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
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
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
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 picture).
For the next 15 or 20 million years the rivers
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
Gravel deposits indicate the Gunnison River
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
Canyon’s history might look like:
About 25 to 30 million years ago the ancestral
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
began to rise. The rivers begin to cut down to form Unaweep Canyon and
About 7 to 10 million years ago the Colorado River
an easier route around the north end of the Uncompahgre Plateau. The
Gunnison River continued to follow through Cactus Park and Unaweep
About 7 to 9 million years ago the Uncompahgre
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
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
About 4 to 6 million years ago, the Gunnison
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 course.
About 4 to 5 million years ago. The Colorado River
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
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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