Hi Jberry,
I know exactly what you're talking about. They almost look like
hiking trails on the hillsides, but they occur in a branching pattern
and cover the entire hillside. They are known as "terracettes," and
indicate a geologic process known as soil creep. Terracettes are
accentuated by animals walking over them and grazing. Here is a
website with a picture from Southern Australia and another from
Mission Pass in California showing the lines you're talking about.
(This is a HUGE page because of all the pictures. It's over 1.5Meg,
so be prepared...after the page loads, scroll about 3/4 of the way
down or search for "Mission Pass"):
http://www.ngdc.noaa.gov/seg/image/geohazards_v3/document/647006.htm
*I see that the author of the web page called the ridges "laterial
lines." I'm certain that this a typo, reproduced by copying text
during the creation of the page. I did a pretty thorough search and
couldn't substantiate this as a geologic term.*
Some more websites with info on terracettes:
http://lois.co.uk/pasture/geology.shtml
(Scroll to bottom of page)
___________________________________________
Another HUGE page...(what are people thinking when they do this!? 7.5
Megs and 35 pictures later)...but with a couple of nice diagrams of
terracettes:
http://www.geology.ohio-state.edu/~vonfrese/gs100/lect18/
(1/4 of the way down page, search for "terracettes") Only go to this
site with broadband. I've got cable access and it still took about 5
minutes for the page to load.
___________________________________________
The author of this site maintains that they are not caused by animals
but only by soil creep:
http://home.uleth.ca/vft/lethbridge/north.html
___________________________________________
http://www.ahs.cqu.edu.au/humanities/geography/GEOG11023/week_5_textbook.htm
"Human activities often accelerate mass wasting processes.
Over-grazing on steep slopes in seasonally wet climatic areas readily
leads to the creation of terracettes."
___________________________________________
So, the stories you've heard about erosion and grazing are true,
however the erosion process is not wind-related, but rather gravity
and moisture related.
On any hillside, the top layer or upper "soil horizon" will creep
downward in response to gravity, and in the clay rich soils of the
rainy Coast Ranges, this downward motion results in "rippling" of the
moist soils, and the ridges you've noticed. Viola! Terracettes.
This process occurs with or without grazing, but the removal of
grasses and their stabilizing roots weakens the cohesion properties of
the soil and accelerates the creeping motion, *possibly* resulting in
a greater number of ridges in a given area. The heavy rains that come
off the Pacific add significant moisture to the clayey soils, and you
can almost "sense" how the upper soil layer kind of "squeaks" down and
bunches up.
I found a study by a grad student at UC Davis in the Geography
department that addresses issues of hillside stability and the effects
of logging. A particular chapter addresses the issues of vegetation
removal on slope stability. Grazing by cattle is a significant
compacting/vegetation removal mechanism, and parallels can be drawn
between the information on logging in the report and grazing. I've
copied a little of the chapter below:
Logging Impacts on Jackass Creek; Mendocino, California
http://bss.sfsu.edu/jdavis/geog810/papers/s01/bill'spdf_joint.pdf
Vegetation & Hillside Stability
"Vegetation on steep, unstable slopes is crucial in limiting slope
failure and increased erosion. Once vegetation has been removed,
exposed soils are highly susceptible to erosional processes. Not only
does timber harvest, particularly clear cutting, remove protective
cover, but it compacts soils, reducing infiltration capacity.
According to Gray (1970), vegetation acts to preserve hillside
integrity through mechanical reinforcement from the root system and
modification of soil moisture distribution and pore water pressures.
"In addition to soil erosion, broad expanses of shallow water moving
downslope, or infiltration-excess overland flow (Clark 1990), is
dramatically increased by soil compaction. Johnson and Beschta (1980)
attribute the majority of this compaction to tractor logging, tractor
windrowing of slash, and burning of slash, all common logging
practices. Once vegetation is removed from a hillside, mass wasting is
much more likely to occur. Mass wasting is spontaneous downward
movement of rock and soil (Clark 1990). This can include small
movement such as creep, or large movements such as mud/earthflows,
slumps, and landslides. These types of movement can deliver large
amounts of sediment directly or indirectly into streams and creeks.
This sedimentation
effectively alters channel morphology once it is introduced to the
stream."
I also wrote to geology professor Andre Lehre at Humboldt State
University who studies geomorphology, hydrology and hillslope
processes to request information. Following is Professor Lehre's
response:
"If we are thinking of the same thing, path-like features that run
approximately along contour --they are called "terracettes" and are
created and maintained primarily by trampling by cows, sheep, goats,
and perhaps deer. I have noticed them most prominently on grassy
hillsides in Marin and Sonoma Co. They don't necessarily reflect
"soil damage" but are rather a natural response to having animals walk
regularly along soil that is plastic when wet.
A brief description of terracettes, along with some references, is
given in M.J. Selby, Hillslope Materials and Processes (Oxford, 1993),
p. 258."
So Professor Lehre feels they are only due to animals, however it
seems to me that there must be the addtional issue of down-slope
transport to account for the proliferation of these features.
Hope this answers your question and lightens your bucket a little!
Kutsavi
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