Delicate Arch, Arches National Park, Moab, Utah

   Delicate Arch is one of over 2000 arches in Arches National Park.  The arches are made of sandstone, a rock composed of rock and mineral grains cemented together.  But not all sandstones are alike - their cementing agents can vary.  

 

   The base of Delicate Arch is formed of the Entrada sandstone that is cemented together with calcium carbonate, CaCO3.  The arch is capped with the more erosion-resistant Curtis Formation sandstone that is cemented with dolomite, CaMg(C)3)2.  Nonetheless, all sandstones are susceptible to weathering.  Rain water, charged with carbon dioxide, forms weak carbonic acid the effectively dissolves the CaCO3 cement of the Entrada quartz sandstone.  When the cement dissolves, the individual grains accumulate and blow around to carve the cliffs, hoodoos and arches.  

 

   The Entrada sandstone is mostly clear quartz while the more varied Curtis formation features orange, gray-orange or light blue-gray grains and grades from medium-coarse sandstone to calcareous green-gray mudstone seen in this sample.

Lisa Zolly, USGS. Public domain.

USGS public domain