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Swish, swish, swish go the cars as they pass me on the National Park roadways. More
than a million and a half people visit the Canyonlands of southeastern Utah each year
to marvel at rocks, mostly sandstone rocks. Most of us that visit this dry and thirsty
land have rocks where we come from, but here the rock displays are over the top and
almost beyond belief.
These are sedimentary rocks, most of them sandstones or their close kin. Sand, the
basic ingredient of sandstone, is both a mineral and a size class. Typically sand
is made of pieces of silica from the size of a pinhead to tiny pieces that are gritty
when you rub your fingernail across them. In lakes and oceans sand, silt and clay
settles out at different rates and in different places in the water column. Larger
sand grains fall out first, then the smaller silt sized particles settle next to create
siltstone. The last to precipitate are tiny clay particles which are the final weathering
product of metamorphic rocks. These accumulate in deeper water and turn into shale
deposits when they become rock.
Turning a loose assemblage of particles into rock is a process known as lithification.
It usually involves the percolation of some cementing agent into the matrix such as
calcite (calcium carbonate) or something to firm up the matrix. And then a long steady
application of pressure and sometimes heat.
Sandstones, siltstones, mudstones and shales can be tinted various colors by infusion
of minerals such as hematite that turn the rocks red or limonite that turns them yellow
or tan. Blends of the two forms of iron oxide can create an array of colors in the
rocks. Near-white sandstone is pure silica sand that accumulates on a beach or originated
as a sand dune that became frozen in place when buried by more sediments.
The deposition of these sedimentary rocks has been going on for billions of years
but most of the exposed rocks here are in the 200 to 250 million year old range. This
part of the American west, not unlike Arkansas, spent a lot of time as a part of a
shallow sea that was surrounded by uplifted mountains. These ancient mountain systems
wore down, filling the seas with their sediments, building thicker and thicker layers
of sedimentary rocks. Tectonic forces were frequently, in a geologic sense, lifting
the land out of the sea only to submerge it again.
This portion of Canyonland was lifted out of the water for the last time about 60
million years ago. It is included as a part of a big, thick chunk of land called the
Colorado Plateau - land in western Colorado, southeastern Utah, northeastern Arizona
and northwestern New Mexico - that mostly all drains into the Colorado River. When
the Rocky Mountains began forming about 70 million years ago, the Colorado Plateau
lands were mostly unaffected because of the thick depth of the basement rocks.
From where I write this in The Island in the Sky, I’m on a plateau surrounded by red,
thousand foot sandstone and siltstone cliffs on all sides that drop away to broad
valleys formed in softer shale and mudstone deposits. In these softer rocks the Green
River (to the west) and the Colorado (to the east) has cut a thousand foot deep trench
in the valley floor to reach their current levels. They join up about 20 miles further
south and begin the process of cutting the mile deep trench we call the Grand Canyon.
Surprisingly, all of this erosion is thought to have occurred only in the past six
The amazing thing to consider is that most of us live atop thousands and thousands
of feet of sedimentary rocks. In many places these deposits are five miles thick,
but they haven’t been eroded away — yet. Something to ponder as another magnificent
display of rock art flashes by.
Gerald Klingaman is a retired Arkansas Extension Horticulturist and retired Operations
Director for the Botanical Garden of the Ozarks. After more than two decades of penning the popular Plant of the Week column, he’s taking a new direction, offering views on nature as he pokes about the
state and nation. Views and opinions reflect those of the author and are not those
of the University of Arkansas System Division of Agriculture. If you have questions
or comments for Dr. Klingaman about these articles contact him at email@example.com.