FOLDS: ANTICLINES, SYNCLINES, MONOCLINES

“Living on a layer cake
Way down under the ground
Living on a layer cake
Take a slice and look around.”

                  (Chris Rawlings)

Growing up in Kansas I was quite used to term “layer cake geology”—the rocks are relatively flat, at least on the surface, and it is fairly easy to predict the ages of outcrops.  After stripping off the glacial debris in the northeastern one-fifth of the state, one finds the youngest rocks in the west and the oldest in the eastern part of the state. As an example, in driving I-70 from the Colorado-Kansas line east to Kansas City the traveler would traverse rocks ranging from the Pleistocene/Holocene sediments and late Cenozoic Ogallala Formation through a nice section of Cretaceous rocks, to fantastic exposures of fossiliferous Permian (the Flint Hills) and Pennsylvanian rocks (Osage Cuestas).  Triassic and Jurassic strata are missing (on the surface) and the oldest rocks cropping out in Kansas are of Mississippian age in the extreme southeastern corner.  One must look in the subsurface to locate older rocks.  The traveler also will not notice any appreciable tilting of the beds---layer cake geology at its best; however, erosion by streams has produced fine exposures.  

 

Generalized geologic map of Kansas showing outcrops of Cenozoic (C), Cretaceous (K), Permian (Pm), Pennsylvanian (P), Mississippian (M), glacial (G).  Map courtesy of Kansas Geological Survey. 

You can imagine my surprise, then, on my first trip to Colorado (in grade school) when I noticed that some rock layers were not flat but were actually “standing up”!  How in the world did that happen?  In the pre-internet days the only possibility for an answer was waiting until school started in the fall and then consulting an ancient set of “encyclopedias”. 

In the meantime, I continued to collect rocks and minerals with the most interesting specimens being those from local sand and gravel quarries.  Little did I realize that “those most interesting” minerals (mostly jasper, chert, and quartz) had a source area near the “standing up rocks”.  At any rate, I was hooked on geology.

TWO roads diverged in a yellow wood,

And sorry I could not travel both

And be one traveler, long I stood

And looked down one as far as I could

To where it bent in the undergrowth;

Then took the other, as just as fair,

And having perhaps the better claim,

Because it was grassy and wanted wear;

