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There is a river near Lake City that is rather unusual. In some
places, it is over 1000 feet wide and 300 feet deep. Yet you can
stand in the middle of it and not get wet. . At just four miles
long, it is one of the shortest rivers around. It has a vertical
drop of 600 feet per mile, yet its fastest moving sections are
flowing only about eighteen feet per year. Still, it is
responsible for forming the second largest natural lake in
Colorado. None of this likely makes sense if you are still
thinking water. The most unusual thing about this river is that
it is made of flowing earth.
The Slumgullion earth flow
The Slumgullion earth flow, just a few miles east and south of
Lake City, starts near the top of Mesa Seco and ends at Lake San
Cristobal. Among earth flows, it is sort of a celebrity; its
photo (accompanied, unfortunately, by only a sentence or two of
explanation) appears in numerous geology textbooks. It not one
continuous flow, but is actually a series of three flows built
on top of each other. Because of its ease of access and unusual
nature, it has been studied by many geologists, particularly
over the last thirty years.
Radiocarbon dating of trees buried in the first of three flows
shows the initial flow occurred about 1200 years ago. This first
flow stopped short of damming the Lake Fork of the Gunnison. A
second earth flow occurred about 800 years ago damming the Lake
Fork of the Gunnison and causing the formation of Lake San
Cristobal. Both of those older flows are now inactive (contrary
to occasional lore, cabins built on the old parts of the slide
are not headed for a dunking in the lake). For the past 300
years, the active part of the slide has been flowing down from
the base of Mesa Seco over the top of the older slides.
Look out below?
Currently, the toe (bottom) of the two-and-a-half mile long
active slide is about 1/8 mile above the road where highway 149
crosses the older portion of the slide. While the fastest parts
of the earth flow move eighteen feet per year, recent studies
have shown the toe to be advancing about five feet per year. At
this rate, the highway won't be in jeopardy for another hundred
years or so. At some point though, the earth flow will likely
cause some interesting problems for the highway department. At
the same rate of advancement, the cabins near Lake San Cristobal
in the path below will not be in danger for a thousand years. Of
course, there have been three major flows in the past in about
as much time, so there is little accuracy in projecting that far
ahead.
Research on the flow
The Slumgullion earth flow is a natural laboratory for
geologists and a wide variety of research projects have been
conducted there in the past. Many of the studies relate to
mapping the movement of the flow. The old method of using a grid
of survey markers across the active and inactive parts of the
flow and measuring them periodically was time consuming and
required the assistance of many people. Recent studies with GPS
technologies have shown that a GPS unit placed on the slide
surface and left for a few hours will record a noticeable change
in position as the slide moves. (GPS is short for Global
Positioning System--by detecting the exact distance to any four
GPS satellites a GPS unit can calculate where on the Earth's
surface it is. Unlike my retail GPS which lets me know where I
am in the world within about fifty meters or so, the GPS units
available to government geologists in combination with some
powerful data processing computers allow detection of changes in
position of less than a centimeter!)
A current experiment is testing another method of monitoring
movement of the earth flow that involves using radar in small
aircraft. Radar reflectors (small, triangular-shaped aluminum
pieces) have been placed on the active and inactive parts of the
flow. By periodically flying over the flow with a small aircraft
and a special (but fairly inexpensive) radar, it is hoped
movements as little as one to two centimeters will be detected.
A recent project used the knowledge that sound travels at
different speeds through different materials to make a
three-dimensional map of what is under the surface of the slide.
By detonating charges at various places on the slide and
listening to the echo with sophisticated recording equipment,
the depth of the slide was measured. Hmmm ... detonating charges
on a landslide area ... I appreciate the knowledge gained, but
I'm not sorry I missed out on that one!
Another project recently conducted used parts of the Slumgullion
earth flow as a natural but miniature model of what is happening
at a much larger scale along the San Andreas Fault in
California. The earth flow has many places where the ground is
sliding past itself. These areas may give clues to help predict
or explain movements along major faults.
Explore the flow
The main reason I write this column is because I think too often
science is not accessible to non-scientists. Yet there are many
fascinating things of scientific interest all around us.
Technical scientific papers may contain interesting discoveries
but be difficult or very dry to read. If the purpose of science
is to explain the world around us, then why not make those
explanations a little more accessible? But another reason I
write this column is more selfish. Being dedicated to both a
career and a family makes it difficult to find time for pursuing
personal interests. Committing myself to a monthly column
provides a sort of obligated opportunity to keep up with the
field of science and further explore the wonders I see around
me. This "obligation" has led to many rewarding experiences. The
most recent was spending time with my brother exploring the
Slumgullion earth flow.
The drive down Slumgullion Pass towards Lake City on highway 149
is, in my opinion, one of the most scenic in the state. The view
from the overlook at Windy Point is spectacular, and includes a
glimpse of the earth flow. Nearly two miles further down the
highway we found an unmarked pullover that has an easy to miss
sign pointing out the earth flow. But the best view of the
Slumgullion earth flow comes after rounding the hairpin curve
(you can't miss it--or at least won't want to!), where the flow
appears as a yellow river carrying scattered trees below the
base of the Mesa Seco scarp (curved cliff) flowing right towards
you.
Where the road crosses the slide (just look for yellow, powdery
clay), a small pull off on the right offers a temporary parking
spot while exploring the slide. It is an eerie and fascinating
experience. Although the trees look familiar, the terrain on the
slide is very different from the surrounding aspen-spruce or
high meadow areas. As you climb onto the active slide, you will
notice that you sink slightly with each step in the fine, yellow
clay (old, weathered volcanic rock and ash that used to make up
a gentler Mesa Seco), leaving tracks reminiscent of those still
on the moon. Pick up some dry clay and you can funnel the
powdery dust from hand to hand. But get it wet, and it instantly
swells, becoming moldable enough to make a nice lump for
launching at your brother, should he happen to be exploring
nearby (fortunately for him, and perhaps for me, my aim has not
improved with age and the unnoticed missile lands harmlessly in
the powder). That this clay drastically changes properties when
wet is part of the reason the earth is moving slowly beneath
your feet here.
As you continue your climb, you will notice that the trees,
understandably, have a hard time growing straight up when they
are rooted in a migrating foundation, and seem to swagger every
which way. There is virtually no topsoil here, as the ground has
not been stable long enough to support its growth. When you find
a wet spot, look closely at the ground. Mineral salts have
crystallized on the surface forming a pretty yet eerie crust.
Looking around, you will notice that spring runoffs have carved
deep channels that are now dry. You might find a young aspen
tree growing out of a sucker root that is suspended like a
clothes line across a deep channel carved by erosion. No doubt,
the tree started growing before the channel formed.
Spending time in an area that seems so out of place yet
perfectly natural is a worthwhile experience. If you go for a
drive to look at the aspen leaves this fall, why not head to
Lake City? The Slumgullion earth flow is definitely worth a
look. |
