The La Plata Mountains as seen from above the authorís

Durango Bill's

Paleogeography (Historical Geology) Research

San Juan River - Goosenecks and Monument Upwarp, Utah

   Contour intervals are 100 feet. This view to the north shows the San Juan River near Mexican Hat, Utah with U. S. Route 163 crossing the river at Mexican Hat. The highest mesa at the top center of the picture is the Cedar Mesa portion of the Monument Upwarp. Structurally the upwarp continues across the river to include Douglas Mesa on the south side. Near the exact center of the picture there as a faintly visible side road (State Route 316) that goes west and then southwest from the main road and leads to the famous overlook at The Goosenecks.

A view of the
            Goosenecks in the San Juan River near Mexican Hat, Utah.

   Comb Ridge forms a north-south hogback near the right edge. At one time the hard Navajo Sandstone completely covered Cedar Mesa, but erosion has worn it down to the point where only the upturned edge is left. The result is a classic western hogback.

   Of significance to the geologic history of the area, river gravels are found on the high mesas to the northwest of the highway in the lower left quadrant of the picture. (These are due south of the highest portion of Douglas Mesa.)

   Finally, the San Juan River cuts through a small anticline to the southeast of the highway (right of center) to produce the upper portion of San Juan Canyon.
   The San Juan River established a route westward through this area about late Oligocene or early Miocene time when it was forced to abandon its earlier route toward the northwest across southwestern Colorado. Its original route through this area was somewhat south of its current path as it tried to find the lowest spot between the Monument Upwarp to the north and the Defiance Plateau to the south. The old gravel deposits on the south side of Douglas Mesa are remnants from this former route. After getting west of the Monument Upwarp, this early version of the San Juan turned north toward the Colorado River.

    When the rising Wasatch Ranges began to block the ancestral westward course of the Colorado River into western Utah, a large backup system began to form. Silt brought down by the San Juan covered everything up to at least the 6,300-foot level by 5.4 million years ago. After the Colorado overflowed across the Kaibab Plateau and started cutting the Grand Canyon, canyon cutting began to work its way back upstream. The San Juan at that time happened to be over the southern portion of the Monument Upwarp, and that is where it started cutting downward. The meanders are typical of any river traversing a silt plane, and they were preserved when the river started cutting straight downward. The meanders are particularly concentrated in this area upstream from the Monument Upwarp as the upwarp helped preserve the old silt layers. Finally, the silt backup system also covered the small anticline to the northeast of Mexican Hat and the river happened to be on top of it when it started cutting downward.

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