South Valley Park and the Future

            South Valley Park open space is a unique geographic region.  The hogback ridge is an anticline, the valley a syncline, and the foothills rising to the west a monocline.  The hogback was formed hundreds of millions years ago, during the Jurassic and Cretaceous periods, due to tectonic forces pushing up the sandstone rocks of the Dakota Group.  The final upthrusting was the Laramide Orogeny, beginning 70 million years ago, and probably caused by the North American plate slamming into the northeast-moving Pacific Plate.  The Laramide uplifts rose up vertically as narrow, north-south upfolds (anticlines), elevating the formerly flat layers of sedimentary rock in a series of wave-like ripples.  Most of this sedimentary rock has eroded, their broken ends forming the hogback escarpment.  These sandstone features are porous and permeable; the clastic sediments were formed in a subtropical climate, when a shallow ocean covered the land.  Today, the environment of South Valley Park is deeply shaped by the drought conditions that affect the entire state of Colorado. 

1,000 years from now, South Valley Park will look very much like Utah: red, treeless, nearly grassless.  Global temperatures are expected to rise dramatically in the next hundred years.  The melting polar ice caps and methane escaping from the tundra permafrost will create a positive feedback cycle that will make much of the world, including South Valley Park, uninhabitable by mammals.  Geologically speaking, South Valley park will look much the same.  Rock decay and chemical weathering will continue to wear down the intrusive sandstone boulders, but not so much that changes are visible to the naked eye.  What will change is the vegetation; greasewood will replace the mountain mahogany and sagebrush.  The water table will continue to drop, and even seasonal spring streams and marsh regions will disappear. 

10,000 years from now, South Valley Park will again be carpeted with short-grass prairie, rich summer marshes, seasonal river floods, and the few small mammals that survived the Anthropocene Mass Extinction Event.  There will be no human presence affecting the hogback, because humans will be extinct, the last few families dying thousands of years earlier while attempting to farm in Siberia and Alaska.  Global warming, by now, has burned itself out, and an intensifying Ice Age has swung global temperatures back to what they were during the Last Glacial Maximum.  The region will still be characterized by aridity, but instead of the dramatic seasonal temperature differences of the Holocene, the climate is more like it was during the Pleistocene; the aggregate temperature, that is, will be several degrees colder than today, but there will be less seasonal variation.  Also, landscapes will not be striped as it today, but a mosaic of interlinked habitats.  This means that wild horses and elk will exist side by side with musk oxen.    

1,000,000 years from now, South Valley Park will not exist in its present form at all.  Due to renewed tectonic uplifting and a long balmy interglacial, the current valley will be an enormous, Everest-sized mountain encircled by a shallow semi-tropical lake.  A new type of animal -- neither lizard nor mammal -- will frolic in the marshes and the forests mountain slopes.  As in the ancient past, the lowland areas will be covered with confers, eucalyptus, magnolia, and fig trees.  Oak, walnut, and ash will flourish in the upper montane.     

Climate Talk

Located squarely in the mid-continent, hundreds upon hundreds of miles away from wet maritime air masses, South Valley Park’s landlocked position very much determines its climate and the varieties of its weather, notably its large diurnal and seasonal differences in temperature, its persistent aridity, its sunshine and lack of clouds.

 Three maritime sources and two continental sources influence our mid-continental climate: the maritime polar (the wet and cool Pacific Northwest); maritime tropical (the warm Pacific from the Baja area of Southern California); maritime tropical (the Gulf of Mexico); the continental tropical (Mexico); and the polar continental (north-central Canada).

                The driving force behind the movement is air is a term called atmospheric convection.  When air is heated, it increases in volume and then rises because of its greater buoyancy.  These vertical air currents deplete the surface of some its air, thus creating regions of slightly lower pressure at the bottom of the rising air column.  As air is forced to move from areas of higher pressure toward the newly created low-pressure area, a horizontal pressure gradient develops.  These air movements vary in size and in their significance for the climate, ranging from local winds, such as dust devils and warm southerly chinook winds, to large-scale air currents in the upper levels of the atmosphere.

