MadSci Network: Earth Sciences

Re: Arches

Date: Thu Apr 9 10:16:30 1998
Posted By: Robert Chesson, Other (pls. specify below), Working Geologist (Certified Profssional Geologist), FEC
Area of science: Earth Sciences
ID: 891452088.Es

Contrary to popular belief, the natural sandstone arches found in the 
southwestern United States (such as Arches NP) are not entirely the result 
of wind erosion by wind-blown sand.  Their formation has more to due with 
differential weathering of the rocks (sandstones) by water and the effects 
of freeze-thaw cycles than erosion by wind-blown sand.

Sandstone rock is made up of small grains of sand-sized rock (usually 
quartz sand but, it can also be other rock material) that are cemented 
together by another rock material (such as silica, iron oxide, or calcium 
carbonate).  It is common to have varying degrees of cementation within any 
mass of rock; that is,  some rock will be “harder” and some will be 
“softer."   The softer rock will tend to be easier to erode by water than 
the harder rock.  The differential weathering between the hard and soft 
rock over long periods of time can eventually  lead to creating alcoves 
(caves) in a rock face in the softer rock.  These caves may  eventually 
develop into the natural stone arches common at Arches National Park. 

Along with weathering by water, the desert regions of the southwest United 
States also experience a high number of days of freeze-thaw cycles.  
Freeze-thaw cycles are the daily freezing on moisture at

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