due to increased surface area and water tension. This warm period was short and followed by the next period of glaciation.

      The third, and most recent, period of glaciation the Fraser Glaciation, occurred approximately twenty to ten thousand years B.P. during the late Wisconsin (Maynard 10). During this period it is most likely that the wide U-shaped valley of the Lynn Valley flood plain was formed (stop one, stop five) and the erratic boulders were deposited (stop two, stop six) as the glacier moved to the Pacific Ocean. Other evidence of the passage of this glacier include the till deposits found at stop three under the boardwalk in the peat wall above the layer of peat and the layers of coarse till found in the cliff wall above the fine clay silt in stop seven. This most recent period of glaciation carved the Coast Range landscape to the present form and shape.

      The final shaping of the Coast Range occurred during the third inter- glacial period, the post Fraser Glaciation period from approximately ten thousand years to present day during the Holocene epoch (Maynard 10). The climate warmed, and the Fraser Glacier retreated. The meltwaters carried alluvium into the Lynn Valley eroding the valley floor and transporting boulders and sediment. Evidence of the high level of the waters is found in Lynn Valley in the soil profiles at stop three (peat wall) and stop seven (cliff wall). The large rounded boulders are mixed in the soil profile below the tree root level. There are large round smooth boulders deposited along the flood plain and up at higher elevations around the box canyon. The water levels rose ten metres forming the Fraser River Delta and the fjord coastline that is presently familiar.

      The Coast Range that formed a hundred million years ago has undergone the cyclic advance and retreat and glaciation, part of the Earth’s long term cycle of cooling and warming periods. The Coast Range is a relatively young range in terms of Earth’s geomorphic history. The retreat of the last period of glaciation and the alluvial erosion of the post Fraser period has exposed a geomorphic time record. The igneous dike in the box canyon is from the period of volcanic formation. The region is above a subduction zone of converging plates. Associated with subduction zones is earthquakes and fault lines. The erosion of the canyon granitic base rock most likely lies along a fault crack in the bedrock.

      Much of the history of Lynn Canyon has been altered due to stratification inversion and erosion.

      Piecing together the history of the Lynn Valley is like putting together the pieces of a puzzle that have been all shook up and spread out haphazard across the playing field.

Table 1: Geologic History of Lynn Canyon

Armstrong, Dr. John E. Vancouver Geology. Vancouver, B.C. Geological Association of Canada, 1990.

Christopherson, Robert W. Geosystems : An Introduction to Physical Geography. 2nd ed. New York: MacMillan, 1994.

Maynard, Denny. "Guidebook for Geologic Field Trips in the Lynn Canyon- Seymour Area of North Vancouver." Vancouver, B.C. Department of Geological Sciences, University of British Columbia, 1977.

Wright, Judith. "Lynn Canyon Field Trip Assignment".Langara College. Vancouver,B.C. 1986.

Stop One:The Canyon

Stop One

---

Stop Two:Canyon Base and Creek Flood Plain

Stop Two

---

Stop Three:Under the Boardwalk

Stop Three

---

Stop Four:Lynn Creek Channel

Stop Four

---

Stop Five:Lynn Creek Braided Channel

Stop Five:

---

Stop Six:Erratic Boulder

Stop Six

---

Stop Seven:Cliff

Stop Seven

---

Return to Index at top of Tour

---

---

Appendix III

Sketch Gallery

Stop One:The Canyon Sketch

Stop One

---

Stop Two:Canyon Base and Creek Flood Plain Sketch

Stop Two

---

Stop Three:Under the Boardwalk Sketch

Stop Three

---

Stop Four:Lynn Creek Channel Sketch

Stop Four

---

Stop Five:Lynn Creek Braided Channel Sketch

Stop Five

---

Stop Six:Erratic Boulder Sketch

Stop Six

---

Stop Seven:Cliff Sketch

Stop Seven

---

Return to Index at top of Tour

---

email Waterose

Please Sign My Guestbook
Please View My Guestbook

Search this site powered by FreeFind

Site Map What's New Search

---

Articles	Projects	Resume	Cartoons	Windsurfing	Paintings	Album