Make your own free website on Tripod.com

Monument Valley

Home
Geology
Cultural History
Way Cool Facts

How Was It Made? The Geological History of Monument Valley

ut_mvalley.jpg
Regional map of Monument Valley (8)

   Monument Valley is an interesting geological region that is situated on the Colorado Plateau, a region that was first formed during the Permian era about 270 million years ago.  As long as 2 billion years ago, this area was underwater as part of an ancient sea and by the end of the Permian era, approximately 160 million years ago, what is now Monument Valley was part of the ocean floor (1).  Over the millenia this ocean floor emerged and resubmerged and it was through these geological processes that the many different types of rock were formed.  All three types of rock, sedimentary, igneous, and metamorphic, are present in Monument Valley.  The most predominant types of rock present are red sandstone and shale.  These rocks made up the sediments of the ancient ocean floor and through the numerous uplifts and erosion in the region, they now make up the landforms that Monument Valley is so well-known for.
     Monument Valley is situated on top of an anticline known as the Monument Upwarp.  An anticline is an upward-arching fold, somewhat like a ridge, however, unlike a ridge an anticline forces the oldest rock up towards the centre and the younger rock that originally constituted the upper layers of sediments downwards on what is known as the
synclinal hinge (2).  It was through this process that the ancient sandstone and shale sediments were brought to the surface.  After this uplift had occurred, volcanic activity took place along the surrounding area, which allowed molten lava to rise from deep underground and create igneous rocks (3).  Through erosion and weathering that took place slowly over the millenia, the landforms that now make Monument Valley famous were formed.
     The landforms that Monument Valley is best known for are the massive red sandstone buttes and spires.  These landforms began as part of the layered sediments of the ancient ocean floor.  After the uplift occurred, this broad layer of sediment was affected by erosion and over time this layer was broken down into mesas.  As more time passed, these mesas formed into buttes, and some of these eventually became spires.  As more and more time passes, these spires will eventually erode into nothing (4).  These landforms are made of sandstone and shale.  Both of these types of rock are sedimentary, meaning that they began as small grains and were cemented together due to pressure in a process known as lithification.  As the tectonic uplift occurred, vertical joints formed in the sedimentary layers.  These joints are widely spaced and run through a thick layer of sediment, therefore, when one part of the original sedimentary bed collapses, huge blocks of sediment are still left (this is how the original layer formed into mesas, then to buttes, and then to spires) (5). Sandstone is a much harder rock than the shale, and through erosion, the softer shale layers were worn off of
the sandstone layers, which now constitutes the landforms that are present in Monument Valley. 
  
 

One of the buttes Monument Valley is famous for
mvbutte2.jpg
You can see the different layers of rock present in the top, middle, and bottom layers (9)

     As previously mentioned, all the rock present in Monument Valley's various landforms dates back to the Permian era.  The two main kinds of rock present in these buttes and spires are sandstone and shale, however, these types of rocks can be broken down further.  In the picture above, three layers can be seen in the butte: the top, middle, and bottom.  The top layer is called a caprock.  This layer is made of a harder shale known as Red Organ Rock shale.  This caprock is more resistant to erosion, due to its being a harder rock than the ones below, and this allows for a slower weathering process because it protects the softer layers underneath.  The middle layer of the butte is made of a type of sandstone known as Cedar Mesa sandstone.  This sandstone is also quite hard and it has many strong vertical joints that hold the layers together.  The bottom layer, also known as the pediment, is made of Halgaito shale (6).  This shale is softer than the layers of rock above it, however, the pediment is the largest part of the butte because that is where the eroded material from above accumulates.
     Another prominant type of rock in Monument Valley is igneous rock.  As uplift occurred, molten rocks surfaced from underneath, however, the only landforms that remain from these ancient volcanoes are the igneous dikes, which are the hardened cores of the volcanoes.  The most well-known volcanic monolith in Monument Valley is El Capitan which rises approximately 1500 feet in the air towards the south end of the Monument Upwarp (7).
     Monument Valley is a geologically fascinating region of the earth.  Its numerous landforms are a testament to both the earth's tectonic forces and the erosion and weathering processes as the different types of landforms present show how rock that started out as an ocean floor can become towering mesas, buttes, and spires, depending on how long the erosion process has been at work.  As the millenia continue to pass, these landforms will slowly disappear due to the same geological forces that created them, however, until that distant future, these landforms will continue to stand out as a breath-taking reminder to the power of the earth and its forces.

tworocks_buttes.jpg
Distant view of buttes rising out of the desert floor (10)

1. NState, Arizona: The Geography of Arizona, January 2006, <http://www.netstate.com/states/geography/az_geography.htm> (5 January 2006).
 
2. Charles C. Plummer, et al.,  Physical Geography and The Environment.  1st ed.  (Toronto: McGraw-Hill Ryerson, 2004).
 
3.  NState, Arizona...
 
4.  Plummer et al., Physical Geography....
 
5.  Stephen Marshak, Earth: Portrait of a Planet (New York:  W.W. Norton & Company, 2001).
 
6.  NState, Arizona....
 
7.  Ibid.
 
8.  Go-Utah.com, Utah's Largest Travel Site, 2006, <http://www.go-utah.com/Monument-Valley-Area-Map> (5 January 2006).
 
9.  John Crossley, The American Southwest: Utah: Monument Valley, 2005, <http://www.americansouthwest.net/utah/monument_valley/index.html> (5 January 2006).
 
10.  Tom Phillips, Keyah Hozhoni Monument Valley Tours, 2005, <http://www.monumentvalley.com/Pages/english_tours.html> (30 December 2005).