Category Archives: Colorado

Arkose, Sand, and Loess

Geologists like to give common things fancy names.  Aeolian Deposition means wind-blown stuff from the mountains on the land.  Here in Eastern Colorado the South Platte and the Arkansas Rivers are in the process of hauling everything to the Mississippi Delta.  This is kind of a slow process, but a lot of stuff is already there.  Lots more is on the way.   

The material coming off the mountains is in three general categories: gravel, sand and dirt.  The stuff gets deposited, may get buried long enough to form rock (Castle Rock), but most of it sits around for a while, maybe millions of years, before it is washed into the streams and heads east. 

Castle Rock

Castle Rock

Along the base of the mountains are two kinds of deposits.  As the Rockies were rising, they eroded almost as fast.  Out mountains are mostly the roots of what was once there.  During wet periods, like when glaciers were melting, the chunks coming down were pretty angular.  The geologists call the deposits arkosic.  The Castle Rock Conglomerate is a good example.  At other times the erosion was so rapid that a mixture of angular rock and rounded river gravel were deposited together.  Rocky Flats between Golden and Boulder is an example.   

There are huge gravel deposits at the mouths of the many canyons emptying onto the flatter land where all the people live today.  Boulder didn’t get its name by accident.  I briefly worked for the telephone company in Boulder.  I was on a crew burying telephone cable in new subdivisions.  I ran a backhoe and a cable plow, a small bulldozer with a ripping tooth in back digging down about thirty inches and paying telephone cable out the back of the tooth (or plow).  In some places, we had to bring in a big D8 Caterpillar dozer with the power to rip through all the hard packed gravel so my little John Deere 450 dozer could do its job.  It’s sort of a Mini Cooper versus a Hummer. 

All those lakes you see near all the streams exiting the mountains are old gravel pits converted into water storage reservoirs.  You can find gravel in the South Platte River bottom in Nebraska that came out of the Rockies.  As the gravel is carried along, it erodes from angular pieces to progressively more rounded rocks, eventually becoming sand, clay, or just plain dirt. 

The flat country at the base of the Rockies is a patchwork of older rocks exposed by erosion, gravels and arkose near the mountains, then lots of sand, then dirt farther out.  A geologic map shows the patchwork.  Nature is relentless in its processes, but they are not uniform. 

Denver's Sand Creek

Denver’s Sand Creek

Eastern Colorado has several Sand Creeks, carrying the sand that blew out onto the flats to the South Platte or the Arkansas.  You can identify the sand deposits in Eastern Colorado because they are cow country, not suitable for farming.  My favorite Sand Creek runs from northern Aurora through some of the old Stapleton Airport property and on west to the Platte.  The Bluff Lake Nature Center can give you a good look at the sand and the loess.  Bluff lake itself is down along Sand Creek where you can play in the sand.  The trail leading down to the creek and lake drops down the bluff from the parking lot.  The bluff is loess.   

Bluff at Bluff Lake Nature Center. Loess

Bluff at Bluff Lake Nature Center. Loess

Under most of the eastern plains is the Pierre Shale or the Ogalla Formation.  The shale can be farmed, and the Ogalla holds all that rapidly diminishing irrigation water.  The surface is mostly stuff the wind blew in.  The dirt the wind carries is called loess, a German word.  The soil is fine, and in some places can be hundreds of feet thick.  That dust on your car after it sits out in Denver?  It will either be Mississippi mud or loess.  Well, even the loess will be mud someday, it is just being delayed for a while.

Weather

Climate Change

Climate Change

2013 Flood

There is currently a lot of controversy about climate change and whether humankind has a role in the warming trend.  While I think it is true that pouring huge amounts of sequestered carbon is the culprit, I don’t think it matters much for us here in Colorado and much of the west. 

We live in a land of extremes except for the rainy Pacific Northwest, but, they have their earthquakes and volcanos.  Here in Colorado, we dwell in a land of extremes.  The west is dry, it snows in the mountains, the Front Range is kind of a mix, and it is pretty dry in the east.  That varies from year to year.  It varies a lot.   

In the late nineteenth century it was a wet cycle in the eastern prairie, and the railroads made millions enticing settlers to buy their land and get rich farming.  The population in eastern Colorado peaked then and has been declining ever since.  The mountain ski areas have lots of snow some years and almost no snow other years.  The western desert country looks dry and desolate most of the time, but I have seen it bloom in a stunning variety of color.   

Then there are the floods, blizzards, and tornados, often followed by drought.  The one thing we can count on is change.  There are long term trends.  Most archeologists think one reason the ancestral Pueblo Indians left southwestern Colorado was a prolonged drought cycle.  Anyone who tries to raise dry land beans in that country can tell you not much has changed. 

2013 Flood

2013 Flood

Here along the base of the mountains we have the extremes as well. There was the drought of 2002, and the floods of 2013.  The mountains create an unusual weather pattern that stalls along the mountain front, bringing more moisture than the land can handle.  That is when lots of the mountains wash out into the flat country.  It has been going on for more than sixty million years.  The gravel in the Platte River in Nebraska is Rocky Mountain gravel.  Some of the Louisiana mud is Long’s peak mud.   

Some climate models say climate change is going to dry Colorado out, other models say it will be wetter.  My money is on more extreme weather.  Longer, more violent wet periods and long droughts.  Look for more frequent floods, not the thirty or forty year cycle we have had since the first European-American settlers and miners arrived.  Think about the tornados and hailstorms recently.   

I like the extremes.  We have our regular four seasons here but the winters are milder than in Iowa.  It can get hot but there are few days over one hundred degrees, but not like southeastern Utah.  I think that may change, hotter in the summer.  I don’t think the winters will be colder.  I can remember forty below in Boulder when I was flunking out of CU.  Twenty below seems to be more the cold winter norm now.  What I do not like is the hailstorms.  I don’t think the insurance companies like them much either.  Homeowners insurance costs keep rising.  That hail is hard on the garden as well. We had only one tomato plant survive last year. 

One of the big impacts of climate change will be on water supplies.  The amount of precipitation may not change, but if it is warmer, the snowpacks will not last as long in the spring.  That means more spring floods and a shorter runoff period, which will impact water storage.  That could be bad news for the populated Front Range.  People keep coming, but there will not be more water, and a lot of the big spring runoff will go out of the state.  That will be good for the Sandhill Cranes in Nebraska, but bad for Parker and Highlands ranch. 

I spent a long time in the water business, and it always disturbed me watching all that high quality drinking water being used to attempt to replicate Surrey or Connecticut foliage in the Great American Desert.   All that bluegrass will have to go. The urban forest will have more drought-hardy trees.  Denver Water’s customers have done a good job of conserving since the big drought of 2002, but the bluegrass model of landscaping continues.  In Denver, daily water consumption is about 110 million gallons per day in winter.  I the hot part of summer, it’s over 400 million gallons per day, most of it run out onto the ground. 

At our house, we have significantly reduced the size of our lawn, but we still have a lot of crabgrass.  It should be buffalo grass and blue grama, both native drought-resistant grasses.  They don’t stay green all summer, so we are stalling and paying the water bill.  Marijuana legalization is bringing lots of people to Colorado, and the economy is booming.  Those people use water, and lots of water is used growing the stuff.  One of the unintended consequences of legalizing pot is increased water consumption. 

Myself, I am not too concerned about climate change for myself.  After all I am 73 years old and don’t live on the coast.  Long term change is a reality, but as John Maynard Keynes said, “In the long range we are all dead”.