The Physical Geology of the Black Hills of South Dakota.

The Physical Geology of the Black Hills of South Dakota.

Bird’s-eye view of the Black Hills. Jenny-Newton survey, 1875 (Gries 1996 pg 203)

The Black Hills of South Dakota have a very fascinating and very old geological history. To begin with, the name “Black Hills” is actually based on trees. At a far away distance, the hills look black because of all the pine and spruce trees. The area has been described as an “island of trees in a sea of grass”, (Wikipdeia). South Dakota is basically flat prairie land with a few rolling hills near the Missouri River that winds down through the center of the state. The Black Hills covers an area approximately 100 miles long and 50 miles wide, (Schwartz). The average elevation of the Black Hills is 3000 feet above sea level, (Wikipedia). Harney Peak, at 7242 feet above sea level, is not only the tallest peak in the Black Hills but also the tallest peak east of the Rocky Mountains, (Peakbagger). Also the Black Hills boost about having some of the oldest rocks in the western United States dating back to 1.6 and 2.5 billion years with the beginning of Precambrian sediment and granite. The area contains 22 different formations presented in all geological time periods except the Silurian and Devonian, (Schwartz). Due to the sedimentary rocks and many time periods, there are many wonderful fossil finds still being discovered today. It is reported that the Black Hills region contains 182 minerals, which makes this area of economic interest.

Geology map of the Black Hills (1965-1969). Note the oval-circular shape of the various outcrops which is characteristic of strata exposed in a dome structure. (Map courtesy of South Dakota Geological Survey). http://www.northern.edu/natsource/EARTH/Geolog1.htm


Approximately 1.6 to 2.5 billion years ago, during the Precambrian Eon, the Black Hills area was at times a swallow water sea or a sea shoreline. Due to lack of vegetation, erosion rapidly wore down the sand and clay, depositing large amounts of sediments. Over time, the pressure from it’s own weight turned the sediments into sandstone and shale, (NPS). Following this was the Trans-Hudsonian Orogeny. This was caused by the collision of the North American continent and a terrane (tract of land or continental crust). The original sedimentary rocks were folded and twisted over millions of years into a mountain range. There is evidence of this today where metamorphic rock ends in an angular unconformity below younger sedimentary layers, exposed by erosion, (Wikipedia), (see Geological Map above).

During the Paleozoic Era, approximately 543 to 248 million years ago, begins with the Deadwood formation being laid down and ending with the Spearfish formation. The seas were advancing and retreating during this period and adding new layers of sandstone, shale and limestone. There were also periods of erosion and weathering; evidence of this can be found in sinkholes and caverns that intersperse the area. Trilobite and other marine fossils found in this area are also evidence of the seas. Around this period, magma deep within the earth began to rise. It pushed it’s way through existing cracks and areas of weakness creating dikes of metaphoric rocks that are visible today at the Cathedral Spires (Gries) (photo below and Geological Map above).

Cathedral Spires created from weathering processes acting on granite. (Location is along Needles Highway in the Custer State Park).
http://plantsci.sdstate.edu/woodardh/Geology/Black_Hills/General_Geology/cathedral_spires.htm


By the end of the Mesozoic and the beginning of the Cenozoic Era, the Black Hills region rose to form an asymmetrical or oval dome (see photograph on cover). It is estimated that this rise could of reach the height of 1500 feet above sea level, (Iles). This uplift was related to the uplift that created the Rocky Mountains. Over the years, erosion has exposed the “crystalline core” which consists of the metamorphic rocks mostly slate, schist and quartzite and intrusive rocks of granite, rhyolite, phonolite and pegmatite, that are visible today. Mount Rushmore is carved out of this core, a large block of fine-grained granite, (NPS) (photo below and Geological Map).


Mt. Rushmore National Memorial was carved out of the granite, which is part of the central crystalline rock core of the Black Hills.
http://plantsci.sdstate.edu/woodardh/Geology/Black_Hills/General_Geology/mount_rushmore.htm


The Black Hills of South Dakota probably contains the largest wealth of mineralogy in the United States. The magma intrusions that occurred during the Paleozoic Era were rich in a watery solution. They melted some of the country rock and with varied cooling rates, formed 182 different minerals found in the Black Hills and surrounding area. The most common one is gold. Black Hills gold, (which was mined from the largest United States gold mine, the Homestake Mine in Lead, SD), is world famous. The gold was found in Precambrian rocks that had mineralized nearly two billion years ago. Silver was also found and sometimes mixed with the gold (known as dore'). Silver deposits were found more times in Tertiary deposits than Precambrian deposits, (Gries). A break down of the minerals includes:
7 native elements
22 sulphides, tellurides, and arsenides
8 sulphosalts
7 haloids
26 oxides
13 carbonates
65 silicates
2 niobates and tantalates
24 phosphates and arsenates
22 sulphates

“The Harney Peak granite is apparently only the top of a large batholith and much of the main mass is pegmatite. Extending outward into the schists in all directions are dikes and irregular masses of pegmatite. Some of these are several miles horizontally from the main granite mass. The most interesting and economically important are these masses in the schists”, (Schwartz). The largest crystal recorded came from the Harney Peak group. It was a forty-two feet long, three to six feet in diameter and contained thirty seven tons of spodumene. There have been other crystals almost this size also found. This area (Etta mine near Keystone, SD) has contained large masses of nearly pure microcline, amblygonite, mica, crystals of black tourmaline. There also has been some rare minerals found like apatite and tourmaline, (Schwartz). The South Dakota state gem is the Fairburn Agate (see photo below). Typically it consist of white quartz, iron oxide (the red color) and manganese oxide (the black color). Agates are formed under sedimentary conditions that were prevalent in this area. Fairburn, SD is located on the very eastern side of the Black Hills (Clark).