Though as for that the passing there

Had worn them really about the same,... Today (August 2015) I thought about this early Colorado trip in relationship to an NPR story about Robert Frost and his most famous poem—The Road Not Taken.  I don’t know about schools today but virtually every kid in my time studied the poem in English class(es).  The poem reminded me of, not two roads diverging, but two layers of rocks diverging--one being flat the other bending.  I know, strange analogy, but that is the way my mind works!  What I did find in the “encyclopedia” was that mountain building, and igneous rock events, pushing up from below, had tilted and bent the overlying sedimentary rocks.  Of course, in the days before our understanding of plate tectonics the “encyclopedias” really did not explain how these mountains formed.  As I grew older, and with an additional understanding of geology, I became fascinated with the bending and folding of rock layers, especially those that formed topographic or geographic features.  Colorado and the Mountain West are fortunate to have a wide variety of folded rocks that are described by Matthews and others (2003) as “metamorphic folds, basement cored folds, salt-cored folds, monoclines, syn-depositional folds, anticlines, synclines, domes, basins, refolded folds, evaporate-flowage folds, collapse folds, disharmonic folds, and forced folds”.  This small article will focus on some of the larger geographic features that readers might locate.     Cartoon sketch of an anticline (with oldest rocks in the center and beds dipping away from the axis) and a syncline with youngest rocks in the center).  Public Domain sketch courtesy of Pearson Scott Foresman.  Anticlines are folds where the limbs dip away from the axis (convex-up) and where the oldest rocks are in the center of the fold.  The cartoon above shows a nice symmetrical fold while in reality most anticlines are asymmetrical, plunging, or even overturned.  In addition, erosion often planes off the top of the structure so that the fold does not form a topographic high and one must examine geologic maps and/or aerial photos to determine size and extent.  A structural high (anticline) does not always produce a topographic high (hill/mountain).  At other times the structure is quite visible as resistant rocks in the limbs form an impressive outline of the fold.  Most anticlines in Colorado are the result of compression associated with the Laramide Orogeny (building of Rocky Mountains).  Imagine piling several carpets, of different colors, on top of each other and then pushing them against a wall.  The carpets would “bulge up” into several “anticlines” due to the compressive forces. Many of the larger mountain ranges in Colorado, such as the Front Range, are actually large anticlines where steeply dipping sedimentary rocks are exposed along the flanks and Precambrian basement rocks crop out in the center of the fold.  The original extent of the sedimentary rocks was “over” the Precambrian rocks; however, with uplift the sedimentary rocks were eroded off the top.  Geologists often term this type of large fold as a “basement-cored anticline”.  The Black Hills of South Dakota represent a miniature version of a Laramide basement-cored anticline and one that is easy to see and understand.  Here in South Dakota vertical movement has produced a topographic high corresponding with a structural high.  In the Black Hills, at the center of the anticline (sometime referred to as a dome), are rocks of Precambrian age.  As one moves out from the center the sedimentary rocks of Paleozoic and Mesozoic age grow progressively younger.  What makes the Black Hills nice to study is the lack of large faults that often complicate the geological understanding of Colorado’s mountain ranges.   Sketch of the Black Hills showing the Precambrian-cored center of the anticline (Strahler and Strahler 1978).  Smaller anticlines (non-basement-cored), also due to compression and folding, are often found off the flanks of the mountain folds.  One of my favorite vest pocket anticline is Split Mountain at Dinosaur National Monument along the CO-UT state line, a fold associated with the much larger Uinta Mountain Range. At Split Mountain the Green River cuts a fantastic canyon right through the heart of the anticline and a raft ride takes the paddler through both limbs and the core of the fold.  Aerial photograph of Split Mountain Anticline at Dinosaur National Monument.  Resistant beds nicely outline the nose of the fold; all beds dip away from the core.  Photo from mapcard.com.   Dipping rocks (Weber Formation) on the flank of Split Mountain Anticline with Green River in foreground. Some subsurface anticlines are commercially important as they serve as traps for petroleum (oil and gas).  The Raven Park Anticline (aka Rangely Anticline), the major petroleum trap at the giant Rangely Field in northwestern Colorado, was discovered in 1901 and has produced between 800-900 million barrels of oil, mostly from the Weber Sandstone at subsurface depths of ~6400 feet.  Secondary Recovery (water injection) started in 1957 while Tertiary Recovery (injection of  CO2) commenced in 1986 and the anticline is still producing. It is interesting to note that the Weber is subsurface at Rangely but is well exposed and surficial at Dinosaur National Monument, a mere ~30 miles away.     Gently dipping limbs of the Cub Creek or Jensen Syncline (Dinosaur National Monument).  Foreground rocks are the Jurassic Morrison Formation.  Red rocks in distance (showing the fold) are Triassic and Jurassic in age. Synclines are the opposite of anticlines with the limbs dipping toward the center of the axis, convex-down, and where the youngest rocks are in the center of the fold.  Most synclines associated with the Colorado mountains are small in stature and rarely form topographic features.  The Cub Creek Syncline at Dinosaur National Monument is an easy fold to observe. Large scale basins are the opposite of the basement-cored anticlines and include Laramide features such as the Denver-Julesburg Basin east of the Front Range (70,000sq. mi.) and the Piceance Basin in northwest Colorado.  Both of these are structural basins (large synclines) and do not form topographic lows and therefore do not show up as “landscape features”.  There is a formation elevation change (in the subsurface) of about 9000 feet from the edge of the Denver-Julesburg Basin to the center.   Blue Mountain at Dinosaur National Monument.  Notice the horizontal beds on top of the mountain which then bend and dip steeply down the flank.  The massive sandstone is the Weber Formation which reappears in the subsurface (-6400 feet) at the Rangely Oil Field approximately 30 miles to the southeast. The last type of the major folding structures is the monocline, often described as “half an anticline”—there is only one dipping limb coming off flat or horizontal layers.  The Colorado Plateau has some of the most impressive monoclines in the world with the “bend” commonly associated with subsurface faulting in the underlying Precambrian rocks.  Monoclines at Colorado National Monument and Dinosaur National Monument are especially impressive.    The great monocline at Colorado National Monument.  The lower gray rocks are Precambrian in age, the slope forming unit is the Chinle Formation (Triassic) while the upper massive sandstone is the Wingate Formation (Jurassic). This small article cannot begin to cover all of the standing, contorted, disturbed, folded, and bent rocks in Colorado.  Virtually any of the rocks in the western one-half of the state are folded and faulted and a drive on most of the roads leading west will reveal some of the magic.  Purchase a geologic map from the State Survey and take a road trip! As for my circuitous route from Kansas to Colorado—I came here from the city, a thousand miles away; Now I sing a mountain song of the night wind in the pines; I've seen the quiet splendor of a field of columbine (The Mountain Song; John Denver). REFERENCES CITED Matthews,V., K. KellerLynn, and B. Fox.  2003. Messages in Stone. Denver: Colorado Geological Survey. Strahler, A. N., and A. H. Strahler. 1978. Modern Physical Geography. New York: John Wiley & Sons.