View to the
north with contour intervals of 100 feet. The La Plata
Mountains are in the upper right portion of the picture.
Durango, Colorado is near the right edge. Near the left edge
Mancos, Weber, and East Canyons merge to form the main Mancos
River, and a piece of Mesa Verde National Park is visible on
the left edge.
The source of the La
Plata River is at the northern end of the La Plata Mountains
and then it splits the range lengthwise as it flows southward.
Bridge Timber Mountain shows up as a faint fuzzy label just
above the lower right edge. Bridge Timber Mountain is more
than 1,000 feet higher than any other point of land within 25
miles both to the east and west of it including the large flat
area shown in the picture. Its summit also has river gravels
including boulders up to 6 feet long. How and when did they
This area to the west and southwest of Durango
has one of the most complex histories of any area in the
Rockies. There was an initial uplift of the La Plata Mountains
due to igneous intrusions some 65 to 70 million years ago.
When the San Juan Basin (to the south and southeast off the
lower edge of the picture) rebounded in the Eocene, the
ancestral San Juan River was born and flowed from lower right
to upper left across this area. (See the
) Then, regional uplift about the end of the
Oligocene forced the San Juan River south into New Mexico. The
uplift also changed the tilt across most of the view area from
down-to-the-north to down-to-the-south. This set up the
drainage for the Mancos River system.
The sequence that we propose in this model
attributes the Bridge Timber gravel deposits to remnants of
the San Juan/Animas/La Plata River system that prevailed about
the time the San Juan abandoned this ancient route some 25 to
30 million years ago.
The currently published model for these
deposits credits them to an outwash fan by the La Plata River
as it washed down from the La Plata Mountains. The age of the
deposits via this alternate model would be within the last 5
In either case there was once a river that
crossed the top of Bridge Timber Mountain. Then all the
surrounding terrain was worn down to what we see now. A key
question is how long would it take to erode over 1,000 feet
from the terrain on all sides of the mountain, and then leave
the flat surface stretching over 20 miles to the west (to the
left of the mountain)? We leave it to future research to
settle the issue.
Also please see “Bridge Timber Mountain and the
Ancestral San Juan River”
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