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Desert Diary
Fossils/River Terraces


Drive along the floodplain of Southwestern river valleys and you may notice a series of sedimentary benches or terraces rising one after another above the valley floor. Such terraces can be formed by two different processes. Untamed rivers, such as the Rio Grande before upstream dams, meander back and forth across the floodplain. During times of steady down-cutting by the river, meanders may excavate the floor on one side of the valley while leaving the opposite side untouched. This results in sporadic terraces standing above the current floodplain.

There's another source of river terraces, though. Geologists sometimes call them paired terraces, because they tend to come in matched pairs on opposite sides of the valley. They tend to extend long distances up and down the valley. From a stable or rising river bed, a change in climate produces a faster flowing river that quickly entrenches, leaving the old floodplain high and dry. A sharp eye in El Paso will pick out three or four such terraces. Rivers, like people, tend to have their ups and downs.
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Listen to the Audio (mp3 format) as recorded by KTEP, Public Radio for the Southwest.


Contributor: Arthur H. Harris, Laboratory for Environmental Biology, Centennial Museum, University of Texas at El Paso.

Desert Diary is a joint production of the Centennial Museum and KTEP National Public Radio at the University of Texas at El Paso.