Symmetry in Glaciated Landscape
In Southeast Alaska, adjacent basins show drainage patterns that
are approximate mirror images. Corresponding portions of paired
basins are equal or proportional in planar area.
These “twin” basins share a common structure:
- Bilaterally symmetrical basins
are separated by a ridge
extending from a mountain mass
which
- forms the upper boundary of the
basins,
- each of which is in turn divided
by a similar ridge.
Twin drainage basins from different lithic terranes are similar in
shape and size. The most perfectly equal and regular patterns face southwest and run at
approximately right angles to local fault patterns and across terrane boundaries.
Top to bottom:
San Alberto Bay, Prince of Wales Island, Alexander Terrane 50k wide
Rowan Bay/Bay of Pillars, Kuiu Island, Alexander Terrane 25k wide
Nakwasina/Katlian Bay, Baranof Island, Chugach Terrain 30k wide
Hood Bay, Admiralty Island, Alexander Terrane 17k wide
What percent of the Landscape Shows Symmetry?
Dr. Ronald Shreve (letter: April, 2000)
“At a minimum you should determine what fraction of all the basins in a certain
size range in a defined reasonably large area are members of “pairs.”
Prince of Wales Island
(193km x 65km)
(USGS geologic map)
• Variations on
the same
basic bilateral
theme.
• The same
process
formed all.
• Entire
landscape
formed by
this process.
Examination of Prince of Wales Island (193km x 65km)
reveals that the island is organized into four symmetrical
basins, and shows that the phenomenon applies to the entire
landscape. Faults are paralell to the boundaries of these
basins, the upper margins in particular.
Prince of Wales Island faults (black) are parallel with upper drainage basin boundaries
(red).
What Causes Symmetry?
Cross Section Glacial Channel
L. Reynaud, 1973 J. Glaciology
- Shear forces are symmetrically distributed.
- Maxima are found near the edges of the channel.
- These maxima are at least twice that found in the center.
- If it takes a threshold amount of force to drive erosion,
that threshold will first be met on the outside of the channel.
- Two troughs will be created, with a ridge between them.
Once a ridge is established, two smaller troughs may form in each of the original troughs.
It seems that 4 channels is the most that are ever needed to do the required work,
otherwise each of these 4 troughs could divide again.
All Cirques (hundreds observed?)
show bi-laterally Symmetrical Drainage.
Look at areas of the world that were “never” glaciated to see if this pattern was
present. However, who knows what wasn’t glaciated?
Instead, look at places that were “always” glaciated. Cirques have had nothing but ice
in them.
All criques showed symmetrical drainage patterns.
Cirque
near Haines, Alaska
Observations of symmetrical drainage patterns in cirques, often miniatures of patterns
that are hundreds of square kilometers larger, support the conclusion that this bi-laterally
symmetrical drainage pattern is formed by erosion of bedrock under a lobe of flowing ice.
Cirque near Haines, Alaska, 3k
wide
Hood Bay, Admiralty Island, Alaska,
17k
wide
Symmetrical Branching Records?:
•
Climatic condition
of sufficient duration to erode the
bedrock.
•
Footprint or scar
formed at a glacial terminus…
• Erosion event may be recorded in sediments
on the continental margins.
Grindstone Creek and Rhine Creek…
about 8 miles south of Juneau,
Alaska
New Paradigm
for Glacial Erosion
- A uniform symmetrical pattern indicates that
basins were formed two and four at-a-time
by some over-arching process.
- That process appears to be sub glacial
erosion.
The Old Paradigm
Flashback… 202 years ago: Playfair’s Law
A. J. Playfair. Illustrations of the Huttonian Theory
of the Earth, William Creech, Edinburgh,
1802.
“Every river appears to consist of a main
trunk, fed from a variety of branches, each
running in a valley proportioned to its size,
and all of them together forming a system
of vallies, communicating with one another,
and having such a nice adjustment of
their declivities, that none of them join the
principal valley, either on too high or too
low a level;
a circumstance which would
be infinitely improbable, if each of these
vallies were not the work of the stream that
flows in it (p. 102).”
Note: Playfair’s first observation.. that “vallies” form a system “communicating with one
another” is correct.
His assertion that “vallies” are “the work of the stream that flows in” them, reflects his
friend James Hutton’s ideas of Uniformitarianism…. that currently operating processes
are responsible for the landscape we see. This idea is 202 years old this year…………
though Dury (1964) showed that most rivers are “underfit”.
In southeastern Alaska, the granite that we see now, miles in the air, formed about 50
million years ago in-place, but some number of miles underground. Therefore, if we
can see the granite, miles of the original bedrock overburden have been removed.
What did the original surface look like? Were there ever antecedent river valleys? How
can anybody know?
It is evidence for a terrific amount of erosion in a relatively short time. Materials
removed are no longer anywhere nearby. This is the kind of thing glaciers do well.
……………………………………………………………………..
References
Connor, C and D. O’Haire, 1988. The Roadside Geology of Alaska, Mountain Press
Publishing, Missoula, Montana.
Dury, 1964. Principles of underfit streams. US Geol. Survey Prof. Paper 452A.
Gherels, G. and Berg, H., 1992. Geologic Map of Southeastern Alaska. USGS,
Reston, VA.
Plafker, G., L.M. Gilpin, and J.C. Lahr, 1986. Plate 12: Neotectontic Map of Alaska. in
Geology of North America Vol.
G-1, Geological Society of North America, Boulder, Colorado.
Playfair, J., 1802. Illustrations of the Huttonian Theoryof the Earth, William Creech,
Edinburgh.
Reynaud, L., 1963. Flow of a Valley Glacier with a Solid Friction Law., Journal of
Glaciology, Vol. 12, No.65. p257.
Shreve, R. L., 2000. Letter reviewing some of these demonstrations of landscape
symmetry.
Shreve, R. L., 1969. Stream Lengths and Basin Areas in topologically random channel
networks.
Journal of Geology. V3, p527-529.
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Thankyou
Gunalchéesh Geological Society of America.
It was an honor to be with you in Boise.
Thanks to to Cathy Connor, Robert Anderson,
James Kirchner, Bruce Molnia, Ronald Shreve,
Luna Leopold, David Rosgen, and everyone else
that has listened and offered criticism and encouragement.
Thanks to Gary Fisher and the US Forest Service Tongass Forest GIS
for Prince of Wales Island mapping and analysis.