http://www.sdgs.usd.edu/

The Black Hills are rich in fossil finds. Known as the “Island in the Plains” the Black Hills were rugged mountains surround by vast expanses of grassland. The mammals that had lived here were isolated on the “island” after the last glaciations (approximately 125,000 years ago), (Wagner). Some of the best preservations are the sinkholes found in the area. “Dissolution of gypsum and anhydrite in four stratigraphic units in the Black Hills, South Dakota, and Wyoming has resulted in development of sinkholes,” (Epstein). A famous one is the “Mammoth Site” in Hot Springs, SD. About 26,000 years ago a cavern in the Minnelusa limestone collapsed causing a breccia pipe (a vertical shaft) to form. The breccia pipe allowed the underlying artesian spring to fill the hole, approximately 120 feet by 150 feet and 65 feet deep. This type of sinkhole is called a karst. Mammoths would go to the water hole to drink and swim and get trapped due to the steep shale sides, and die. As time went on, the sinkhole was eventually filled with sediment and the artesian spring was diverted to another spot. The mammoths were well preserved in the mud and sediment, (The Mammoth Site), (see photo below). The “Parker’s Pit” and “Gooseberry Pit” are also sinkholes that have provided thousands of fossils.

http://www.mammothsite.com/MammothSiteGeology.html

Fossils dating back to the Cambrian period have been found. Some examples are Trilobites, Brachiopods, Conodonts and Marclurites. Through the ages, this area has inhabited mollusks, echinoderms, arthropods, amphibians, fish, birds, mammals, and dinosaurs. The most famous dinosaur of all, a Tyrannosaurus rex – “Sue” was found just east of the Black Hills in an area named Hell Creek by Susan Hendrickson. This was the world’s largest and most complete fossil of a Tyrannosaurus rex. It was located in the Hell Creek sediment, approximately 65 million years old. Currently, “Sue” is on exhibit at Chicago’s Field Museum, (Larson).

Besides the wealth of minerals and fossils, The Black Hills is also abundant in water. The artesian aquifers in the upper part of the Minnelusa formation supply most of western South Dakota. Broad outcrop areas at higher elevations in the Black Hills allow intake or recharge for the aquifers. The water then sinks into caverns and is returned to the surface through large springs, (Gries). As mentioned before, the Homestake Mine use to be the largest gold mine in the United States, recovering over $4,000,000 annually since the start of the gold rush, 1875 – 1878. Also other precious stones are mined for use in jewelry. Limestone and gypsum are mined for cement. Feldspars and micas are mined for the manufacturing of glass and for electrical and ceramic industries, (Jarret). Today the biggest industry in the Black Hills is tourism. People come from all parts of the world to see Mount Rushmore and the Crazy Horse monument. There are countless caves and mines to tour. Some examples are The Black Hills Caverns, The Wind Caves, and the Jewel Caves. There are also several fossil viewing areas including The Mammoth Sinkhole, and the South Dakota School of Mines and Technology Museum of Geology, in Rapid City, SD.

The Black Hills are a Geologists and Mineralogist playground. There is a wealth of information to be discovered here. Research and fossil finds in the Black Hills area begin the story in the Precambrian Eon and evolve through today. Another interesting point is human’s history in this area. The Black Hills region is considered scared by the American natives. Their association dates back thousands of years. The entire area is beautiful with the hills covered in evergreen trees, the many streams that wind their way through the hills and the picturesque lakes they pour into. Also there is abundant wildlife living on this “island of trees in a sea of grass”.

References:

Clark, Roger “South Dakota’s Fairburn Agate” 1998

Epstein, Jack “Hydrology, Hazards, and Geomorphic Development of Gypsum Karst in the Northern Black Hills, South Dakota and Wyoming” 2001
http://water.usgs.gov/ogw/karst/kigconference/jbe_hydrologyhazards.htm

Gries, John “Roadside Geology of South Dakota” 1996

Iles, Derric “Geological Survey Program” 02/2007 http://www.sdgs.usd.edu/

Jarret, Martin “The Geology of South Dakota” 1994 http://www.northern.edu/natsource/EARTH/Geolog1.htm

Larson, Peter & Kristin Donnan “Rex Appeal” 2002

NPS (National Park Service) U.S. Department of the Interior “Mount Rushmore
National Memorial” 06/2004 http://www.nps.gov/archive/moru/park_history/geology.htm

Peakbagger http://www.peakbagger.com/list.aspx?lid=1 2004

Schwartz, G.M. “American Mineralogist” Volume 13 pg 56 – 63, 1928
http://www.minsocam.org/msa/collectors_corner/arc/black_hills.htm

S.D. (South Dakota) Department of Environment and Natural Resources. 02/2007
http://www.sdgs.usd.edu/

The Mammoth Site, 03-2007
http://www.mammothsite.com/mammothsitegeology.html

Wagner, Steve “Paleocurrents.com”, 09/2004
http://www.paleocurrents.com/docs/parkers_pit_2004.html

Wikipedia, the free encyclopedia 03/2007
http://en.wikipedia.org/w/index.php?title=Black_Hills