View looking north
where Unaweep Canyon cuts across the Uncompahgre Plateau.
Contour intervals are 100 feet. Today there is no river in
this canyon. The high saddle point within the canyon (just
northwest of the Gill Creek label) forms a divide with local
drainage flowing down each side. However, up to about 7 or 8
million years ago, a large river flowed from east-northeast
(right) to west-southwest (left) through this canyon.
Gill Creek (see
label) is of interest as it uses an ancient tributary to this
ancestral river. For the first 2 miles of its path, Gill Creek
flows eastward following the downhill slope of the terrain.
When it gets to the local road, it does something strange.
Instead of continuing into La Fair Creek/Canyon, Gill Creek
turns uphill (toward the foreground), and then loops to the
west through Casto Reservoir. From there it continues
west-northwest (a combination of up strata/slope, across
strata/slope) to join West Creek which goes down the west side
of Unaweep Canyon. Actually Gill Creek is using a remnant of a
tributary stream system that was here before the Uncompahgre
Plateau started to rise some 10 to 15 million years ago.
(Note: Gill Creek’s path is a natural phenomenon. It is not a
product of a manmade diversion system.)
As mentioned above, the highest point
within Unaweep is just northwest of the Gill Creek label. West
of the high point, the gradient is only gradually downhill for
the first 15 miles indicating the ancestral river abandoned
this route before canyon cutting from the Grand Canyon event
had backed up this far. (Otherwise it would have rapidly cut
down through the west side of the Uncompahgre Plateau.) This
means the canyon had to be abandoned at least 4-5 million
years ago. On the other hand, the canyon walls appear to have
undergone little erosion which means it can't be very old. A
ballpark estimate would indicate that a river was here less
than 10 million years ago. Thus we get an estimated age of 7
(+/- 3) million years ago when the canyon was abandoned
Prior to 30 million years ago, the ancestral
Colorado River flowed north from Colorado into Wyoming. About
25 to 30 million years ago the Rabbit Ears Range rose in
northern Colorado blocking the former path of the ancestral
Colorado River. The river turned westward and established a
new route across this area. The Gunnison River joined the
ancestral Colorado on the east side of Unaweep Canyon just to
the northwest of Cactus Park (off the right edge of the
For the next 15 or 20 million years the rivers
maintained these routes. During the mid Miocene (about 10 to
15 million years ago), the Uncompahgre Plateau underwent an
uplift. For a while, the river tried to maintain this course
and dug Unaweep Canyon. However, it eventually found an easier
route around the north end where it is found today.
Gravel deposits indicate the Gunnison River
continued to flow through the canyon for a short while, but
before long, it too abandoned the route and turned more
northwesterly to join the Colorado. The Uncompahgre Plateau
continued to rise a few hundred feet more after these events
to reach its present height. The old abandoned canyon remains
as a reminder that at one time things were a little different
than they are today.
A (tentative and somewhat speculative) summary of
Unaweep Canyon’s history might look like:
About 25 to 30 million years ago the ancestral
Colorado turned westward and established a westward course
over the Uncompahgre Plateau. The plateau was thousands of
feet lower than it is today. The present East and West Creeks
mark this ancestral course. The Gunnison River flowed westward
from the current Black Canyon area, and then northwestward
across Cactus Park to join the Colorado River at the northwest
end of Cactus Park.
About 10 to 15 million years ago the Uncompahgre
Plateau began to rise. The rivers begin to cut down to form
Unaweep Canyon and Cactus Park.
About 7 to 10 million years ago the Colorado
River found an easier route around the north end of the
Uncompahgre Plateau. The Gunnison River continued to follow
through Cactus Park and Unaweep Canyon.
About 7 to 9 million years ago the Uncompahgre
Plateau was still rising and the Gunnison River abandoned
Unaweep Canyon. Canyons on the northeast side of the
Uncompahgre Plateau were entrenched. Hence, from the northwest
end of Cactus Park, the Gunnison flowed northeastward through
the Colorado's former canyon (the current lower half of East
Creek). The Gunnison eroded this route deeper before the
backup system reached this far.
About 5.4 to 7 million years ago. The backup
system from Utah's rising Wasatch reached the Unaweep/Cactus
Park area. Unaweep had risen high enough so that it stayed
high and dry. However, the Gunnison River filled Cactus Park
(and the lower East Creek Canyon) with thick gravel deposits.
About 4 to 6 million years ago, the Gunnison
abandoned Cactus Park. Sediment brought in from the east by
the Colorado and Gunnison Rivers forced the Gunnison to remain
on the west side of the Grand Valley. The Gunnison meandered
northwestward across the Grand Valley establishing its present
About 4 to 5 million years ago. The Colorado
River had started to cut the Grand Canyon. As the exit
elevation for the Colorado Plateau was lowered, canyon cutting
and generalized erosion worked back upstream. The two rivers
became entrenched in their current paths (including meanders
in the Gunnison to the east of Cactus Park).
About 4 million years ago to present. Grand
Valley eroded down to its present elevation. East Creek
removed the sediments and gravel deposits that were left in
the Colorado's path to the north and northeast of Cactus Park,
and then continued to erode downward to form its own canyon.
In Cactus Park, most but not all of the stream deposits left
by the Gunnison were also washed away by Gibbler Creek. In the
high areas of Unaweep Canyon erosion is very slow due to the
very hard Precambrian rock. For example, Gill Creek continues
to cut down but only at a very slow rate.
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