One of the things that makes our mountains so interesting is the variety of material they're made of—rock that tells a story. Basalt, andesite, or other igneous types mark the sites of ancient (or not so ancient) volcanic activity. Equally as interesting, though, are the layers of sedimentary rocks displayed in so many of our desert ranges. Although most of these were laid down in marine waters, local forces currently active illuminate many of the principles.
High-energy waters rushing down arroyos deposit a mixture of gravel and
sand. Cement it together, and we have a conglomerate. As the water slows on gentler
slopes, the larger stones can no longer be transported. Now carrying sand and finer
particles, the arroyo flood, reaching leveler ground, becomes increasingly sluggish,
and drops the sand to become the progenitor of sandstone. As the water pools on the
valley floor, the fine suspended particles slowly settle to form sheets of
clay—tomorrow's shale. Substitute wave action and rivers entering the sea, and
it's not hard to read the history frozen in our desert mountains.
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.