 The largest-scale wind patterns are sometimes called “the winds aloft."  These are initiated by pressure systems set up by latitudinal variations in insolation (incoming solar radiation).  In the Northern Hemisphere, the winds aloft are deflected to the right by the Earth’s rotation, spiraling counterclockwise around the lows and clockwise around the highs.  Centers of low pressure are called cyclones; centers of high pressure are called anticyclones.  As a result of the Coriolis effect, large-scale air motions move perpendicular to the pressure of gradient, that is, around regions of low and high pressure, but not directly to or from them. 

The sharp north-to-south temperature gradient that exists in the Northern Hemisphere during the winter months causes the upper-air westerlies to intensify seasonally and with increasing altitude.  Consequently, winds in the southern Rockies are nearly twice as strong in the winter as in summer.  During the summer, when westerly airflow is weak, the main air current and its associated polar front jet stream stray far to the north, typically over Canada.  By winter, the westerlies and the main axis of the polar front jet tend to be positioned over the Southern Rockies, guiding Pacific storm systems through the region fairly regularly. However, during those winters when a large ridge in the upper atmosphere keeps the polar front jet along or to the north of the Canadian border, the southern Rockies experience a lack of snow, severe drought, and unseasonably warm weather.

Forms of Weathering on a Single Sandstone Outcrop in South Valley Park

The little sandstone eco-community in the photo above is an example of two different kinds of weathering.  The moss creates a form of chemical weathering, as the acid in the moss dissolves minerals in the sandstone.  The roots of the miniature tree, meanwhile, are a form of physical or organic weathering, its roots reaching down through slender cracks in the rock and expanding them. 

In the picture (above) we see moss, plant, and water weathering, and also erosion in the seasonal transport stream running down a narrow seasonal rill in the sandstone outcrop. The water penetrates through cracks in rocks, further contributing to the slow disintegration of the rock.

The photo above is a good example of mechanical weathering.  It is hard to discern from the picture, but the roots of this tree extend down into a joint in the outcropping.  Also, the action of frost wedging has eroded the sharp edges and corners of the outcrop, making the sandstone appear spheroidal. 

In the photo above we see a form of mechanical weathering called frost action or frost shattering.  The melted snow seeps into cracks and joints in the rock.  Then when the water freezes its mass expands, jarring the cracks a little wider.  Thus over time sections of rock split off into smaller rocks or gravel.  

The whitish portion of the sandstone is due to a form of mechanical rock decay called salt weathering.  The salt can come from many sources, including pollution and rising ground water.  Salt weathering tends to occur in arid climates.  
The decay is caused by the salt crystals precipitating in pore spaces in the rock, thus exerting pressure on the rock and causing mineral breakdown.   

This patch of green lichen is a form of both mechanical weathering and chemical weathering.  The former is caused by microscopic rootlets pushing down through grains in the rock; once there they swell and shrink, enlarging the gaps.  The latter is caused by the humic acids in the lichen breaking down the rock into the beginnings of soil.    

The View East from Crescent Rockshelter

                 Leave your car or bike in the south parking lot of SV Open Space, walk the trail uphill a hundred yards of so, and stop at the crest and look to the East.  The giant red sandstone rockshelter or overhang staring back at you, called Crescent Rockshelter by the archeologists who sifted dirt here a few years ago, belongs to what geologists call the Fountain Formation, a sedimentary rock composed of sand and gravel that crumbled off the Ancestral Front Range as it began to rise between 315 and 300 million years ago. 
           The existence of this Ancestral Front Range is something of a mystery.  315 million years ago, when tectonic activity pushed Pangea together, the continental collision pushed up a chain of mountains on the eastern seaboard of modern America, from Appalachia to the Ozarks; but from there, inexplicably, the chain bent northward, running north-west through New Mexico and Colorado.  This Ancestral Front Range stood in the same place the modern Front Range stands, except that Colorado was an island in a great inland shallow tropical sea, something we know because in the Colorado Springs area scientists found Fountain Formation gravel and sand interbedded with mudstone and limestone containing marine fossils.

