Tag Archives: Loess

Eolian Deposition

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Loess Caves in China

While studying a geography unit in grade school I was fascinated by the extensive deposits of loess in China.  Loess is fine wind-deposited soil.  In China, the loess covers a huge area in central China and is an important agricultural region.  The soil is fertile, easy to work, and when it erodes it can form steep cliffs.  People have carved homes into the cliffs for centuries, long enough to be included in geography books in western Colorado.  I lived in western Colorado cliff country, probably responsible for my interest in loess.

Loess is eolian soil, meaning it is wind deposited.  There is also wind deposited sand.  The Colorado National Monument, across the Colorado River from Fruita where I grew up has lots of eolian sandstone.  The cliff forming Wingate and the arch forming Entrada Sandstone are ancient sand dunes turned to stone.  My fascination for wind deposited cliffs comes naturally.

I now live east of the Rocky Mountains.  The mountains are tall and rugged, but are in the process of wearing away.  The Rockies have had several glacial periods.  Glaciers form, grind the mountains, then melt and leave their grinding as sand, silt, and gravel called glacial till. The more coarse debris often was carried in huge quantities onto the plains.  The  smaller particles, sand and silt, were blown onto the plains east of the eroding mountains.

The sand has created extensive sand hills on much of the plains in Colorado.  The sandy soil is thin and fragile, poor for farming, but fine for livestock grazing.  Interspersed among the sand hills are loess deposits now farmed extensively with water drawn from the Ogallala Aquifer in the Ogallala Sandstone, which was washed out of the mountains as the glaciers melted. The Ogallala extends from the mountains into Nebraska and south into the Texas Panhandle.

You may be aware the wind blows in Wyoming.  The entire atmosphere passes through Wyoming in any 24 hour period.  You have seen the Wyoming Wind Gauge?  It’s a logging chain hanging from a post.  I may exaggerate here, but not much.

While blowing, wind carries sand eastward out of the mountains. It also erodes the prairies in eastern Wyoming.  The sand ends up in the Western Nebraska Sand Hill country, covering over a quarter of the state.  The silt blew farther east, building loess deposits along the rivers.

In the dust bowl years in southeastern Colorado, northeastern New Mexico, southern Kansas, and the Texas and Oklahoma panhandles, the dust storms sent silt as far as Washington D.C.  Wind blown stuff is a big deal everywhere.  In North Africa, the sandy deserts are getting sandier as the silt is picked up and blows across the Atlantic Ocean, fertilizing the Amazon Basin.

In Denver, a car sitting outside for several days acquires a significant coat of wind blown dirt.  A couple of years ago, the dirt was red, meaning it originated in eastern Utah.   I notice my sidewalk is lower than the lawn by a couple of inches.  How much of it is organic accumulation and how much is eolian silt and sand?

I went for a long time thinking water is responsible for most of erosion.  It may be true, but don’t discount the wind, especially in Wyoming.

Posted in Eolian deposits, Geology, Rocky Mountains, Sand Hills, Stories, Wind

Dirty Denver

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Windstorm

Windstorm

The wind blows here in Denver, and it is not all air.  Those mountains just west of us are wearing down.  Most of the sand and dirt ends up in the rivers, put the wind blows some of the mountains away.  If you park your car outside, you already know this.  The dust covers everything on the car, and you have to wash the damn thing.  Mostly the dust is that tan/brown dirt color, but sometimes it is red when parts of Utah decide to take a visit.

The constant deposition of wind-blown dirt is why ancient cities get buried.  The process is slow, but relentless.  Denver is being buried, but we haul a lot of it away.  Our older house has what seems to be sunken sidewalks.  It is the wind- blown dust combined with organic matter (grass clippings) to form topsoil.

Carol likes to say that a lot of the stuff that accumulates is spider legs.  True, along with the exoskeletons of millions of insects.  In geologic terms the rate of deposition of all that stuff is fairly rapid.

Here on Colorado’s Front Range we get wind, but nothing like the mountain winds, unless you live in Boulder or other places at the foot of the mountains.  The mountains are high enough to get some of the higher altitude winds that the flatlands don’t get much of.  Also, when a weather system blows in, the mountains act as a barrier, forcing all that moving air up.  As it rises, it cools, gets more dense, and descends on the lee side of the mountains. As it falls, it gains velocity and tends to warm up, creating our famous chinook winds.

Loess Soil Windblown Dirt

Loess Soil
Windblown Dirt

As the wind moves on the plains, it’s velocity decreases and some of the dirt it carries falls out.  Close to the mountains, a lot of it is sand.  That explains the sand hills we have close to the mountains.  You can identify the sand because it does not support plant growth as well as soil.  Sandy areas are cow country, no farming.  A little farther east, the dirt falls down.  A lot of dirt falls down, forming loess, a German term for wind-blown dirt deposits.  Eastern Colorado has thousands of square miles of loess.  Without irrigation, it is usually planted in wheat.

Of course, all the water or wind-borne sediment is headed for Mississippi, Lousiana, or the gulf.  Most of that Mississippi mud will end up as shale.  At some point plate tectonics will shove it up as dry land and the cycle starts over.  Most of the sand will eventually end up in the streams, get buried, and form sandstone like the Dakota sandstone dinosaur fossils are found in.  In other places, tremendous deposits of wind blown sand accumulate, eventually forming the sandstone that blankets the Colorado Plateau.

That dirt accumulating in your lawn and garden is part of a recycling process going on all over our planet for millions of years.  The process will continue long after we are all gone, as long as there is air and water on the planet.  To me, the whole thing is a miracle, all these geologic processes creating conditions existing long enough for the evolutionary mistake known as humanity to develop.

Posted in Stories, Weather, Wind