My next post about the Geology of the National Parks Through Pictures is from our move across the country from Utah to New York. Along the way we visited 13 National Parks as well as some other sites. This was the 10th National Park along the way.
You can find more Geology of the National Parks Through Pictures as well as my Geological State Symbols Across America series at my website Dinojim.com.
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Obligatory entrance sign photo
Minuteman Missile National Historic Site protects two locations (plus the Visitor's Center), as representatives of the once sprawling Minuteman Missile field that covered the far western portion of South Dakota. In total, there were 15 Launch Control Facilities and 150 Missile Silos the held the Minuteman Intercontinental Ballistic Missiles. The two locations preserved are one of the Launch Control Facilities, Delta-01, and the corresponding Missile Sile (Launch Facility), Delta-09. Delta-01 was only accessible via a tour, and since we were pressed for time, we only visited Delta-09 and the Visitor's Center.
The Minuteman Missile Silo Delta-09 sits within the American Great Plains, a expansive region with little topography. However, the site here does sit very close to the Badlands National Park just to the south. Per the National Park's page:
Landforms in the Great Plains are unglaciated and retain soils formed by shallow seas that covered the region approximately seventy-five million years ago.... The seas that once covered South Dakota's Western Plains deposited limestone and sandstone overlain by soft Pierre shale.
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| Looking at the missile silo cap at Delta-09 |
The geological description of the park is a little simplistic, however there is not much that can be discerned from other online sources. They even state in their combined Historic Structures Report, Cultural Landscape Report, and Environmental Assessment report, which often contains geological information on many of the parks within the NPS, that since any proposed work would not disturb the bedrock that "further analysis of geology will be dismissed". So while they have no intention of getting into the geology of the site, if we look back into the original structure, the missile silo was obviously dug into the geological layers of the region.
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| Looking down at the now defunct missile, within Missile Silo Delta-09 |
The missile silo itself is 12 feet in diameter and 80 feet deep. You can take a look down it, as seen above, when visiting the site. The Visitor's Center also has the nice diagram below, however, it also is lacking in geological context.
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| Diagram of the Delta-09 Missile Silo |
Looking at a geological map of the region does elucidate some of the geology. The map below shows a snippet of the regional geology map with a close up view of the geology of the Delta-09 site, marked by the "X". Delta-09 sits within the Qe deposit. This is wind blown sand (eolian), that dates back to the Eocene (~34 million years old). This sand is far younger than the denoted Pierre Shale at 75 million years old listed on the website. The sand here is predominantly wind blown sediment that is reworked sands from the older rocks below.
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| Snippet of the Delta-09 Geology Map. Full map courtesy of the National Geologic Map Database. |
On the map to the south of the Delta-09 site, you can see a different, darker shade of orange, Qoa. Qoa, the Older Alluvium, represents a different form of Eocene deposit that also formed from the reworking of even older rocks, soils, and sediments. Both of these "Q" (Quaternary), layers sit upon the thicker bedrock of the region. However, through the Qoa deposit, we can catch a glimpse of some of the true bedrock of the region. The description of the geological layers to the right of the map lists them in reverse chronological order. This means that the oldest layers are located at the bottom of the legend, with progressively younger layers stacked on top. The Pierre Shale is indeed the oldest, and thickest, bedrock layer within this region and although it only peaks out a little bit in the area of Delta-09, it is definitely a major rock unit below the surface. The Pierre Shale is Cretaceous in Age (~75 to 69 million years old) and formed when this area, and much of central North America, was covered in a vast inland sea known as the Western Interior Seaway. Within the Pierre Shale, several fossils can be found in nearby Badlands National Park, such as ammonites and mosasaurs. These lived at the same time as the dinosaurs, but since this area was covered in water during the Age of the Dinosaurs, dinosaurs were not to be found.
The next rock unit above the Pierre Shale within this region, and likely can be found along the walls of the missile silo, is the Chadron Formation. The Chadron Formation is Eocene in age (~37 to 34.3 million years old) and was formed along an ancient river floodplain. The Chadron Formation is made up of light gray claystone beds and represent an environment similar to the Everglades today. Fossils that can be found within Badlands NP within the Chadron are alligators, early horses, and the large brontothere (AKA titanothere) mammals. Within this area though, the Chadron Formation is likely a thin layer, especially compared to the outcropping within Badlands NP.
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