              Crescent Rockshelter was uplifted from the Golden Fault.  It contains loose sand cobbles of quartz and granite that were combined in riverbeds to form the distinctive sandstone of the Fountain Formation.  The red color comes from a high concentration of iron within the rock.  Look beyond Crescent and you will see a grayish, pink-veined Hogback Ridge.  This is the second-to-last ridge crest before the Great Plains sweep toward the East.  It is 280 million years old and was deposited on top of the Fountain Formation about 20 million years after the uplifting of the Ancestral Front Range.  The Lyons outcrop consists mainly of sandstone, and the pebbles are much smaller than in the Fountain Formation, which indicates that in those intervening 20 million years a period of acute aridity had slowed and narrowed the huge, tumbling rivers draining the Ancestral Front Range.  By then the mountains had probably been eroded to knobby stumps or crumpled foothills, greatly mellowing the rivers’ gradients.  Originally, the dense sandstone layers of the Lyons Formations were sand dunes.  The slow-motion process of lithification locked the wind-stirred crossbeds of sand into stone; since this is seen from Montana to Arizona, geographers believe the desert was once the size of the Sahara.  The Ancestral Front Range, however, continued to collect enough moisture to produce a few straggly creeks meandering through the dunes, depositing the tiny pebbles that can be seen today.  

References

Lon Abbott and Terri Cook, Geology Underfoot: Along Colorado’s Front Range (Mountain Press Publishing Company: Missoula, MT, 2012).

Ann M. Johnson, Archaeological Investigations at the Ken-Caryl Ranch, Colorado (Memoir Number 6 of the Colorado Archeological Society: Denver, 1997.)

Link
This takes you to the homepage of an archeological dig at Crescent Rockshelter.  The archeologists, of course, don't want us to poke around their sites, but it's pretty clear where this outcrop is located.  This is a great little website for anyone curious about the people who wintered in the Front Range for thousands and thousands of years before we pillaged and ruined everything.  To get a good look at Crescent Rockshelter (and the tawny reds and salmon pinks of the Fountain Formation), click on Contexts.http://archaeology.csumb.edu/SlideShows/Crescent/RockShelter.swf

Finally, photos!

Crescent Rockshelter, Fountain Formation

Lyons Formation, north-south running ridge

 Crescent with Mom for scale

 Crescent looking North

 Lyons up Close; micro pebbles

 Fountain Formation up close: big pebbles or cobbles

 Fountain Formation up close: chunk of quartz

 Fountain Formation outcrop, Lyons Formation ridge looming behind

 Crescent, with Mom for scale

 Crescent, presumably sleeping area for families thousands of years ago

Intro to Geo 1202 Blog

Hello!

My name is Andrew Saltarelli, and after thirteen years out of school, I'm back to finish my degree.  I have chosen South Valley Park Open Space as the location for my blog.  Back in the Bitterroot Mountains of western Montana, where I lived before moving to Denver, I became very interested in the hunter-gatherers who hiked the mountains long before me, and far more wisely, sensitively, and adventurously.  At any rate, this interest carried over to Denver, and I spend as much time as I can in the summer wandering round the Indian Peaks looking for ancient rock walls and cairns.  Essentially, the high country of the Front Range has some of the best archeology in the world.  The game drive rock walls -- one of which you can actually see from Google Earth -- were built for the great big horn sheep hunts during the rutting season in early fall.  At any rate, some of the people who summered in the Indian Peaks -- and Middle Park and North Park -- wintered in the hogback valleys at the extreme western edge of the Great Plains.  To my mind the best place for a winter encampment would not have been Red Rocks but a little farther south, in SV Open Space.  There are some amazing south facing rockshelters and overhangs with great views of the valleys and the foothills.  This blog, I realize, is about physical geography, not ancient humans, but I'm pretty sure those ancient humans were far more in touch with that geography than anyone today.  So hopefully as class progresses I'll be able to better understand all the complex processes that make this area such a unique and variegated ecotone, capable of fostering so much life (I do not include that d--d Lockheed building!) even as the country to the immediate west and east -- the mountains, the Great Plains -- is so often harsh and inhospitable.