1. Glaciofluvial Infilling and Scour of the Puget Lowland
By: Alexander Pacubas
*Presentation based on paper by Derek B. Booth (1994)

2. Pleistocene/Fraser Glaciation
Advancement of the Puget Lobe of the Cordilleran Ice Sheet During The Vashon Stade
The Puget Lowland has well-preserved records of glacial sediment deposition
Below an altitude of 1000m, almost the entire land surface of the Lowland was shaped by glacier processes

3. Vashon Stade
The most recent glacier advance period or the last Glacial Maximum (15ka)
Much of the sediment deposition in the Lowland is from glacial advance
The Advancing Ice blocked the Strait of Juan de Fuca

4. The Blocking the Strait
Aggradation farther South of the Strait of Juan de Fuca would have been limited
Much of the sediment supply would have been emptied there
Once blocked, sediment build up became easier in the Puget Lowland

5. “The Great Lowland Fill”
Sediment accumulated in Ice Dammed Lakes
Two main members:
1. The Lawton Clay Member
2. Esperance Sand Member
Outwash would have prograded as deltas
significant sediment build-up during the glacial advance created an extensive outwash plain AKA “The Great Lowland Fill”

6. Continued Advance
The advancing ice overridden the outwash sediment and deposited basal till
From dating the basal till, the outwash of the advancing ice-sheet must have finished prior to ka
The Advance sediment consists of the clays, outwash and overlying till

7. Puget Lowland Topography
Elevations of most of the topographic features in the Puget Lowland is determined by thickness of the advance outwash
It is possible to reconstruct the surface of the outwash sediment

8. Basin fill represents the lower topography in the foreground ( m above sea level). The Olympic Mountains are seen the background. Image taken from Whidbey Island, WA looking west.

9. Reconstructing the Proglacial Fill
Plotting location and altitudes of hilltops underlain by glacial sediment -> Develop Smooth topographic map
Typical elevations of m
Outwash deposit averaged to 100m thick

10. Drainage out the South end of the Puget Lowland into Chehalis River
Blocked drainage out of the Strait
*The slope of the drainage is not corrected for isostatic rebound. Depression occurred after outwash deposition.

11. Notable Morphologies
The Eastern and Western Margins are concave
Topographic Relief on the Eastern Side due to drainage from high elevation Cascade Range

12. Notable Morphologies
The Southwest Corner has a broad southeast sloping plain of till-mantled outwash
The Maximum elevation (+180m) extends along and beyond the Hood Canal trough in the Southwest
These two characteristics infer a strong erosional process

13. Trough Formation Sub-parallel alignment
Extend to depths up to 400m below the glaciated uplands
Thought to have been formed by ice tongues or the subaerial channel erosion. Both reasoning are IMPOSSIBLE.

14. Trough Formation
Ice tongues would have no bed contact since proglacial lakes would have floated them
Subaerial channels could not erode the deepest trough, which run 300m below the basin outlet
SUBGLACIAL MELTWATER IS THE ANSWER
*The troughs here match dimensions and studies of troughs in New York, Ontario, Nova Scotia, and Germany.

15. Trough Formation Erosion into the Lowland fill occurred:
After advance and outwash deposition
Before subaerial exposure of the glacier bed by ice recession
Most of the troughs are several times deeper than the original fill
Most of the eroded sediment was pre-Vashon age
-Much of the troughs are mantled with basal till

16. Erosion Volume
The difference in altitude between model surface and modern surface Calculated the volume of sediment eroded to be about 1000 km3 or 100 m of ice-occupied area *Trough Erosion accounts for well over three-fourths of this volume

17. Erosion Rate
The magnitude of erosion must be very large for such deep erosion
Glacial meltwater is notorious for having high sediment content
m3/m3 (sediment/water concentration)
Median suspended load of 5000 mg/L
Avg. water discharge 3000 m3/yr
Transport rate of 0.2 km3/yr
*These are estimates

18. Erosion Power
To erode 1000 km3 at a rate of 0.2 km3/yr would take 5,000 years!
This estimate is about twice the available time
Sediment concentrations must be double what is estimated here!
Sediment may have came from catastrophic discharges from elsewhere

19. Conclusion
Although direct ice interaction may have stronger geomorphic impact, much of the formation of the Puget Lowland was caused by fluvial processes.

20. Any Questions?

21. Image Source: https://en.wikipedia.org/wiki/Glacier